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a Department of Animal Health and Biomedical Sciences, University of Wisconsin, Madison, Wisconsin 53706
b Department of Animal Science, Federal University of Viçosa, Viçosa, MG 31570-000, Brazil
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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20 mm (Day 0). The larger follicle became the dominant follicle in all mares in which both follicles (n = 8) or only the larger follicle (n = 10) was retained. When only the smaller follicle (n = 9) was retained, it became dominant and ovulated in six mares and became atretic in three mares; the difference in diameter between the two follicles on Day 0 was less (p < 0.01) in mares in which the retained smaller follicle grew and ovulated (2.2 ± 0.6 mm) than in the mares in which the follicle became atretic (5.9 ± 1.2 mm). A decline (p < 0.0001) in FSH concentrations occurred over Days -4 (8.4 ± 0.7 ng/ml) to 0 (5.9 ± 0.3 ng/ml), averaged over all groups, and the decline continued for several more days in the groups with both follicles or with only the larger follicle retained. In the group with only the smaller follicle retained, compared to the group with both follicles retained, FSH concentrations and diameter of the smaller follicle increased between Days 0 and 1 (significant interaction for each end point). After Day 1, FSH concentrations continued to increase when the smaller retained follicle became atretic; concentrations decreased when the smaller retained follicle became dominant. An increase (p < 0.0001) in LH concentrations occurred over Days -4 (12.2 ± 1.1 pg/ml) to 0 (21.1 ± 2.0 pg/ml), averaged over the three groups. In 23 of 27 mares, a transient peak in LH concentrations occurred within 2 days of Day 0. In the groups with both follicles or with only the larger follicle retained, an increase (p < 0.0001) in systemic estradiol concentrations occurred between Day 0 (5.3 ± 0.6 pg/ml) and Day 2 (7.5 ± 0.4 pg/ml). When only the smaller follicle was retained, estradiol did not begin to increase until Day 2, and it increased only when the retained follicle grew and became dominant. The beginning of an increase in estradiol and continued decrease in FSH at the expected beginning of deviation were attributable to the future dominant follicle; there was no indication that the smaller follicle was involved.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Although the deviation mechanism is poorly understood, the following reported findings in mares may be related to the factors that control it. 1) Daily injections of FSH begun when follicles were first detected at > 20 mm (presumably before deviation) overrode deviation as indicated by an increased ovulation rate [8]; 2) endogenous circulating FSH was at low concentrations at the beginning of deviation [2, 3]; 3) LH was at increasing or plateaued concentrations, sometimes transiently, at the time of deviation [3]; 4) follicular-fluid estradiol concentrations in the future dominant follicle differentially increased before the beginning of deviation [3]; 5) LH receptors were higher in the theca when the mean diameter of the largest follicle was 29 mm (presumably after deviation [9]); and 6) deviation began and was established in less than 1 day as indicated by a 1-day size advantage of the largest follicle over the next largest follicle [3]. The latter is the basis for the proposal [3] that when the largest follicle reaches a critical diameter, it inhibits the next largest follicle before it reaches a similar critical diameter.
The purpose of the present experiment in mares was to obtain further information on the nature of the deviation mechanism, using the two-follicle model. The hypothesis was tested that the smaller follicle is capable of becoming dominant if the larger follicle is removed at the expected beginning of deviation. Circulating FSH, LH, and estradiol-17ß concentrations following ablation of none or either of the two follicles were used to assess the hormonal aspects of the deviation mechanism.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Nonlactating cyclic pony mares (n = 29) between 4 and 12 yr of age and weighing 204–426 kg were used from August to September. The mares were kept in partially sheltered outdoor paddocks and were maintained on alfalfa/grass hay with access to water and trace-mineralized salt. The ultrasound scanner was equipped with a 5-MHz linear-array transrectal transducer (Tokyo Keiki LS300; Products Group International, Lyons, CO) for examinations of the ovaries and uterus. All mares were determined to be in the ovulatory season, as indicated by detection (using ultrasonography) of ovulation and the formation of a corpus luteum [10]. Ultrasonic examinations of the ovaries and uterus were done daily or every other day as described [10], and mares were not used if they had indications of ovarian or uterine abnormalities. Before the start of the study, mares were synchronized with prostaglandin F2
(5 mg, i.m., Lutalyse; Pharmacia & Upjohn Co., Kalamazoo, MI). Follicles 30 mm in diameter were monitored daily until ovulation. The experiment was started 10 days after ovulation.
Follicle Ablations
The experiment used the two-follicle model as in previous studies [3, 4]. Follicle ablations were done transvaginally by ultrasound-guided follicle entry, using an ultrasound scanner (Aloka SSD-500V; Aloka, Wallingford, CT) with a 5-MHz convex-array transvaginal transducer (Aloka UST974V-5) as described [3]. Briefly, 10 days after ovulation, all follicles ( 5 mm) were ablated by aspiration of follicular contents through a 17-gauge needle using a vacuum pump (250–300 mm Hg). Follicle ablation was defined by collapse of the antrum after evacuation of follicular contents. Growing follicles ( 4 mm) of the new or postablation wave were tracked daily to maintain individual identity [10]. Subsequent ablation sessions were done to establish the two-follicle model for the new follicular wave. When the largest follicle reached 15 mm, all follicles 5 mm were ablated except the two largest. Ablation sessions were repeated whenever a new follicle reached 15 mm until the larger of the two retained follicles reached 25 mm or until 3 days after the beginning of reduction in diameter of the subordinate follicle.
Follicular-Wave Definitions
Emergence of a follicle was defined as occurring on the day before the follicle first exceeded 6 mm [3, 4]. The dominant follicle (one that grew to a preovulatory diameter of 30 mm) and subordinate follicle (one that grew to a moderate diameter and regressed) were chosen retrospectively according to the maximum attained diameters. The beginning of deviation between the future dominant and subordinate follicles was defined as the day the two follicles began to differ in growth rates [3]. Thus, the beginning of deviation refers to the examination preceding the first change in differences in diameters between the two follicles. For purposes of treatments, the first day that the larger follicle reached 20 mm was used as the expected day of the beginning of deviation as in a previous study [4] and was designated Day 0.
Ultrasound End Points
The relative location of follicles, corpus luteum, and follicle ablation sites (echoic areas) were used as references for identifying and tracking follicles. Height and width of the two follicles of the model were taken from three images during a scanning session, and the average of the six measurements per follicle was used as the follicle diameter for each day. The diameter of the corpus luteum was evaluated daily as described [10], and mares in which the corpus luteum did not regress by 24 days after ovulation were removed from the experiment.
Blood Sampling and Hormone Assays
Blood samples were collected daily, beginning 10 days after ovulation and ending on the day of the next ovulation or 24 days after the pretreatment ovulation. Samples were assayed for Days -4 to +4. Samples were collected by jugular venipuncture into heparinized tubes and held for 1–2 h at 4°C until sedimentation. Plasma was decanted and placed in polyethylene vials for cold storage (-20°C) until assay.
Circulating concentrations of FSH [11] and LH [12] were determined using RIAs previously validated in this laboratory for this species. The intraassay coefficients of variation and the sensitivity for the FSH and LH assays were as follows: FSH, 4.19% and 0.18 ng/ml; LH, 4.22% and 0.23 ng/ml, as determined over one assay each for FSH and LH.
Estradiol-17ß (estradiol) concentrations were determined using a modification of a commercially available RIA kit (double-antibody estradiol; Diagnostic Products Corporation, Los Angeles, CA) that has been validated for analysis of bovine samples [13] and has been adapted for use with equine plasma in our laboratory. Duplicate aliquots of 200 µl of plasma were extracted with 2 ml of diethyl ether, frozen in a dry ice/methanol bath, and decanted into assay tubes. Estradiol standards (1.0–400 pg/ml) were prepared in 100% ethanol. Before the assay, the ether and the ethanol of the assay tubes were removed by evaporation in a hot water bath. The dried samples and standards were resuspended with 100 µl of assay buffer (0.1% gelatin in PBS). For the assay, 30 µl of estradiol antiserum, 75–100 µl of 125I-estradiol (approximately 22 000 cpm), and 1 ml of precipitating solution were used. The recovery of known amounts of estradiol (0.1–20 pg) from ovariectomized mare plasma was, on average, 80.4%, and the slopes of the diestrus and estrus plasma pools (10–200 µl) were similar to that of the estradiol standard. In the present study, plasma concentrations of estradiol were expressed relative to the standard curve, and no correction was made for recovery. The intraassay and interassay coefficients of variation were 11.4% and 9.1%, respectively (calculated from extracted samples from 3 assays). The sensitivity of the assay was 0.57 pg/ml (2 standard deviations below the mean cpm at maximum binding).
Follicular-fluid estradiol concentrations were evaluated with a specific ELISA, as previously described [14], and modified for direct use with follicular fluid [4, 15]. Briefly, follicular-fluid samples were diluted 1:1000 in assay buffer and analyzed directly with the ELISA. The standard curve was made in assay buffer containing a 1:1000 dilution of charcoal-treated equine follicular fluid with estradiol concentrations from 31 to 2000 pg/ml. The intraassay and interassay coefficients of variation were 11.6% and 12.3%, respectively. The sensitivity (2 standard deviations from maximum bound) was equivalent to a concentration of 0.91 pg/ml.
Experimental Groups
Follicle ablations were done when the larger follicle of the two-follicle model reached 20 mm. Mares were randomized into three groups: 1) both follicles retained (neither follicle ablated, n = 9), 2) larger follicle retained (smaller follicle ablated, n = 10), and 3) smaller follicle retained (larger follicle ablated, n = 10). The latter group was partitioned for some of the analyses into mares in which the smaller retained follicle became dominant versus atretic. Before ablation, the follicular fluid was sampled with a syringe attached to the ablation needle. A 1-ml portion of the aspirated follicular fluid was inserted into tapered microtubes after centrifugation and stored at -20°C. Thereafter, the sampled follicle was completely ablated by attaching the vacuum pump to the needle. The retained follicles were monitored daily until ovulation or 24 days after the pretreatment ovulation.
Statistical Analyses
Data were normalized to the day of follicle ablation (Day 0). End points for Days -4 to 4 were analyzed using a group-by-day factorial ANOVA for sequential data. In addition, the pretreatment (Days -4 to 0) changes were analyzed by one-way ANOVA for sequential data using all mares as a single group, and the posttreatment (Days 0–4) changes were analyzed by a factorial ANOVA. A 2-by-2 factorial ANOVA was used for diameter of the smaller follicle and FSH concentrations between Days 0 and 1 in the presence and absence of the larger follicles. Duncan's multiple-range tests were used to locate the significant differences after Day 0 within each hormone. Unpaired t-tests were used to compare single-point differences. Significance was indicated by a probability of p < 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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20 mm were 21.2 ± 0.2 mm and 17.7 ± 0.4 mm combined for all groups. In all mares with both follicles retained (n = 8), the first follicle to reach 20 mm became dominant and ovulated. In all mares with only the larger follicle retained (n = 10), the retained follicle grew and ovulated. In six of nine mares with only the smaller follicle retained, the follicle grew and ovulated, and in the remaining three mares the retained follicle underwent atresia; ovulation occurred from a follicle of a subsequent wave. On Day 0, the diameter of the larger follicle (p < 0.001) and the difference in diameter between the two follicles (p < 0.01) were greater in the mares in which the retained smaller follicle became atretic (23.3 ± 0.3 mm and 5.9 ± 1.2 mm, respectively) than in the mares in which the follicle ovulated (20.6 ± 0.3 mm and 2.2 ± 0.6 mm). The diameter of the smaller follicle on Day 0 was not significantly different between the mares with atresia (17.4 ± 1.4 mm) and the mares with ovulation (18.4 ± 0.5 mm).
The mean growth profiles of the follicles that became dominant and ovulated and the results of the statistical analyses normalized to Day 0 are shown for each of the three groups (Fig. 1). The growth and regression profiles of the smaller follicle in mares with both follicles retained and in the three mares with atresia of the smaller follicle in the group with only the smaller follicle retained also are shown (Fig. 1). The means for maximum diameter of the dominant follicle (36.6 ± 0.7 mm combined for all groups), preovulatory diameter on the day before ovulation (35.8 ± 0.8 mm), length of the interval from Day 0 to ovulation (7.3 ± 0.3 days), and length of the interovulatory interval (24.9 ± 0.3 days) were not significantly different among the three groups.
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Significant day and group effects were obtained for FSH concentrations over Days -4 to 4, and the interaction approached significance (p < 0.09; Fig. 2). The FSH concentrations decreased (p < 0.0001) during Days -4 to 0 averaged over all groups. An interaction (p < 0.003) on Days 0–4 involved a decrease (p < 0.0001) after Day 0 in the groups with both follicles retained or with only the larger follicle retained. Partitioning of the mares with ablation of the larger follicle into those in which the retained smaller follicle eventually ovulated and those in which it regressed demonstrated an interaction resulting from a decrease versus increase, respectively, in FSH concentrations after Day 1 (Fig. 3). Comparisons of diameters of the smaller follicle and concentrations of FSH for Days 0 and 1 are shown for the groups with both follicles retained or with only the smaller follicle retained (Fig. 4); the group-by-day interaction was significant for both end points.
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The day effect for concentrations of LH on Days -4 to 4 was significant and was attributable to an increase (p < 0.0001) for all groups on Days -4 to 0 (Fig. 2). A significant interaction was primarily due to an increase (p < 0.01) between Days 0 and 1 in the group with only the smaller follicle retained. Only the day effect was significant when mares with ablation of the larger follicle were partitioned into those with ovulation versus atresia of the retained smaller follicle (Fig. 3). In 23 of 27 (85%) mares, a progressive increase in LH concentrations occurred for 2–4 days, leading to a peak value on Days -2 to 2. The peak value occurred earlier in the group with both follicles retained (Day -0.5 ± 0.4) than in the groups with only the larger follicle retained (Day 0.9 ± 0.4; p < 0.03) or only the smaller follicle retained (Day 1.1 ± 0.3; p < 0.0007). The peak was followed by a decrease and then by an increase in 21 of the 23 mares.
Circulating estradiol concentrations showed a significant day effect for Days -4 to 4 and a significant interaction. The interaction was primarily due to a delayed increase after Day 0 in the group with only the smaller follicle retained (Fig. 2). An interaction occurred after partitioning of mares with the smaller follicle retained and resulted from an increase after Day 2 (p < 0.04) in mares in which the retained smaller follicle ovulated but not in the mares in which the retained follicle became atretic (Fig. 3). The mean concentration of estradiol in the follicular fluid on Day 0 just before ablation was greater (p < 0.006) for the larger follicle (1268 ± 110 ng/ml; diameter, 21.5 ± 0.5 mm) than for the smaller follicle (738 ± 126 ng/ml; diameter, 17.3 ± 0.9 mm).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Actual mean diameter (21.2 mm) of the larger follicle when it reached 20 mm can be considered close to mean diameters at the beginning of deviation in growth rates between the two follicles. Mean diameter of the larger follicle at the beginning of deviation, based on inspection of follicle-diameter profiles in individual mares, was 20.6 mm in the eight mares with both follicles retained and 21.0, 21.9, and 23.1 mm in various groups in previous studies that used the two-follicle model [3, 4]. In the group with both follicles retained, the first follicle to reach 20 mm became the ovulatory follicle in all mares, in agreement with earlier studies [3, 4]. The larger follicle ovulated in all mares in which the smaller follicle was ablated.
When the larger follicle was ablated at 20 mm, the smaller retained follicle continued to grow, and ovulated in six (67%) mares. In the remaining three mares, the retained smaller follicle became atretic, and the subsequent mean diameter changes were similar to those for the smaller follicle in the group with both follicles retained. Apparently, the three follicles were irreversibly destined to become subordinate follicles before the larger follicle reached 20 mm. In these three mares, the larger follicle was larger at ablation, and the difference in diameter between follicles was greater than for the mares in which the follicle ovulated. The mean diameter difference between follicles was 0.2 mm on Day -1 and 4.5 mm on Day 0 for one of the three mares; deviation had already occurred. In the remaining two mares, the difference in diameters was exaggerated (5.0 and 8.3 mm on Day 0) consistently during Days -4 to 0. In comparison, the differences were 1.0–3.2 mm in the six ovulatory mares. The hypothesis that the smaller follicle is capable of becoming the dominant follicle if the larger follicle is removed at the expected time of deviation was supported. Apparently, however, the difference in diameter between the two follicles must not be exaggerated. The ability of a smaller follicle to assume dominance if the larger follicle is removed also has been demonstrated in cattle; a 5-mm follicle selected at random became dominant when the other 5-mm follicles were aspirated [5], and another follicle sometimes became the dominant follicle when the largest follicle was cauterized apparently soon after expected deviation [6, 7].
Growth rate of the dominant follicle for the 4 days after ablation of the other follicle was comparable between the two ablation groups. However, a dominant follicle that originated from a smaller follicle after ablation of the larger follicle did not compensate for its smaller diameter when compared to a dominant follicle that originated from a larger follicle. In the group with both follicles retained, the dominant follicle did not grow as rapidly after Day 0 as in the two ablation groups. This was indicated by the group-by-day interaction between Days 0 and 4. The reason for this result is perplexing, especially because there were no significant differences among groups in maximal and preovulatory diameters or in the length of the interval from Day 0 to ovulation.
A decline in FSH concentrations occurred over Days -4 to 0, for all groups, and the decline continued for several more days in the groups with both follicles retained or with only the larger follicle retained. The influence of the larger follicle between Days 0 and 1 was consistent; the FSH values decreased in 17 of 18 mares with the larger follicle intact. In the group with only the smaller follicle retained, FSH increased between Days 0 and 1. After Day 1, concentrations continued to increase when the smaller retained follicle became atretic and decreased when the smaller follicle grew and became dominant. These results indicated that when the larger follicle reached 20 mm, it was responsible for the continuing decrease in FSH concentrations, whereas the companion smaller follicle had no detected FSH-suppressing capability. The deviation mechanism apparently involved suppression of the FSH concentrations by the larger follicle below the requirements of the smaller follicle. Low concentrations of FSH have been postulated to play a role in deviation (reviewed in [22]). However, this is apparently the first report in mares that the larger follicle alone is responsible for the depressed FSH concentrations at the beginning of deviation or before the detection of a reduced growth rate by the second-largest follicle. A role for the low concentrations of FSH in the deviation mechanism is consistent with the findings that exogenous FSH increased ovulation rates when given daily, beginning early in the estrous cycle [23] or when the follicles were > 20 mm [8].
Whether all follicles contribute to the FSH decline before the largest follicle reaches 20 mm or before deviation has not been studied in mares. However, in cattle, results of experimentally decreasing the number of follicles indicated that all growing follicles contributed to the decline in FSH after wave emergence but before deviation [5]. In the present study in mares, the relationship in kind or quantity between the factor produced by growing follicles that causes the major decline in the FSH surge that initiated the follicular wave and the hypothetical factor that apparently is produced primarily by the largest follicle at the time of deviation and continues the FSH depression is not known and was not considered.
A close temporal effect of a change in FSH concentrations on a change in diameter of the smaller follicle was demonstrated by comparing Days 0 and 1 for the groups with both follicles retained versus only the smaller follicle retained. Only the mares in both groups with a difference in diameter between the two follicles on Day 0 of < 4 mm were used because of nonovulation when the difference was 4 mm in mares with only the smaller follicle retained. A day- (Days 0 and 1) by-group (both follicles retained or smaller follicle retained) interaction for FSH resulted from a decrease in concentrations between days when both follicles were retained versus an increase when only the smaller follicle was retained. An interaction for the associated changes in diameter of the smaller follicle resulted from a greater increase in the group with only the smaller follicle retained. The FSH increase after Day 0 and the resulting increase in follicle diameter occurred within 24 h for both end points. In the group with the larger follicle ablated, the smaller follicle on Day 1 (21.1 ± 0.7 mm) had a diameter equivalent (no significant difference) to the diameter of the larger follicle just before ablation (20.6 ± 0.2 mm). The growth of the smaller follicle between Days 0 and 1 would account for its acquisition of FSH-suppressing capability after Day 1. These findings support the proposal for cattle [22, 24] and mares [3] that when the larger follicle reaches a critical diameter it suppresses the next largest follicle before it can reach a similar critical diameter. The establishment of follicle suppression occurred within 1 day, which is equivalent to the reported difference in diameter between the two follicles, extending, on average, from emergence to deviation [3].
A significant increase in LH concentrations involved all groups over Days -4 to 0. Upon ablation of the larger follicle (smaller follicle retained), mean LH concentrations increased above the means in mares with both follicles or only the larger follicle. The increased LH concentrations are not attributable to FSH, since the assay cross-reactivity was low [12]. This unexpected result indicated that the larger follicle at this time produced a substance that had a depressing effect on LH concentrations. Ablation of the smaller follicle did not significantly increase LH concentrations. The identities of the follicular LH stimulants and depressants under these circumstances are not known. Exogenous estrogens can have either a positive or negative effect on LH, as well as FSH, depending on the dosage and physiologic status of the animal [25, 26]. Studies are needed to clarify the relationships between circulating LH and its control by follicular products at this specific time.
Inspection of LH profiles for individual mares was informative and indicated a later occurrence of the LH peak when the larger follicle was ablated. This result may be related to the higher means after Day 0 in this group. The peak was followed by a decrease and then by an increase in most mares. The nadir observed in individuals was masked in the mean profiles by the occurrence of the low values on different days after Day 0. Thus, a peak in LH concentrations occurred within 2 days of Day 0 and was followed in a few days by a nadir. Although not further studied, the LH concentrations after the nadir would be expected to continue increasing as a continuation of the prolonged equine preovulatory LH surge, which reaches a peak, on average, 1 day after ovulation [12, 27]. These results are consistent with a previous observation [3] that transiently high or plateaued concentrations of LH encompassed deviation in 11 of 14 mares, and in the remaining mares increased progressively without a transient elevation or plateau. In cattle, a small but significant plateau in LH concentrations encompassed the time of deviation [28]. These results in the two species are consistent with a postulated role for LH in a shift in gonadotropic responsiveness of the larger follicle from FSH to LH near the time of deviation (reviewed in [22]). Support for a role of LH in deviation in cattle includes the findings that follicles did not grow beyond 7 to 9 mm when circulating LH concentrations were suppressed [29], and LH receptors began to develop in the granulosa cells of the largest follicle at about the expected time of deviation [15, 30, 31]. Similar receptor studies have not been done in mares until well after the expected time of deviation [9].
The concentrations of circulating estradiol did not increase before Day 0, agreeing with the results of follicular-fluid estradiol assessment in a previous study in cattle [32]. However, in a previous study in mares [4], systemic concentrations began to increase before the beginning of diameter deviation. In the present study, circulating estradiol concentrations increased progressively immediately after Day 0 in both groups with the larger follicle retained. In the group with only the smaller follicle retained, the concentrations did not increase until after Day 2, and the increase was attributable to the six mares that ovulated. The increase after Day 0 only when the larger follicle was present indicated that after the larger follicle reached 20 mm, it was primarily responsible for systemic estradiol concentrations. Blood sampling more often than once every 24 h will be needed to clarify the temporal relationship between increased estrogen productivity by the future dominant follicle and the beginning of diameter deviation. The estradiol concentrations in the follicular fluid of the two follicles were similar to concentrations previously reported when the larger follicle reached 20 mm [4]. In the previous study, the difference in follicular-fluid estradiol concentrations between the two follicles increased before deviation because of a greater increase in the future dominant follicle than in the future subordinate follicle. Estradiol remains a candidate for involvement in the mechanism of deviation and could function both by systemic inhibition of circulating FSH concentrations and by intrafollicular facilitation of LH responsiveness by the larger follicle (reviewed for cattle in [22]).
In conclusion, ablating the larger or smaller follicle of a two-follicle model at the expected day at the beginning of follicle deviation indicated that the smaller follicle was viable and capable of becoming dominant when the larger follicle was removed. Assessment of the resulting hormonal profiles indicated that the larger follicle alone was responsible for an increase in estradiol and a continuing decrease in systemic FSH at the expected beginning of deviation; no changes were detected after ablation of the smaller follicle.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Correspondence: O.J. Ginther, Department of Animal Health and Biomedical Sciences, University of Wisconsin-Madison, 1656 Linden Drive, Madison, WI 53706. FAX: 608 262 7420; ojg{at}ahabs.wisc.edu
Accepted: April 21, 1999.
Received: March 3, 1999.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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O.J. Ginther, D.R. Bergfelt, L.J. Kulick, and K. Kot Selection of the Dominant Follicle in Cattle: Role of Two-Way Functional Coupling Between Follicle-Stimulating Hormone and the Follicles Biol Reprod, April 1, 2000; 62(4): 920 - 927. [Abstract] [Full Text]
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E.L. Gastal, D.R. Bergfelt, G.P. Nogueira, M.O. Gastal, and O.J. Ginther Role of Luteinizing Hormone in Follicle Deviation Based on Manipulating Progesterone Concentrations in Mares Biol Reprod, December 1, 1999; 61(6): 1492 - 1498. [Abstract] [Full Text]
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