- Email: [email protected]
Uterine morphology and function in postpartum mares
~.~~
~
Refereed
UTERINEMORPHOLOGYAND FUNCTION IN POSTPARTUMMARES P. G. Griffin, DVM, MS and O. J. Ginther, VMD, PhD
SUMMARY
Ultrasonically detectable characteristics of the uterus and embryo and palpable uterine tone were assessed in 10 postpartum mares. A bright fern-like pattern of ultrasonic uterine echogenicity, outlining the endometrial folds, was observed for an average of 2.1 _+0.2days following parturition (range, 1 to 3 days). Unexpectedly, the uterus was quiescent throughout the postpartum interval, based on daily one-minute contractility scans. Contractilitywas maximal on Days 12 to 15 of pregnancy in both postpartum (n--7) and nonparturient (n-7) mares. The mean diameter of ultrasonically detectable intrauterine fluid collections increased (P<0.05) abruptly between days 1 and 2 postpartum and gradually decreased (P<0.05) between days 4 and 7; no collections were detected after day 16. There was no effectof day on echogenicity of the intrauterine fluid collections; on all days, fluid was relatively black or nonechogenic, suggesting that puerperal endometritis was not a problem in this group. Because the increase in intraluminal fluid occurred after parturition and in temporal association with a decrease in diameter and tone of the uterus, the fluid Authors' address: Department of Veterinary Science, University of Wisconsin-Madison, 1655 Linden Drive, Madison, W153706. Acknowledgments: Supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison. The authors thank Dr. G. P. Adams, R. Gneiser, P. Krcma and L. Kulick for technical assistance.
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collections apparently represented a physiologic influx from the involuting uterus rather than residual placental fluid. Involution of the homs was completed by day 27 (formerly nongravid horn) and day 31 postpartum (formerly gravid horn), based on failure to detect further significant decreases in diameter. However, the formerly gravid horn was larger (P<0.05) in diameter than the formerly nongravid horn on each of Days 1 to 35 postpartum (end of experiment), indicating residual effects on uterine size. When averaged over both horns, uterine diameters were larger on Days 0 to 24 (Day 0=day of ovulation) of pregnancy in postpartum mares than in nonparturient mares; by Day 25, diameters were similar between statuses~ By approximately Day 6 of pregnancy, uterine contractility and ultrasonic endometrial exhotexture were similar between postpartum mares and nonparturient mares. Uterine tone was greater (P<0.05) in postpartum mares than in nonparturient mares on all days between Day 0 and 25. An unexpected, transient increase in uterine tone was detected on Day 5 of pregnancy in both postpartum mares and nonparturient mares. No differences were found between reproductive statuses in patterns of embryo mobility, the day of fixation of the embryonic vesicle (postpartum, Day 15.3 _+0.4; nonparturient, Day 15.0 _+0.3), and diameter of the embryonic vesicle on the day of fixation (postpartum, 22.1 +1.4 mm; nonparturient, 19.4 +l.6mm). However, mean uterine tone and mean horn diameters on the side of fixation were greater (cranial and middle comual segments; P<0.05) EQUINE VETERINARY SCIENCE
or tended to be greater (caudal segment; P<0.1 ) on the day of fixation in postpartum mares than in nonparturient mares. In all postpartum mares, fixation occurred in the formerly nongravid horn. Enhanced uterine tone in postpartum mares may account for the occurrence of fLxationon the same day for the two reproductive statuses, despite the larger uterus in postpartum mares. INTRODUCTION The mare is unique among large domestic animals (e.g., cow, sow, ewe) in establishing and maintaining pregnancy despite a remarkably short (e.g., 9 day) interval between parturition and the first postpartum ovulation (postpartum interval). 1°,17 Given the ll-month gestation in mares, postpartum reproductive events are crucial to maintenance ofa 12month foaling interval. Examination ofdma from four reported surveys indicated that 51 to 62% of mares bred each year were postpartum. 10This consideration further emphasizes the importance of the postpartum interval to breeding programs. After parturition, the uterus undergoes a dynamic involution with a reduction in overall uterine size, repair of uterine tissues, and re-establishment of a luminal environment favorable to conceptus development. Histologic, 13 ultrastructural,25 cytologic,21 bacteriologic,13 biochemical,18 clinical3O and ultrasonic 2° aspects of involution in mares have been described. However, there is a lack of information on several uterine characteristics (e.g., uterine conUactility, tone, and ultrasonic echotexture) from parturition to fast ovulation and during the resulting early pregnancy. The objectives of the present study were as follows: 1) to characterize patterns of uterine contractility, tone, and ultrasonic echotexture during the postpartum interval (parturition to first ovulation); 2) to compare patterns of uterine contractility, tone, echotexture, and diameter during early pregnancy (Days 0 to 25) between postpartum mares and nonparturient (barren) mares; 3) to further characterize changes in uterine diameter and intrauterine fluid collections after parturition (days 1 to 35 postpartum); and 4) to characterize and compare to nonparturient mares some aspects of embryo-uterine interactions (embryo mobility, fixation). In addition, considering greater uterine diameter, the hypothesis was tested that fixation of the embryonic vesicle (cessation of embryo mobility) occurs later in postpartum mares than in nonparturient mares. Transrectal ultrasonography was used to evaluate all end points, except that uterine tone was evaluated by wansrectal palpation. MATERIALS AND METHODS Ten postpartum and 7 nonpartmient (barren) pony mares, 4-12 years-of-age and weighing 200-300 kg, were used contemporaneously. A capital "D" is used in the text to designate number of days after ovulation (e.g., Day 0 = day of ovulation) and a small "d" is used for the number of days after Volume 11, Number6, 1991
parturition (e.g., day 0 = day of parturition). The postpartum mares delivered apparendy normal foals without complication (i.e., no dystocias or retained placentae) between midJune and mid-July and were examined once daily, beginning on day 1 and continuing a n d Day 25 of pregnancy or until the end of the first postpartum interovulatory interval. Nonparturient mares were examined daily from Day 0 to Day 25. All mares were artificially inseminated with approximately 20 m l of raw semen every other day, beginning on the day of ulu:asonic detection of a >35 mm follicle and continuing until ovulation; no mares were bred on or after the day of ovulation. Ultrasonic embryo detection and monitoring began on Day 11. Daily examinations in both groups included a systematic transrectal ultrasound scan of the reproductive tract9 and transrectal digital palpation of the uterus. Ultrasound examinations were done with a real-time, B-mode scanner" equipped with a 3.5 or 7.5 MHz linear-array transducer; examinations were recorded, with oral comments, ttsinga3/4" videotape recorder connected to the scanner. For all end points, scores were assigned or measurements obtained without reference to or consideration of values for preceding days. Uterine Contractility Uterine contractility was estimated from a 1-minute continuous scan of a longitudinal section of the uterine body, as described. 4,5 The extent of contractility was scored from 1 to 5 (minimal to maximal) based on the degree of to and fro movements of endometrial tissue reflectors and the presence and extent of waves or indentations of the ventral surface of the uterus. Fractional scores (e.g., 1.5) were assigned when appropriate. Contractility scoring was done at the time of examination from the viewing screen of the scanner. Uterine Echotexture The ultrasonic echotexture of the endometrium was assessed and scored from the viewing screen at the time of examination, as described. 15A scale of 1 to 4 was used based on the degree of endometrial folding (minimal to maximal); fractional scores were assigned when appropriate. Uterine Diameter A 3.5 MHz transducer was used in postpartum mares for assessing uterine diameter when the horns were too large (>50 m m in width) to be imaged completely in cross section with a 7.5 MHz transducer; this generally occurred during the first week postpartum (range, 6 to 10 days). Each uterine horn was divided into approximately equal cranial, middle, and caudal segments. 9 The outer cross-sectional diameter (ram) of each comual segment was measured at the estimated midpoint and defined as the mean of the largest vertical and horizontal diameters. After fixation of the embryonic vesicle, diameter of the caudal cornual segment was determined immediately cranial to the vesicle on the side of fixation and at the aTokyo Keiki, LS 300, Products Group International, Inc., Boulder, CO.
331
Table 1. Reproductive characteristics in postpartum (n=10) and nonparturient in--7) mares and mean (+SEM) length (days) of components of the postpartum interval,
End point
Postpartum
Parturition to loss of uterine firmness#
5.3 _+0.4 range, 4-8
Parturition to detection of estrus echotexture
6.5 + 0.3 range, 5-8
Nonparturlent
5.4 ± 1.0 range, 2-13
Parturition to first ovulation
11.6±1.3 range, 7-19
Pregnancy rate at Day 11
7/10 (70%)
7/11 (64%)
Embryo-loss rate by Day 40
1/8 (13%)++
0/7 (0%)
Days of fixation of embryonic vesicle
15.3±0.4 range, 14-16
15.0±0.3 range, 14-16
Embryonic vesicle diameter (ram) on day of fixation
22.1 ± 1.4 range, 16-27
19.4 ± 1.6 range, 14-24
24.4±0.4 27.4±0.5 31.3±1.4
22.2±0.5 24.6±1.2 28.7±0.9
24.9±0.8 30.4±0.9 34.5±1.4 3.1±0.2
22.3±0.9 24.9±1.2 28.6±1.6 2.6±0.1
Middle* Caudal+ Horn contralateral to fixation Cranial+
Middle* Caudal, Uterine tone score on day of fixation*
Location Formerly Formerly Uterine body gravid horn nongravid horn No. of collections*+
Duration of estrous echotexture
Diameter (mm) of cornual segments on day of fixation## Horn ipsilateral to fixation Cranial*
Table 2. Frequency of location of intrauterine fluid collections between days 1 and 15 postpartum in= 10 mares and 121 fluid collections).
*Significant(P<0.05) differencebetweenreproductivestatusesfor a given
end point. +Tendency(P <0.1) for a differencebetweenstatusesfor a givenend point.
#Uterinetone score=3or 4. **Prominentendometrialfolding,echotexturescore=3or 4. ++Onevesiclewasfirstdetectedon Day14and subsequentlylostby Day 18. #:#Forpostpartummares,significant(P<0.05)differencesbetweenhornsfor diameterof middleand caudalcornualsegments.
corresponding point on the opposite horn. Measurements were obtained from the videotapes using a high-resolution video monitor and an overlay grid calibrated to the centimeter scale on the ultrasound image. For purposes of analysis, the formerly gravid horn in postpartum mares was defined as the horn occupied by the fetus during late gestation, as determined by ultrasonography. For all foaling mares, the horn which contained the fetus at term was the same as the horn of fixation and umbilical attachment.
69/121 (57%)
44/121 (36%)
8/121 (7%)
Uterine segment Cranial 15/69 (22%) a Middle 36/69 (52%) b Caudal 18/69 (26%) a
13/44 (30%)a 9/44 (20°/o)a 22/44 (50%)b
5/8 (63%) a 0/8 (0%)b 3/8 (37%) =
*More (P<0.05)fluidcollectionswere detected in the uterinebody (69/121) than in both uterinehornscombined(52/121). +Frequenciesare different(P
examination was determined and assigned to 1 of 9 uterine segments (each comual segment and cranial, middle, and caudal segments of uterine body). The ultrasonic echogenicity (whiteness) of each fluid collection was assessed and scored on a scale of 1 to 4 ( 1=black or nonechogenic; 4=white or hyperechogenic). For mares with more than one collection on a given day, echogenicity scores were averaged.
Embryonic Vesicle The size of the embryonic vesicle was obtained by averaging its largest vertical and horizontal diameters. Location of the vesicle at the start of the examination was determined and assigned to 1 of the 9 uterine segments. On Day 13, a 2-hour embryo mobility trial was conducted in each mare, as described,s Atrial consisted of 1 determination of embryo location (among the 9 segments of the uterus) every 5 minutes for a total of 25 determinations per mare. Daily mobility trials continued after Day 13, and the day of fixation was defined as the first day in which no change in vesicle location was detected during a mobility trial and between subsequent daily examinations. Uterine Tone
Uterine tone was assessed digitally at the approximate mid-point of the uterine horns at each examination, and scored on a scale of 1 to 4) l=minimal tone, flaccid; 4=maximal tone, turgid or filTU).16Fractional scores were assigned when appropriate. Statistical A n a l y s e s
Intrauterine Fluid Collections The diameter of intrauterine fluid collections was estimated from the videotapes by averaging maximal height and width. When more than one discrete fluid collection was detected in a mare on a given day, the mean values for each collection were summed to obtain one value for the examination. The location of each collection at the start of the
332
Sequential data were analyzed by split plot analyses of variance using the Statistical Analysis System General Linear Models procedure.22 When significant (P<0.05) effects or interactions were detected, paired t-tests, t-tests, or least significant difference were used to further examine differences. Frequency data were examined by Chi-square analyses. Differences in single point data due to reproductive status
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Fig ure 1. Ultrasonograms from one mare of the formerly nongravid horn (upper series of images) and formerly gravid horn (lower series of images). The n umber of days postpartum is indicated in the lower right hand corner of each image. All images are crosssectional and the short arrows indicate the ventral surface of the uterus. In the upper series of images, the long arrows indicate an echogenic steel bead which was sutured to the uterine serosa as part of an earlier study and which serves to indicate that the images are from the same area. Note the bright, fern-like pattern of endometrial echogenicity in the horns on day 1 postpartum and the decrease in its extent on day 2. In this mare, the pattern was not detected after day 2. The nonechogenic (black) areas on the dorsal surface of the uterus on day 1 represent dilated blood vessels. Note also the decrease in size of the horns as the postpartum interval progressed. By day 8, the mare was entering her first postpartum estrus, as evidenced by the characteristic prominent endometrial folding shown here in the formerly gravid uterine horn.
(postpartum versus nonparturient) were examined by t-tests; paired t-tests were used to detect differences within each status. RESULTS
Mean (+SEM) lengths (days) of components of the postpartum interval (parturition to first ovulation) and a comparison of reproductive characteristics between postpartum mares and nonparturient mares are summarized in Table l. In postpartum mares, adistinctive fem-likepattern of bright endometrial echogenicity was observed for an average of 2. l _+0.2days following parturition (range, 1-3 days; Fig. l). The
Volume 11, Number 6, 1991
pattern seemed to outline the luminal surface of the endometrial folds and was noted throughout all uterine segments. Mean uterine contractility, tone, and echotexture scores, uterine horn diameters, and diameters of intrauterine fluid collection for days I to l0 for postpartum mares and for Days 0 to 25 of pregnancy for postpartum mares and nonparturient mares are shown in Figure 2, along with results of the statistical analyses. No day effect on echogenicity scores for intrauterine fluid collections was detected between days 1 and 14 postpartum; mean scores ranged from 1.0_+0.0to 1.4_+0.2. The frequency of location of intrauterine fluid collections between days 1 and 15 is summarized in Table 2. Mean uterine horn diameters, averaged over all cornual segments, for postpartum mares between Days 1 and 35
333
l~t ii \
1
Postpartum
i~\/
~)0 ~O 0~
(..7)
O: P
I`o.,, l .2 E
50'
aa
]
40'
m
i
30'
"oE "5 .to n-,'o
20' bc'
be }
10" 0' 4" "h
eb
3"
=8 I-ca O
2"
c
i
I%'
t.]_~
x'f ~
~,d,. ~-'~o=i~-
~-Y ~ _.~' ' D: P
~'~i 4{"
1"
r'tl
i'i
5~
J=
illl
8"--
o
I
i
Formorly f 60'
r'- "..,-"
oh
'°I
50'
l •
40'
",',~ l"~-~,.O~,
S: P<0.0005 D: P<0.0001
SD: P
30' 20'
g • i 1 5
] Nonparturient • i iJ - , - l - i - i - ~ - l . 9 lOV 4 8 12 16 20 24
Days postpartum
Days of pregnancy
Figure 2. Mean (+SEM) uterine echotexture, tone, and contractility scores, uterine diameters averaged over all cornual segments, and diameters of intrauterine fluid collections for postpartum mares (n= 10) and for pregnant postpartum (n=7) and nonparturient mares (n=7). S, effect of reproductive status; D, effect of day; SD, reproductive status x day interaction; NS, not significant. For echotexture scores, means were different (P<0.05) between statuses on Days 0 to 4 and on Days 7, 14, and 20. For fluid collection diameters and uterine tone scores between days 1 and 10 postpartum, there were significant effects of day; within each end point, means without at least one common superscript are different (P<0.05). For contractility scores, means were different (P<0.05) between statuses on Days 0 and 2 and tended (P<0.1) to be different on Day 4. For tone scores and uterine diameter averaged over both horns, means were different (P<0.05) between statuses on each day, except on Day 25 for horn diameter. For postpartum mares, the formerly gravid horn was greater (P<0.05) in diameter than the formerly nongravid horn on each day. 334
(including data for pregnant and nonpregnant mares) and for pregnant mares between Days 0 and 25 were analyzed separately. In both instances (days 1 to 35 and Days 0 to 25),results of statistical analyses were similar among the individual comual segments; therefore, horn diameters averaged over all segments and statistical results are presented for illustrative purposes (Fig. 2). For both analyses, there were significant effects of day and side (formerly gravid versus nongravid horn) on horn diameters; for days 1 to 35, there was a significant interaction between side and day.Additionally, for both analyses, there were significant effects of cornual segment (cranial, middle, caudal) on horn diameter. Averaged over days 1 to 35, diameter (mm) progressively increased (P<0.05) within each uterine horn from cranial to caudal segments (formerly gravid horn: cranial, 31.9 _+0.3, middle, 38.9 + 0.5, caudal, 47.7 _+0.7; formerly nongravid horn: 28.5 _+0.3, 34.1 _+0.4, 39.1 _+0.5,respectively). Similarly, averaged over Days 0 to 25, segmentdiameter increased from cranial to caudal (formerly gravid horn: cranial, 27.9_+0.3, middle, 32.9 _+0.4, caudal, 38.6 _+0.5; formerly nongravid horn: 25.6 _+0.3, 29.7 _+0.3, 34.1 _+0.4,respectively). For nonparturient mares, no effect of side (ipsilateral versus conlralateral to side of fixation) on horn diameters was detected for any cornual segment or when all segments were averaged. However, there was an effect (P<0.0001) of cornual segment on horn diameter; averaged over both horns and Days 0 to 25, diameters (ram) increased (P<0.05) from cranial (22.8 _+0.2) to middle (26.0 _+0.2) to caudal (29.8 _+0.2) segments. Additionally, in nonparturient mares, there was an effect (P<0.0001 ) of day for each segment and averaged over all segments; diameters were larger (P<0.05) on each of Days 0 to 7 than on Days 8 to 25. Involution of the uterine horns was complete on day 27 (formerly nongravid horn) and day 31 (formerly gravid horn), based on failure to detect further significant decreases in diameter. During involution, diameters decreased in a curvilinear fashion, best fitting a quadratic profile (formerly gravid horn, R2=0.96, P<0.001; formerly nongravid horn, Ra=0.94, P<0.001), with 2 approximately linear decreases between days 1 and 10 and between day 11 and the completion of involution. Therefore, for each cornual segment and mare, the rate for each linear decrease (involution rate) was calculated as the mean decrease in diameter (mrn/day) between days 1 and 10 and days 11 and 27 (formerly nongravid horn) or 31 (formerly gravid horn); mean involution rates are shown in Table 3. Based on Day 13 mobility trials, the mean numbers of vesicle movements between successive location determinations were not different between postpartum mares and nonparturient mares (segment-to-segment movements; 12.0 + 1.3 versus 10.1 + 1.1, body-to-horn or vice-versa; 5.0 + 1.2 versus 4.4 _+0.8, horn-to-hom; 0.7 _+0.5 versus 0.1 _+0.1,respectively). Likewise, there were no differences between or within statuses in the number of times per mobility trial that the vesicle was detected in the uterine body (postpartum, 11.5 +3.0; nonparturient, 7.3 +_2.3) or homs (postpartum, 13.5
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Figure 3. Ultrasonograms from two postpartum mares of intrauterine fluid collections located in the uterine body. Both images are longitudinal and the number of days postpartum is indicated in the lower right hand corner of each image. The ventral surfaces
of the uteri are indicated by the long arrows. The short arrows indicate the borders of the fluid collections within the uterine lumen. In both images, note the nonechogenic (black) nature of the intrauterine field.
+3.0; nonparturient, 17.7 +_2.3).Within each status, the vesicle was equally likely to be found in either hom (postpartum, formerly gravid horn, 6.8 +1.7, formerly nongravid horn, 6.7 + 1.4; nonparturient, right horn, 11.0 +_2.7,left horn, 6.7 +3.2). In 7 of 7 postpartum mares, fixation of the embryonic vesicle occurred in the caudal segment of the formerly nongravid horn. In nonparturient mares, 5 of 7 vesicles became fixed in the right horn and 2 of 7 in the left (no significant difference); all fLxations occurred in the caudal comual segment. DISCUSSION
Uterine Contraetnlty Minimal levels of ultrasonically detectable uterine contractility were observed between the day after parturition and the first ovulation (Fig. 2). This is in sharp contrast to the commonly held assumption of high levels of contractility during the postpartum interval, a,19Presumably, this assumption is based on t~e belief that enhanced contractility is an important facet of involution and facilitates evacuation of postpartum debris. Results of the present study suggest that ultrasonically detectable phases of uterine contractility play a minimal role in involution and lochial discharge during the puerperium. However, due to the length and schedule of the contractility scans, episodic bursts of contractility may have been undetected. Additionally, because of limitations of the technique, tonic or sustained myometrial contractions were not observable. There is little additional information on postpartum uterine contractility in mares. The extent of contractility and its role in uterine involution and clearance between parturition and day 1 is not clear. In an electromyographic study of
Volume 11, Number 6, 1991
myometrial activity in 2 mares during the periparturient period, 14 no consistent pattern of electrical activity was detected at 1, 3, and 5 hours postpartum. No quantitative information was presented for the period beyond 5 hours, except to note a decrease in long bursts of electrical activity (> 120 secs in duration) over the next few days; medium bursts of activity (7 to 120 secs) then became predominant. In cows, based on acute studies utilizing intrauterine pressure recordings (review27), contraction frequency decreased from approximately 12-18 contraction-relaxation cycles per hour following parturition to 0-6 cycles per hour by 48 hours postpartum. Thereafter, the postpartum uterus was relatively quiescent. It has been suggested that, in mares, considering low concentrations of progesterone and estrogen, the period between parturition and the first postpartum estrus is anestruslike. la In this regard, the process of involution in mares is apparently independent of ovarian function; ovariectomy had no effect on histologic, bacteriologic or palpable aspects of involution.a3.a4 Perhaps the observed low levels of contractility also reflect an anestrus-like condition; in seasonallyanovulatory mares, ultrasonically detectable uterine contractility was minimal. 4 The indications from the present experiment of a quiescent postpartum uterus emphasize the need for additional study of postpartum uterine conlractility. This is particularly evident in view of the advocacy of injections of myometrial stimulants as aids to preparation of the postpartum uterus for breeding. 19 Levels of uterine contractility were depressed in postpartum mares following the first ovulation and extending until Day 5 of pregnancy, when levels approached those ofnonparturient mares (Fig. 2). This occurred at approximately the time the blastocyst is expected to enter the uterus (Day 5 or 6). l° Subsequently, patterns of contractility were strikingly similar
335
between statuses until the end of the study (Day 25). The postpartum uterus, therefore, had regained its pre-gravid contractility by Day 5 or 6 of the subsequent pregnancy, presumably creating an appropriate environment for conceptus development. The overall patterns of contractility for both statuses (maximal contractility detected on Days 12 to 15 or 16) agreed with previously reported patterns for pregnant nouparturient mares, s.1 Uterine Tone and Embryo-uterine Interactions The uterus was extremely firm upon palpation for several days following parturition (Fig. 2). The firmness and consistency were qualitatively different from the characteristic turgid uterine tone of early pregnancy and likely reflected considerable postpartum edema and extensive uterine blood flow developed during gestation. The uterus was heavy and difficult to retract, hard to the touch, and often seemed to pit in response to digital pressure. The firmness gradually decreased over the postpartum interval. In contrast, it has been reported that uterine tone and "tubularity" in Thoroughbreds increased over the first 5 days postpartum.21 The reasons for the discrepancy between studies are unclear, but may be related to differences in criteria for tone assessment and to the relative inaccessibility of the uterus to palpation early in the postpartum period in larger mares. In one study, a° it was stated that the uterus could not be circumferentially palpated until approximately 3 days postpartum in horse mares. In the present study using pony mares, both uterine horns were defined and palpated thoroughly by day 1; this is consistent with an earlier report2 which indicated that the uteri of pony mares can be outlined by rectal palpation by about 12 hours postpartum. Uterine tone scores were consistently higher between Days 0 and 25 in pregnant postpartum mares than in nonparturient mares (Fig. 2). The reasons for the enhanced level of tone in postpartum mares are not known. However, there is rationale to hypothesize that it may be compensatory for larger uterine diameter during the puerperium, so that important embryo-uterine interactions, particularly fixation of the embryonic vesicle, can occur at the appropriate time. In this regard, results did not support the hypothesis that fixation occurs later in postpartum mares than in nonpartufient mares; there was no significantdifferencebetween statuses for day of fixation. Factors thought to be involved in fixation include progressively increasing embryonic vesicle diameter, increasing uterine tone, and decreasing uterine size. 7There was no difference in embryo mobility patterns or embryonic vesicle size on the day of fixation between postpartum mares and nonparturient mares. However, on the day of fixation, diameter of the comual segments on the side of fixation were larger (cranial and middle segments) or tended to be larger (caudal segment) in postpartum mares than in nonparturient mares, and uterine tone scores were greater in postpartum mares (Table 1). Perhaps, considering similar vesicle size and embryo mobility patterns, the larger diameter of the postpar-
336
tum uterine horns was negated by enhanced uterine tone, resulting in similar days of fixation between postpartum mares and nonparturient mares. In all postpartum mares, the embryonic vesicle became fixed in the caudal portion of the formerly nongravid horn, consistent with previous reports which indicated that the vesicle fixes more frequently on the formerly nongravid side? .8The number of times the vesicle was detected in either of the two horns during 2-hour mobility trials was not different, confirming an earlier study.8 Differential patterns of embryo mobility, therefore, were not responsible for the observed fixation in the formerly nongravid horn. Instead, fixation on the nongravid side was attributable to smaller diameter of the formerly nongravid horn on the day of fixation, as has been postulated,a For both statuses, the overall profiles of increasing uterine tone scores during early pregnancy were consistent with previously reported profiles for nonparturient mares. 18,2a However, for both statuses, significant transient increases in tone scores were detected several days after ovulation, with peak levels occurring on Day 5 (Fig. 2); similar increases have not been described previously and confirmation is needed. The increases in tone were temporally related to the expected entry of the blastocyst into the uterus. Uterine Echotexture
A bright"fem-like" patternof endometrial echogenicity was observed following parturition in all 10 mares at least once during days 1 to 3 postpartum (Fig. 1). This distinctive pattern apparently has not been described previously, and its origin is not known. Given its ultrasonic characteristics, however, the pattern may have been due to small amounts of air trapped between the endometrial folds. Air may have entered the uterus through the widely dilated cervix after foaling. The echogenicity and regularity of the pattern decreased over time, probably reflecting the stage of involution. The presence and intensityof this pattern may have diagnostic value, assuming that delayed involution would be reflected by a prolonged fern-like pattern. The first day postpartum of detection of an estrous echotexture (i.e., prominent endometrial folding) corresponds with reported mean days to onset of behavioral estrus (7 to 9 days; review1°). In 5 of 10 mares, echotexture scores remained estrus-like until the day of the first postpartum ovulation; in the remaining 5 mares, scores were elevated until Day -1 (3 mares), Day -2 (1 mare), and Day -4 (1 mare) prior to first ovulation. On average, the last day that postpartum uterine tone was firm preceded by approximately 1 day the first appearance of an eslrous echotexture. In nonparturient mares, estrus is associated with flaccid uterine tone. 16~ Minimal levels of uterine tone were detected on the days preceding the first postpartum ovulation (Fig. 2). However, tone scores did not decline to levels expected for nonpartufient mares preceding ovulation,16perhaps reflecting residual postpartum edema. Despite a differencein degree, the present
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Table 3. Mean (+SEM) involution rates (mm/day) for cornual segments (n=10).
Cornual segment location*+
Type of uterine horn Formerly Formerly gravid horn nongravid horn
Cranial days 1 to 10 days 11 to 27 or 31"* Middle days 1 to 10 days 11 to 27or 31 Caudal days 1 to 10++ days 11 to 27 or 31
All segments days 1 to 10++ days 11 to 27or 31
1.1 +0.2a 0.4+0.1
0.9+0.1 a
1.5 +0.2 a 0.3_+0.1
1.4 +0.1 b 0.3±0.1
2.8+0.4 b 0.5_+0.1
2.0 ±0.1 c 0.4 +0.1
1.8 _+0.2 0.4_+0.1
1.4 ±0.1 0.3_+0.1
0.3+0.1
*Mean rates for days 1 to 10 are different (P<0.05) than rates for day 11 to completion of involution for each cornual segment location. +For days 1 to 10 there was a significant (P<0.009) effect of cornual segment within each type of uterine horn. **Completion of involution; day 27 (formerly nongravid horn), day 31 (formerly gravid horn). ++Means tended (P<0.1) to be different between horns. a,",cMeans for days 1 to 10 with different superscripts are different (P<0.05) within each type of uterine horn.
study indicated that the same relative association between estrus and uterine tone exists in postpartum mares during their first ovulatory period, as in nonlactating mares during the estrous cycle. There was significant interaction between reproductive status and day on endometrial echotexture scores between Days 0 and 25 of pregnancy, due primarily to higher scores in postpartum mares than in nonparturient mares on Days 0 to 4 and on Day 7 (Fig. 2). The enhanced endometrial folding on these days may be indicative of residual postpartum edema. Interestingly, and in common with contractility scores, echotexture scores began to converge between statuses around Day 6, temporally related to the reported day of conceptus entry into the uterus. These results indicate that the uterus had regained its pre-gravid ulWasonic characteristics by this important deadline. Beginning on Days 14 (nonparturient mares) and 15 (postpartum mares), an irregular pattern of increased endometrial scores was detected. This pattern is consistent with that previously reported for pregnant mares9.12and may be related to exposure to estrogens or other compounds (e.g., prostaglandins), perhaps of embryonic origin. Intrauterine Fluid Collections The number of mares with ultrasonically detected intrauterine fluid collections and the average diameter of the fluid collections increased considerably between days 1 and 2 postpartum (Fig. 2). Five of 10 mares had fluid collections on day 1 (diameter range, 5-28 ram) compared with 9 of 10 on day 2 (range, 2-84 ram). By day 3, all mares had fluid collections and, in most cases, there was dramatic increase (>20 ram) in the diameter of collections between days i and 2 (5 mares) or between days 2 and 3 (4 mares). The abrupt
Volume 11, Number 6, 1991
increase in intrauterine fluid observed between days 1 and 2 has apparently not been described previously. This pattem of fluid accumulation suggests that the collections were not residual placental fluids but instead developed in association with involution. In this regard, the extent of fluid collections decreased temporally with the decrease in uterine tone and size. There was no effect of day on echogenicity of the fluid collections during the postpartum interval. On all days, mean echogenicity scores were <1.4, indicating relatively black or nonechogenic fluid (Fig. 3). Increasing echogenicity of intrauterine fluid has been associated with increasing content of inflammatory cells2s and, presumably, cellular debris. Perhaps, the lack of echogenicity of the postpamun fluids observed in the present study suggests that the fluiddevelopment was a physiologic phenomenon reflecting the involutionary process, rather than a pathologic or inflammatory response (e.g., due to contamination at foaling). In contrast, a previous ultrasonic studya° reported a pattem of decreasing fluid echogenicitybetween days 3 and 15 postpartum. Perhaps, the discrepancy between studies is related to the use in the present study of young mares (< 12 years), apparently healthy and free of reproductive abnormalities, which had foaled without complication; information in this regard was not presented for the previous study. The amount of intrauterine fluid gradually decreased between days 4 and 7 postpamun.Afterday 9, only small fluid collections (2-10 mm) were detected and none were found after 16 days. Similarly, the earlier ultrasonic study2° indicated that the number of mares with detectable intrauterine fluid decreased after day 5 postpartum; fluid was not detected in any mare after day 15. In the present study, fluid collections were not detected in postpartum mares after Day 3 postovulation and after Day 1 in nonparturient mares (Fig. 2). The observed distribution of intrauterine fluid collections (Table 2) may reflect differences in size of uterine segments; more collections were detected in the uterine body than in the horns and in the formerly gravid horn than in the nongravid horn. It is emphasized, however, that the observed locations of fluid collections were those detectedat the onsetofan examination; the distribution and sizes of individual collections were dynamic and often changed during an examination as fluids moved and individual collections merged. Uterine Diameter
Based on failure to detect further significantdecreases in diameter, uterine horn involution was completed by day 27 (formerly nongravid horn) and day 31 (formerlygravid horn). In the previous ultrasonic study,2°involution wasjudged to be complete in an average of 23 days postpartum (range 13-29 days). The earlier study also indicated that the formerly gravid horn was recognizable by ultrasonic assessment of size for an average of 21 days postpartum (range, 15-25 days). A study based on transrectal palpation indicated that both uterine horns returned to their pre-gravid size by day 32 p o s s . 1 3 In the present study, the formerly gravid horn was larger
337
(P<0.05) in diameter than the formerly nongravid hom, averaged over all cornual segments, on every day between days 1 and 35 postpartum and between Days 0 and 25 of pregnancy (Fig. 2). Similar results were obtained when each segment was considered individually, except for apparently randomly occurring days on which horn diameters were not different (P> 0.1). Therefore, averaged over all segments, the formerly gravid horn did not return to it pre-gravid diameter by 35 days postpartum orDay 25 of the subsequent pregnancy. The residual effects of pregnancy on horn diameter beyond 35 days postpartum are not known. Significant side (formerly gravid horn versus nongravid horn) by day interactions between days 1 and 35 postpartum were detected for diameters of the middle and caudal comual segments and when diameters of all segments were averaged. This can be attributed to the larger initial diameter and faster initial involution rates (days 1 to 10; Table 3) for cornual segments of the formerly gravid horn. Within each horn, the caudal cornual segment involuted (decreased in diameter) at a faster rate than either of the other 2 segments, while the cranial segment had the slowest rate. The patterns of involution were likely due to greater distension of the gravid horn and segments of both horns closest to the fetus. There was a rapid decrease in uterine diameter between days 1 and 10; however, between day 11 and the completion of involution, rates slowed significantly for each comual segment and averaged ovet all segments. The degree of in volution between parturition and day 1 was not examined and is in need of critical study. However, considering the extensive hypertrophy, hyperplasia, and distension of the uterus late in gestation, and the relatively small diameter of the uterine horns observed on day 1 postpartum (e.g., mean diameter of caudal segment of formerly gravid horn=approximately 77mm), profound size decreases must have occurred during the first 24 hours postpartum. Changes in diameter of uterine body were not monitored in the present study and the extent and rate of involution of the uterine body is not known. However, during prepartum scanning late in gestation, the body (along with the gravid uterine horn) contained a substantial proportion of the fetus, as previously noted, a9As a result, distension, and subsequent involution, of the uterine body (particularly the cranial segment near thecorpus-comualjunctions) may be similar to that of the caudal segment of the gravid horn. Average uterine horn diameter was larger (P<0.05) in postpartum mares than in nonparturient mares on all days between Days 0 and 24; no difference in diameter was detected on Day 25 (Fig. 2). The general profiles of decreasing horn diameters early in pregnancy are consistent with that previously reported for nonparturient mares. 12 Diameters in postpartum mares decreased to a greater extent and apparently at a faster rate than in nonparturient mares. In postpartum mares, therefore, decreasing horn diameter during early pregnancy was compounded by diameter decreases attributable to involution. By Day 25, despite differences in diameter be-
338
tween horns (formerly gravid versus nongravid; Fig. 2), the average size of the postpartum uterine horns approached the average size of the horns of nonparturient mares.
REFERENCES 1. Allen WE, Newcombe JR: Relationship between early pregnancy site in consecutive gestations in mares. Equine Vet J 13:51-52,1981. 2. Arthur GH, Noakes DE, Pearson H: VeterinaryReproduction and Obstetrics. 6th Ed. Baili~re Tindall:London. p167,1989. 3. Blanchard TL, Varner DD, Brinsko SP, Meyers SA, Johnson L: Effects of postparturient uterine lavage on uterine involution in the mare. Therio 32:527-535,1989. 4. Cross DT, Ginther OJ: The effect of estrogen progesterone and prostaglandin F2a on uterine contractions in seasonally anovulatory mares. Dom Anim Endocrin 4:271-278,1987. 5. Cress DT, Ginther OJ: Uterine contractions in nonpregnant and early pregnant mares and jennies as determined by ultrasonography. J Anim Sci 66:250-254,1988. 6. Feo JCSA: Contralateral implantation in mares mated during postpartum estrus. Vet Rec 106:368,1980. 7. Ginther OJ: Fixation and orientation of the early equine conceptus. Therio 19:613-623,1983. 8. Ginther OJ: Intrauterine movement of the early conceptus in barren and postpartum mares. Therio 21:633-644,1984. 9. Ginther OJ: Ultrasonic Imaging and Reproductive Events in the Mare. Equiservices, Cross Plains, WI, 1986. 10. Ginther OJ: Reproductive Biology of the Mare: Basic and App/iedAspects. 2nd Ed. Equiservices, Cross Plaines, Wl, 1991. 11. Griffin PG, Ginther OJ: Uterine contractile activity in mares during the estrous cycle and early pregnancy. Therio 34:4756,1990. 12. Griffin PG, Ginther OJ: Dynamics of uterine diameter and endometrial morphology during the estrous cycle and early pregnancy in mares. Anim ReprodSci 25:133-142, 1991. 13. Gygax AP, Ganjam VK, Kenney RM: Clinical, microbiological and histological changes associated with uterine involution in the mare. J ReprodFert (Suppl) 27:571-578,1979. 14. Haluska GJ, Lowe JE, Currie WB: Electromyographic properties of the myometrium correlated with the endocrinology of the pre-partum and post-partum periods and parturition in pony mares. J ReprodFert (Suppl)35:553-564, 1987. 15. Hayes KEN, Pierson RA, Scraba ST, Ginther OJ: Effects of estrous cycle and season on ultrasonic uterine anatomy in mares. Therio 24:465-477,1985. 16. Hayes KEN, Ginther OJ: Role of progesterone and estrogen in development of uterine tone in mares. Therio 25:581590,1986. 17. Kiracofe GH: Uterine involution: Its role in regulating postpartum intervals. JAnim Sci (Suppl II)51:16-28,1980. 18. LeBlanc MM, Hansen PJ, Buhi WC: Uterine protein secretion in postpartum and cyclic mares. Therio 29:13031316,1988. 19. Ley WB, Purswell BJ, Bower JM: The effects of prostalene and alfaprostol as uterine myotonics, and the effect on postpartum pregnancy rate in the mare following daily treatment with prostalene. Therio 29:1113-1121,1988. 20. McKinnon AO, Squires EL, Harrison LA, Blach EL, Shideler RK: Ultrasonographic studies on the reproductive tract of mares after parturition: Effect of involution and uterine fluid on pregnancy rates in mares with normal and delayed first postpartum ovulatory cycles. JAm VetMedAssoc 192:350-353,1988. 21. Saltiel A, Gutierrez A, de Buen-Llado N, Sosa C: Cervicoendometrial cytology and physiological aspects of the postpartum mare. J Reprod Fert (Suppl) 35:306-309,1987. 22. SAS: SAS User's Guide: Statistics. SAS Institute, Inc., Cary, NC, 1985. 23. Sertich PL, Hinrichs K, Schiereck DE, Kenney RM: Periparturient events in ovariectomized embryo transfer recipient
EQUINE VETERINARY SCIENCE
mares. Theriogenology30:401-409,1988. 24. SertichPL, Hinrichs K, Kenney RM: Histological aspects of uterine involution in the post parturient, ovariectomised, embryorecipient mare: a model for the study of involution. Equine Vet J (Suppl) 8:56-58,1989. 25. Steven DH, Jeffcott LB, Mallon KA, Ricketts SW, Rossdale PD, Samuel CA: Ultrastructural studies of the equine uterus and placenta following parturition. J ReprodFert (Suppl) 27:579-586,1979. 26. Squires EL, Barnes CK, Rowley HS, McKinnon AO, Pickett BW, Shideler RK: Effect of uterine fluid and volume of extender on fertility. Proc 35th Ann Conv of Am Assoc Equine Practnr, Boston, pp25-30,1989. 27. Taverne MAM: Uterine motility in the post partum female. l Oth International Congress on Animal Reprod and AI, June 10-14, Urbana-Champaign, IL, 4:X1-1,1984. 28. van Niekerk CH: Early clinical diagnosis of pregnancy in mares. J S Afr Vet Med Assn 36:51-58,1965. 29. VandeplasscheM: The normal and abnormal presentation, position and posture of the foal-fetus during gestation and at parturition. VlammsDiergeneesk Tijdschr (Suppl 1)26:1-68,1957. 30. VandeplasscheM, Bouters R, Spincemaille J, Bonte P, Coryn M: Observations on involution and puerperal endometritis in mares. Irish Vet J 37:126-132,1983.
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Volume 11, Number6, 1991
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~
Refereed
UTERINEMORPHOLOGYAND FUNCTION IN POSTPARTUMMARES P. G. Griffin, DVM, MS and O. J. Ginther, VMD, PhD
SUMMARY
Ultrasonically detectable characteristics of the uterus and embryo and palpable uterine tone were assessed in 10 postpartum mares. A bright fern-like pattern of ultrasonic uterine echogenicity, outlining the endometrial folds, was observed for an average of 2.1 _+0.2days following parturition (range, 1 to 3 days). Unexpectedly, the uterus was quiescent throughout the postpartum interval, based on daily one-minute contractility scans. Contractilitywas maximal on Days 12 to 15 of pregnancy in both postpartum (n--7) and nonparturient (n-7) mares. The mean diameter of ultrasonically detectable intrauterine fluid collections increased (P<0.05) abruptly between days 1 and 2 postpartum and gradually decreased (P<0.05) between days 4 and 7; no collections were detected after day 16. There was no effectof day on echogenicity of the intrauterine fluid collections; on all days, fluid was relatively black or nonechogenic, suggesting that puerperal endometritis was not a problem in this group. Because the increase in intraluminal fluid occurred after parturition and in temporal association with a decrease in diameter and tone of the uterus, the fluid Authors' address: Department of Veterinary Science, University of Wisconsin-Madison, 1655 Linden Drive, Madison, W153706. Acknowledgments: Supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison. The authors thank Dr. G. P. Adams, R. Gneiser, P. Krcma and L. Kulick for technical assistance.
330
collections apparently represented a physiologic influx from the involuting uterus rather than residual placental fluid. Involution of the homs was completed by day 27 (formerly nongravid horn) and day 31 postpartum (formerly gravid horn), based on failure to detect further significant decreases in diameter. However, the formerly gravid horn was larger (P<0.05) in diameter than the formerly nongravid horn on each of Days 1 to 35 postpartum (end of experiment), indicating residual effects on uterine size. When averaged over both horns, uterine diameters were larger on Days 0 to 24 (Day 0=day of ovulation) of pregnancy in postpartum mares than in nonparturient mares; by Day 25, diameters were similar between statuses~ By approximately Day 6 of pregnancy, uterine contractility and ultrasonic endometrial exhotexture were similar between postpartum mares and nonparturient mares. Uterine tone was greater (P<0.05) in postpartum mares than in nonparturient mares on all days between Day 0 and 25. An unexpected, transient increase in uterine tone was detected on Day 5 of pregnancy in both postpartum mares and nonparturient mares. No differences were found between reproductive statuses in patterns of embryo mobility, the day of fixation of the embryonic vesicle (postpartum, Day 15.3 _+0.4; nonparturient, Day 15.0 _+0.3), and diameter of the embryonic vesicle on the day of fixation (postpartum, 22.1 +1.4 mm; nonparturient, 19.4 +l.6mm). However, mean uterine tone and mean horn diameters on the side of fixation were greater (cranial and middle comual segments; P<0.05) EQUINE VETERINARY SCIENCE
or tended to be greater (caudal segment; P<0.1 ) on the day of fixation in postpartum mares than in nonparturient mares. In all postpartum mares, fixation occurred in the formerly nongravid horn. Enhanced uterine tone in postpartum mares may account for the occurrence of fLxationon the same day for the two reproductive statuses, despite the larger uterus in postpartum mares. INTRODUCTION The mare is unique among large domestic animals (e.g., cow, sow, ewe) in establishing and maintaining pregnancy despite a remarkably short (e.g., 9 day) interval between parturition and the first postpartum ovulation (postpartum interval). 1°,17 Given the ll-month gestation in mares, postpartum reproductive events are crucial to maintenance ofa 12month foaling interval. Examination ofdma from four reported surveys indicated that 51 to 62% of mares bred each year were postpartum. 10This consideration further emphasizes the importance of the postpartum interval to breeding programs. After parturition, the uterus undergoes a dynamic involution with a reduction in overall uterine size, repair of uterine tissues, and re-establishment of a luminal environment favorable to conceptus development. Histologic, 13 ultrastructural,25 cytologic,21 bacteriologic,13 biochemical,18 clinical3O and ultrasonic 2° aspects of involution in mares have been described. However, there is a lack of information on several uterine characteristics (e.g., uterine conUactility, tone, and ultrasonic echotexture) from parturition to fast ovulation and during the resulting early pregnancy. The objectives of the present study were as follows: 1) to characterize patterns of uterine contractility, tone, and ultrasonic echotexture during the postpartum interval (parturition to first ovulation); 2) to compare patterns of uterine contractility, tone, echotexture, and diameter during early pregnancy (Days 0 to 25) between postpartum mares and nonparturient (barren) mares; 3) to further characterize changes in uterine diameter and intrauterine fluid collections after parturition (days 1 to 35 postpartum); and 4) to characterize and compare to nonparturient mares some aspects of embryo-uterine interactions (embryo mobility, fixation). In addition, considering greater uterine diameter, the hypothesis was tested that fixation of the embryonic vesicle (cessation of embryo mobility) occurs later in postpartum mares than in nonparturient mares. Transrectal ultrasonography was used to evaluate all end points, except that uterine tone was evaluated by wansrectal palpation. MATERIALS AND METHODS Ten postpartum and 7 nonpartmient (barren) pony mares, 4-12 years-of-age and weighing 200-300 kg, were used contemporaneously. A capital "D" is used in the text to designate number of days after ovulation (e.g., Day 0 = day of ovulation) and a small "d" is used for the number of days after Volume 11, Number6, 1991
parturition (e.g., day 0 = day of parturition). The postpartum mares delivered apparendy normal foals without complication (i.e., no dystocias or retained placentae) between midJune and mid-July and were examined once daily, beginning on day 1 and continuing a n d Day 25 of pregnancy or until the end of the first postpartum interovulatory interval. Nonparturient mares were examined daily from Day 0 to Day 25. All mares were artificially inseminated with approximately 20 m l of raw semen every other day, beginning on the day of ulu:asonic detection of a >35 mm follicle and continuing until ovulation; no mares were bred on or after the day of ovulation. Ultrasonic embryo detection and monitoring began on Day 11. Daily examinations in both groups included a systematic transrectal ultrasound scan of the reproductive tract9 and transrectal digital palpation of the uterus. Ultrasound examinations were done with a real-time, B-mode scanner" equipped with a 3.5 or 7.5 MHz linear-array transducer; examinations were recorded, with oral comments, ttsinga3/4" videotape recorder connected to the scanner. For all end points, scores were assigned or measurements obtained without reference to or consideration of values for preceding days. Uterine Contractility Uterine contractility was estimated from a 1-minute continuous scan of a longitudinal section of the uterine body, as described. 4,5 The extent of contractility was scored from 1 to 5 (minimal to maximal) based on the degree of to and fro movements of endometrial tissue reflectors and the presence and extent of waves or indentations of the ventral surface of the uterus. Fractional scores (e.g., 1.5) were assigned when appropriate. Contractility scoring was done at the time of examination from the viewing screen of the scanner. Uterine Echotexture The ultrasonic echotexture of the endometrium was assessed and scored from the viewing screen at the time of examination, as described. 15A scale of 1 to 4 was used based on the degree of endometrial folding (minimal to maximal); fractional scores were assigned when appropriate. Uterine Diameter A 3.5 MHz transducer was used in postpartum mares for assessing uterine diameter when the horns were too large (>50 m m in width) to be imaged completely in cross section with a 7.5 MHz transducer; this generally occurred during the first week postpartum (range, 6 to 10 days). Each uterine horn was divided into approximately equal cranial, middle, and caudal segments. 9 The outer cross-sectional diameter (ram) of each comual segment was measured at the estimated midpoint and defined as the mean of the largest vertical and horizontal diameters. After fixation of the embryonic vesicle, diameter of the caudal cornual segment was determined immediately cranial to the vesicle on the side of fixation and at the aTokyo Keiki, LS 300, Products Group International, Inc., Boulder, CO.
331
Table 1. Reproductive characteristics in postpartum (n=10) and nonparturient in--7) mares and mean (+SEM) length (days) of components of the postpartum interval,
End point
Postpartum
Parturition to loss of uterine firmness#
5.3 _+0.4 range, 4-8
Parturition to detection of estrus echotexture
6.5 + 0.3 range, 5-8
Nonparturlent
5.4 ± 1.0 range, 2-13
Parturition to first ovulation
11.6±1.3 range, 7-19
Pregnancy rate at Day 11
7/10 (70%)
7/11 (64%)
Embryo-loss rate by Day 40
1/8 (13%)++
0/7 (0%)
Days of fixation of embryonic vesicle
15.3±0.4 range, 14-16
15.0±0.3 range, 14-16
Embryonic vesicle diameter (ram) on day of fixation
22.1 ± 1.4 range, 16-27
19.4 ± 1.6 range, 14-24
24.4±0.4 27.4±0.5 31.3±1.4
22.2±0.5 24.6±1.2 28.7±0.9
24.9±0.8 30.4±0.9 34.5±1.4 3.1±0.2
22.3±0.9 24.9±1.2 28.6±1.6 2.6±0.1
Middle* Caudal+ Horn contralateral to fixation Cranial+
Middle* Caudal, Uterine tone score on day of fixation*
Location Formerly Formerly Uterine body gravid horn nongravid horn No. of collections*+
Duration of estrous echotexture
Diameter (mm) of cornual segments on day of fixation## Horn ipsilateral to fixation Cranial*
Table 2. Frequency of location of intrauterine fluid collections between days 1 and 15 postpartum in= 10 mares and 121 fluid collections).
*Significant(P<0.05) differencebetweenreproductivestatusesfor a given
end point. +Tendency(P <0.1) for a differencebetweenstatusesfor a givenend point.
#Uterinetone score=3or 4. **Prominentendometrialfolding,echotexturescore=3or 4. ++Onevesiclewasfirstdetectedon Day14and subsequentlylostby Day 18. #:#Forpostpartummares,significant(P<0.05)differencesbetweenhornsfor diameterof middleand caudalcornualsegments.
corresponding point on the opposite horn. Measurements were obtained from the videotapes using a high-resolution video monitor and an overlay grid calibrated to the centimeter scale on the ultrasound image. For purposes of analysis, the formerly gravid horn in postpartum mares was defined as the horn occupied by the fetus during late gestation, as determined by ultrasonography. For all foaling mares, the horn which contained the fetus at term was the same as the horn of fixation and umbilical attachment.
69/121 (57%)
44/121 (36%)
8/121 (7%)
Uterine segment Cranial 15/69 (22%) a Middle 36/69 (52%) b Caudal 18/69 (26%) a
13/44 (30%)a 9/44 (20°/o)a 22/44 (50%)b
5/8 (63%) a 0/8 (0%)b 3/8 (37%) =
*More (P<0.05)fluidcollectionswere detected in the uterinebody (69/121) than in both uterinehornscombined(52/121). +Frequenciesare different(P
examination was determined and assigned to 1 of 9 uterine segments (each comual segment and cranial, middle, and caudal segments of uterine body). The ultrasonic echogenicity (whiteness) of each fluid collection was assessed and scored on a scale of 1 to 4 ( 1=black or nonechogenic; 4=white or hyperechogenic). For mares with more than one collection on a given day, echogenicity scores were averaged.
Embryonic Vesicle The size of the embryonic vesicle was obtained by averaging its largest vertical and horizontal diameters. Location of the vesicle at the start of the examination was determined and assigned to 1 of the 9 uterine segments. On Day 13, a 2-hour embryo mobility trial was conducted in each mare, as described,s Atrial consisted of 1 determination of embryo location (among the 9 segments of the uterus) every 5 minutes for a total of 25 determinations per mare. Daily mobility trials continued after Day 13, and the day of fixation was defined as the first day in which no change in vesicle location was detected during a mobility trial and between subsequent daily examinations. Uterine Tone
Uterine tone was assessed digitally at the approximate mid-point of the uterine horns at each examination, and scored on a scale of 1 to 4) l=minimal tone, flaccid; 4=maximal tone, turgid or filTU).16Fractional scores were assigned when appropriate. Statistical A n a l y s e s
Intrauterine Fluid Collections The diameter of intrauterine fluid collections was estimated from the videotapes by averaging maximal height and width. When more than one discrete fluid collection was detected in a mare on a given day, the mean values for each collection were summed to obtain one value for the examination. The location of each collection at the start of the
332
Sequential data were analyzed by split plot analyses of variance using the Statistical Analysis System General Linear Models procedure.22 When significant (P<0.05) effects or interactions were detected, paired t-tests, t-tests, or least significant difference were used to further examine differences. Frequency data were examined by Chi-square analyses. Differences in single point data due to reproductive status
EQUINE VETERINARY SCIENCE
Fig ure 1. Ultrasonograms from one mare of the formerly nongravid horn (upper series of images) and formerly gravid horn (lower series of images). The n umber of days postpartum is indicated in the lower right hand corner of each image. All images are crosssectional and the short arrows indicate the ventral surface of the uterus. In the upper series of images, the long arrows indicate an echogenic steel bead which was sutured to the uterine serosa as part of an earlier study and which serves to indicate that the images are from the same area. Note the bright, fern-like pattern of endometrial echogenicity in the horns on day 1 postpartum and the decrease in its extent on day 2. In this mare, the pattern was not detected after day 2. The nonechogenic (black) areas on the dorsal surface of the uterus on day 1 represent dilated blood vessels. Note also the decrease in size of the horns as the postpartum interval progressed. By day 8, the mare was entering her first postpartum estrus, as evidenced by the characteristic prominent endometrial folding shown here in the formerly gravid uterine horn.
(postpartum versus nonparturient) were examined by t-tests; paired t-tests were used to detect differences within each status. RESULTS
Mean (+SEM) lengths (days) of components of the postpartum interval (parturition to first ovulation) and a comparison of reproductive characteristics between postpartum mares and nonparturient mares are summarized in Table l. In postpartum mares, adistinctive fem-likepattern of bright endometrial echogenicity was observed for an average of 2. l _+0.2days following parturition (range, 1-3 days; Fig. l). The
Volume 11, Number 6, 1991
pattern seemed to outline the luminal surface of the endometrial folds and was noted throughout all uterine segments. Mean uterine contractility, tone, and echotexture scores, uterine horn diameters, and diameters of intrauterine fluid collection for days I to l0 for postpartum mares and for Days 0 to 25 of pregnancy for postpartum mares and nonparturient mares are shown in Figure 2, along with results of the statistical analyses. No day effect on echogenicity scores for intrauterine fluid collections was detected between days 1 and 14 postpartum; mean scores ranged from 1.0_+0.0to 1.4_+0.2. The frequency of location of intrauterine fluid collections between days 1 and 15 is summarized in Table 2. Mean uterine horn diameters, averaged over all cornual segments, for postpartum mares between Days 1 and 35
333
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Days postpartum
Days of pregnancy
Figure 2. Mean (+SEM) uterine echotexture, tone, and contractility scores, uterine diameters averaged over all cornual segments, and diameters of intrauterine fluid collections for postpartum mares (n= 10) and for pregnant postpartum (n=7) and nonparturient mares (n=7). S, effect of reproductive status; D, effect of day; SD, reproductive status x day interaction; NS, not significant. For echotexture scores, means were different (P<0.05) between statuses on Days 0 to 4 and on Days 7, 14, and 20. For fluid collection diameters and uterine tone scores between days 1 and 10 postpartum, there were significant effects of day; within each end point, means without at least one common superscript are different (P<0.05). For contractility scores, means were different (P<0.05) between statuses on Days 0 and 2 and tended (P<0.1) to be different on Day 4. For tone scores and uterine diameter averaged over both horns, means were different (P<0.05) between statuses on each day, except on Day 25 for horn diameter. For postpartum mares, the formerly gravid horn was greater (P<0.05) in diameter than the formerly nongravid horn on each day. 334
(including data for pregnant and nonpregnant mares) and for pregnant mares between Days 0 and 25 were analyzed separately. In both instances (days 1 to 35 and Days 0 to 25),results of statistical analyses were similar among the individual comual segments; therefore, horn diameters averaged over all segments and statistical results are presented for illustrative purposes (Fig. 2). For both analyses, there were significant effects of day and side (formerly gravid versus nongravid horn) on horn diameters; for days 1 to 35, there was a significant interaction between side and day.Additionally, for both analyses, there were significant effects of cornual segment (cranial, middle, caudal) on horn diameter. Averaged over days 1 to 35, diameter (mm) progressively increased (P<0.05) within each uterine horn from cranial to caudal segments (formerly gravid horn: cranial, 31.9 _+0.3, middle, 38.9 + 0.5, caudal, 47.7 _+0.7; formerly nongravid horn: 28.5 _+0.3, 34.1 _+0.4, 39.1 _+0.5,respectively). Similarly, averaged over Days 0 to 25, segmentdiameter increased from cranial to caudal (formerly gravid horn: cranial, 27.9_+0.3, middle, 32.9 _+0.4, caudal, 38.6 _+0.5; formerly nongravid horn: 25.6 _+0.3, 29.7 _+0.3, 34.1 _+0.4,respectively). For nonparturient mares, no effect of side (ipsilateral versus conlralateral to side of fixation) on horn diameters was detected for any cornual segment or when all segments were averaged. However, there was an effect (P<0.0001) of cornual segment on horn diameter; averaged over both horns and Days 0 to 25, diameters (ram) increased (P<0.05) from cranial (22.8 _+0.2) to middle (26.0 _+0.2) to caudal (29.8 _+0.2) segments. Additionally, in nonparturient mares, there was an effect (P<0.0001 ) of day for each segment and averaged over all segments; diameters were larger (P<0.05) on each of Days 0 to 7 than on Days 8 to 25. Involution of the uterine horns was complete on day 27 (formerly nongravid horn) and day 31 (formerly gravid horn), based on failure to detect further significant decreases in diameter. During involution, diameters decreased in a curvilinear fashion, best fitting a quadratic profile (formerly gravid horn, R2=0.96, P<0.001; formerly nongravid horn, Ra=0.94, P<0.001), with 2 approximately linear decreases between days 1 and 10 and between day 11 and the completion of involution. Therefore, for each cornual segment and mare, the rate for each linear decrease (involution rate) was calculated as the mean decrease in diameter (mrn/day) between days 1 and 10 and days 11 and 27 (formerly nongravid horn) or 31 (formerly gravid horn); mean involution rates are shown in Table 3. Based on Day 13 mobility trials, the mean numbers of vesicle movements between successive location determinations were not different between postpartum mares and nonparturient mares (segment-to-segment movements; 12.0 + 1.3 versus 10.1 + 1.1, body-to-horn or vice-versa; 5.0 + 1.2 versus 4.4 _+0.8, horn-to-hom; 0.7 _+0.5 versus 0.1 _+0.1,respectively). Likewise, there were no differences between or within statuses in the number of times per mobility trial that the vesicle was detected in the uterine body (postpartum, 11.5 +3.0; nonparturient, 7.3 +_2.3) or homs (postpartum, 13.5
EQUINE VETERINARY SCIENCE
Figure 3. Ultrasonograms from two postpartum mares of intrauterine fluid collections located in the uterine body. Both images are longitudinal and the number of days postpartum is indicated in the lower right hand corner of each image. The ventral surfaces
of the uteri are indicated by the long arrows. The short arrows indicate the borders of the fluid collections within the uterine lumen. In both images, note the nonechogenic (black) nature of the intrauterine field.
+3.0; nonparturient, 17.7 +_2.3).Within each status, the vesicle was equally likely to be found in either hom (postpartum, formerly gravid horn, 6.8 +1.7, formerly nongravid horn, 6.7 + 1.4; nonparturient, right horn, 11.0 +_2.7,left horn, 6.7 +3.2). In 7 of 7 postpartum mares, fixation of the embryonic vesicle occurred in the caudal segment of the formerly nongravid horn. In nonparturient mares, 5 of 7 vesicles became fixed in the right horn and 2 of 7 in the left (no significant difference); all fLxations occurred in the caudal comual segment. DISCUSSION
Uterine Contraetnlty Minimal levels of ultrasonically detectable uterine contractility were observed between the day after parturition and the first ovulation (Fig. 2). This is in sharp contrast to the commonly held assumption of high levels of contractility during the postpartum interval, a,19Presumably, this assumption is based on t~e belief that enhanced contractility is an important facet of involution and facilitates evacuation of postpartum debris. Results of the present study suggest that ultrasonically detectable phases of uterine contractility play a minimal role in involution and lochial discharge during the puerperium. However, due to the length and schedule of the contractility scans, episodic bursts of contractility may have been undetected. Additionally, because of limitations of the technique, tonic or sustained myometrial contractions were not observable. There is little additional information on postpartum uterine contractility in mares. The extent of contractility and its role in uterine involution and clearance between parturition and day 1 is not clear. In an electromyographic study of
Volume 11, Number 6, 1991
myometrial activity in 2 mares during the periparturient period, 14 no consistent pattern of electrical activity was detected at 1, 3, and 5 hours postpartum. No quantitative information was presented for the period beyond 5 hours, except to note a decrease in long bursts of electrical activity (> 120 secs in duration) over the next few days; medium bursts of activity (7 to 120 secs) then became predominant. In cows, based on acute studies utilizing intrauterine pressure recordings (review27), contraction frequency decreased from approximately 12-18 contraction-relaxation cycles per hour following parturition to 0-6 cycles per hour by 48 hours postpartum. Thereafter, the postpartum uterus was relatively quiescent. It has been suggested that, in mares, considering low concentrations of progesterone and estrogen, the period between parturition and the first postpartum estrus is anestruslike. la In this regard, the process of involution in mares is apparently independent of ovarian function; ovariectomy had no effect on histologic, bacteriologic or palpable aspects of involution.a3.a4 Perhaps the observed low levels of contractility also reflect an anestrus-like condition; in seasonallyanovulatory mares, ultrasonically detectable uterine contractility was minimal. 4 The indications from the present experiment of a quiescent postpartum uterus emphasize the need for additional study of postpartum uterine conlractility. This is particularly evident in view of the advocacy of injections of myometrial stimulants as aids to preparation of the postpartum uterus for breeding. 19 Levels of uterine contractility were depressed in postpartum mares following the first ovulation and extending until Day 5 of pregnancy, when levels approached those ofnonparturient mares (Fig. 2). This occurred at approximately the time the blastocyst is expected to enter the uterus (Day 5 or 6). l° Subsequently, patterns of contractility were strikingly similar
335
between statuses until the end of the study (Day 25). The postpartum uterus, therefore, had regained its pre-gravid contractility by Day 5 or 6 of the subsequent pregnancy, presumably creating an appropriate environment for conceptus development. The overall patterns of contractility for both statuses (maximal contractility detected on Days 12 to 15 or 16) agreed with previously reported patterns for pregnant nouparturient mares, s.1 Uterine Tone and Embryo-uterine Interactions The uterus was extremely firm upon palpation for several days following parturition (Fig. 2). The firmness and consistency were qualitatively different from the characteristic turgid uterine tone of early pregnancy and likely reflected considerable postpartum edema and extensive uterine blood flow developed during gestation. The uterus was heavy and difficult to retract, hard to the touch, and often seemed to pit in response to digital pressure. The firmness gradually decreased over the postpartum interval. In contrast, it has been reported that uterine tone and "tubularity" in Thoroughbreds increased over the first 5 days postpartum.21 The reasons for the discrepancy between studies are unclear, but may be related to differences in criteria for tone assessment and to the relative inaccessibility of the uterus to palpation early in the postpartum period in larger mares. In one study, a° it was stated that the uterus could not be circumferentially palpated until approximately 3 days postpartum in horse mares. In the present study using pony mares, both uterine horns were defined and palpated thoroughly by day 1; this is consistent with an earlier report2 which indicated that the uteri of pony mares can be outlined by rectal palpation by about 12 hours postpartum. Uterine tone scores were consistently higher between Days 0 and 25 in pregnant postpartum mares than in nonparturient mares (Fig. 2). The reasons for the enhanced level of tone in postpartum mares are not known. However, there is rationale to hypothesize that it may be compensatory for larger uterine diameter during the puerperium, so that important embryo-uterine interactions, particularly fixation of the embryonic vesicle, can occur at the appropriate time. In this regard, results did not support the hypothesis that fixation occurs later in postpartum mares than in nonpartufient mares; there was no significantdifferencebetween statuses for day of fixation. Factors thought to be involved in fixation include progressively increasing embryonic vesicle diameter, increasing uterine tone, and decreasing uterine size. 7There was no difference in embryo mobility patterns or embryonic vesicle size on the day of fixation between postpartum mares and nonparturient mares. However, on the day of fixation, diameter of the comual segments on the side of fixation were larger (cranial and middle segments) or tended to be larger (caudal segment) in postpartum mares than in nonparturient mares, and uterine tone scores were greater in postpartum mares (Table 1). Perhaps, considering similar vesicle size and embryo mobility patterns, the larger diameter of the postpar-
336
tum uterine horns was negated by enhanced uterine tone, resulting in similar days of fixation between postpartum mares and nonparturient mares. In all postpartum mares, the embryonic vesicle became fixed in the caudal portion of the formerly nongravid horn, consistent with previous reports which indicated that the vesicle fixes more frequently on the formerly nongravid side? .8The number of times the vesicle was detected in either of the two horns during 2-hour mobility trials was not different, confirming an earlier study.8 Differential patterns of embryo mobility, therefore, were not responsible for the observed fixation in the formerly nongravid horn. Instead, fixation on the nongravid side was attributable to smaller diameter of the formerly nongravid horn on the day of fixation, as has been postulated,a For both statuses, the overall profiles of increasing uterine tone scores during early pregnancy were consistent with previously reported profiles for nonparturient mares. 18,2a However, for both statuses, significant transient increases in tone scores were detected several days after ovulation, with peak levels occurring on Day 5 (Fig. 2); similar increases have not been described previously and confirmation is needed. The increases in tone were temporally related to the expected entry of the blastocyst into the uterus. Uterine Echotexture
A bright"fem-like" patternof endometrial echogenicity was observed following parturition in all 10 mares at least once during days 1 to 3 postpartum (Fig. 1). This distinctive pattern apparently has not been described previously, and its origin is not known. Given its ultrasonic characteristics, however, the pattern may have been due to small amounts of air trapped between the endometrial folds. Air may have entered the uterus through the widely dilated cervix after foaling. The echogenicity and regularity of the pattern decreased over time, probably reflecting the stage of involution. The presence and intensityof this pattern may have diagnostic value, assuming that delayed involution would be reflected by a prolonged fern-like pattern. The first day postpartum of detection of an estrous echotexture (i.e., prominent endometrial folding) corresponds with reported mean days to onset of behavioral estrus (7 to 9 days; review1°). In 5 of 10 mares, echotexture scores remained estrus-like until the day of the first postpartum ovulation; in the remaining 5 mares, scores were elevated until Day -1 (3 mares), Day -2 (1 mare), and Day -4 (1 mare) prior to first ovulation. On average, the last day that postpartum uterine tone was firm preceded by approximately 1 day the first appearance of an eslrous echotexture. In nonparturient mares, estrus is associated with flaccid uterine tone. 16~ Minimal levels of uterine tone were detected on the days preceding the first postpartum ovulation (Fig. 2). However, tone scores did not decline to levels expected for nonpartufient mares preceding ovulation,16perhaps reflecting residual postpartum edema. Despite a differencein degree, the present
EQUINE VETERINARYSCIENCE
Table 3. Mean (+SEM) involution rates (mm/day) for cornual segments (n=10).
Cornual segment location*+
Type of uterine horn Formerly Formerly gravid horn nongravid horn
Cranial days 1 to 10 days 11 to 27 or 31"* Middle days 1 to 10 days 11 to 27or 31 Caudal days 1 to 10++ days 11 to 27 or 31
All segments days 1 to 10++ days 11 to 27or 31
1.1 +0.2a 0.4+0.1
0.9+0.1 a
1.5 +0.2 a 0.3_+0.1
1.4 +0.1 b 0.3±0.1
2.8+0.4 b 0.5_+0.1
2.0 ±0.1 c 0.4 +0.1
1.8 _+0.2 0.4_+0.1
1.4 ±0.1 0.3_+0.1
0.3+0.1
*Mean rates for days 1 to 10 are different (P<0.05) than rates for day 11 to completion of involution for each cornual segment location. +For days 1 to 10 there was a significant (P<0.009) effect of cornual segment within each type of uterine horn. **Completion of involution; day 27 (formerly nongravid horn), day 31 (formerly gravid horn). ++Means tended (P<0.1) to be different between horns. a,",cMeans for days 1 to 10 with different superscripts are different (P<0.05) within each type of uterine horn.
study indicated that the same relative association between estrus and uterine tone exists in postpartum mares during their first ovulatory period, as in nonlactating mares during the estrous cycle. There was significant interaction between reproductive status and day on endometrial echotexture scores between Days 0 and 25 of pregnancy, due primarily to higher scores in postpartum mares than in nonparturient mares on Days 0 to 4 and on Day 7 (Fig. 2). The enhanced endometrial folding on these days may be indicative of residual postpartum edema. Interestingly, and in common with contractility scores, echotexture scores began to converge between statuses around Day 6, temporally related to the reported day of conceptus entry into the uterus. These results indicate that the uterus had regained its pre-gravid ulWasonic characteristics by this important deadline. Beginning on Days 14 (nonparturient mares) and 15 (postpartum mares), an irregular pattern of increased endometrial scores was detected. This pattern is consistent with that previously reported for pregnant mares9.12and may be related to exposure to estrogens or other compounds (e.g., prostaglandins), perhaps of embryonic origin. Intrauterine Fluid Collections The number of mares with ultrasonically detected intrauterine fluid collections and the average diameter of the fluid collections increased considerably between days 1 and 2 postpartum (Fig. 2). Five of 10 mares had fluid collections on day 1 (diameter range, 5-28 ram) compared with 9 of 10 on day 2 (range, 2-84 ram). By day 3, all mares had fluid collections and, in most cases, there was dramatic increase (>20 ram) in the diameter of collections between days i and 2 (5 mares) or between days 2 and 3 (4 mares). The abrupt
Volume 11, Number 6, 1991
increase in intrauterine fluid observed between days 1 and 2 has apparently not been described previously. This pattem of fluid accumulation suggests that the collections were not residual placental fluids but instead developed in association with involution. In this regard, the extent of fluid collections decreased temporally with the decrease in uterine tone and size. There was no effect of day on echogenicity of the fluid collections during the postpartum interval. On all days, mean echogenicity scores were <1.4, indicating relatively black or nonechogenic fluid (Fig. 3). Increasing echogenicity of intrauterine fluid has been associated with increasing content of inflammatory cells2s and, presumably, cellular debris. Perhaps, the lack of echogenicity of the postpamun fluids observed in the present study suggests that the fluiddevelopment was a physiologic phenomenon reflecting the involutionary process, rather than a pathologic or inflammatory response (e.g., due to contamination at foaling). In contrast, a previous ultrasonic studya° reported a pattem of decreasing fluid echogenicitybetween days 3 and 15 postpartum. Perhaps, the discrepancy between studies is related to the use in the present study of young mares (< 12 years), apparently healthy and free of reproductive abnormalities, which had foaled without complication; information in this regard was not presented for the previous study. The amount of intrauterine fluid gradually decreased between days 4 and 7 postpamun.Afterday 9, only small fluid collections (2-10 mm) were detected and none were found after 16 days. Similarly, the earlier ultrasonic study2° indicated that the number of mares with detectable intrauterine fluid decreased after day 5 postpartum; fluid was not detected in any mare after day 15. In the present study, fluid collections were not detected in postpartum mares after Day 3 postovulation and after Day 1 in nonparturient mares (Fig. 2). The observed distribution of intrauterine fluid collections (Table 2) may reflect differences in size of uterine segments; more collections were detected in the uterine body than in the horns and in the formerly gravid horn than in the nongravid horn. It is emphasized, however, that the observed locations of fluid collections were those detectedat the onsetofan examination; the distribution and sizes of individual collections were dynamic and often changed during an examination as fluids moved and individual collections merged. Uterine Diameter
Based on failure to detect further significantdecreases in diameter, uterine horn involution was completed by day 27 (formerly nongravid horn) and day 31 (formerlygravid horn). In the previous ultrasonic study,2°involution wasjudged to be complete in an average of 23 days postpartum (range 13-29 days). The earlier study also indicated that the formerly gravid horn was recognizable by ultrasonic assessment of size for an average of 21 days postpartum (range, 15-25 days). A study based on transrectal palpation indicated that both uterine horns returned to their pre-gravid size by day 32 p o s s . 1 3 In the present study, the formerly gravid horn was larger
337
(P<0.05) in diameter than the formerly nongravid hom, averaged over all cornual segments, on every day between days 1 and 35 postpartum and between Days 0 and 25 of pregnancy (Fig. 2). Similar results were obtained when each segment was considered individually, except for apparently randomly occurring days on which horn diameters were not different (P> 0.1). Therefore, averaged over all segments, the formerly gravid horn did not return to it pre-gravid diameter by 35 days postpartum orDay 25 of the subsequent pregnancy. The residual effects of pregnancy on horn diameter beyond 35 days postpartum are not known. Significant side (formerly gravid horn versus nongravid horn) by day interactions between days 1 and 35 postpartum were detected for diameters of the middle and caudal comual segments and when diameters of all segments were averaged. This can be attributed to the larger initial diameter and faster initial involution rates (days 1 to 10; Table 3) for cornual segments of the formerly gravid horn. Within each horn, the caudal cornual segment involuted (decreased in diameter) at a faster rate than either of the other 2 segments, while the cranial segment had the slowest rate. The patterns of involution were likely due to greater distension of the gravid horn and segments of both horns closest to the fetus. There was a rapid decrease in uterine diameter between days 1 and 10; however, between day 11 and the completion of involution, rates slowed significantly for each comual segment and averaged ovet all segments. The degree of in volution between parturition and day 1 was not examined and is in need of critical study. However, considering the extensive hypertrophy, hyperplasia, and distension of the uterus late in gestation, and the relatively small diameter of the uterine horns observed on day 1 postpartum (e.g., mean diameter of caudal segment of formerly gravid horn=approximately 77mm), profound size decreases must have occurred during the first 24 hours postpartum. Changes in diameter of uterine body were not monitored in the present study and the extent and rate of involution of the uterine body is not known. However, during prepartum scanning late in gestation, the body (along with the gravid uterine horn) contained a substantial proportion of the fetus, as previously noted, a9As a result, distension, and subsequent involution, of the uterine body (particularly the cranial segment near thecorpus-comualjunctions) may be similar to that of the caudal segment of the gravid horn. Average uterine horn diameter was larger (P<0.05) in postpartum mares than in nonparturient mares on all days between Days 0 and 24; no difference in diameter was detected on Day 25 (Fig. 2). The general profiles of decreasing horn diameters early in pregnancy are consistent with that previously reported for nonparturient mares. 12 Diameters in postpartum mares decreased to a greater extent and apparently at a faster rate than in nonparturient mares. In postpartum mares, therefore, decreasing horn diameter during early pregnancy was compounded by diameter decreases attributable to involution. By Day 25, despite differences in diameter be-
338
tween horns (formerly gravid versus nongravid; Fig. 2), the average size of the postpartum uterine horns approached the average size of the horns of nonparturient mares.
REFERENCES 1. Allen WE, Newcombe JR: Relationship between early pregnancy site in consecutive gestations in mares. Equine Vet J 13:51-52,1981. 2. Arthur GH, Noakes DE, Pearson H: VeterinaryReproduction and Obstetrics. 6th Ed. Baili~re Tindall:London. p167,1989. 3. Blanchard TL, Varner DD, Brinsko SP, Meyers SA, Johnson L: Effects of postparturient uterine lavage on uterine involution in the mare. Therio 32:527-535,1989. 4. Cross DT, Ginther OJ: The effect of estrogen progesterone and prostaglandin F2a on uterine contractions in seasonally anovulatory mares. Dom Anim Endocrin 4:271-278,1987. 5. Cress DT, Ginther OJ: Uterine contractions in nonpregnant and early pregnant mares and jennies as determined by ultrasonography. J Anim Sci 66:250-254,1988. 6. Feo JCSA: Contralateral implantation in mares mated during postpartum estrus. Vet Rec 106:368,1980. 7. Ginther OJ: Fixation and orientation of the early equine conceptus. Therio 19:613-623,1983. 8. Ginther OJ: Intrauterine movement of the early conceptus in barren and postpartum mares. Therio 21:633-644,1984. 9. Ginther OJ: Ultrasonic Imaging and Reproductive Events in the Mare. Equiservices, Cross Plains, WI, 1986. 10. Ginther OJ: Reproductive Biology of the Mare: Basic and App/iedAspects. 2nd Ed. Equiservices, Cross Plaines, Wl, 1991. 11. Griffin PG, Ginther OJ: Uterine contractile activity in mares during the estrous cycle and early pregnancy. Therio 34:4756,1990. 12. Griffin PG, Ginther OJ: Dynamics of uterine diameter and endometrial morphology during the estrous cycle and early pregnancy in mares. Anim ReprodSci 25:133-142, 1991. 13. Gygax AP, Ganjam VK, Kenney RM: Clinical, microbiological and histological changes associated with uterine involution in the mare. J ReprodFert (Suppl) 27:571-578,1979. 14. Haluska GJ, Lowe JE, Currie WB: Electromyographic properties of the myometrium correlated with the endocrinology of the pre-partum and post-partum periods and parturition in pony mares. J ReprodFert (Suppl)35:553-564, 1987. 15. Hayes KEN, Pierson RA, Scraba ST, Ginther OJ: Effects of estrous cycle and season on ultrasonic uterine anatomy in mares. Therio 24:465-477,1985. 16. Hayes KEN, Ginther OJ: Role of progesterone and estrogen in development of uterine tone in mares. Therio 25:581590,1986. 17. Kiracofe GH: Uterine involution: Its role in regulating postpartum intervals. JAnim Sci (Suppl II)51:16-28,1980. 18. LeBlanc MM, Hansen PJ, Buhi WC: Uterine protein secretion in postpartum and cyclic mares. Therio 29:13031316,1988. 19. Ley WB, Purswell BJ, Bower JM: The effects of prostalene and alfaprostol as uterine myotonics, and the effect on postpartum pregnancy rate in the mare following daily treatment with prostalene. Therio 29:1113-1121,1988. 20. McKinnon AO, Squires EL, Harrison LA, Blach EL, Shideler RK: Ultrasonographic studies on the reproductive tract of mares after parturition: Effect of involution and uterine fluid on pregnancy rates in mares with normal and delayed first postpartum ovulatory cycles. JAm VetMedAssoc 192:350-353,1988. 21. Saltiel A, Gutierrez A, de Buen-Llado N, Sosa C: Cervicoendometrial cytology and physiological aspects of the postpartum mare. J Reprod Fert (Suppl) 35:306-309,1987. 22. SAS: SAS User's Guide: Statistics. SAS Institute, Inc., Cary, NC, 1985. 23. Sertich PL, Hinrichs K, Schiereck DE, Kenney RM: Periparturient events in ovariectomized embryo transfer recipient
EQUINE VETERINARY SCIENCE
mares. Theriogenology30:401-409,1988. 24. SertichPL, Hinrichs K, Kenney RM: Histological aspects of uterine involution in the post parturient, ovariectomised, embryorecipient mare: a model for the study of involution. Equine Vet J (Suppl) 8:56-58,1989. 25. Steven DH, Jeffcott LB, Mallon KA, Ricketts SW, Rossdale PD, Samuel CA: Ultrastructural studies of the equine uterus and placenta following parturition. J ReprodFert (Suppl) 27:579-586,1979. 26. Squires EL, Barnes CK, Rowley HS, McKinnon AO, Pickett BW, Shideler RK: Effect of uterine fluid and volume of extender on fertility. Proc 35th Ann Conv of Am Assoc Equine Practnr, Boston, pp25-30,1989. 27. Taverne MAM: Uterine motility in the post partum female. l Oth International Congress on Animal Reprod and AI, June 10-14, Urbana-Champaign, IL, 4:X1-1,1984. 28. van Niekerk CH: Early clinical diagnosis of pregnancy in mares. J S Afr Vet Med Assn 36:51-58,1965. 29. VandeplasscheM: The normal and abnormal presentation, position and posture of the foal-fetus during gestation and at parturition. VlammsDiergeneesk Tijdschr (Suppl 1)26:1-68,1957. 30. VandeplasscheM, Bouters R, Spincemaille J, Bonte P, Coryn M: Observations on involution and puerperal endometritis in mares. Irish Vet J 37:126-132,1983.
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