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Fetal and placental growth in young, primiparous and old, multiparous rats
E.werimental Gerontolo,o', Vol. 22, pp. 257-261, 1987 Printed in the USA, All rights reserved.
0531-5565/87 $3.00 + .00 Copyright '~ 1987 Pergamon Journals Ltd
FETAL AND PLACENTAL G R O W T H IN YOUNG, PRIMIPAROUS AND OLD, M U L T I P A R O U S RATS A. RAHIMA* and N. W. BRUCE Department of A n a t o m y and H u m a n Biology, University of W e s t e r n Australia, N e d l a n d s , W e s t e r n Australia 6009, Australia
Abstract - - Fetal and placental weights and peripheral blood progestagen concentrations were examined on days 13, 16 and 22 of gestation in young (3-5 month) nulliparous and old (9-12 month) multiparous, albino Wistar rats. There was no difference in fetal weights at any stage examined whereas placental weights were significantly heavier in the old rats at days 16 (71%) and 22 (38%). Blood progesterone concentrations were similar in the two groups at days 16 and 22 although concentrations of the less biologically active progestagen, 20~-dihydroprogesterone, were higher (61%) at day 16 in the old rats. The relative hypertrophy of the placentas in the old rats could not be explained by reduced litter size or progestagen concentrations: it may represent a compensatory effect in response to a less favorable uterine environment. Key Words: rat, hypertrophy, age, growth, progesterone, placenta, fetus, litter-size, uterus
INTRODUCTION A DECLINE in litter size with increasing maternal age has been described in a number of polytocous laboratory mammals (reviewed by Talbert, 1968). This decline is thought to be due to increased embryonic wasteage rather than to a reduction in ovulation rate in the older animals (Talbert, 1968: Adams, 1970). An inadequate maternal environment appears to be an important factor in this wasteage (Biggers, 1969) since it has been shown that in mice, hamsters and rabbits, fertilised eggs harvested fl'om young animals survive more poorly when transferred to uteri of old rather than young recipient animals (Biggers, 1969). Despite the general decline in fertility in aged laboratory animals, few studies have examined the effects of age on fetal growth. Furthermore, there have been no studies on the effects of age on placental growth even though placental inadequacy is a major factor contributing to embryonic and fetal wasteage and growth retardation. The present work was directed at establishing fetal growth patterns in young rats at the beginning of their reproductive career and in older rats, culled from commercial breeding stock as exceeding the limit of their efficient reproductive capacity. Placental C o r r e s p o n d e n c e to: Dr. N. W. Bruce. *Present address: Dept. of H u m a n Biology, Curtin University of Technology, Kent St., Bentley, W e s t e r n Australia 6102, Australia. ~Rcceived 4 N o v e m b e r 1986: Accepted 23 February 1987) 257
258
A. RAHIMA and N.W BRUCE
weights were examined as one index of the maternal e n v i r o n m e n t likely to affect fetal growth. The blood concentrations of the two major progestagens secreted by the rat, namely progesterone and 20~-dihydroprogesterone were measured. There is s o m e evidence that progesterone concentrations are raised in older animals (Albrecht. 1985) but no information was available on peripheral blood concentrations of 2()c~dihydroprogesterone. MATERIALS AND METHODS The young rats were nulliparous, 3-5 months old and weighed 232 +_ 4.9 g (mean + s.e.m.) at mating. The old rats were multiparous (6 or more litters), 12-15 months old and weighed 308 _+ 7.1 g at mating. All rats were obtained from a closed colony of albino Wistars supplied by the Animal Resources Centre, Murdoch, Western Australia. Before use in this e x p e r i m e n t they were acclimatized in an environmentally controlled r o o m (temperature, 17-23°C; relative humidity, 50-70~/~; light on 07:00 to 21:00 h). Food and water were freely available. Rats were mated overnight and the day on which s p e r m a t o z o a were found in a vaginal s m e a r was called day 1 of gestation: rats in this colony normally litter on the morning of day 23. Rats were killed with an o v e r d o s e of sodium pentobarbitone on the morning of days 13, 16 or 22 of pregnancy. The entire uterus of each rat, together with attached oviducts, ovaries, cervix and vagina, was removed. Each uterine horn was dissected free from its m e s o m e t r i u m and from its attachment to the vaginal wall and contralateral horn. The n u m b e r and position of each implanatation site was recorded and an incision was made along the anti-mesometrial b o r d e r for the full length of the horn taking care not to rupture the conceptual sacs. The conceptuses (fetuses, fetal m e m b r a n e s and placentas) were peeled a w a y from the e n d o m e t r i u m by undercutting their a t t a c h m e n t to the underlying uterus with the smooth side of curved iris forceps. T h e y were c o v e r e d with plastic film to prevent dehydration and after the uterine horn had been examined, the fetal m e m b r a n e s and umbilical cord were r e m o v e d , the fetus and placenta uniformly wiped on a plastic surface to r e m o v e excess fluids and weighed to the nearest milligram (Norman and Bruce, 1979). Progestagen concentrations were examined in rats at days 16 and 22 of gestation only. A blood sample was taken fi'om the dorsal aorta of each rat before it had fully succ u m b e d to the o v e r d o s e of anaesthetic. The sample was hemolysed with twice the sample volume of double-distilled water and stored at - 1 0 ° C until analysed. Progesterone and 20~-dihydroprogesterone concentrations were determined by RIA as described by M e y e r and Bruce (1979) and Dharmarajan et al. (1986), respectively. All samples were m e a s u r e d in one assay run: the within assay coefficients of variation were 8.0% and 5.1% for the progesterone and 20oe-dihydroprogesterone, respectively. Results are e x p r e s s e d as means +_ s.e.m., c o m p a r i s o n s between groups were made by unpaired t tests. RESULTS General
Some of the older rats initially allocated to the e x p e r i m e n t had abnormal vaginal smears with excessive mucus and failed to exhibit an oestrous s m e a r or mate. Only rats
259
AGE, MULTIPARITY AND FETAL GROWTH
T A B L E 1. F E T A l AN D P L A C E N T A L W E I G H T S IN Y O U N G AND O L D RATS E X A M I N E D ON DAYS
13, 16 A N D 22 OF
GESTATION
No. ~[" rats
No. o f implant sites
No. o1" live fetuses
5 5
13.2 _+ 0.4 9.4_+ 1.9
12.8 _+ 0.2 8.0+_ 1.8"
7 7
12.4 _+ 0.9 9.7 _+ 0.5*
11.4_+ 0.9 9.7 +_ 0.5
7 10
12.9 _+ 0.8 11.2 _+ 0.6
12.4 + 0.9 11.2 _+ 0.6
Fetal ~'eig,hts (mg,)
Place,tal ~'eig,hts (tilL')
Fetal weight Place,tal ~'ei~,ht ratio
Day 13 Young Old
25.3 _+ 24.2_+
1.1 I.l
58.5 _+ 2.4 53.2_+ 0.5
0.4 0.5
223.4 _+ 222.1 _+
6.0 8.7
194.5 + 4.4 332.3 _+ 41.3"*
1.2 0.7***
4571.2 + 69.9 4544.1 _+ 126.7
413.0 + 8.3 571.1 _+ 31.9"*
11.1 8.1"**
Day 16 You ng Old Day 22 You ng Old
Values given are m e a n s _+ s.e.m. *p<0.05; **p<0.01; ***p<0.001: Old value significantly different to young value (unpaired t test).
with normal smears were included in the study. O f the three groups of old rats, those examined at days 13 and 16 had f e w e r implantation sites and live fetuses per litter than the c o m p a r a b l e young rats but there was no difference between those examined at day 22 (Table 1). There was no evidence o f increased fetal mortality subsequent to the establishment of implantation sites in any of the groups examined.
Fetal-placental weights Fetal weight was not significantly affected by age at any stage examined. Placental weight, on the other hand, though similar in the young and old rats at day 13, was 70% heavier in the old rats by day 16 and remained 38% heavier in the old rats near term at day 22.
Peripheral blood progestagen concentrations The blood concentrations of progesterone were similar in the young and old rats at day 16 (48.3 _+ 4.0 and 47.8 _+ 9.5 ng/ml, respectively) and at day 22 (28.9 + 8.3 and 30.6 _+ 1.8 ng/ml). The concentration o f 20~-dihydroprogesterone, on the other hand, was 64% higher in the old rats at day 16 (15.5 _+ 0.5 and 25.0 _+ 2.6 ng/ml) but similar in young and old rats at day 22 (27.6 _+ 7.2 and 30.6 _+ 1.8 ng/ml, respectively).
DISCUSSION Despite the well-documented reduction in litter size and fertility, little has been published on the changes in birth weight with advancing maternal age. An inadequate maternal e n v i r o n m e n t , sufficient to c o m p r o m i s e fertility might be e x p e c t e d also to affect fetal and perhaps placental growth.
260
A. R A H I M A and N.W. B R U C E
In the present work, the old rats had already been discarded from the commercial breeding unit as having reached the age when their fertility was likely to be impaired. This was borne out by the finding that in 2 of the three groups of older rats there was a decrease of 20-30c~ in the number of implantation sites and live fetuses per litter. Despite this, there was no indication that fetal weight was reduced in any of the three groups of old rats examined. Indeed, the pattern of growth over days 13-22, during which fetal weight increases about 180-fold, was identical in the old and young rats. It would therefore seem that once implantation and development to day 13 is accomplished, subsequent fetal growth is normal in old rats. This would imply that any adverse uterine environment which may have limited the initial number of successful implantations either was no longer a significant factor later in gestation, or that some c o m p e n s a t o r y mechanism was brought into action. The finding of hypertrophied placentas at days 16 and 22 could be pertinent here. A similar degree of placental hypertrophy has been observed in rats either hemorrhaged (Bruce and Cabral, 1975) o1" in which some of the major vessels supplying the uterus were ligated early in gestation (Bruce, 1976). It was postulated that the hypertrophy observed with these stimuli was to compensate for a reduced oxygen delivery to the placentas in the hemorrhaged rats and a reduced placental perfusion pressure in the ligated rats. There is good evidence that the uterine circulation is impaired, at least early in gestation, in aged laboratory animals (Larson and Foote, 1972; Rahima and Soderwall, 1978) and so the placental hypertrophy in the older rats in the present work might again represent a c o m p e n s a t o r y response in an attempt by the mother to ensure an adequate supply of oxygen and nutrients to the fetuses. It is also feasible that the placental hypertrophy has an endocrinological basis. Csapo and Weist (1973) have reported that high levels of oestradiol retard placental growth in rats, and Larson et al. (1972) have shown that at least in in vitro the uptake ofoestradiol by uterine tissues is up to four times greater in young than old rabbits. If the latter p h e n o m e n o n occurs also in rats then placental growth might be greater in the older animals with their lower concentration of oestradiol in their uterine tissues. The present results show that differences in progesterone concentrations were unlikely to have contributed to the placental hypertrophy in the older rats. There was no difference in progesterone concentrations between young and old rats at either days 16 or 22. This finding is consistent with the report of Miller and Riegle (1980) who examined cycling and pseudopregnant rats. Although in a recent paper by Albrecht (1985), progesterone concentrations were found to be higher at some stages in older rats. Even if the progesterone levels were higher in the old rats, this would not have affected placental weights since progesterone concentrations can be elevated up to five times normal endogenous levels with no apparent effect on placental weights (Bartholomeusz and Bruce, 1976). The significant increase in 20c~-dihydroprogesterone in the old rats at day 16 is again unlikely to have affected placental weight given the relative lack of potency of this progestagen (Talwalker et al., 1966). In conclusion, given that the hypertrophied placentas in the old rats could not be satisfactorily explained by changes in litter size or hormone concentration, the possibility that it simply compensates for a less favorable maternal environment seems most likely. Further work is now needed to examine these aspects of the maternal environment which elicited the hypertrophic response.
AGE, MULTlPARITYAND FETAL GROWTH
261
Acknowledgments - - We thank Ms. J. Kreibich and M. A. Stuart for expert technical assistance. This work was supported by the Australian Research Grants Scheme and the Mary Raine Memorial Research Foundation.
REFERENCES Adams, C.E. Ageing and reproduction in the female mammal with particular reference to the rabbit../. Reprod. Fert. Suppl. 12, 1-16, 1970. Albrecht, E.D. Pregnancy in young and aged rats: II. Peripheral serum progesterone concentrations. Biol. Reprod. 33,432-435, 1985. Bartholomeusz, R.K. and Bruce, N.W. Effects of maternal progesterone supplementation on fetal, placental and corpus luteal weights in the rat. Biol. Reprod. 15, 84-89, 1976. Biggers, J.D. Problems concerning the uterine causes of embryonic death, with special reference to the effects of ageing of the uterus. J. Reprod. Fert. Suppl. 8, 27-43. 1969. Bruce, N.W. The effect ofligating a uterine artery on fetal and placental development in the rat. Biol. Reprod. 14, 246-247. 1976. Bruce, N.W. and Cabral, D.A. Effects of maternal blood loss on embryonic and placental development in the rat. J. Reprod. Fert. 45, 349-356, 1975. Csapo, A.I. and Wiest, W.G. Plasma steroid levels and ovariectomy-induced placental hypertrophy in rats. Endocrinolo~,,y 93, 1173-1177, 1973. Dharmarajan, A.M., Bruce. N.W., and McArdle. H.J. Comparison of flow rates and composition of ovarian lymph and blood in the Day-16 pregnant rat. J. Reprod. Fert. 77, 169-176. 1986. Larson, L.L., Spilman, C.H., and Foote, R.H. Uterine uptake of progesterone and estradiol in young and aged rabbits. Proc. Soc. E.~p. Biol. Med. 141,436-466, 1972. Larson, L i . and Foote, R. H. Uterine blood flow in young and aged rabbits. Proc. Soc. E.~7). Biol. Med. 141, 67-69, 1972. Meyer, G.T. and Bruce, N.W. Corpus luteum growth and plasma progesterone levels in unilaterally ovariectomized pregnant rats. Anat. Rec. 195, 311-316, 1979. Miller, A.E. and Riegle, G.D. Temporal changes in serum progesterone in ageing female rats. Endocrinolo.~,y 106, 1579-1583, 1980. Norman, N.A. and Bruce, N.W. Fetal and placental weight relationships in the rat at Days 13 and 17 of gestation. J. Reprod. Fert. 57, 345-348, 1979. Rahima, A. and Soderwall, A.L. Endometrial blood flow in pregnant young and senescent female golden hamsters. E.~p. Geront. 13, 47-49, 1978. Talbert, G.B. Effect of maternal age on reproductive capacity. Amer. J. Obstet. Gynec. 10213), 451-477, 1968. Talwalker, P.K., Krahenbuhl, C., and Desaulles, P.A. Maintenance of pregnancy in spayed rats with 20c~Hydroxypregn-4-ene-3-one and 20/3-Hydroxypregn-4-ene-3-one. Nature 209, 86-87, 1966.
0531-5565/87 $3.00 + .00 Copyright '~ 1987 Pergamon Journals Ltd
FETAL AND PLACENTAL G R O W T H IN YOUNG, PRIMIPAROUS AND OLD, M U L T I P A R O U S RATS A. RAHIMA* and N. W. BRUCE Department of A n a t o m y and H u m a n Biology, University of W e s t e r n Australia, N e d l a n d s , W e s t e r n Australia 6009, Australia
Abstract - - Fetal and placental weights and peripheral blood progestagen concentrations were examined on days 13, 16 and 22 of gestation in young (3-5 month) nulliparous and old (9-12 month) multiparous, albino Wistar rats. There was no difference in fetal weights at any stage examined whereas placental weights were significantly heavier in the old rats at days 16 (71%) and 22 (38%). Blood progesterone concentrations were similar in the two groups at days 16 and 22 although concentrations of the less biologically active progestagen, 20~-dihydroprogesterone, were higher (61%) at day 16 in the old rats. The relative hypertrophy of the placentas in the old rats could not be explained by reduced litter size or progestagen concentrations: it may represent a compensatory effect in response to a less favorable uterine environment. Key Words: rat, hypertrophy, age, growth, progesterone, placenta, fetus, litter-size, uterus
INTRODUCTION A DECLINE in litter size with increasing maternal age has been described in a number of polytocous laboratory mammals (reviewed by Talbert, 1968). This decline is thought to be due to increased embryonic wasteage rather than to a reduction in ovulation rate in the older animals (Talbert, 1968: Adams, 1970). An inadequate maternal environment appears to be an important factor in this wasteage (Biggers, 1969) since it has been shown that in mice, hamsters and rabbits, fertilised eggs harvested fl'om young animals survive more poorly when transferred to uteri of old rather than young recipient animals (Biggers, 1969). Despite the general decline in fertility in aged laboratory animals, few studies have examined the effects of age on fetal growth. Furthermore, there have been no studies on the effects of age on placental growth even though placental inadequacy is a major factor contributing to embryonic and fetal wasteage and growth retardation. The present work was directed at establishing fetal growth patterns in young rats at the beginning of their reproductive career and in older rats, culled from commercial breeding stock as exceeding the limit of their efficient reproductive capacity. Placental C o r r e s p o n d e n c e to: Dr. N. W. Bruce. *Present address: Dept. of H u m a n Biology, Curtin University of Technology, Kent St., Bentley, W e s t e r n Australia 6102, Australia. ~Rcceived 4 N o v e m b e r 1986: Accepted 23 February 1987) 257
258
A. RAHIMA and N.W BRUCE
weights were examined as one index of the maternal e n v i r o n m e n t likely to affect fetal growth. The blood concentrations of the two major progestagens secreted by the rat, namely progesterone and 20~-dihydroprogesterone were measured. There is s o m e evidence that progesterone concentrations are raised in older animals (Albrecht. 1985) but no information was available on peripheral blood concentrations of 2()c~dihydroprogesterone. MATERIALS AND METHODS The young rats were nulliparous, 3-5 months old and weighed 232 +_ 4.9 g (mean + s.e.m.) at mating. The old rats were multiparous (6 or more litters), 12-15 months old and weighed 308 _+ 7.1 g at mating. All rats were obtained from a closed colony of albino Wistars supplied by the Animal Resources Centre, Murdoch, Western Australia. Before use in this e x p e r i m e n t they were acclimatized in an environmentally controlled r o o m (temperature, 17-23°C; relative humidity, 50-70~/~; light on 07:00 to 21:00 h). Food and water were freely available. Rats were mated overnight and the day on which s p e r m a t o z o a were found in a vaginal s m e a r was called day 1 of gestation: rats in this colony normally litter on the morning of day 23. Rats were killed with an o v e r d o s e of sodium pentobarbitone on the morning of days 13, 16 or 22 of pregnancy. The entire uterus of each rat, together with attached oviducts, ovaries, cervix and vagina, was removed. Each uterine horn was dissected free from its m e s o m e t r i u m and from its attachment to the vaginal wall and contralateral horn. The n u m b e r and position of each implanatation site was recorded and an incision was made along the anti-mesometrial b o r d e r for the full length of the horn taking care not to rupture the conceptual sacs. The conceptuses (fetuses, fetal m e m b r a n e s and placentas) were peeled a w a y from the e n d o m e t r i u m by undercutting their a t t a c h m e n t to the underlying uterus with the smooth side of curved iris forceps. T h e y were c o v e r e d with plastic film to prevent dehydration and after the uterine horn had been examined, the fetal m e m b r a n e s and umbilical cord were r e m o v e d , the fetus and placenta uniformly wiped on a plastic surface to r e m o v e excess fluids and weighed to the nearest milligram (Norman and Bruce, 1979). Progestagen concentrations were examined in rats at days 16 and 22 of gestation only. A blood sample was taken fi'om the dorsal aorta of each rat before it had fully succ u m b e d to the o v e r d o s e of anaesthetic. The sample was hemolysed with twice the sample volume of double-distilled water and stored at - 1 0 ° C until analysed. Progesterone and 20~-dihydroprogesterone concentrations were determined by RIA as described by M e y e r and Bruce (1979) and Dharmarajan et al. (1986), respectively. All samples were m e a s u r e d in one assay run: the within assay coefficients of variation were 8.0% and 5.1% for the progesterone and 20oe-dihydroprogesterone, respectively. Results are e x p r e s s e d as means +_ s.e.m., c o m p a r i s o n s between groups were made by unpaired t tests. RESULTS General
Some of the older rats initially allocated to the e x p e r i m e n t had abnormal vaginal smears with excessive mucus and failed to exhibit an oestrous s m e a r or mate. Only rats
259
AGE, MULTIPARITY AND FETAL GROWTH
T A B L E 1. F E T A l AN D P L A C E N T A L W E I G H T S IN Y O U N G AND O L D RATS E X A M I N E D ON DAYS
13, 16 A N D 22 OF
GESTATION
No. ~[" rats
No. o f implant sites
No. o1" live fetuses
5 5
13.2 _+ 0.4 9.4_+ 1.9
12.8 _+ 0.2 8.0+_ 1.8"
7 7
12.4 _+ 0.9 9.7 _+ 0.5*
11.4_+ 0.9 9.7 +_ 0.5
7 10
12.9 _+ 0.8 11.2 _+ 0.6
12.4 + 0.9 11.2 _+ 0.6
Fetal ~'eig,hts (mg,)
Place,tal ~'eig,hts (tilL')
Fetal weight Place,tal ~'ei~,ht ratio
Day 13 Young Old
25.3 _+ 24.2_+
1.1 I.l
58.5 _+ 2.4 53.2_+ 0.5
0.4 0.5
223.4 _+ 222.1 _+
6.0 8.7
194.5 + 4.4 332.3 _+ 41.3"*
1.2 0.7***
4571.2 + 69.9 4544.1 _+ 126.7
413.0 + 8.3 571.1 _+ 31.9"*
11.1 8.1"**
Day 16 You ng Old Day 22 You ng Old
Values given are m e a n s _+ s.e.m. *p<0.05; **p<0.01; ***p<0.001: Old value significantly different to young value (unpaired t test).
with normal smears were included in the study. O f the three groups of old rats, those examined at days 13 and 16 had f e w e r implantation sites and live fetuses per litter than the c o m p a r a b l e young rats but there was no difference between those examined at day 22 (Table 1). There was no evidence o f increased fetal mortality subsequent to the establishment of implantation sites in any of the groups examined.
Fetal-placental weights Fetal weight was not significantly affected by age at any stage examined. Placental weight, on the other hand, though similar in the young and old rats at day 13, was 70% heavier in the old rats by day 16 and remained 38% heavier in the old rats near term at day 22.
Peripheral blood progestagen concentrations The blood concentrations of progesterone were similar in the young and old rats at day 16 (48.3 _+ 4.0 and 47.8 _+ 9.5 ng/ml, respectively) and at day 22 (28.9 + 8.3 and 30.6 _+ 1.8 ng/ml). The concentration o f 20~-dihydroprogesterone, on the other hand, was 64% higher in the old rats at day 16 (15.5 _+ 0.5 and 25.0 _+ 2.6 ng/ml) but similar in young and old rats at day 22 (27.6 _+ 7.2 and 30.6 _+ 1.8 ng/ml, respectively).
DISCUSSION Despite the well-documented reduction in litter size and fertility, little has been published on the changes in birth weight with advancing maternal age. An inadequate maternal e n v i r o n m e n t , sufficient to c o m p r o m i s e fertility might be e x p e c t e d also to affect fetal and perhaps placental growth.
260
A. R A H I M A and N.W. B R U C E
In the present work, the old rats had already been discarded from the commercial breeding unit as having reached the age when their fertility was likely to be impaired. This was borne out by the finding that in 2 of the three groups of older rats there was a decrease of 20-30c~ in the number of implantation sites and live fetuses per litter. Despite this, there was no indication that fetal weight was reduced in any of the three groups of old rats examined. Indeed, the pattern of growth over days 13-22, during which fetal weight increases about 180-fold, was identical in the old and young rats. It would therefore seem that once implantation and development to day 13 is accomplished, subsequent fetal growth is normal in old rats. This would imply that any adverse uterine environment which may have limited the initial number of successful implantations either was no longer a significant factor later in gestation, or that some c o m p e n s a t o r y mechanism was brought into action. The finding of hypertrophied placentas at days 16 and 22 could be pertinent here. A similar degree of placental hypertrophy has been observed in rats either hemorrhaged (Bruce and Cabral, 1975) o1" in which some of the major vessels supplying the uterus were ligated early in gestation (Bruce, 1976). It was postulated that the hypertrophy observed with these stimuli was to compensate for a reduced oxygen delivery to the placentas in the hemorrhaged rats and a reduced placental perfusion pressure in the ligated rats. There is good evidence that the uterine circulation is impaired, at least early in gestation, in aged laboratory animals (Larson and Foote, 1972; Rahima and Soderwall, 1978) and so the placental hypertrophy in the older rats in the present work might again represent a c o m p e n s a t o r y response in an attempt by the mother to ensure an adequate supply of oxygen and nutrients to the fetuses. It is also feasible that the placental hypertrophy has an endocrinological basis. Csapo and Weist (1973) have reported that high levels of oestradiol retard placental growth in rats, and Larson et al. (1972) have shown that at least in in vitro the uptake ofoestradiol by uterine tissues is up to four times greater in young than old rabbits. If the latter p h e n o m e n o n occurs also in rats then placental growth might be greater in the older animals with their lower concentration of oestradiol in their uterine tissues. The present results show that differences in progesterone concentrations were unlikely to have contributed to the placental hypertrophy in the older rats. There was no difference in progesterone concentrations between young and old rats at either days 16 or 22. This finding is consistent with the report of Miller and Riegle (1980) who examined cycling and pseudopregnant rats. Although in a recent paper by Albrecht (1985), progesterone concentrations were found to be higher at some stages in older rats. Even if the progesterone levels were higher in the old rats, this would not have affected placental weights since progesterone concentrations can be elevated up to five times normal endogenous levels with no apparent effect on placental weights (Bartholomeusz and Bruce, 1976). The significant increase in 20c~-dihydroprogesterone in the old rats at day 16 is again unlikely to have affected placental weight given the relative lack of potency of this progestagen (Talwalker et al., 1966). In conclusion, given that the hypertrophied placentas in the old rats could not be satisfactorily explained by changes in litter size or hormone concentration, the possibility that it simply compensates for a less favorable maternal environment seems most likely. Further work is now needed to examine these aspects of the maternal environment which elicited the hypertrophic response.
AGE, MULTlPARITYAND FETAL GROWTH
261
Acknowledgments - - We thank Ms. J. Kreibich and M. A. Stuart for expert technical assistance. This work was supported by the Australian Research Grants Scheme and the Mary Raine Memorial Research Foundation.
REFERENCES Adams, C.E. Ageing and reproduction in the female mammal with particular reference to the rabbit../. Reprod. Fert. Suppl. 12, 1-16, 1970. Albrecht, E.D. Pregnancy in young and aged rats: II. Peripheral serum progesterone concentrations. Biol. Reprod. 33,432-435, 1985. Bartholomeusz, R.K. and Bruce, N.W. Effects of maternal progesterone supplementation on fetal, placental and corpus luteal weights in the rat. Biol. Reprod. 15, 84-89, 1976. Biggers, J.D. Problems concerning the uterine causes of embryonic death, with special reference to the effects of ageing of the uterus. J. Reprod. Fert. Suppl. 8, 27-43. 1969. Bruce, N.W. The effect ofligating a uterine artery on fetal and placental development in the rat. Biol. Reprod. 14, 246-247. 1976. Bruce, N.W. and Cabral, D.A. Effects of maternal blood loss on embryonic and placental development in the rat. J. Reprod. Fert. 45, 349-356, 1975. Csapo, A.I. and Wiest, W.G. Plasma steroid levels and ovariectomy-induced placental hypertrophy in rats. Endocrinolo~,,y 93, 1173-1177, 1973. Dharmarajan, A.M., Bruce. N.W., and McArdle. H.J. Comparison of flow rates and composition of ovarian lymph and blood in the Day-16 pregnant rat. J. Reprod. Fert. 77, 169-176. 1986. Larson, L.L., Spilman, C.H., and Foote, R.H. Uterine uptake of progesterone and estradiol in young and aged rabbits. Proc. Soc. E.~p. Biol. Med. 141,436-466, 1972. Larson, L i . and Foote, R. H. Uterine blood flow in young and aged rabbits. Proc. Soc. E.~7). Biol. Med. 141, 67-69, 1972. Meyer, G.T. and Bruce, N.W. Corpus luteum growth and plasma progesterone levels in unilaterally ovariectomized pregnant rats. Anat. Rec. 195, 311-316, 1979. Miller, A.E. and Riegle, G.D. Temporal changes in serum progesterone in ageing female rats. Endocrinolo.~,y 106, 1579-1583, 1980. Norman, N.A. and Bruce, N.W. Fetal and placental weight relationships in the rat at Days 13 and 17 of gestation. J. Reprod. Fert. 57, 345-348, 1979. Rahima, A. and Soderwall, A.L. Endometrial blood flow in pregnant young and senescent female golden hamsters. E.~p. Geront. 13, 47-49, 1978. Talbert, G.B. Effect of maternal age on reproductive capacity. Amer. J. Obstet. Gynec. 10213), 451-477, 1968. Talwalker, P.K., Krahenbuhl, C., and Desaulles, P.A. Maintenance of pregnancy in spayed rats with 20c~Hydroxypregn-4-ene-3-one and 20/3-Hydroxypregn-4-ene-3-one. Nature 209, 86-87, 1966.