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Biology of Reproduction 66, 266-271 (2002)
© 2002 Society for the Study of Reproduction, Inc.

Regular Article

Effect of the Regressing Corpus Luteum of Pregnancy on Ovarian Folliculogenesis after Parturition in Cattle1

I. Martin Sheldon2,a, David E. Noakesa, and Hilary Dobsonb

a Department of Farm Animal and Equine Medicine and Surgery, Royal Veterinary College, University of London, Hatfield AL9 7TA, United Kingdom b Department of Veterinary Clinical Science and Animal Husbandry, University of Liverpool, Chester CH64 7TE, United Kingdom


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In cattle, the first postpartum dominant follicle has a predilection for the ovary contralateral to the previously gravid uterine horn, possibly due to a local inhibitory effect of the regressing corpus luteum of pregnancy in the ipsilateral ovary. The aim of the present study was to test the hypothesis that the regressing corpus luteum of pregnancy suppresses folliculogenesis in the ipsilateral ovary after parturition. Dairy cows were treated with prostaglandin F2{alpha} between 190 and 220 days of gestation to cause luteolysis without inducing parturition (n = 14) or were untreated controls (n = 32). Follicular growth and function were monitored by daily transrectal ultrasonography and collection of plasma samples for estimation of FSH, estradiol, and progesterone concentrations. The proportion of first dominant follicles in the ipsilateral ovary was similar for treated and control animals (4/14 vs. 8/32), as was the time interval between calving and establishment of a dominant follicle (mean ± SEM, 10.1 ± 0.4 vs. 10.7 ± 0.5 days). Furthermore, no significant effect of treatment on dominant follicle growth or function was found as determined by plasma hormone concentrations. Although greater folliculogenesis was found in the ovary contralateral to the previously gravid uterine horn, once the location of the future first dominant follicle was selected, the timing of events was independent of location. We suggest that the corpus luteum of pregnancy does not have a local effect on postpartum ovarian folliculogenesis and that, instead, an effect of the previously gravid uterine horn shortly after parturition should be considered.

corpus luteum, follicular development, ovary, parturition, uterus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
After parturition in cattle, an increase in plasma FSH concentration occurs, which is followed by the emergence of a follicular wave and subsequent selection of a dominant follicle [1, 2]. Use of sequential transrectal ultrasonography has revealed a preference for the first postpartum dominant follicle to be selected in the ovary contralateral to the previously gravid uterine horn and, thus, the ovary bearing the corpus luteum (CL) of pregnancy [3–5]. This is important, because the presence of a large follicle in the ovary ipsilateral to the previously gravid uterine horn within 4 wk of parturition, although less frequent, is associated with improved subsequent fertility [6–8].

Suppression of folliculogenesis in the ipsilateral ovary decreases as the postpartum interval advances, concurrent with disappearance of the CL of pregnancy and uterine involution [6, 9]. This could be explained by a local inhibitory effect of the regressing CL of pregnancy or by a regional effect of the previously gravid uterine horn [10]. Although luteolysis and regression of the CL is rapid during the estrous cycle, luteolysis following parturition is protracted, with physical remnants of luteal cells being detectable up to 35 days postpartum [9]. In addition, evidence for a nonsteroidal aromatase inhibitor of luteal origin that can influence follicular steroidogenesis has been found [11].

The aim of the present study was to test the hypothesis that the regressing CL of pregnancy suppresses folliculogenesis in the ipsilateral ovary after parturition. Specific objectives were, first, to remove the CL of pregnancy without disruption of the fetus by administering prostaglandin F2{alpha} (PG) during the last trimester of pregnancy [12]. Then, the effect of the absence of the CL of pregnancy on ovarian follicle structure and function, after parturition, would be determined using sequential ultrasonography and plasma hormone assays.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals

All procedures were carried out under the Animals (Scientific Procedures) Act 1986 regulations for experiments on living animals, administered by the UK Home Office. In addition, experimental protocols were approved by the Royal Veterinary College Ethical Review Board.

A dairy herd of 90 Holstein-Friesian cows, with an annual average milk yield of 6800 L, was selected for the study on the basis of accurate farm records. A total of 53 cows that were due to calve within a 5-mo period were included in the study. All cows had been artificially inseminated with Holstein-Friesian semen. The PG analogue (500 µg of cloprostenol; Schering-Plough Animal Health, Uxbridge, UK) was administered i.m. between 190 and 220 days of gestation to 17 animals selected using a randomization chart (http://www.randomizer.org).

Examination

The genital tracts of all cows were examined daily by transrectal palpation and ultrasonography using a 7.5-MHz, linear-array transducer (Aloka SSD 210 DXII; BCF Technology, Livingstone, UK) starting on Postpartum Day 6 and continuing for 21 days. The side of the previous pregnancy was determined by assessing which uterine horn was longest and of greatest diameter on Postpartum Day 6. Follicles were defined as nonechogenic (black), spherical structures with a clear demarcation between the follicular wall and antrum. Corpora lutea were defined as grainy, echogenic structures having a well-defined border with the less echogenic ovarian stroma; in some CL, a nonechogenic lacuna was observed. After freezing the image on the screen, the number of ovarian follicles >=4 mm in diameter and of CL in each ovary were counted, and their maximum diameter was measured using the internal calipers of the machine. When the image of the structure being scanned was not spherical, the diameter was estimated by averaging two 90° dimensions.

A dominant follicle was defined as the largest follicle in the ovary with an internal diameter of >=10 mm in the absence of other growing follicles [13]. A dominant follicle and cohorts were defined as a follicular wave [13]. The first day of dominance within a follicular wave was determined, retrospectively, as the first day on which the dominant follicle was >=10 mm in diameter. The number of follicles >=4 mm or >=10 mm in a wave was based on the emergence of the follicles at the same, or consecutive, examinations [14]. Day of ovulation was defined as the day when a dominant follicle was last scanned before subsequent appearance of a CL in the same location and was confirmed, retrospectively, by a subsequent increase in plasma progesterone to a concentration of >1 ng/ml. A persistent follicle was defined as a follicle with an internal diameter of >=10 mm that persisted for more than 5 consecutive days in the absence of new follicular growth and that did not ovulate [13].

Blood Sampling and Hormone Assays

Blood samples were collected daily for 21 days, starting on Postpartum Day 6, from the coccygeal vein or artery into evacuated, heparinized tubes (Vacutainer; Becton Dickinson, Meylan, France) and transported on ice to the laboratory. Within 30 min, plasma was separated by centrifugation (2200 x g for 10 min), harvested, and stored frozen at -20°C.

Estradiol-17ß concentration was estimated in duplicate using a previously characterized RIA (Estradiol MAIA; Serono Diagnostics Ltd., Woking, UK) following diethyl ether extraction of the plasma samples [15]. The mean intraassay (n = 12 samples) and interassay (n = 3 assays) coefficients of variation were 8.1% and 13.1%, respectively, for a sample of 0.9 pg/ml, and the sensitivity was 0.24 pg/ml. Progesterone concentration was estimated in duplicate using a commercial ELISA kit (Ridgeway Science, Gloucester, UK). The intraassay (n = 10 samples) and interassay (n = 3 assays) coefficients of variation were 6.5% and 11.2%, respectively, for a sample of 1.7 ng/ml, and the sensitivity was 0.6 ng/ml. The FSH concentration was estimated in duplicate using a previously validated RIA [13]. The standard used for the FSH assay was AFP 5679C RP-1. The intraassay (n = 20 samples) and interassay (n = 3 assays) coefficients of variation were 3.4% and 4.7%, respectively, for a sample of 1.2 ng/ml, and the sensitivity was 0.12 ng/ml.

Statistical Analysis

Data analysis was performed on 46 animals (14 treated and 32 control) using the SAS version 8.01 computer program (SAS Institute Inc., Cary, NC). Three treated cows were excluded from the analysis: one aborted at 243 days of gestation, one had a cesarean operation, and one had a physical injury. Four control cows were excluded: 2 had cesarean operations, and 2 had mastitis. Data are quoted as the arithmetic mean ± SEM, and significance was attributed at P < 0.05. Continuous data were examined for normality using the Kolmogorov-Smirnoff test and for equality of variance using Levene test [16]. Plasma hormone data were log10 transformed before statistical analysis.

The duration of gestation, time interval from insemination to PG administration, intervals between parturition and dominance or ovulation, and interval between ovulation and increase in plasma progesterone concentration were examined by Kaplan-Meier survival analysis [17]. Differences between the treated and control groups were compared using the log rank test [17]. The maximum diameter of follicles was compared between ovaries and between treatments using unpaired t-tests and between different follicular fates using ANOVA. Comparisons of the location or the fate of ovarian structures were tested using the chi-square test or, when a cell had an expected frequency of less than five, Fisher Exact test. Data for the number of follicles in each ovary were compared using the nonparametric Wilcoxon signed rank test, and differences between treatment groups were tested using the Mann-Whitney test [18].

Follicle diameters and plasma hormone concentrations were examined using ANOVA mixed models for repeated measures [19]. Response variables tested were day postpartum, treatment group, fate of the first dominant follicle (ovulated, regressed, persistent follicle), location of the first dominant follicle (ipsilateral, contralateral ovary), and their interactions. Variables were removed from the model following examination for correlation with remaining variables until those variables remaining were significant. Model fitting and selection of the covariance structure were determined using Akaike information criterion [19]. Correlation between diameter of the first dominant follicle and plasma hormone concentrations were tested using the Pearson correlation coefficient.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
The interval from conception to administration of PG for treated cows was 207.5 ± 3.4 days (range, 190–220 days). The interval between administration of PG and parturition was 66.4 ± 3.5 days (range, 48–82 days). The gestation interval was shorter (P < 0.001) for treated cows (274.6 ± 2.5 days; range, 255–289 days) compared with control animals (284.2 ± 0.9 days; range, 276–296 days).

Location of Events

The CL of pregnancy was not detected postpartum by ovarian ultrasonography in any cow administered PG, although it was observed in all control animals between Postpartum Days 6 and 14. A wave of follicular development, with the emergence of a dominant follicle, was observed in all cows within 14 days of parturition.

Fewer follicles of >=4 mm in diameter were found in the ipsilateral ovary compared with the contralateral ovary in the first follicular wave after parturition (2.27 ± 0.23 vs. 3.34 ± 0.20 mm, P < 0.001). However, when comparisons were made between treated and control animals, no significant differences were found in the numbers of follicles >=4 mm in diameter in the first postpartum follicular wave in the ipsilateral ovary (2.71 ± 0.37 vs. 2.03 ± 0.28 mm) or contralateral ovary (3.36 ± 0.40 vs. 3.33 ± 0.22 mm). Fewer first postpartum dominant follicles were observed in the ipsilateral ovary compared with the contralateral ovary (12 vs. 34, P < 0.01), but again, the proportions in the ipsilateral ovary did not differ between treated and control animals (4/14 vs. 8/32). A smaller proportion of dominant follicles, compared to follicles of >=4 mm in diameter, was observed in the ipsilateral ovary (12/46 vs. 104/257, P < 0.05).

The first dominant follicle ovulated in 35 of 46 cases. Fewer ovulations occurred in the ipsilateral ovary compared with the contralateral ovary (11 vs. 24, P < 0.01). No statistical difference was found in the proportion of dominant follicles that ovulated between the ipsilateral and contralateral ovaries (11/12 vs. 24/34), and no statistical difference was found in the frequency of ovulations from the ipsilateral ovary between the treated and control groups (4/14 vs. 7/32).

A second follicular wave was detected in 36 animals, 33 of which had ovulated and 3 in which the first dominant follicle had regressed. Similar numbers of follicles of >=4 mm in diameter were found in the ipsilateral ovary compared with the contralateral ovary (2.40 ± 0.26 vs. 2.43 ± 0.18). No significant difference was observed between the number of second dominant follicles in the ipsilateral and contralateral ovaries (13 vs. 23), and the frequency of second dominant follicles in the ipsilateral ovary did not differ between treated and control animals (6/14 vs. 7/22).

Timing of Events

The interval between calving and achieving dominance for the first dominant follicle was similar for the treated and control animals (10.1 ± 0.4 vs. 10.7 ± 0.5 days). The interval from calving to dominance did not differ between dominant follicles located in the ipsilateral or contralateral ovaries (10.6 ± 0.5 vs. 10.2 ± 0.4 days).

For those animals in which the first dominant follicle ovulated, the interval from calving to ovulation did not differ between the treated and control groups (16.1 ± 1.0 vs. 17.0 ± 0.7 days). The interval from calving to ovulation from the ipsilateral and contralateral ovaries was also similar (16.5 ± 0.88 vs. 16.8 ± 0.8 days). After each ovulation, an increase in plasma progesterone concentration to >1 ng/ml was detected 3.8 ± 0.2 days later, and this interval did not differ between those animals with a first dominant follicle in the ipsilateral or the contralateral ovary (3.7 ± 0.5 vs. 3.9 ± 0.2 days). The interval from ovulation to progesterone increase for cows treated with PG more than 60 days earlier did not differ significantly from that of control animals (4.6 ± 0.4 vs. 3.5 ± 0.2 days).

The interval from parturition to dominance of the second dominant follicle postpartum did not differ between treated and control animals (19.9 ± 1.0 vs. 20.8 ± 0.8 days) or between the ipsilateral and contralateral ovaries (20.8 ± 1.0 vs. 20.6 ± 0.8 days).

Follicular Growth and Function

The diameter of the first dominant follicle increased between Postpartum Days 6 and 16 (P < 0.001). However, follicular growth rates did not differ between treatment and control groups, and the interaction of group x day postpartum was not significant (Fig. 1a). The maximum diameter of the first dominant follicle of the treated and control groups was 14.9 ± 1.1 and 16.9 ± 0.8 mm, respectively. No significant difference was found between the diameter of dominant follicles in the ipsilateral and contralateral ovaries between Postpartum Days 6 and 16 (Fig. 1b). The maximum diameter of the first dominant follicle of the ipsilateral and contralateral ovaries was 14.8 ± 1.1 and 16.8 ± 0.8 mm, respectively.



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  		FIG. 1. Diameter (mm, mean ± SEM) of the first postpartum dominant follicle between Postpartum Days 6 and 16 in cows a) administered prostaglandin F2{alpha} at approximately 200 days of gestation ({blacksquare}) and control animals ({diamond}) and b) when the first postpartum dominant follicle was in the ipsilateral (•) or contralateral ({triangleup}) ovary

No significant effect of treatment group or location of the first dominant follicle was observed on plasma estradiol or FSH concentration, so the combined data are illustrated in Figure 2. Plasma estradiol concentration increased between Postpartum Days 6 and 16 (P < 0.001), and FSH concentration decreased between Postpartum Days 6 and 11 (P < 0.001), as follicular diameter increased. Between Postpartum Days 6 and 16, a significant correlation was observed between first dominant follicle diameter and plasma estradiol concentration (r = 0.53, P < 0.001) or FSH concentration (r = -0.44, P < 0.001). Plasma estradiol and FSH concentration also were correlated (r = -0.26, P < 0.001).



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  		FIG. 2. Diameter (mm, mean ± SEM) of the first postpartum dominant follicle ({blacksquare}), plasma estradiol (•) concentration (pg/ml) and FSH ({diamond}) concentration (ng/ml) between Postpartum Days 6 and 16 using data pooled from all animals

Ultrasonography revealed three possible fates for the first dominant follicle: ovulation; regression, followed by a second follicular wave; or formation of a persistent follicle (Fig. 3). The frequencies of these different fates of the first dominant follicle did not differ significantly between the treated and control animals. The dominant follicle ovulated, regressed, or formed a persistent follicle in 10, 3, and 1 treated animals and in 25, 3, and 4 control animals, respectively. Using pooled data across treatment groups, the interaction of fate of the first dominant follicle x day postpartum was significant for follicle diameter (P < 0.01), plasma FSH concentration (P < 0.01), and plasma estradiol concentration (P < 0.05). The diameters of first dominant follicles before regression were smaller than those of ovulatory or persistent follicles between Postpartum Days 9 and 16 (P < 0.01), and plasma estradiol concentrations were also lower between Postpartum Days 14 and 16 (P < 0.01). Plasma estradiol concentrations were correlated with diameter of the first dominant follicle for those that ovulated (r = 0.60, P < 0.001) or formed a persistent follicle (r = 0.36, P < 0.01), but not for those that regressed. Furthermore, unlike ovulatory or persistent follicles, plasma estradiol concentration in those animals with a first dominant follicle that subsequently regressed did not exceed 1 pg/ml. Plasma FSH concentration was lower in animals with a persistent follicle on Postpartum Day 15 compared to animals with a first dominant follicle that ovulated or regressed (0.56 ± 0.07 vs. 0.76 ± 0.08 or 0.72 ± 0.06 ng/ml, P < 0.05).



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  		FIG. 3. Diameter (mm) of the first ({blacksquare}) and, where observed, the second ({square}) postpartum dominant follicles, plasma estradiol (•) concentration (pg/ml), and FSH ({diamond}) concentration (ng/ml) between Postpartum Days 6 and 25 for typical cases in which the first dominant follicle a) ovulated, b) regressed, or c) persisted


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Administration of PG at approximately 200 days of gestation caused luteolysis without disturbance of the fetus, which remained supported by extragonadal sources of progesterone [12, 20]. The present study also confirmed that the CL of pregnancy was not detectable by ultrasonography after parturition in cows treated with PG early in the third trimester. Treated cows had a shorter gestation, although it was not possible in the present study to determine how this might affect the results. However, because the absence of the CL of pregnancy had no effect on postpartum follicular growth or function, or on the timing or location of ovarian events, an effect of the previously gravid uterine horn and its contents should be considered.

A postpartum, transient increase in plasma FSH concentration precedes emergence of the first postpartum follicular wave and subsequent selection of a dominant follicle during the following phase of decreasing FSH concentration [1, 2, 21, 22]. Similarly, in the present study, each animal after parturition developed a cohort of approximately 6 follicles of >=4 mm in diameter, and as plasma FSH concentration decreased, selection of the first dominant follicle occurred. However, plasma FSH concentration did not differ between those cows in which the first dominant follicle developed in the ipsilateral or contralateral ovary or between the treated and control animals.

The preference for the first postpartum dominant follicle to be selected in the contralateral ovary has been established by repeated transrectal ultrasonography after parturition [3, 4]. Additionally, in the present study, fewer follicles of >=4 mm in diameter were found in the ipsilateral ovary during the first postpartum follicular wave, and fewer ovulations were observed, reflecting fewer first dominant follicles in the ipsilateral ovary. However, the preference for structures to occur in the contralateral ovary diminishes with increasing interval from parturition [6]. Two possible explanations for these observations have been suggested. First, there could be a local luteal inhibitory effect [4, 10]; however, in the present study, the presence of a regressing CL of pregnancy did not affect several aspects of follicular growth. Furthermore, during normal estrous cycles, more dominant follicles are found in the CL-bearing ovary [23]. A second explanation is that there could be a regional effect of the involuting, previously gravid uterus on folliculogenesis in the ipsilateral ovary [6, 10, 24]. A possible mechanism would be the transfer of products from the uterus to the ovary via the countercurrent mechanism, as established for PG during the process of luteolysis [25].

The observation of fewer dominant follicles in the ipsilateral ovary after parturition appears to be a consequence of two components. First, fewer follicles of >=4 mm in diameter emerged in the ipsilateral ovary during the first postpartum follicular wave. Second, a smaller proportion of these first-wave follicles were selected to achieve dominance in the ipsilateral ovary. As FSH concentration gradually decreases, selection of the dominant follicle occurs, with subsequent transfer of gonadotropin dependence from FSH to LH [26, 27]; LH probably plays a minor role until the point of selection [28]. After parturition, we suggest that further control on follicle emergence and selection is exerted at the level of the ovary, acting to modulate the response of follicles to FSH secreted by the pituitary. However, inhibition of postpartum follicular growth in the ipsilateral ovary can be overcome by administration of eCG, which has both FSH- and LH-like activity [5]. Interestingly, a similar multilevel control mechanism has been suggested for endotoxin disruption of the follicular phase in ewes [29]. It is possible that immune/inflammatory challenges could also be involved in the control of folliculogenesis after parturition, because uterine involution and elimination of bacterial contamination provoke an acute-phase protein response [30].

Once the location of the first dominant follicle had been determined, the timing of events was independent of location in the ipsilateral or contralateral ovary and the presence of the CL of pregnancy. In addition, their ovarian location, or the CL of pregnancy, did not affect the growth rate of the first dominant follicles or their maximum diameter. Therefore, the mechanism responsible for imbalance between the ipsilateral and contralateral ovaries after parturition acts before and/or at the time of dominant follicle selection, and the regressing CL of pregnancy is not involved. Furthermore, first dominant follicles were as competent in the ipsilateral ovary as in the contralateral ovary as determined by their ability to secrete estradiol and to ovulate.

In the absence of an effect of the CL of pregnancy on postpartum follicular development, the influence of uterine involution on postpartum folliculogenesis should be considered. Postpartum folliculogenesis parallels observations of decreased follicular activity in the ovary ipsilateral to the gravid uterine horn during early and midpregnancy [31, 32]. In addition, the suppressive effect of the pregnant uterus and/or its contents on ovarian follicular growth is removed following hysterectomy [33]. Researchers studying sheep have concluded that the negative effects on folliculogenesis were exerted by the gravid uterine horn, and that the CL of pregnancy had a positive effect on follicle numbers in the ipsilateral ovary [34, 35].

The fates of the first dominant follicles observed in the present study and their function support previous descriptions [1]. In particular, first dominant follicles that regressed secreted less estradiol, irrespective of follicle diameter; this may be a consequence of inadequate LH support [28]. The frequency with which dominant follicles ovulated, regressed, or formed a persistent follicle did not differ between the treated and control cows, suggesting that fate is dependent on other factors, probably LH pulse frequency [36–38].

In conclusion, removal of the CL of pregnancy by administration of PG before parturition did not influence first postpartum follicular wave location, timing of ovarian events, or dominant follicle growth or function. Although greater folliculogenesis was observed in the ovary contralateral to the previously gravid uterine horn, once the location of the future first dominant follicle was selected, the timing of events was independent of location. We suggest that the CL of pregnancy does not have a local effect on postpartum ovarian function; instead, an effect of the previously gravid uterine horn shortly after parturition should be considered.


   ACKNOWLEDGMENTS
 
We thank Vicky Harrison, Hilary Purcell, and Jean Routley for technical assistance with hormone assays and Prof. Dirk Pfeiffer for statistical advice.


   FOOTNOTES
 
First decision: 15 August 2001.

1 Supported by the Royal College of Veterinary Surgeons (Wilson) Scholarship in Production Animal Medicine to I.M.S. Back

2 Correspondence: I.M. Sheldon, Department of Farm Animal and Equine Medicine and Surgery, Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield AL9 7TA, UK. FAX: 44 1707 666239; msheldon{at}rvc.ac.uk Back

Accepted: September 5, 2001.

Received: June 29, 2001.


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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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