- Email: [email protected]
1 The biological basis of the menopause
1 The biological basis of the menopause SANDRA
J. R I C H A R D S O N
Age-related changes in the pattern of menstrual cyclicity, hormonal secretion and relative fertility occur throughout a woman's reproductive lifespan, culminating in the 'menopause' or last menses. In a society where more couples are delaying childbearing for career or financial reasons, an understanding of the mechanisms underlying the relation between age and the reproductive capacity of older women has gained importance (Hansen, 1986). Factors affecting the age of menopause also have clinical implications because of the association of early menopause with an increased risk of cardiovascular disease and osteoporosis and delayed menopause with breast and endometrial cancer (Whelan et al, 1990). Of particular interest is a recent report of an association between age at menopause and age of death which suggests that age at menopause itself may be a strong biomarker of the general ageing state of the individual (Snowdon et al, 1989; Snowdon, 1990). AGE AND THE PATTERN OF MENSTRUAL CYCLICITY The pattern of menstrual cyclicity changes throughout a woman's reproductive lifespan. This is best documented in Treloar et al's (1967) mammoth prospective longitudinal study begun in 1934. Yearly records of menses and related reproductive and health events were collected from college students at the University of Minnesota. Treloar analysed the records of over 2700 women who had accumulated records for 25 825 woman years. He observed that, contrary to popular belief, lack of perfect regularity was the rule. In all of the records collected, only one woman had completely regular menses for over a year. Figure i presents the combined data; they are divided into three zones of menstrual life in order to standardize the ages of menarche and menopause. Note that the period of irregular menses preceding the menopause (the perimenopause) is almost a mirror image of the early years following menarche. Menses tend to be fairly regular in the middle menstrual years. During this period, the median menstrual interval falls steadily from 28 days at age 20 years to 26 days at age 40 years. This progressive shortening was later shown to be due to a shortening of the follicular rather than the luteal phase of the cycle (Sherman and Korenman, 1975; Lenton et al, 1984a,b). Bailli~re' s Clinical Endocrinology and Metabolism--
Vol. 7, No. 1, January 1993 ISBN 0-7020-1697-7
1
Copyright © 1993, by Bailli~re Tindall All rights of reproduction in any form reserved
2
S . J . RICHARDSON
85 80
;',' I',I"
70
"!!,
~9',~, 9o
9 5 ,,/~h9 9
ill:
• jr
~,',i", '," i
o
~6o ~55
"' ' I~,'"
All menstrual intervals
,,
~
o
/80
,,,'"' ',
'
,,~
90
t/)
=
; ~' t~'~
..o
,',;t
45
• '/~'" 99',
\'%
o
~, 40
. , .
80,
~
,\q 70, ,
13 "~
60, ,,,,,~_ ~
30,
5o, •
25
3o,-z~-"
'
.. . . .
...... ~--4_
-
.
.
.
-~'~-
;
~..~,._ _
.
.
.... 98
i, ," ,"/ .
.
...-
-'~!i ii ._-~. ..........
~~
~
.
~.-~
~
-~" ~ ~
~~ '
",• ~-~_
....
70 I
,,,',/// //
.9,o
°
60 0o
i~"~'/' 7 "
50
.-
4o
~ ~
g
SJ~o,g
"1 . , - - "-~-.'~ . . ~ . ~ .
1'-~-"
10 I
I , l l l , l , l l l , l ~ l l l ~ l , l l l l l l l ~ l 0 2 4 6 20 22 24 26 28 30 32 34 36 38 4 0 - 8
Menstrual year
Chronological age
,I,1,1~1
-6 -4-2 0 Premenopausal year
Figure 1. Contours for the frequency distribution of all menstrual intervals in three zones of experience. From Treloar et al (1967).
The period of greatest regularity occurs just before the onset of perimenopausal irregularity. AGE AND FECUNDABILITY 'Fecundability' is defined as the ability of a currently married woman to have a live-born child (Hansen, 1986). In studies of non-contracepting populations, fertility is at a peak between 20 and 29 years of age and declines steadily thereafter, while the rate of spontaneous abortion rises with maternal age (Nelson and Felicio, 1985; Hansen, 1986). In this regard, however, the effects of ageing in the male partner or a decline in the frequency of sexual intercourse in older couples cannot be ruled out. These potentially confounding factors were eliminated in the CECOS report of the relation between maternal age and pregnancy success in 2193 women undergoing
THE BIOLOGICAL BASIS OF THE MENOPAUSE
3
artificial insemination with sperm from young healthy donors. After 12 attempts, pregnancy, which was defined by rising circulating hCG levels, was 73% in women under 31 years of age, 62% in those 31-35 years, and 54% in those over 35 years after 12 attempts. The 16 women over 40 years of age also had a success rate of 54% after 12 cycles. The spontaneous abortion rate was not reported. AGE AND THE HYPOTHALAMIC-PITUITARY-OVARIAN AXIS FSH and age in women with regular menses
Sherman and Korenman (1975) and Reyes et al (1977) first reported that serum FSH was selectively increased in older women who were still menstruating regularly. These observations have been confirmed more recently by Lee et al (1988) (Figure 2). Daily blood samples for one cycle were obtained from 94 women with regular menses who ranged in age from 24 to 50 years. When the subjects were divided into the age groups 24-35, 36-40, 41-45 and 46-50 years of age, FSH was observed to be elevated in the midfollicular and early postovulatory phase in the 40-45-year-old group. In the 46-50-year-old group serum FSH levels were even higher while serum LH concentration had also become elevated. Whereas Sherman et al (1975, 1976) had previously reported lower mean oestradiol levels in the older women, Reyes et al (1977) and Lee et al (1988) found no significant agerelated changes in either serum oestradiol or progesterone. The dynamics of the progressive increase in FSH with age are more precisely delineated in another report by the same group. Mean follicular gonadotropin levels (days LH-10 to LH-5 inclusive) were calculated for 127 subjects aged 23-49 years (day 0 is the day of the LH surge.) When the subjects were grouped by age in two-year bands, a significant increase in follicular FSH was found in the 40-41 year group with a progressive increase throughout the forties. Serum LH only became significantly elevated in the oldest group (Lenton et al, 1988). Two hypotheses have been suggested to account for the selective increase in FSH seen in older women. Sherman and Korenman (1975) first proposed the existence of an ovarian hormone, 'an inhibin' which would exert negative feedback control over FSH secretion. As a consequence of a diminished number of follicles in older women, FSH would become elevated in the absence of changes in serum oestradiol. Reyes et al suggested the alternative possibility that there may be an age-related decrease in sensitivity to feedback inhibition at the level of the hypothalamus-pituitary, a hypothesis later advocated by Metcalf et al (1981a) and Lenton et al (1988). Follicle number, menstrual pattern and age
The relationship between follicle number, menstrual pattern and age was clarified in a study published by Richardson et al (1987). Previous studies of follicle counts indicated that follicle number is maximal in the 7-month fetus
. 35o o
4 (A)
.L
a
S . J . ~CHARDSON
b
"1" _1 lO
3°1 ::)
15
.lco I.l.
10
- 4''
-4 ' o
. . 4. . . . .8. . . .12 .
~,,
-8-4
, 8 , 12 ,,b . . . . -.8. - 4
4
o''
' 8 ' ' 12 '
4
5
0 . % ' - ; ' o . . ;,. . a. .12'
-; . .-4. . . . .o.
(B)
a
E
;,50-
"
5oo-
i~j
250-
,
from
Days
;
l i . . -.a -., . .6':~ . . .;
~'2
LH s u r g e
b
c
0 _~
i
i
-4
¢-
el
3O 15 0
=
,
0
i
I
4
J
,
8
,
i
12
~
i
,
-8
f
i
,
-4
~
0
i
,
i
4
=
I
8
i
i
12
A A
'-~'-;.';'~'~'17'
'4'-~';'~';';2'
Days
from
LH
i
i
-8
s
i
-4
i
,
0
i
i
4
~
t
8
,
=
12
'-~ '-~ ' ~' ;~'~ '1'2'
surge
Figure 2. (A) Comparison of the daily geometric mean concentrations of LH and FSH (with 68% confidence intervals) in 41 women aged 24-35 years (O O, the control group, profiles repeated in each section) with (a), 19 women aged 36-40 years; (b), 18 women aged 41-45 years and (c), 16 women aged 46-50 years (all shown as © O). (B) Comparison of the daily geometric mean concentrations of oestradiol (E2) and progesterone (P) (with 69% confidence intervals) in 41 women aged 24-35 years (© O, the control group, profiles repeated in each section) with (a), 19 women aged 36-40 years; (b), 18 women aged 41-45 years and (c), 16 women aged 46-50 years (all shown as © O). From Lee et al (1988).
THE BIOLOGICAL BASIS OF THE MENOPAUSE
5
after which there is a steady logarithmic decline. At birth an average of 700000 follicles remain in the two ovaries (Block, 1953; Baker, 1963). In Block's classical study (1952) of follicle counts from autopsy specimens from 43 girls and women aged 7-44 years, the number of primordial follicles declined logarithmically. He also observed a wide range of follicle counts between women of the same age. For example, one 31-year-old subject had 8000 follicles in both ovaries whereas a 37-year-old subject had 208 000. In Gougeon's later study (1984) -of follicle counts in ovaries obtained at hysterectomy in 35 women with regular menses, aged 19-52 years, the slope of the curve relating age to follicle number as well as the range of follicle counts between women of the same age are similar to the data of Block. If one extrapolates from either study, assuming that the depletion curve is a straight line, there would be between 2000 and 5000 follicles remaining in the ovaries at age 50, the mean age of menopause (Nelson and Felicio, 1985). In order to (i) clarify the relationship between follicle number and the transition from regular menses to the irregular cycles of the perimenopause, and (ii) estimate the number of follicles remaining at the time of menopause, Richardson et al (1987) estimated follicle numbers in 17 healthy women, 44-55 years of age. For the purpose of analysis, they were divided into three age-matched groups according to the pattern of their menses in the previous 12 months:
1. regular menses with intervals between 3 and 5 weeks with no hot flushes; 2. perimenopausal; i.e. irregular menses with intervals of less than 3 weeks and/or more than 5 weeks for more than 12 months, with or without hot flushes; 3. postmenopausal; i.e. no menses for at least a year.
1000 0 0..
100
.-_o o
10
0
E
-t" 13-
1 1
01-
7-
,1
Regular
r-z-i-rq Peri
Post
Figure 3. The relationship between primordial follicle number and menstrual status in agematched women, 45-55 years of age (n = 17). The effect of menstrual status on follicle number was significant (p < 0.001, by ANOVA; age was the covariate). Mean follicle counts in each group differed significantly from those in the other groups (p <0.05, by Student-NewmanKeuls test). From Richardson et al (1987).
6
s . J . RICHARDSON
As seen in Figure 3, follicles were virtually absent f r o m the ovaries of the four p o s t m e n o p a u s a l w o m e n . T h e follicle counts in the w o m e n with regular menses were tenfold greater than in the p e r i m e n o p a u s a l w o m e n , with little overlap b e t w e e n the two groups. These observations indicate that the size of the follicular reserve is the m a j o r determinant of b o t h the transition f r o m regular menses to the p e r i m e n o p a u s e as well as the subsequent m e n o p a u s e itself.
• Block (1952) [] Gougeon (1984) Richardson, Nelson (1987) 106
o Regular
|
z~ Perimenopausal • Postmenopausal • []e
105
rn
[] r.1°
o•
~ 8•
ooo
D • [] 18rn rn ~ 121
•
0 0 o
°121
E 104
ch
•
oQ
•
o•
o rn
[] 0
103 O
zx
E
E_
•
0
10 2 Z~
Zx
Z~
A
10
1
L
10
I
20
I
30
i
40
•
I
• • • 50
//
Age in years Figure 4. The relationship between age and primordial follicle number is compared using data from the studies of Block (1952, 1953), Gougeon (1984) and Richardson et al (1987).
THE BIOLOGICALBASIS OF THE MENOPAUSE
7
W h e n this data is combined with that of Block (1952) and G o u g e o n (1984), both of w h o m restricted their study to w o m e n with regular menses, it can be seen that the subjects with regular menses fall into the same range of follicle counts as those of Block and G o u g e o n (Figure 4). The combined data from the three studies also suggests an acceleration in the rate of follicular decline in the last decade before menopause, a tendency which can be seen in both Block's and G o u g e o n ' s data. W e have hypothesized that the elevated F S H seen in the last decade of menstrual life stimulates a greater proportion of follicles to enter the growing phase and hence be destined to atresia. Although little is known about the factors affecting the rate of atresia, studies in rodents have shown that the rate of depletion is retarded by techniques which lower gonadotropin levels such as hypophysectomy, pharmacological suppression of gonadotropin secretion and food restriction (Richardson et al, 1987; Richardson and Nelson, 1990).
The ageing oocyte and endometrium The decline in fecundability and the progressive increase in rate of spontaneous abortion with advancing maternal age suggests an age-related defect in either the hormonal milieu, the oocytes or the uterus and endometrium of older w o m e n (Hansen, 1986; Biggers, 1988; Nelson and Felicio, 1985). Recent reports on in vitro fertilization (IVF), gamete intrafallopian transfer ( G I F T ) and oocyte donation provide some insight into the relationship between maternal age, follicle n u m b e r and/or quality and the ageing endometrium. As in natural fertility and artificial insemination procedures, maternal age is an important determinant of pregnancy outcome in 'assisted fertilization'. In a report of the combined performance of 180 clinics throughout the United States using the technology of assisted fertilization, there was a decrease in pregnancy rate and an increase in the rate of spontaneous abortions with advancing age with both I V F and G I F T (Medical Research International, 1992) (Table 1). In both procedures, the highest pregnancy and delivery rates occurred in the 25-34-year-old women.
Table 1. Results from the IVF ET Registry, 1990: outcome by maternal age§. Stimulation cycles
Pregnancies*
Spontaneoust abortions
Deliveries~
Age (years)
IVF(n)
GIFT
IVF
GIFT
IVF
GIFT
IVF
GIFT
25-29 30-34 35-39 40+
569 1769 1890 660
228 737 659 313
20% 24 19 11
34% 36 27 17
18% 16 22 29
9% 16 18 39
16% 19 15 7
30% 29 21 9
*Clinical pregnancy rates expressed as a percentage of embryo transfer cycles. ? Spontaneous abortion rates expressed as a percentage of clinical pregnancies. ~:Delivery rate expressed as a percentage of embryo transfer cycles. §Adapted from Medical Research International (1992), Tables 3 and 6.
8
S. J. RICHARDSON
Oocyte donation from younger women corrects this age-related decline in fecundability and results in pregnancy and delivery rates which are similar to those of younger women. Serhal and Craft (1989) first reported a 55% pregnancy rate with oocyte donation in nine women aged 40-48 years whom they classified as menopausal, and a 43% pregnancy rate in 20 women aged 42-47 years who were still menstruating. Similar results have been obtained by Sauer et al (1990) and Navot et al (1991). Both groups observed a much higher pregnancy and delivery rate in women over the age of 40 when they received donated oocytes, than in those using their own oocytes (pregnancy rates of 62% vs. 8% and 56% vs. 3%, respectively). In Navot's study the delivery rates were similar in the young donors and the older recipients of oocytes from these donors (23% vs. 30%). These observations indicate that the relative decline in fecundability seen in older women is related to an age-associated deficiency in the oocyte and that the endometrium of older women has not lost its capacity to support a pregnancy. For each embryo transferred, fewer pregnancies and deliveries resulted in older women who received their own oocytes when compared with those who received donor oocytes (Serhal and Craft, 1989; Sauer et al, 1990; Navot et al, 1991). Navot interpreted this to indicate an age-related decline in oocyte quality. It is difficult, however, to separate the negative effect of oocyte quality from oocyte number in these studies. In the older women using their own oocytes, fewer oocytes were retrieved and fewer embryos transferred. For example, in Navot's study the number of embryos transferred in the older women using their own ova was 1.0 (confidence interval [CI] 0.7-1.3) whereas they later received a mean of 4.5 embryos (CI 4.4-4.8) from donor oocytes. Since it has been well documented that the rate of successful pregnancies increases with the number of embryos transferred up to six (Medical Research International, 1992), the adverse effect of the declining number of embryos transferred in the older women cannot be separated from the effect of oocyte quality in these studies. There is, however, other evidence that older women have a greater proportion of abnormal ova. Chromosomal analysis of oocytes obtained for IVF found 47% of ova were abnormal in women over the age of 35 years versus 25% in those younger (Planchot et al, 1988). There is also a strong association between maternal age and the incidence of chromosomal abnormalities found in human fetuses, whether studied from induced abortions, spontaneous abortions, midtrimester amniocenteses or live births (Hansen, 1986). In these circumstances, however, other factors such as paternal age may play a role. Serum follicular FSH as an indicator of ovarian follicle number and/or quality
Elevated follicular FSH concentrations are seen, not only in normal older women with regular menses, but also in some younger women with problems of conception. Usually these women respond poorly in programmes of assisted fertilization. Scott et al (1989) analysed 758 stimulated cycles in 441 women participating in an IVF programme. Patients with basal FSH con-
THE BIOLOGICAL BASIS OF THE MENOPAUSE
9
centrations < 15 IU 1-1 on day 3 of the cycle had ongoing pregnancy rates twice those of women with day 3 FSH between 15 and 24.9 and six times those whose FSH was ~>25IU1 -a. There was no consistent relationship between pregnancy rates and basal oestradiol or LH concentrations. The number of follicles aspirated and embryos transferred was highest in those with basal FSH concentration of < 15 IU 1-1 and lowest in those whose basal FSH was/> 25. Scott concluded that the basal FSH on day 3 reflects the reproductive potential of that cycle. Others have also found basal follicular FSH levels to be predictive of outcome in IVF programmes (Cameron et al, 1988; Fenichel et al, 1989). More recently, Toner et al (1991) analysed 1368 stimulation cycles for IVF. He concluded that basal FSH on day 3 is a better predictor than maternal age of number of oocytes retrieved and transferred, as well as ongoing pregnancies. He proposed that basal FSH is an index of functional ovarian reserve or 'ovarian age'. Serum inhibin as an indicator of follicle number and/or quality
Inhibin, the ovarian factor postulated by Sherman in 1975 to be a negative feedback inhibitor of FSH secretion, has since been isolated from follicular fluid. A radioimmunoassay has been developed and daily concentrations throughout the normal menstrual cycle documented (McLachlan et al, 1987). Inhibin is secreted by the human granulosa cell in response to FSH during the follicular phase (Hillier et al, 1991) and by the corpus luteum during the luteal phase in response to LH (McLachlan et al, 1989; Roseff et al, 1989; Hillier et al, 1991; Illingworth et al, 1991). We have recently observed levels to be lower during both the follicular and luteal phases of the cycle, in women with regular menses who are 45 years of age and over, than in younger controls (Richardson, Smith and Nelson, unpublished data). MacNaughton et al (1992) also reported lower inhibin levels in a similar group of women, noting a progressive decline in serum inhibin as a function of increasing age in regularly cycling subjects. They observed that follicular phase serum FSH levels remained constant up to the age of 43 years and then increased linearly at a rate of 2.4 IU 1-1 per year whilst serum oestradiol levels also remained constant up to the age of 38 years and then fell at a rate of 10.8 pmol 1-1 per year. Luteal phase inhibin and FSH levels declined with age. Lenton's recent study of the relationship between serum inhibin levels and age was inconclusive (Lenton et al, 1991). This can be attributed to the fact that the inhibin levels throughout the cycle were significantly different in the two young control groups. Thus the six older women, aged 40-48 years, had lower inhibin levels than one of the control groups but not the other. The reason for the discrepancy between the control groups could not be explained by the authors. The finding of lower inhibin levels in older women is not unexpected. Plasma inhibin concentrations are undetectable after menopause (McLachlan et al, 1986). There is also evidence that inhibin responsiveness to hyperstimulation in women undergoing in vitro fertilization is decreased in women 35 years of age and older, although basal levels are unchanged
10
S . J . ~CHARDSON
(Hughes et al, 1990). Plasma inhibin levels have also been reported to be diminished in younger women assumed to be nearing premature follicular depletion on the basis of elevated FSH levels and infertility (Buckler et al, 1991), although not all studies are in agreement (Cameron et al, 1988). Thus, it appears that inhibin is also a biomarker of the number and/or quality of follicles that remain in the ovary. Because of confounding factors such as the presence of other ovarian regulatory peptides (Ling et al, 1990), the distinction between immunoreactive and biologically active FSH and inhibin (Robertson et al, 1988; Biliar et al, 1991) and the additive effect of oestrogen and progesterone in the feedback regulation of FSH (Roseff et al, 1989), the precise relationship between inhibin and FSH remains to be elucidated. Although there is no direct evidence, the possibility that there may also be a loss of sensitivity of the hypothalamic-pituitary axis to feedback inhibition by oestradiol cannot be ruled out (Biliar et al, 1989). THE P E R I M E N O P A U S E
The 'perimenopause' is the term commonly used to describe the period of irregular menses which precedes the menopause or 'last menses'. It has been best characterized clinically by Treloar (1981) who, as part of his longitudinal study (Figure 1), analysed 393 cases whose menstrual records terminated in spontaneous menopause (Treloar, 1981). The median age upon entry into the phase of irregular menses was 45.5 years. The youngest woman was 41; the oldest 59 years of age. For most women the period of irregular menses lasted between 2 and 7 years, the range being 0-11 years. The endocrine profile of this period was first documented by Sherman et al (1976) and has been verified in Metcalf's later study of weekly urinary steroid and gonadotropin levels in 30 perimenopausal women (Metcalf et al, 1981b). In these women, cycles varied in both length and hormonal profile. Some cycles had high gonadotropins with menopausal oestrogen concentrations and no evidence of ovulation. These tended to be longer cycles. About one third of the cycles were ovulatory with oestrogen and gonadotropin levels similar to those found in younger women. Most cycles fell between these two extremes. There was no steady progression from one type of cycle to another. Rather, the pattern of cyclicity and hormonal secretion was unpredictable. THE LAST MENSES
Menopause or the final menses can only be determined retrospectively. Most studies use the definition of 12 months or more of amenorrhea (Wallace et al, 1979; Metcalf et al, 1982). In an analysis of the data from Treloar's longitudinal study, Wallace et al (1979) calculated the probability of menopause based on age and the duration of amenorrhea. For a given duration of amenorrhea, the possibility of being menopausal increases with a woman's age. For example, the probability of being menopausal after 6 months of amenorrhea is 46% in women aged 45-49 years, 65% in women
THE BIOLOGICAL BASIS OF THE MENOPAUSE
11
50-52 years old and 72% in those over 52 years of age. Ten percent of the women studied had a further bleed after 12 months of amenorrhea. Interestingly, there are no clear endocrine markers of the last cycle. As part of a larger study, Metcalf et al (1982) obtained weekly urine specimens for analysis of gonadotropins, oestrogens and pregnanediol in eight women, for 1-15 weeks prior to the last menses to 22-30 weeks following it. The last menses was identified retrospectively after 12 months of amenorrhea. Ovulation, as indicated by urinary pregnanediol levels, occurred as late as the last menses in three of the eight women, but there was no evidence of ovulation once menses ceased. Urinary gonadotropin levels were elevated both before and after the last cycle. There were eight episodes of elevated oestrogen excretion, compatible with follicular development, in five of the women during the postmenopausal period (from the menopause till death). THE POSTMENOPAUSAL OVARY
Cross-sectional studies of hormonal levels around the time of menopause suggest that 20-40% of women have oestrogen concentrations consistent with the presence of functioning follicles in the first 6-12 months after cessation of menses (Longcope et al 1986; Rannevik et al, 1986; Trevoux et al, 1986). None of the subjects had progesterone levels indicative of ovulation. By 12-24 months after menopause, oestrogen levels had fallen into the postmenopausal range in most women (Longcope et al, 1986; Trevoux et al, 1986). However, studies comparing ovarian vein with peripheral plasma samples in women at least 3 years postmenopause, indicate that some women still secrete significant amounts of oestradiol (Longcope et al, 1980; Lucisano et al, 1984). Does this indicate that functioning follicles are still present? There have been isolated reports of follicles seen in postmenopausal ovaries (Sauroma, 1952; Guraya, 1976; Costoff and Mahesh, 1975). There have even been two reports of 'fresh corpora lutea' in a 53-year-old woman 3 years postmenopause (Novak, 1970) and in a 63year-old woman 12 years postmenopause (Dawood et al, 1980). It is likely, however, that in most instances, the oestrogen measured from ovaries more than a few years postmenopause is secreted by ovaries which show evidence of stromal hyperplasia. Dennefors et al (1980) has shown that specimens of postmenopausal ovarian stroma which have evidence of stromal hyperplasia produce more androstenedione and oestrogens than stroma without evidence of hyperplasia. Lucisano also concluded that postmenopausal ovaries showing marked stromal hyperplasia had much higher levels of ovarian vein androgens and oestrogens than those without hyperplasia (Lucisano et al, 1986). Recently, Inkster and Brodie (1991) found immunoreactive aromatase in the stromal compartment of 3 of 7 postmenopausal ovaries in which there were no follicles observed. Taken together, it is likely that at the time of the last menses, follicles remain in the ovaries of some women which are capable of oestrogen secretion but, with rare exception, not of ovulation. After 2-3 years, the most likely source of oestrogen secretion is ovarian stromal hyperplasia.
12
S. J. RICHARDSON
AGE AT MENOPAUSE
Menopause occurs at around 50 years of age in most women in Western industrialized societies, with some indication that this has not changed over the past century (WHO Scientific Group, 1981; Stanford et al, 1987). Although few studies have been done on menopausal age in women living in underdeveloped countries, there is some evidence that they may have a later menarche and an earlier menopause (Beal, 1983). Studies done in industrialized societies have failed to show a relationship between age of menarche and age of menopause (Stanford et al, 1987; Whelan et al, 1990). Factors believed to reduce the age of menopause are smoking, being single or nulliparous and of low socio-economic status. Factors believed to be associated with a later menopause are parity and increased body weight. However, Brambilla and McKinley (1989), in a recent prospective study of a random sample of the population of the State of Massachusetts found that only smoking had an effect on the age of menopause. They reviewed 16 studies by others and found inconsistent results for all of the factors mentioned above except smoking. Age at menarche was not addressed. Smoking was associated with an earlier menopause in seven of the eight studies cited. Bramilla and McKinley attributed the lack of agreement between studies to self-selection of subjects, recall bias and univariant analysis in many of the studies which did not control for confounding factors. The most recent and most complete prospective study is by Whelan et al (1990). He analysed the records of 561 women who enrolled in Treloar's study between 1935 and 1939 and returned yearly records of menses and health for at least 20 years. They reported a striking relationship between menstrual pattern and age at menopause amongst these women. When the women were categorized according to cycle length and variability in the years between ages 20 and 35, those women whose mean cycle length was < 26 days reached menopause 1.4 years earlier than those with cycles between 26 and 32 days, and 2.2 years before those with cycles longer than 32 days. The results also confirmed the lack of relationship between age at menarche and age at menopause. Women who had never been pregnant reached menopause slightly, but significantly, earlier than women who had ever been pregnant and there was a trend to later age of menopause with increasing parity. One possible explanation for the association between cycle length and age at menopause might be that short cycles indicate a relatively smaller residual follicular pool which, in turn, would predict an earlier menopause. This would be consistent with the observation that the progressive shortening of cycle length with age as documented by Treloar (Figure 1) occurs concurrent with a decline in follicle count, as reported by Block (1952, 1953) and Gougeon (1984) (Figure 4). MENOPAUSAL AGE AS A BIOMARKER
It has been suggested that reproductive ageing may be coupled to, and
T H E B I O L O G I C A L BASIS OF T H E M E N O P A U S E
13
therefore serve as, a biomarker of the physiological changes which increase the likelihood of dying (Nelson JF, 1988). The report of menopausal age and mortality in Seventh-Day Adventists (Snowdon et al, 1989) suggests that age at menopause may be such a biomarker. The 5287 subjects, all of whom had a natural menopause, were between 55 and 100 years of age when the study began in 1976. At that time they completed a lifestyle questionnaire which included their age at menopause. Mortality was documented until the end of 1982. The mean age of menopause for the group was 49.2 years with a median age of 50. When the 50-54 age group was taken as the reference group, the age-adjusted odds-ratio of death in those whose menopause occurred before the age of 40 was 1.95; for those 40-44 years, the odds-ratio was 1.39; and for those 45-49 years, the ratio was 1.03. The use of replacement oestrogen did not weaken the association between age at natural menopause and mortality. Coronary heart disease and stroke mortality were higher in those with an earlier menopause while cancer mortality was slightly lower. Snowdon (1990) later examined the relative contribution of years of premenopausal versus years of postmenopausal life to age of death and found that mortality was not related to the relative duration of exposure to either the favourable premenopausal or unfavourable postmenopausal milieu. Rather, he interpreted his findings to suggest that age of natural menopause in a potent biological marker of ageing. SUMMARY
Age-related changes occur throughout the reproductive lifespan of normal healthy women. From the age of 20, the menstrual interval gradually shortens and becomes increasingly regular until the perimenopause. This is related to a shortening of the follicular rather than the luteal phase of the cycle. Serum FSH concentration is elevated during the follicular phase in older women who are still menstruating regularly, while serum inhibin levels are decreased in both the follicular and luteal phases. The relationship between FSH secretion, ageing and feedback inhibition by oestradiol, inhibin, or other presently unmeasured factors in women with regular menses, remains to be elucidated. The primary factor influencing the transition from regular menses to the perimenopause and subsequent menopause appears to be the size of the residual primordial follicle pool. Fecundability begins to decline by the age of 29 years. There is considerable evidence from the experience gained in assisted fertilization procedures that this is related to the effect of age on the quality of the oocyte rather than on the endometrium. At the time of the last menses, few follicles remain. There are no endocrine markers to signal the last cycle. Hence, menopause can only be identified retrospectively when there is no further menses. The probability of being menopausal increases with the duration of amenorrhea and age. Considerable evidence suggests that both serum FSH and serum inhibin
14
s. J. RICHARDSON
are biomarkers of the number and/or quality of follicles remaining in the ovary. There is also evidence that the age of menopause, itself, is a potent biomarker of the general ageing state of the individual. Acknowledgements I am deeply indebted to J. F. Nelson, Department of Physiology, San Antonio, Texas for his teaching insights and guidance. Also to Pat Smith, Royal Victoria Hospital who coordinated and made possible our study of inhibin and its relationship to FSH. The enthusiasm and cooperation of the participants in the inhibin study is gratefully acknowledged.
REFERENCES Baker TG (1963) A quantitatfve and cytological study of germ cells in human ovaries. Proceedings of the Royal Society of London (Biological Sciences) 158: 417--433. Beal CM (1983) Age at menopause and menarche in a high-altitude Himalayan population. Annals of Human Biology 10: 365-370. Biggers JD (1988) Fecundability. Annals of the New York Academy of Science 541: 706-714. Biliar RB, Richardson DW & Little B (1989) Escape from chronic estrogenic suppression of ovarian function in the adult Rhesus monkey: evidence for changing sensitivity of gonadotropin secretion to estrogen inhibition. Journal of Clinical Endocrinology and Metabolism 124: 2373-2382. Biliar RB, Richardson DW & Little B (1991) Reduced serum inhibin concentrations during ovulatory cycles of estrogen-treated Rhesus monkeys: and indicator of FSH bioactivity. Journal of Clinical Endocrinology and Metabolism 128: 2280-2284. Block E (1952) Quantitative morphological investigations of the follicular system in women. Acta Anatomica 14: 108-123. Block E (1953) A quantitative morphological investigation of the follicular system in newborn female infants. Acta Anatomica 17: 201-206. Bramilla DJ & McKinlay SM (1989) A prospective study of factors affecting age at menopause. Journal of Clinical Epidemiology 42: 1031-1039. Buckler HM, Evans CA, Mamtora H et al (1991) Gonadotropin, steroid, and inhibin levels in women with incipient ovarian failure during anovulatory and ovulatory rebound cycles. Journal of Clinical Endocrinology and Metabolism 72: 116-124. Cameron IT, O'Shea FC, Rolland JM et al (1988) Occult ovarian failure: a syndrome of infertility, regular menses, and elevated follicle-stimulating hormone concentrations. Journal of Clinical Endocrinology and Metabolism 67:1190-1194. Costoff A & Mahesh VB (1975) Primordial follicles with normal oocytes in the ovaries of postmenopausal women. Journal of the American Geriatric Society 23: 193-196. Dawood MY, Strongin M, Kramer EE & Wieche R (1980) Recent ovulation in a postmenopausal woman. International Journal of Gynaecology and Obstetrics 18" 192-194. Dennefors BL, Janson PO, Knutson F & Hamberger L (1980) Steroid production and responsiveness to gonadotropin in isolated stromal tissue of human postmenopausal ovaries. American Journal of Obstetrics and Gynecology 136: 997-1001. Fenichel P, Donzeau M, Grimbaldi M et al (1989) Predictive value of hormonal profiles before stimulation for in vitro fertilization. Fertility and Sterility 51: 845-849. Gougeon A (1984) Caract~re qualitative et quantatif de la population folliculaire dans l'ovaire humain adulte. Contraception, Fertilit~Sexualit~ 12: 527-535. Guraya SS (1976) Histochemical observations on the corpus luteum atreticum of the human postmenopausal ovary with reference to steriod hormone synthesis. Archivio Italiano di Anatomia e di Embriologia 81: 434-455. Hansen JP (1986) Older maternal age and pregnancy outcome: a review of the literature. Obstetrical and Gynecological Survey 41: 726-742. Hillier SG, Wickings EJ, Illingworth PI et al (1991) Control of immunoactive inhibin production by human granulosa ceils. Clinical Endocrinology 35: 71-78.
THE BIOLOGICAL BASIS OF THE MENOPAUSE
15
Hughes EG, Robertson DM, Handelsman DJ et al (1990) Inhibin and estradiol responses to ovarian hyperstimulation: effects of age and predictive value for in vitro fertilization outcome. Journal of Clinical Endocrinology and Metabolism 70: 358-364. Illingworth P J, Reddi K, Smith KB & Baird DT (1991) The source ofinhibin secretion during the human menstrual cycle. Journal of Clinical Endocrinology and Metabolism 73: 66%673. Inkster SE & Brodie AMH (1991) Expression of aromatase cytochrome P-450 in premenopausal and postmenopausal human ovaries: an immunocytochemical study. Journal of Clinical Endocrinology and Metabolism 73: 717-726. Lee SJ, Lenton EA, Sexton L & Cooke ID (1988) The effect of age on the cyclical patterns of plasma LH, FSH, oestradiol and progesterone in women with regular menstrual cycles. Human Reproduction 3: 851-855. Lenton EA, Landgren BM, Sexton L & Harper R (1984a) Normal variation in the length of the follicular phase of the menstrual cycle: effect of chronological age. British Journal of Obstetrics and Gynaecology 91: 681-684. Lenton EA, Landgren & Sexton L (1984b) Normal variation in the length of the luteal phase of the menstrual cycle: identification of the short luteal phase. British Journal of Obstetrics and Gynaecology 91" 685-689. Lenton EA, Sexton L, Lee S & Cooke ID (1988) Progressive changes in LH and FSH and LH: FSH ratio in women throughout reproductive life. Maturitas 10: 3543. Lenton EA, De Kretser DM, Woodward AJ & Robertson DM (1991) Inhibin concentrations throughout the menstrual cycles of normal, infertile, and older women compared with those during spontaneous conception cycles. Journal of Clinical Endocrinology and Metabolism 73: 1180-1190. Ling N, DePaolo LV, Bicsak TA & Shimasaki S (1990) Novel ovarian regulatory peptides: inhibin, activin, and follistatin. Clinical Obstetrics and Gynecology 33: 690-702. Longcope C, Hunter R & Franz C (1980) Steroid secretion by the postmenopausal ovary. American Journal of Obstetrics and Gynecology 138: 564-568. Longcope C, Franz C, Morella C et al (1986) Steroid and gonadotropin levels in women during the peri-menopausal years. Maturitas 8: 189-196. Lucisano A, Acampora MG, Russo N e t al (1984) Ovarian and peripheral plasma progestogens, androgens and oestrogens in post-menopausal women. Maturitas 6: 45-53. Lucisano A, Russo N, Acampora MG et al (1986) Ovarian and peripheral androgen and oestrogen levels in post-menopausal women: correlations with ovarian histology. Maturitas 8: 57-65. MacNaughton J, Bangah M, McCloud Pet al (1992) Age related changes in follicle stimulating hormone, luteinizing hormone, oestradiol and immunoreactive inhibin in women of reproductive age. Clinical Endocrinology 36: 339-345. McLachlan RI, Robertson DM, De Kretser DM & Burger HG (1986) Plasma inhibin levels during gonadotropin-induced ovarian hyperstimulation for IVF: a new index of follicular function. Lancet i: 1233. McLachlan RI, Robertson DM, Healy DL et al (1987) Circulating immunoreactive inhibin levels during the normal human menstrual cycle. Journal of Clinical Endocrinology and Metabolism 65: 954-961. McLachlan RI, Cohen NL, Vale WW et al (1989) The importance of luteinizing hormone in the control of inhibin and progesterone secretion by the human corpus luteum. Journal of Clinical Endocrinology and Metabolism 68: 1078-1085. Medical Research International, Society for Assisted Reproductive Technology & American Fertility Society (1992) In vitro fertilization-embryo transfer (IVF-ET) in the United States: 1990 results from the IVT-ET Registry. Fertility and Sterility 57: 15-24. Metcalf MG (1988) The approach of menopause: a New Zealand study. The New Zealand Medical Journal 101: 103-106. Metcalf MG, Donald RA & Livesay JH (1981a) Pituitary-ovarian function in normal women during the menopausal transition. Clinical Endocrinology 14: 245-255. Metcalf MG, Donald RA & Livesay JH (1981b) Classification of menstrual cycles in pre- and perimenopausal women. Journal of Endocrinology 91: 1-10. Metcalf MG, Donald RA & Livesay JH (1982) Pituitary-ovarian function before, during and after the menopause: a longitudinal study. Clinical Endocrinology 17: 489-494. Navot D, Bergh PA, Williams MA et al (1991) Poor oocyte quality rather than implantation failure as a cause of age-related decline in female fertility. Lancet 337: 1375-1377.
16
S. J. RICHARDSON
Nelson JF (1988) Puberty, gonadal steroids and fertility: potential reproductive markers of aging. Experimental Gerontology 23: 359-367. Nelson JF & Felicio LS (1985) Reproductive aging in the female: an etiological perspective. Review of Biological Research in Aging 2: 251-314. Novak ER (1970) Ovulation after fifty. Obstetrics and Gynecology 36" 903-910. Planchot M, De Grouchy J, Junca AM et al (1988) Chromosome analysis of human oocytes and embryos in an in vitro fertilization program. Annals of the New York Academy of Science 541: 384-397. Rannevik G, Carlstrom K, Jeppson Set al (1986) A prospective long-term study in women from pre-menopause to post-menopause: changing profiles of gonadotrophins, oestrogens and androgens. Maturitas 8: 297-307. Reyes FI, Winter JSD & Faiman C (1977) Pituitary-ovarian relationships preceding the menopause. American Journal of Obstetrics and Gynecology 129: 557-563. Richardson SJ & Nelson JF (1990) Follicular depletion during the menopausal transition. Annals of the New York Academy of Sciences 592: 8-12. Richardson SJ, Senikas V & Nelson JF (1987) Follicular depletion during the menopausal transition: evidence for accelerated loss and ultimate exhaustion. Journal of Clinical Endocrinology and Metabolism 65: 1231-1237. Robertson DM, Tsonis CG, McLachlin RI et al (1988) Comparison of inhibin immunological and in vitro biological activities in human serum. Journal of Clinical Endocrinology and Metabolism 67: 438-443. Roseff SJ, Bangah ML, Kettel LM et al (1989) Dynamic changes in circulating inhibin levels during the luteal-follicular transition of the human menstrual cycle. Journal of Clinical Endocrinology and Metabolism 69: 1033-1039. Sauer MV, Paulson RJ & Lobo R A (1990) A preliminary report on oocyte donation extending reproductive potential to women over 40. New England Journal of Medicine 323: 11571160. Sauroma H (1952) Histology, histopathology, and function of the senile ovary. Annales Chirugiae et Gynaecologiae Fenniae 41 (supplement): 1-62. Scott RT, Oehninger S, Toner JP et al (1989) Follicle-stimulating hormone levels on cycle day 3 are predictive of in vitro fertilization outcome. Fertility and Sterility 51: 651-654. Serhal PF & Craft IL (1989) Oocyte donation in 61 patients. Lancet i: 1185-1187. Sherman BM & Korenman SG (1975) Hormonal characteristics of the human menstrual cycle throughout reproductive life. Journal of Clinical Investigation 55: 699-706. Sherman BM, West JH & Korenman SG (1976) The menopausal transition: analysis of LH, FSH, estradiol, and progesterone concentrations during the menstrual cycles of older women. Journal of Clinical Endocrinology and Metabolism 42: 629-636. Snowdon D A (1990) Early natural menopause and the duration of postmenopausal life. Journal of the American Geriatric Society 38: 402-408. Snowdon DA, Kane RL, Beeson WL et al (1989) Is early natural menopause a biological marker of health and aging? American Journal of Public Health 79: 709-714. Stanford JL, Hartge P, Brinton LA et al (1987) Factors influencing the age at natural menopause. Journal of Chronic Disease 40: 995-1002. Toner JP, Philpot CB, Jones GS & Muasher SJ (1991) Basal follicle-stimulating hormone level is a better predictor of in vitro fertilization performance than age. Fertilityand Sterility 55: 784-791. Treloar A E (1981) Menstrual cyclicity and the pre-menopause. Maturitas 3: 249-264. Treloar AE, Boynton RE, Behn BG & Brown BW (1967) Variation of the human menstrual cycle through reproductive life. InternationalJournal of Fertility 12: 77-126. Trevoux R, De Brux J, Castanier M e t al (1986) Endometrium and plasma hormone profile in the peri-menopause and the post-menopause. Maturitas 8: 309-326. Wallace RB, Sherman BM, Bean JA et al (1979) Probability of menopause with increasing duration of amenorrhea in middle aged women. American Journal of Obstetrics and Gynecology 135: 1021-1024. Whelan EA, Sandler DP, McConnaughey DR & Weinberg CR (1990) Menstrual and reproductive characteristics and age at natural menopause. American Journal of Epidemiology 131: 625-632. WHO Scientific Group (1981) Research on the menopause. World Health Organization Technical Report, series 670.
J. R I C H A R D S O N
Age-related changes in the pattern of menstrual cyclicity, hormonal secretion and relative fertility occur throughout a woman's reproductive lifespan, culminating in the 'menopause' or last menses. In a society where more couples are delaying childbearing for career or financial reasons, an understanding of the mechanisms underlying the relation between age and the reproductive capacity of older women has gained importance (Hansen, 1986). Factors affecting the age of menopause also have clinical implications because of the association of early menopause with an increased risk of cardiovascular disease and osteoporosis and delayed menopause with breast and endometrial cancer (Whelan et al, 1990). Of particular interest is a recent report of an association between age at menopause and age of death which suggests that age at menopause itself may be a strong biomarker of the general ageing state of the individual (Snowdon et al, 1989; Snowdon, 1990). AGE AND THE PATTERN OF MENSTRUAL CYCLICITY The pattern of menstrual cyclicity changes throughout a woman's reproductive lifespan. This is best documented in Treloar et al's (1967) mammoth prospective longitudinal study begun in 1934. Yearly records of menses and related reproductive and health events were collected from college students at the University of Minnesota. Treloar analysed the records of over 2700 women who had accumulated records for 25 825 woman years. He observed that, contrary to popular belief, lack of perfect regularity was the rule. In all of the records collected, only one woman had completely regular menses for over a year. Figure i presents the combined data; they are divided into three zones of menstrual life in order to standardize the ages of menarche and menopause. Note that the period of irregular menses preceding the menopause (the perimenopause) is almost a mirror image of the early years following menarche. Menses tend to be fairly regular in the middle menstrual years. During this period, the median menstrual interval falls steadily from 28 days at age 20 years to 26 days at age 40 years. This progressive shortening was later shown to be due to a shortening of the follicular rather than the luteal phase of the cycle (Sherman and Korenman, 1975; Lenton et al, 1984a,b). Bailli~re' s Clinical Endocrinology and Metabolism--
Vol. 7, No. 1, January 1993 ISBN 0-7020-1697-7
1
Copyright © 1993, by Bailli~re Tindall All rights of reproduction in any form reserved
2
S . J . RICHARDSON
85 80
;',' I',I"
70
"!!,
~9',~, 9o
9 5 ,,/~h9 9
ill:
• jr
~,',i", '," i
o
~6o ~55
"' ' I~,'"
All menstrual intervals
,,
~
o
/80
,,,'"' ',
'
,,~
90
t/)
=
; ~' t~'~
..o
,',;t
45
• '/~'" 99',
\'%
o
~, 40
. , .
80,
~
,\q 70, ,
13 "~
60, ,,,,,~_ ~
30,
5o, •
25
3o,-z~-"
'
.. . . .
...... ~--4_
-
.
.
.
-~'~-
;
~..~,._ _
.
.
.... 98
i, ," ,"/ .
.
...-
-'~!i ii ._-~. ..........
~~
~
.
~.-~
~
-~" ~ ~
~~ '
",• ~-~_
....
70 I
,,,',/// //
.9,o
°
60 0o
i~"~'/' 7 "
50
.-
4o
~ ~
g
SJ~o,g
"1 . , - - "-~-.'~ . . ~ . ~ .
1'-~-"
10 I
I , l l l , l , l l l , l ~ l l l ~ l , l l l l l l l ~ l 0 2 4 6 20 22 24 26 28 30 32 34 36 38 4 0 - 8
Menstrual year
Chronological age
,I,1,1~1
-6 -4-2 0 Premenopausal year
Figure 1. Contours for the frequency distribution of all menstrual intervals in three zones of experience. From Treloar et al (1967).
The period of greatest regularity occurs just before the onset of perimenopausal irregularity. AGE AND FECUNDABILITY 'Fecundability' is defined as the ability of a currently married woman to have a live-born child (Hansen, 1986). In studies of non-contracepting populations, fertility is at a peak between 20 and 29 years of age and declines steadily thereafter, while the rate of spontaneous abortion rises with maternal age (Nelson and Felicio, 1985; Hansen, 1986). In this regard, however, the effects of ageing in the male partner or a decline in the frequency of sexual intercourse in older couples cannot be ruled out. These potentially confounding factors were eliminated in the CECOS report of the relation between maternal age and pregnancy success in 2193 women undergoing
THE BIOLOGICAL BASIS OF THE MENOPAUSE
3
artificial insemination with sperm from young healthy donors. After 12 attempts, pregnancy, which was defined by rising circulating hCG levels, was 73% in women under 31 years of age, 62% in those 31-35 years, and 54% in those over 35 years after 12 attempts. The 16 women over 40 years of age also had a success rate of 54% after 12 cycles. The spontaneous abortion rate was not reported. AGE AND THE HYPOTHALAMIC-PITUITARY-OVARIAN AXIS FSH and age in women with regular menses
Sherman and Korenman (1975) and Reyes et al (1977) first reported that serum FSH was selectively increased in older women who were still menstruating regularly. These observations have been confirmed more recently by Lee et al (1988) (Figure 2). Daily blood samples for one cycle were obtained from 94 women with regular menses who ranged in age from 24 to 50 years. When the subjects were divided into the age groups 24-35, 36-40, 41-45 and 46-50 years of age, FSH was observed to be elevated in the midfollicular and early postovulatory phase in the 40-45-year-old group. In the 46-50-year-old group serum FSH levels were even higher while serum LH concentration had also become elevated. Whereas Sherman et al (1975, 1976) had previously reported lower mean oestradiol levels in the older women, Reyes et al (1977) and Lee et al (1988) found no significant agerelated changes in either serum oestradiol or progesterone. The dynamics of the progressive increase in FSH with age are more precisely delineated in another report by the same group. Mean follicular gonadotropin levels (days LH-10 to LH-5 inclusive) were calculated for 127 subjects aged 23-49 years (day 0 is the day of the LH surge.) When the subjects were grouped by age in two-year bands, a significant increase in follicular FSH was found in the 40-41 year group with a progressive increase throughout the forties. Serum LH only became significantly elevated in the oldest group (Lenton et al, 1988). Two hypotheses have been suggested to account for the selective increase in FSH seen in older women. Sherman and Korenman (1975) first proposed the existence of an ovarian hormone, 'an inhibin' which would exert negative feedback control over FSH secretion. As a consequence of a diminished number of follicles in older women, FSH would become elevated in the absence of changes in serum oestradiol. Reyes et al suggested the alternative possibility that there may be an age-related decrease in sensitivity to feedback inhibition at the level of the hypothalamus-pituitary, a hypothesis later advocated by Metcalf et al (1981a) and Lenton et al (1988). Follicle number, menstrual pattern and age
The relationship between follicle number, menstrual pattern and age was clarified in a study published by Richardson et al (1987). Previous studies of follicle counts indicated that follicle number is maximal in the 7-month fetus
. 35o o
4 (A)
.L
a
S . J . ~CHARDSON
b
"1" _1 lO
3°1 ::)
15
.lco I.l.
10
- 4''
-4 ' o
. . 4. . . . .8. . . .12 .
~,,
-8-4
, 8 , 12 ,,b . . . . -.8. - 4
4
o''
' 8 ' ' 12 '
4
5
0 . % ' - ; ' o . . ;,. . a. .12'
-; . .-4. . . . .o.
(B)
a
E
;,50-
"
5oo-
i~j
250-
,
from
Days
;
l i . . -.a -., . .6':~ . . .;
~'2
LH s u r g e
b
c
0 _~
i
i
-4
¢-
el
3O 15 0
=
,
0
i
I
4
J
,
8
,
i
12
~
i
,
-8
f
i
,
-4
~
0
i
,
i
4
=
I
8
i
i
12
A A
'-~'-;.';'~'~'17'
'4'-~';'~';';2'
Days
from
LH
i
i
-8
s
i
-4
i
,
0
i
i
4
~
t
8
,
=
12
'-~ '-~ ' ~' ;~'~ '1'2'
surge
Figure 2. (A) Comparison of the daily geometric mean concentrations of LH and FSH (with 68% confidence intervals) in 41 women aged 24-35 years (O O, the control group, profiles repeated in each section) with (a), 19 women aged 36-40 years; (b), 18 women aged 41-45 years and (c), 16 women aged 46-50 years (all shown as © O). (B) Comparison of the daily geometric mean concentrations of oestradiol (E2) and progesterone (P) (with 69% confidence intervals) in 41 women aged 24-35 years (© O, the control group, profiles repeated in each section) with (a), 19 women aged 36-40 years; (b), 18 women aged 41-45 years and (c), 16 women aged 46-50 years (all shown as © O). From Lee et al (1988).
THE BIOLOGICAL BASIS OF THE MENOPAUSE
5
after which there is a steady logarithmic decline. At birth an average of 700000 follicles remain in the two ovaries (Block, 1953; Baker, 1963). In Block's classical study (1952) of follicle counts from autopsy specimens from 43 girls and women aged 7-44 years, the number of primordial follicles declined logarithmically. He also observed a wide range of follicle counts between women of the same age. For example, one 31-year-old subject had 8000 follicles in both ovaries whereas a 37-year-old subject had 208 000. In Gougeon's later study (1984) -of follicle counts in ovaries obtained at hysterectomy in 35 women with regular menses, aged 19-52 years, the slope of the curve relating age to follicle number as well as the range of follicle counts between women of the same age are similar to the data of Block. If one extrapolates from either study, assuming that the depletion curve is a straight line, there would be between 2000 and 5000 follicles remaining in the ovaries at age 50, the mean age of menopause (Nelson and Felicio, 1985). In order to (i) clarify the relationship between follicle number and the transition from regular menses to the irregular cycles of the perimenopause, and (ii) estimate the number of follicles remaining at the time of menopause, Richardson et al (1987) estimated follicle numbers in 17 healthy women, 44-55 years of age. For the purpose of analysis, they were divided into three age-matched groups according to the pattern of their menses in the previous 12 months:
1. regular menses with intervals between 3 and 5 weeks with no hot flushes; 2. perimenopausal; i.e. irregular menses with intervals of less than 3 weeks and/or more than 5 weeks for more than 12 months, with or without hot flushes; 3. postmenopausal; i.e. no menses for at least a year.
1000 0 0..
100
.-_o o
10
0
E
-t" 13-
1 1
01-
7-
,1
Regular
r-z-i-rq Peri
Post
Figure 3. The relationship between primordial follicle number and menstrual status in agematched women, 45-55 years of age (n = 17). The effect of menstrual status on follicle number was significant (p < 0.001, by ANOVA; age was the covariate). Mean follicle counts in each group differed significantly from those in the other groups (p <0.05, by Student-NewmanKeuls test). From Richardson et al (1987).
6
s . J . RICHARDSON
As seen in Figure 3, follicles were virtually absent f r o m the ovaries of the four p o s t m e n o p a u s a l w o m e n . T h e follicle counts in the w o m e n with regular menses were tenfold greater than in the p e r i m e n o p a u s a l w o m e n , with little overlap b e t w e e n the two groups. These observations indicate that the size of the follicular reserve is the m a j o r determinant of b o t h the transition f r o m regular menses to the p e r i m e n o p a u s e as well as the subsequent m e n o p a u s e itself.
• Block (1952) [] Gougeon (1984) Richardson, Nelson (1987) 106
o Regular
|
z~ Perimenopausal • Postmenopausal • []e
105
rn
[] r.1°
o•
~ 8•
ooo
D • [] 18rn rn ~ 121
•
0 0 o
°121
E 104
ch
•
oQ
•
o•
o rn
[] 0
103 O
zx
E
E_
•
0
10 2 Z~
Zx
Z~
A
10
1
L
10
I
20
I
30
i
40
•
I
• • • 50
//
Age in years Figure 4. The relationship between age and primordial follicle number is compared using data from the studies of Block (1952, 1953), Gougeon (1984) and Richardson et al (1987).
THE BIOLOGICALBASIS OF THE MENOPAUSE
7
W h e n this data is combined with that of Block (1952) and G o u g e o n (1984), both of w h o m restricted their study to w o m e n with regular menses, it can be seen that the subjects with regular menses fall into the same range of follicle counts as those of Block and G o u g e o n (Figure 4). The combined data from the three studies also suggests an acceleration in the rate of follicular decline in the last decade before menopause, a tendency which can be seen in both Block's and G o u g e o n ' s data. W e have hypothesized that the elevated F S H seen in the last decade of menstrual life stimulates a greater proportion of follicles to enter the growing phase and hence be destined to atresia. Although little is known about the factors affecting the rate of atresia, studies in rodents have shown that the rate of depletion is retarded by techniques which lower gonadotropin levels such as hypophysectomy, pharmacological suppression of gonadotropin secretion and food restriction (Richardson et al, 1987; Richardson and Nelson, 1990).
The ageing oocyte and endometrium The decline in fecundability and the progressive increase in rate of spontaneous abortion with advancing maternal age suggests an age-related defect in either the hormonal milieu, the oocytes or the uterus and endometrium of older w o m e n (Hansen, 1986; Biggers, 1988; Nelson and Felicio, 1985). Recent reports on in vitro fertilization (IVF), gamete intrafallopian transfer ( G I F T ) and oocyte donation provide some insight into the relationship between maternal age, follicle n u m b e r and/or quality and the ageing endometrium. As in natural fertility and artificial insemination procedures, maternal age is an important determinant of pregnancy outcome in 'assisted fertilization'. In a report of the combined performance of 180 clinics throughout the United States using the technology of assisted fertilization, there was a decrease in pregnancy rate and an increase in the rate of spontaneous abortions with advancing age with both I V F and G I F T (Medical Research International, 1992) (Table 1). In both procedures, the highest pregnancy and delivery rates occurred in the 25-34-year-old women.
Table 1. Results from the IVF ET Registry, 1990: outcome by maternal age§. Stimulation cycles
Pregnancies*
Spontaneoust abortions
Deliveries~
Age (years)
IVF(n)
GIFT
IVF
GIFT
IVF
GIFT
IVF
GIFT
25-29 30-34 35-39 40+
569 1769 1890 660
228 737 659 313
20% 24 19 11
34% 36 27 17
18% 16 22 29
9% 16 18 39
16% 19 15 7
30% 29 21 9
*Clinical pregnancy rates expressed as a percentage of embryo transfer cycles. ? Spontaneous abortion rates expressed as a percentage of clinical pregnancies. ~:Delivery rate expressed as a percentage of embryo transfer cycles. §Adapted from Medical Research International (1992), Tables 3 and 6.
8
S. J. RICHARDSON
Oocyte donation from younger women corrects this age-related decline in fecundability and results in pregnancy and delivery rates which are similar to those of younger women. Serhal and Craft (1989) first reported a 55% pregnancy rate with oocyte donation in nine women aged 40-48 years whom they classified as menopausal, and a 43% pregnancy rate in 20 women aged 42-47 years who were still menstruating. Similar results have been obtained by Sauer et al (1990) and Navot et al (1991). Both groups observed a much higher pregnancy and delivery rate in women over the age of 40 when they received donated oocytes, than in those using their own oocytes (pregnancy rates of 62% vs. 8% and 56% vs. 3%, respectively). In Navot's study the delivery rates were similar in the young donors and the older recipients of oocytes from these donors (23% vs. 30%). These observations indicate that the relative decline in fecundability seen in older women is related to an age-associated deficiency in the oocyte and that the endometrium of older women has not lost its capacity to support a pregnancy. For each embryo transferred, fewer pregnancies and deliveries resulted in older women who received their own oocytes when compared with those who received donor oocytes (Serhal and Craft, 1989; Sauer et al, 1990; Navot et al, 1991). Navot interpreted this to indicate an age-related decline in oocyte quality. It is difficult, however, to separate the negative effect of oocyte quality from oocyte number in these studies. In the older women using their own oocytes, fewer oocytes were retrieved and fewer embryos transferred. For example, in Navot's study the number of embryos transferred in the older women using their own ova was 1.0 (confidence interval [CI] 0.7-1.3) whereas they later received a mean of 4.5 embryos (CI 4.4-4.8) from donor oocytes. Since it has been well documented that the rate of successful pregnancies increases with the number of embryos transferred up to six (Medical Research International, 1992), the adverse effect of the declining number of embryos transferred in the older women cannot be separated from the effect of oocyte quality in these studies. There is, however, other evidence that older women have a greater proportion of abnormal ova. Chromosomal analysis of oocytes obtained for IVF found 47% of ova were abnormal in women over the age of 35 years versus 25% in those younger (Planchot et al, 1988). There is also a strong association between maternal age and the incidence of chromosomal abnormalities found in human fetuses, whether studied from induced abortions, spontaneous abortions, midtrimester amniocenteses or live births (Hansen, 1986). In these circumstances, however, other factors such as paternal age may play a role. Serum follicular FSH as an indicator of ovarian follicle number and/or quality
Elevated follicular FSH concentrations are seen, not only in normal older women with regular menses, but also in some younger women with problems of conception. Usually these women respond poorly in programmes of assisted fertilization. Scott et al (1989) analysed 758 stimulated cycles in 441 women participating in an IVF programme. Patients with basal FSH con-
THE BIOLOGICAL BASIS OF THE MENOPAUSE
9
centrations < 15 IU 1-1 on day 3 of the cycle had ongoing pregnancy rates twice those of women with day 3 FSH between 15 and 24.9 and six times those whose FSH was ~>25IU1 -a. There was no consistent relationship between pregnancy rates and basal oestradiol or LH concentrations. The number of follicles aspirated and embryos transferred was highest in those with basal FSH concentration of < 15 IU 1-1 and lowest in those whose basal FSH was/> 25. Scott concluded that the basal FSH on day 3 reflects the reproductive potential of that cycle. Others have also found basal follicular FSH levels to be predictive of outcome in IVF programmes (Cameron et al, 1988; Fenichel et al, 1989). More recently, Toner et al (1991) analysed 1368 stimulation cycles for IVF. He concluded that basal FSH on day 3 is a better predictor than maternal age of number of oocytes retrieved and transferred, as well as ongoing pregnancies. He proposed that basal FSH is an index of functional ovarian reserve or 'ovarian age'. Serum inhibin as an indicator of follicle number and/or quality
Inhibin, the ovarian factor postulated by Sherman in 1975 to be a negative feedback inhibitor of FSH secretion, has since been isolated from follicular fluid. A radioimmunoassay has been developed and daily concentrations throughout the normal menstrual cycle documented (McLachlan et al, 1987). Inhibin is secreted by the human granulosa cell in response to FSH during the follicular phase (Hillier et al, 1991) and by the corpus luteum during the luteal phase in response to LH (McLachlan et al, 1989; Roseff et al, 1989; Hillier et al, 1991; Illingworth et al, 1991). We have recently observed levels to be lower during both the follicular and luteal phases of the cycle, in women with regular menses who are 45 years of age and over, than in younger controls (Richardson, Smith and Nelson, unpublished data). MacNaughton et al (1992) also reported lower inhibin levels in a similar group of women, noting a progressive decline in serum inhibin as a function of increasing age in regularly cycling subjects. They observed that follicular phase serum FSH levels remained constant up to the age of 43 years and then increased linearly at a rate of 2.4 IU 1-1 per year whilst serum oestradiol levels also remained constant up to the age of 38 years and then fell at a rate of 10.8 pmol 1-1 per year. Luteal phase inhibin and FSH levels declined with age. Lenton's recent study of the relationship between serum inhibin levels and age was inconclusive (Lenton et al, 1991). This can be attributed to the fact that the inhibin levels throughout the cycle were significantly different in the two young control groups. Thus the six older women, aged 40-48 years, had lower inhibin levels than one of the control groups but not the other. The reason for the discrepancy between the control groups could not be explained by the authors. The finding of lower inhibin levels in older women is not unexpected. Plasma inhibin concentrations are undetectable after menopause (McLachlan et al, 1986). There is also evidence that inhibin responsiveness to hyperstimulation in women undergoing in vitro fertilization is decreased in women 35 years of age and older, although basal levels are unchanged
10
S . J . ~CHARDSON
(Hughes et al, 1990). Plasma inhibin levels have also been reported to be diminished in younger women assumed to be nearing premature follicular depletion on the basis of elevated FSH levels and infertility (Buckler et al, 1991), although not all studies are in agreement (Cameron et al, 1988). Thus, it appears that inhibin is also a biomarker of the number and/or quality of follicles that remain in the ovary. Because of confounding factors such as the presence of other ovarian regulatory peptides (Ling et al, 1990), the distinction between immunoreactive and biologically active FSH and inhibin (Robertson et al, 1988; Biliar et al, 1991) and the additive effect of oestrogen and progesterone in the feedback regulation of FSH (Roseff et al, 1989), the precise relationship between inhibin and FSH remains to be elucidated. Although there is no direct evidence, the possibility that there may also be a loss of sensitivity of the hypothalamic-pituitary axis to feedback inhibition by oestradiol cannot be ruled out (Biliar et al, 1989). THE P E R I M E N O P A U S E
The 'perimenopause' is the term commonly used to describe the period of irregular menses which precedes the menopause or 'last menses'. It has been best characterized clinically by Treloar (1981) who, as part of his longitudinal study (Figure 1), analysed 393 cases whose menstrual records terminated in spontaneous menopause (Treloar, 1981). The median age upon entry into the phase of irregular menses was 45.5 years. The youngest woman was 41; the oldest 59 years of age. For most women the period of irregular menses lasted between 2 and 7 years, the range being 0-11 years. The endocrine profile of this period was first documented by Sherman et al (1976) and has been verified in Metcalf's later study of weekly urinary steroid and gonadotropin levels in 30 perimenopausal women (Metcalf et al, 1981b). In these women, cycles varied in both length and hormonal profile. Some cycles had high gonadotropins with menopausal oestrogen concentrations and no evidence of ovulation. These tended to be longer cycles. About one third of the cycles were ovulatory with oestrogen and gonadotropin levels similar to those found in younger women. Most cycles fell between these two extremes. There was no steady progression from one type of cycle to another. Rather, the pattern of cyclicity and hormonal secretion was unpredictable. THE LAST MENSES
Menopause or the final menses can only be determined retrospectively. Most studies use the definition of 12 months or more of amenorrhea (Wallace et al, 1979; Metcalf et al, 1982). In an analysis of the data from Treloar's longitudinal study, Wallace et al (1979) calculated the probability of menopause based on age and the duration of amenorrhea. For a given duration of amenorrhea, the possibility of being menopausal increases with a woman's age. For example, the probability of being menopausal after 6 months of amenorrhea is 46% in women aged 45-49 years, 65% in women
THE BIOLOGICAL BASIS OF THE MENOPAUSE
11
50-52 years old and 72% in those over 52 years of age. Ten percent of the women studied had a further bleed after 12 months of amenorrhea. Interestingly, there are no clear endocrine markers of the last cycle. As part of a larger study, Metcalf et al (1982) obtained weekly urine specimens for analysis of gonadotropins, oestrogens and pregnanediol in eight women, for 1-15 weeks prior to the last menses to 22-30 weeks following it. The last menses was identified retrospectively after 12 months of amenorrhea. Ovulation, as indicated by urinary pregnanediol levels, occurred as late as the last menses in three of the eight women, but there was no evidence of ovulation once menses ceased. Urinary gonadotropin levels were elevated both before and after the last cycle. There were eight episodes of elevated oestrogen excretion, compatible with follicular development, in five of the women during the postmenopausal period (from the menopause till death). THE POSTMENOPAUSAL OVARY
Cross-sectional studies of hormonal levels around the time of menopause suggest that 20-40% of women have oestrogen concentrations consistent with the presence of functioning follicles in the first 6-12 months after cessation of menses (Longcope et al 1986; Rannevik et al, 1986; Trevoux et al, 1986). None of the subjects had progesterone levels indicative of ovulation. By 12-24 months after menopause, oestrogen levels had fallen into the postmenopausal range in most women (Longcope et al, 1986; Trevoux et al, 1986). However, studies comparing ovarian vein with peripheral plasma samples in women at least 3 years postmenopause, indicate that some women still secrete significant amounts of oestradiol (Longcope et al, 1980; Lucisano et al, 1984). Does this indicate that functioning follicles are still present? There have been isolated reports of follicles seen in postmenopausal ovaries (Sauroma, 1952; Guraya, 1976; Costoff and Mahesh, 1975). There have even been two reports of 'fresh corpora lutea' in a 53-year-old woman 3 years postmenopause (Novak, 1970) and in a 63year-old woman 12 years postmenopause (Dawood et al, 1980). It is likely, however, that in most instances, the oestrogen measured from ovaries more than a few years postmenopause is secreted by ovaries which show evidence of stromal hyperplasia. Dennefors et al (1980) has shown that specimens of postmenopausal ovarian stroma which have evidence of stromal hyperplasia produce more androstenedione and oestrogens than stroma without evidence of hyperplasia. Lucisano also concluded that postmenopausal ovaries showing marked stromal hyperplasia had much higher levels of ovarian vein androgens and oestrogens than those without hyperplasia (Lucisano et al, 1986). Recently, Inkster and Brodie (1991) found immunoreactive aromatase in the stromal compartment of 3 of 7 postmenopausal ovaries in which there were no follicles observed. Taken together, it is likely that at the time of the last menses, follicles remain in the ovaries of some women which are capable of oestrogen secretion but, with rare exception, not of ovulation. After 2-3 years, the most likely source of oestrogen secretion is ovarian stromal hyperplasia.
12
S. J. RICHARDSON
AGE AT MENOPAUSE
Menopause occurs at around 50 years of age in most women in Western industrialized societies, with some indication that this has not changed over the past century (WHO Scientific Group, 1981; Stanford et al, 1987). Although few studies have been done on menopausal age in women living in underdeveloped countries, there is some evidence that they may have a later menarche and an earlier menopause (Beal, 1983). Studies done in industrialized societies have failed to show a relationship between age of menarche and age of menopause (Stanford et al, 1987; Whelan et al, 1990). Factors believed to reduce the age of menopause are smoking, being single or nulliparous and of low socio-economic status. Factors believed to be associated with a later menopause are parity and increased body weight. However, Brambilla and McKinley (1989), in a recent prospective study of a random sample of the population of the State of Massachusetts found that only smoking had an effect on the age of menopause. They reviewed 16 studies by others and found inconsistent results for all of the factors mentioned above except smoking. Age at menarche was not addressed. Smoking was associated with an earlier menopause in seven of the eight studies cited. Bramilla and McKinley attributed the lack of agreement between studies to self-selection of subjects, recall bias and univariant analysis in many of the studies which did not control for confounding factors. The most recent and most complete prospective study is by Whelan et al (1990). He analysed the records of 561 women who enrolled in Treloar's study between 1935 and 1939 and returned yearly records of menses and health for at least 20 years. They reported a striking relationship between menstrual pattern and age at menopause amongst these women. When the women were categorized according to cycle length and variability in the years between ages 20 and 35, those women whose mean cycle length was < 26 days reached menopause 1.4 years earlier than those with cycles between 26 and 32 days, and 2.2 years before those with cycles longer than 32 days. The results also confirmed the lack of relationship between age at menarche and age at menopause. Women who had never been pregnant reached menopause slightly, but significantly, earlier than women who had ever been pregnant and there was a trend to later age of menopause with increasing parity. One possible explanation for the association between cycle length and age at menopause might be that short cycles indicate a relatively smaller residual follicular pool which, in turn, would predict an earlier menopause. This would be consistent with the observation that the progressive shortening of cycle length with age as documented by Treloar (Figure 1) occurs concurrent with a decline in follicle count, as reported by Block (1952, 1953) and Gougeon (1984) (Figure 4). MENOPAUSAL AGE AS A BIOMARKER
It has been suggested that reproductive ageing may be coupled to, and
T H E B I O L O G I C A L BASIS OF T H E M E N O P A U S E
13
therefore serve as, a biomarker of the physiological changes which increase the likelihood of dying (Nelson JF, 1988). The report of menopausal age and mortality in Seventh-Day Adventists (Snowdon et al, 1989) suggests that age at menopause may be such a biomarker. The 5287 subjects, all of whom had a natural menopause, were between 55 and 100 years of age when the study began in 1976. At that time they completed a lifestyle questionnaire which included their age at menopause. Mortality was documented until the end of 1982. The mean age of menopause for the group was 49.2 years with a median age of 50. When the 50-54 age group was taken as the reference group, the age-adjusted odds-ratio of death in those whose menopause occurred before the age of 40 was 1.95; for those 40-44 years, the odds-ratio was 1.39; and for those 45-49 years, the ratio was 1.03. The use of replacement oestrogen did not weaken the association between age at natural menopause and mortality. Coronary heart disease and stroke mortality were higher in those with an earlier menopause while cancer mortality was slightly lower. Snowdon (1990) later examined the relative contribution of years of premenopausal versus years of postmenopausal life to age of death and found that mortality was not related to the relative duration of exposure to either the favourable premenopausal or unfavourable postmenopausal milieu. Rather, he interpreted his findings to suggest that age of natural menopause in a potent biological marker of ageing. SUMMARY
Age-related changes occur throughout the reproductive lifespan of normal healthy women. From the age of 20, the menstrual interval gradually shortens and becomes increasingly regular until the perimenopause. This is related to a shortening of the follicular rather than the luteal phase of the cycle. Serum FSH concentration is elevated during the follicular phase in older women who are still menstruating regularly, while serum inhibin levels are decreased in both the follicular and luteal phases. The relationship between FSH secretion, ageing and feedback inhibition by oestradiol, inhibin, or other presently unmeasured factors in women with regular menses, remains to be elucidated. The primary factor influencing the transition from regular menses to the perimenopause and subsequent menopause appears to be the size of the residual primordial follicle pool. Fecundability begins to decline by the age of 29 years. There is considerable evidence from the experience gained in assisted fertilization procedures that this is related to the effect of age on the quality of the oocyte rather than on the endometrium. At the time of the last menses, few follicles remain. There are no endocrine markers to signal the last cycle. Hence, menopause can only be identified retrospectively when there is no further menses. The probability of being menopausal increases with the duration of amenorrhea and age. Considerable evidence suggests that both serum FSH and serum inhibin
14
s. J. RICHARDSON
are biomarkers of the number and/or quality of follicles remaining in the ovary. There is also evidence that the age of menopause, itself, is a potent biomarker of the general ageing state of the individual. Acknowledgements I am deeply indebted to J. F. Nelson, Department of Physiology, San Antonio, Texas for his teaching insights and guidance. Also to Pat Smith, Royal Victoria Hospital who coordinated and made possible our study of inhibin and its relationship to FSH. The enthusiasm and cooperation of the participants in the inhibin study is gratefully acknowledged.
REFERENCES Baker TG (1963) A quantitatfve and cytological study of germ cells in human ovaries. Proceedings of the Royal Society of London (Biological Sciences) 158: 417--433. Beal CM (1983) Age at menopause and menarche in a high-altitude Himalayan population. Annals of Human Biology 10: 365-370. Biggers JD (1988) Fecundability. Annals of the New York Academy of Science 541: 706-714. Biliar RB, Richardson DW & Little B (1989) Escape from chronic estrogenic suppression of ovarian function in the adult Rhesus monkey: evidence for changing sensitivity of gonadotropin secretion to estrogen inhibition. Journal of Clinical Endocrinology and Metabolism 124: 2373-2382. Biliar RB, Richardson DW & Little B (1991) Reduced serum inhibin concentrations during ovulatory cycles of estrogen-treated Rhesus monkeys: and indicator of FSH bioactivity. Journal of Clinical Endocrinology and Metabolism 128: 2280-2284. Block E (1952) Quantitative morphological investigations of the follicular system in women. Acta Anatomica 14: 108-123. Block E (1953) A quantitative morphological investigation of the follicular system in newborn female infants. Acta Anatomica 17: 201-206. Bramilla DJ & McKinlay SM (1989) A prospective study of factors affecting age at menopause. Journal of Clinical Epidemiology 42: 1031-1039. Buckler HM, Evans CA, Mamtora H et al (1991) Gonadotropin, steroid, and inhibin levels in women with incipient ovarian failure during anovulatory and ovulatory rebound cycles. Journal of Clinical Endocrinology and Metabolism 72: 116-124. Cameron IT, O'Shea FC, Rolland JM et al (1988) Occult ovarian failure: a syndrome of infertility, regular menses, and elevated follicle-stimulating hormone concentrations. Journal of Clinical Endocrinology and Metabolism 67:1190-1194. Costoff A & Mahesh VB (1975) Primordial follicles with normal oocytes in the ovaries of postmenopausal women. Journal of the American Geriatric Society 23: 193-196. Dawood MY, Strongin M, Kramer EE & Wieche R (1980) Recent ovulation in a postmenopausal woman. International Journal of Gynaecology and Obstetrics 18" 192-194. Dennefors BL, Janson PO, Knutson F & Hamberger L (1980) Steroid production and responsiveness to gonadotropin in isolated stromal tissue of human postmenopausal ovaries. American Journal of Obstetrics and Gynecology 136: 997-1001. Fenichel P, Donzeau M, Grimbaldi M et al (1989) Predictive value of hormonal profiles before stimulation for in vitro fertilization. Fertility and Sterility 51: 845-849. Gougeon A (1984) Caract~re qualitative et quantatif de la population folliculaire dans l'ovaire humain adulte. Contraception, Fertilit~Sexualit~ 12: 527-535. Guraya SS (1976) Histochemical observations on the corpus luteum atreticum of the human postmenopausal ovary with reference to steriod hormone synthesis. Archivio Italiano di Anatomia e di Embriologia 81: 434-455. Hansen JP (1986) Older maternal age and pregnancy outcome: a review of the literature. Obstetrical and Gynecological Survey 41: 726-742. Hillier SG, Wickings EJ, Illingworth PI et al (1991) Control of immunoactive inhibin production by human granulosa ceils. Clinical Endocrinology 35: 71-78.
THE BIOLOGICAL BASIS OF THE MENOPAUSE
15
Hughes EG, Robertson DM, Handelsman DJ et al (1990) Inhibin and estradiol responses to ovarian hyperstimulation: effects of age and predictive value for in vitro fertilization outcome. Journal of Clinical Endocrinology and Metabolism 70: 358-364. Illingworth P J, Reddi K, Smith KB & Baird DT (1991) The source ofinhibin secretion during the human menstrual cycle. Journal of Clinical Endocrinology and Metabolism 73: 66%673. Inkster SE & Brodie AMH (1991) Expression of aromatase cytochrome P-450 in premenopausal and postmenopausal human ovaries: an immunocytochemical study. Journal of Clinical Endocrinology and Metabolism 73: 717-726. Lee SJ, Lenton EA, Sexton L & Cooke ID (1988) The effect of age on the cyclical patterns of plasma LH, FSH, oestradiol and progesterone in women with regular menstrual cycles. Human Reproduction 3: 851-855. Lenton EA, Landgren BM, Sexton L & Harper R (1984a) Normal variation in the length of the follicular phase of the menstrual cycle: effect of chronological age. British Journal of Obstetrics and Gynaecology 91: 681-684. Lenton EA, Landgren & Sexton L (1984b) Normal variation in the length of the luteal phase of the menstrual cycle: identification of the short luteal phase. British Journal of Obstetrics and Gynaecology 91" 685-689. Lenton EA, Sexton L, Lee S & Cooke ID (1988) Progressive changes in LH and FSH and LH: FSH ratio in women throughout reproductive life. Maturitas 10: 3543. Lenton EA, De Kretser DM, Woodward AJ & Robertson DM (1991) Inhibin concentrations throughout the menstrual cycles of normal, infertile, and older women compared with those during spontaneous conception cycles. Journal of Clinical Endocrinology and Metabolism 73: 1180-1190. Ling N, DePaolo LV, Bicsak TA & Shimasaki S (1990) Novel ovarian regulatory peptides: inhibin, activin, and follistatin. Clinical Obstetrics and Gynecology 33: 690-702. Longcope C, Hunter R & Franz C (1980) Steroid secretion by the postmenopausal ovary. American Journal of Obstetrics and Gynecology 138: 564-568. Longcope C, Franz C, Morella C et al (1986) Steroid and gonadotropin levels in women during the peri-menopausal years. Maturitas 8: 189-196. Lucisano A, Acampora MG, Russo N e t al (1984) Ovarian and peripheral plasma progestogens, androgens and oestrogens in post-menopausal women. Maturitas 6: 45-53. Lucisano A, Russo N, Acampora MG et al (1986) Ovarian and peripheral androgen and oestrogen levels in post-menopausal women: correlations with ovarian histology. Maturitas 8: 57-65. MacNaughton J, Bangah M, McCloud Pet al (1992) Age related changes in follicle stimulating hormone, luteinizing hormone, oestradiol and immunoreactive inhibin in women of reproductive age. Clinical Endocrinology 36: 339-345. McLachlan RI, Robertson DM, De Kretser DM & Burger HG (1986) Plasma inhibin levels during gonadotropin-induced ovarian hyperstimulation for IVF: a new index of follicular function. Lancet i: 1233. McLachlan RI, Robertson DM, Healy DL et al (1987) Circulating immunoreactive inhibin levels during the normal human menstrual cycle. Journal of Clinical Endocrinology and Metabolism 65: 954-961. McLachlan RI, Cohen NL, Vale WW et al (1989) The importance of luteinizing hormone in the control of inhibin and progesterone secretion by the human corpus luteum. Journal of Clinical Endocrinology and Metabolism 68: 1078-1085. Medical Research International, Society for Assisted Reproductive Technology & American Fertility Society (1992) In vitro fertilization-embryo transfer (IVF-ET) in the United States: 1990 results from the IVT-ET Registry. Fertility and Sterility 57: 15-24. Metcalf MG (1988) The approach of menopause: a New Zealand study. The New Zealand Medical Journal 101: 103-106. Metcalf MG, Donald RA & Livesay JH (1981a) Pituitary-ovarian function in normal women during the menopausal transition. Clinical Endocrinology 14: 245-255. Metcalf MG, Donald RA & Livesay JH (1981b) Classification of menstrual cycles in pre- and perimenopausal women. Journal of Endocrinology 91: 1-10. Metcalf MG, Donald RA & Livesay JH (1982) Pituitary-ovarian function before, during and after the menopause: a longitudinal study. Clinical Endocrinology 17: 489-494. Navot D, Bergh PA, Williams MA et al (1991) Poor oocyte quality rather than implantation failure as a cause of age-related decline in female fertility. Lancet 337: 1375-1377.
16
S. J. RICHARDSON
Nelson JF (1988) Puberty, gonadal steroids and fertility: potential reproductive markers of aging. Experimental Gerontology 23: 359-367. Nelson JF & Felicio LS (1985) Reproductive aging in the female: an etiological perspective. Review of Biological Research in Aging 2: 251-314. Novak ER (1970) Ovulation after fifty. Obstetrics and Gynecology 36" 903-910. Planchot M, De Grouchy J, Junca AM et al (1988) Chromosome analysis of human oocytes and embryos in an in vitro fertilization program. Annals of the New York Academy of Science 541: 384-397. Rannevik G, Carlstrom K, Jeppson Set al (1986) A prospective long-term study in women from pre-menopause to post-menopause: changing profiles of gonadotrophins, oestrogens and androgens. Maturitas 8: 297-307. Reyes FI, Winter JSD & Faiman C (1977) Pituitary-ovarian relationships preceding the menopause. American Journal of Obstetrics and Gynecology 129: 557-563. Richardson SJ & Nelson JF (1990) Follicular depletion during the menopausal transition. Annals of the New York Academy of Sciences 592: 8-12. Richardson SJ, Senikas V & Nelson JF (1987) Follicular depletion during the menopausal transition: evidence for accelerated loss and ultimate exhaustion. Journal of Clinical Endocrinology and Metabolism 65: 1231-1237. Robertson DM, Tsonis CG, McLachlin RI et al (1988) Comparison of inhibin immunological and in vitro biological activities in human serum. Journal of Clinical Endocrinology and Metabolism 67: 438-443. Roseff SJ, Bangah ML, Kettel LM et al (1989) Dynamic changes in circulating inhibin levels during the luteal-follicular transition of the human menstrual cycle. Journal of Clinical Endocrinology and Metabolism 69: 1033-1039. Sauer MV, Paulson RJ & Lobo R A (1990) A preliminary report on oocyte donation extending reproductive potential to women over 40. New England Journal of Medicine 323: 11571160. Sauroma H (1952) Histology, histopathology, and function of the senile ovary. Annales Chirugiae et Gynaecologiae Fenniae 41 (supplement): 1-62. Scott RT, Oehninger S, Toner JP et al (1989) Follicle-stimulating hormone levels on cycle day 3 are predictive of in vitro fertilization outcome. Fertility and Sterility 51: 651-654. Serhal PF & Craft IL (1989) Oocyte donation in 61 patients. Lancet i: 1185-1187. Sherman BM & Korenman SG (1975) Hormonal characteristics of the human menstrual cycle throughout reproductive life. Journal of Clinical Investigation 55: 699-706. Sherman BM, West JH & Korenman SG (1976) The menopausal transition: analysis of LH, FSH, estradiol, and progesterone concentrations during the menstrual cycles of older women. Journal of Clinical Endocrinology and Metabolism 42: 629-636. Snowdon D A (1990) Early natural menopause and the duration of postmenopausal life. Journal of the American Geriatric Society 38: 402-408. Snowdon DA, Kane RL, Beeson WL et al (1989) Is early natural menopause a biological marker of health and aging? American Journal of Public Health 79: 709-714. Stanford JL, Hartge P, Brinton LA et al (1987) Factors influencing the age at natural menopause. Journal of Chronic Disease 40: 995-1002. Toner JP, Philpot CB, Jones GS & Muasher SJ (1991) Basal follicle-stimulating hormone level is a better predictor of in vitro fertilization performance than age. Fertilityand Sterility 55: 784-791. Treloar A E (1981) Menstrual cyclicity and the pre-menopause. Maturitas 3: 249-264. Treloar AE, Boynton RE, Behn BG & Brown BW (1967) Variation of the human menstrual cycle through reproductive life. InternationalJournal of Fertility 12: 77-126. Trevoux R, De Brux J, Castanier M e t al (1986) Endometrium and plasma hormone profile in the peri-menopause and the post-menopause. Maturitas 8: 309-326. Wallace RB, Sherman BM, Bean JA et al (1979) Probability of menopause with increasing duration of amenorrhea in middle aged women. American Journal of Obstetrics and Gynecology 135: 1021-1024. Whelan EA, Sandler DP, McConnaughey DR & Weinberg CR (1990) Menstrual and reproductive characteristics and age at natural menopause. American Journal of Epidemiology 131: 625-632. WHO Scientific Group (1981) Research on the menopause. World Health Organization Technical Report, series 670.