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a Department of Animal Health and Biomedical Sciences, University of Wisconsin, Madison, Wisconsin 53706
ABSTRACT
Follicles of wave 1 were designated F1, F2, and so forth, according to descending diameter at the expected (F1,
8.2 mm) or observed beginning of deviation (Hour 0), as indicated by a reduction in growth rate of F2. During Hours -24 to 0 (experiment 1; n = 34 waves) and Hours -16 to 0 (experiment 2; n = 21), F1 and F2 grew in parallel (no significant differences). During Hours -16 to 0, growth rate was greater (P < 0.05) for F1 (1.4 ± 0.1 mm/16 h) and F2 (1.0 ± 0.1) than for F3 (0.6 ± 0.1) and F4 (0.5 ± 0.1). During Hours 0 to 16, growth rate was greater (P < 0.05) for F1 (1.4 ± 0.2 mm/16 h) than for F2 (0.1 ± 0.1), F3 (0.1 ± 0.1), and F4 (0.1 ± 0.2). In experiment 1, zero, one, two, or three largest follicles were ablated by aspiration of contents at Hour 0 (n = 7/group). For heifers with a single dominant follicle, the dominant follicle formed from the largest retained follicle more often when it was >7.0 mm (14 of 15) than when it was <7.0 mm (0 of 10). When the retained follicles were <7.0 mm, the first follicle to reach 7.0 mm became dominant in seven of eight heifers. Mean hour of observed deviation (occurring after Hour 0 in the ablation groups) increased progressively in groups with increasing number of ablated follicles. Plasma concentrations of FSH for groups with one, two, or three ablated follicles increased to a similar extent between Hours 0 and 12. Results supported the following: 1) during the 24 h before the beginning of deviation, small follicles grew more slowly than large follicles and the largest follicles grew in parallel; 2) after ablation of large follicles, the small retained follicles did not deviate until one reached a diameter characteristic of the beginning of deviation; 3) the potential for dominance at the expected beginning of deviation was greatest for the largest follicle and decreased progressively for the smaller follicles but only when the retained follicles were >7.0 mm; and 4) the three largest subordinate follicles began to deviate simultaneously.
follicle, follicle-stimulating hormone, follicular development, ovary
INTRODUCTION
Follicular wave 1 begins near the time of ovulation in cattle. Follicles of wave 1 continued to emerge up to 48 h after the future dominant follicle was 4 mm, but usually these follicles did not grow beyond 4 or 5 mm [1]. Thus, new follicles continued to emerge until near the end of a common growth phase at about 2.5 days after emergence of the future dominant follicle at 4 mm. The emergence of a wave is stimulated by an FSH surge that reaches a peak at about the time the follicles are 4–5 mm [2] and reaches a nadir 10–24 h after the end of the common growth phase [3, 4]. The follicles that emerge and grow to small diameter (<6 mm) near the end of the common growth phase develop during reduced and declining concentrations of FSH. The future dominant follicle emerged a mean of 6 or 7 h earlier than the future largest subordinate follicle [3, 4]. Although the diameter advantage of the largest follicle is maintained on the average until the end of the growth phase, parallel growth of the two largest follicles throughout the common growth phase is apparent in only about 50% of waves [5]. During the last 24 h of the common growth phase, the two largest follicles begin mean parallel growth or average maintenance of a relative position [1]. Critical study of the nature of the growth profiles, including relative position, has been limited to the two largest follicles.
The two largest follicles begin to deviate in diameter at the end of the common growth phase. Deviation is characterized by continued growth of the largest follicle to become the dominant follicle and a reduction or cessation of growth of the second largest follicle to become the largest subordinate follicle [1]. Based on several studies, mean diameter of the largest follicle in cattle at the beginning of deviation was 8.5 mm [3, 4]. A largest follicle of 8.5 mm has been used to define the expected beginning of deviation in experiments that interfere with locating the beginning of observed deviation by inspecting growth profiles [3, 4]. Regression occurs in all of the follicles of the wave that do not become dominant. However, it is not known whether growth cessation or regression is on average a simultaneous event for follicles of various diameter rankings.
Ablation versus retention of randomly chosen follicles has indicated that when more than one follicle reaches 5 mm at the same examination each follicle of the group is capable of becoming the dominant follicle [5]. The potential for each of the two largest follicles to become dominant has been studied by ablating or retaining the largest follicle when it reached 8.5 mm (expected beginning of deviation [6]). The growth of the second largest follicle after ablation of the largest follicle was accompanied by a surge of FSH that began a few hours after the ablation of the largest follicle. The largest follicle became dominant in all of seven heifers with follicles intact, whereas the second largest follicle became dominant in all of seven heifers with the largest follicle ablated. Thus, a hierarchical position in the potential for dominance was shown at the expected beginning of deviation, with the largest follicle in the most favored position followed by the second largest follicle. It is not known whether hierarchical positioning extends to the smaller follicles and influences their potential for dominance.
The objectives of the present experiments were 1) to characterize the predeviation growth profiles of follicles of different diameters (experiments 1 and 2), 2) to test a hierarchical hypothesis that the potential for dominance at the beginning of deviation is greatest for the largest follicle and decreases progressively for the smaller follicles in the order of their decreasing relative diameter at that time (experiment 1), and 3) to test the hypothesis that the three largest future subordinate follicles begin to deviate at the same time (experiment 2). In addition, the postablation increase in circulating FSH concentrations was examined to obtain information on the role of FSH in the experimental development of dominance by the smaller follicles.
MATERIALS AND METHODS
Animals and Ultrasound Scanning
The experiments were conducted during the wave that emerges during the periovulatory period (wave 1). The animals were Holstein heifers between 24 and 36 mo of age. The feeding program, the prostaglandin (PG)F2
protocol for inducing luteolysis to schedule ovulation, and the equipment and techniques for transrectal ultrasound scanning of the ovaries and measuring the follicles have been described [5]. The scanner was calibrated for linear measurements to the nearest 0.1 mm. Scanning was done every 24 h beginning on the day of induced luteolysis at mid-diestrus and continuing until the largest follicle of a new wave reached a diameter of 6.0 mm. Observed deviation was estimated for individual waves from the profiles of diameter data of the two largest follicles and was defined as beginning at the examination preceding an increase in the difference in diameter between the two follicles [1].
Experiment 1
The ultrasound scanning and collecting of jugular venous samples were done every 12 h beginning when the largest follicle was 6.0 mm and continuing until 48 h after the expected beginning of deviation. Thereafter, the scanning and sampling were done every 24 h until 120 h. Expected hour of deviation in individual waves was taken as the examination when the largest follicle first reached or exceeded 8.2 mm and was designated Hour 0 for all experimental groups. However, in groups in which the 8.2-mm follicle was ablated, expected deviation was based on the examination when the largest retained follicle reached 8.2 mm. The diameter of 8.2 mm was used so that the mean actual diameter would be close to 8.5 mm that was previously established as the mean diameter at the beginning of deviation [3, 4]. To determine if multiple dominant follicles developed, a diameter of >10 mm was used to define a dominant follicle.
When the largest follicle reached 8.2 mm, the follicles were designated as follicle 1 (F1), follicle 2 (F2), follicle 3 (F3), and so forth in descending diameter. Ties in follicle diameters were resolved by averaging the diameters for Hours -12 and 12. Heifers were randomized into five groups (n = 7/group), and follicle ablations were done when F1 was 8.2 mm (Hour 0). The groups were 1) nonablated controls (C); 2) ablation of F1 (AF1); 3) ablation of F1 and F2 (AF12); 4) ablation of F1, F2, and F3 (AF123); and 5) ablation of all follicles 4 mm or larger (AF-all). Group AF-all was included to establish the postablation hour of emergence of a new wave (wave 2) to determine if small follicles without the potential for dominance emerged at a comparable time in the other groups. As a result, newly emerging follicles were assigned to wave 1 if they emerged by Hour 0 and to wave 2 if they emerged at Hours 12–48. Follicle ablations were done by ultrasound-guided transvaginal aspiration of follicle contents, as described [7]. This method of ablation is functionally effective, as discussed [7]. However, in nine heifers, follicles refilled with fluid, and the thoroughness of ablation was considered unreliable. The heifers were replaced, except for one heifer in group AF-all that was removed and not replaced.
Experiment 2
This experiment was done because the number of control heifers in experiment 1 was inadequate for testing the hypothesis that deviation of the smaller subordinate follicles begins at the same time as for the largest subordinate follicle. Ultrasound follicle data were used that were collected during previous studies from control heifers (n = 21) for the four largest follicles at 8-h intervals [6–8]. The objectives of the present experiment were not considered in the previous studies. Not included in the 21 waves were 6 waves with double-dominant follicles. Hour of the beginning of observed deviation (Hour 0) was estimated for individual waves as described above, using the diameter profiles of the two largest follicles. The four largest follicles were designated F1–F4 at Hour 0 as for experiment 1. Only F1 and F2 were considered for determining Hour 0, so that the mean profiles of F3 and F4 could be studied when normalized to Hour 0 of F1 and F2. Complete data were available for Hours -16 to 0 and Hours 0–16, and the growth rates of follicles were compared between these two intervals.
FSH Assay
Blood samples were collected into heparinized tubes, and the plasma was separated by centrifuging, decanted into storage vials, and frozen (-20°C) until assay. Plasma concentrations of FSH were determined, using a validated RIA for cattle [9]. Details on the methodology as used in this laboratory have been reported [2]. Mean assay sensitivity, calculated as 2 SD below the mean counts per minute of maximum binding, was 0.02 ng/ml. The within-assay and between-assay coefficients of variation were 4.3% and 2.1%, respectively.
Statistical Analyses
For sequential follicle and gonadotropin data, split-plot ANOVA were used for determining main effects of group and hour and the interaction. Variation due to sequential data was accounted for by using heifer within group as the error term to test the effect of group. If a significant effect of hour or an interaction of group-by-hour was indicated, differences between groups within hours and among hours within a group were further examined by Duncan multiple range test. In experiment 1, growth rates for Hours -24 to 0 were compared among follicles (F1–F4) for the 34 heifers in the four groups before ablation. In addition, comparisons were made for Hours -24 to 0 for 16 heifers that had seven follicles 4.0 mm at Hour 0 (F1–F7). Concentrations of FSH for heifers with double-dominant follicles in groups AF123 and AF-all and with the development of new small follicles in group AF12 during Hours 12–48 were compared in separate analyses to heifers without these features. In experiment 2, growth rates for the 16 h before and after Hour 0 were compared for these two intervals by Duncan multiple range test as an indication of whether a reduction in growth rates began at different hours for the three largest subordinate follicles (F2, F3, F4). Quantitative data are presented as the mean ± SEM, unless otherwise indicated. Frequency data were examined by chi square. A probability of P < 0.05 was defined as significant. Probabilities between P > 0.06 and P < 0.1 were considered as approaching significance.
RESULTS
Experiment 1
The actual mean diameter of the largest follicle at Hour 0 (expected beginning of deviation) totaled over all groups was 8.5 ± 0.1 mm. Number of wave-1 follicles 5 mm at Hour 0 combined for the 34 heifers was 5.4 ± 1.2 (SD; range 3–10). The diameter profiles for F1–F4 combined for all groups (n = 34 heifers) are shown for Hours -24, -12, and 0 (Fig. 1). Hour -36 was not included because of missing data for most (65%) of the heifers. There were main effects of group and hour and an interaction (P < 0.0001). The group-by-hour interaction when analyzed without F4 was not significant. Diameters were different (P < 0.05) between follicles for all comparisons, except for F3 versus F4 at Hour -24 and at Hour -12. The smallest follicle (F4) had the slowest growth rate.
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Follicle growth rate for Hours -24 to 0 for the 16 heifers with six follicles at Hour 0 are shown (Table 1). The number of observations encompassing Hours -24 to 0 is reduced for F4–F6 (n = 10–15; Table 1); the missing data resulted from not detecting the follicles until after Hour -24. An F7 was detected in only five waves by Hour -24, and data were similar to those shown for F6. The smallest follicle (F6) grew at the slowest rate, intermediate follicles (F4, F5) at an intermediate rate, and the largest follicles (F1–F3) at the fastest rate. The smallest follicle remained static on average (approximately 0 growth rate) during Hours -24 to 0.
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A dominant follicle developed during wave 1 in 21 of 21 heifers combined for groups C, AF1, and AF12 and in 5 of 7 heifers in group AF123 (Table 2). The largest retained follicle became a dominant follicle in more (P < 0.006) heifers in groups C (seven of seven heifers) and AF1 (five of seven) than in groups AF12 (two of seven) and AF123 (two of seven). For heifers that developed a dominant follicle in wave 1, a mean of 1.6 small follicles (maximum diameter, 4.1–8.6 mm) emerged at Hours 12–48 in groups AF12 (three of seven heifers) and AF123 (four of five). Follicles did not emerge at this time in any heifer in groups C and AF1 (0 of 14). Double-dominant follicles developed from wave 1 in group AF123 (3 of 7 heifers) and from wave 2 in group AF-all (3 of 6) but not in groups C, AF1, or AF12 (0 of 21). After ablation of all follicles 4.0 mm or larger (group AF-all), the first follicle of a new wave (wave 2) emerged at Hour 32.0 ± 5.1 (range, Hours 12–48).
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For heifers with a single dominant follicle in groups C, AF1, AF12, and AF123, the dominant follicle formed from the largest retained follicle more often when the largest retained follicle was >7.0 mm (14 of 15) than when the largest retained follicle was <7.0 mm (0 of 10; Table 2). When the largest retained follicle was <7.0 mm, a follicle smaller than the largest retained follicle of wave 1 (n = 8) or a follicle of wave 2 (n = 2) became the dominant follicle. For the eight heifers in which the largest retained follicle was <7.0 mm and the dominant follicle originated from wave 1, the frequency for selection of a follicle for dominance was not affected (P = 0.50) by diameter ranking of the wave-1 retained follicles: first largest follicle, zero of eight became dominant; second largest, three of eight; third largest, two of eight; fourth largest or smaller, three of eight.
Within expected and observed deviation, hours of occurrence were significantly different among groups C, AF1, and AF12 for heifers with a single dominant follicle in wave 1; the means increased progressively for groups with increasing number of ablated follicles (Table 3). The diameter of the future dominant follicle was not different among groups at the hour of observed deviation. The mean growth profiles of the dominant follicles of wave 1 are shown (Fig. 1); heifers that did not develop a dominant follicle during wave 1 were excluded, and in heifers with two dominant follicles the largest follicle was used. There were main effects of group and hour (P < 0.0001), but the interaction was not significant. A similar (not significantly different) increase in diameter occurred from Hour 0 to Hour 12 for groups C, AF1, and AF2, whereas a comparative decrease (P < 0.05) occurred for group AF123.
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For FSH concentrations, there were group (P < 0.01) and hour (P < 0.0001) effects and an interaction (P < 0.0001; Fig. 2). Concentrations decreased (P < 0.05) between Hours 0 and 12 in the controls. Concentrations increased (P < 0.05) in all ablation groups by Hour 12. There was no significant difference among groups AF1, AF12, and AF123 during Hours 0–120, except that concentrations were higher (P < 0.05) at Hour 24 in group AF123 than in group AF1. There were no significant differences between waves with one versus two dominant follicles within the AF123 and AF-all groups or combined for the two groups (data not shown). Within group AF12, concentrations at Hour 48 were higher (P < 0.05) for three heifers that developed small follicles at Hours 12–48 (0.38 ± 0.03 ng/ml) than in four heifers that did not (0.18 ± 0.04 ng/ml). The highest (P < 0.05) means were obtained in group AF-all for Hours 24–48.
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Experiment 2
The actual mean diameter of the largest follicle at Hour 0 (observed beginning of deviation) was 8.4 ± 0.1. For Hours -16 to 16, there were main effects of hour and follicle and an interaction (P < 0.0001; Fig. 3). The growth rates between Hours -16 and 0 and between Hours 0 and 16 are shown for the four follicles (Fig. 3). The growth rates were highest for F1 and F2 for Hours -16 to 0 and for F1 for Hours 0–16, intermediate for F3 and F4 for Hours -16 to 0, and lowest for F2, F3, and F4 for Hours 0–16.
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DISCUSSION
There were differences in follicle growth rates during the last portion of the common growth phase, as calculated retrospectively from their diameter ranking at Hour 0 (expected end of common growth phase or the beginning of deviation). In both experiments, the smaller follicles grew more slowly. The smallest follicles considered (F6, F7) emerged near the end of the common growth phase. The F6 and F7 follicles that emerged before or by Hour -24 remained static on average (0 growth rate) during Hours -24 to 0. These results are consistent with the report that follicles that emerge late in the common growth phase often do not attain >5.0 mm [1]. A relationship between predeviation growth rates and diameter rankings at the beginning of deviation has not been reported, previously.
The two or three largest follicles of wave 1 grew in parallel during the last portion of the common growth phase. Although observations before Hour -24 were limited and not included in the analyses, inspection of the data suggested that parallelism did not develop before Hour -24. These results agree with a report that the two largest follicles grew at a similar rate during the last day but not during the second-last day of the common growth phase, owing to a smaller difference in diameter between the two follicles 2 days before observed deviation [1]. The reason for the slow growth of the smaller follicles and the parallelism of the larger follicles during the last day of the common growth phase is not known. Speculatively, follicles of different diameters may have different degrees of responsiveness to different concentrations of the gonadotropins. In this regard, the decline in FSH concentrations is associated with a small transient LH elevation encompassing deviation [3, 4]. The LH concentrations increase by 24 h before deviation or when the largest follicle is about 6.5–7.0 mm, and these hours and diameters are equivalent to the beginning of parallel growth for the two largest follicles in the present studies.
The follicle hierarchical hypothesis that the potential for dominance at the expected beginning of deviation is greatest for the largest follicle and decreases progressively for the smaller follicles was supported, but only when the retained follicles were >7.0 mm. In all controls, the largest follicle at the expected beginning of deviation (8.2 mm) became dominant, agreeing with a previous report [7]. Following ablation of F1 at the expected beginning of deviation, F2 became dominant in five heifers in which F2 was 7.1–8.1 mm, but not in two heifers in which F2 was 6.3 mm. The hierarchical position or dominance capacity of F2 when F1 is ablated at the expected beginning of deviation has been reported [7]. Within a group of >7.0-mm follicles, the selected follicle was the largest follicle of the group in four of five waves. This is consistent with a previous report that the first follicle to reach 7.0 mm became the dominant follicle in 71% of waves [4]. In heifers with a largest retained follicle of <7.0 mm (5.5–6.9 mm) at Hour 0, diameter ranking of the retained follicles was not involved in determining the follicle that would become dominant. Instead, the first follicle to reach 7.0 mm became the dominant follicle in seven of eight heifers. Several follicles that were 4 or 5 mm at Hour 0 formed a dominant follicle when two or three of the largest follicles were ablated. These results demonstrated that even the small, static follicles and the late-emerging follicles were capable of dominance when the larger follicles were removed.
The hypothesis that the three largest subordinate follicles begin to deviate simultaneously was supported. In experiment 2, F1–F4 increased in diameter between Hours -16 and 0. Thereafter (Hours 0–16), F1 continued to grow and F2–F4 began a reduction in growth rate at Hour 0 that was similar in extent among the three follicles. These results indicated that multiple nondominant follicles were influenced simultaneously on average by the deviation mechanism. This has not been documented previously. Deviation is believed to begin when FSH concentrations decline below the level required by the smaller follicles; the largest more-developed follicle requires less FSH [6]. As noted above, however, the smallest follicles (F6, F7; means at Hour 0, 5.1 and 5.3 mm) reached a mean static growth phase 24 h before deviation; in effect, their deviation occurred early. Apparently, these late-emerging follicles no longer responded to the declining concentrations of FSH, beginning well before deviation of the three largest follicles.
There was no detectable inhibition of mean growth rate in the development of a dominant follicle after ablation of follicles in groups AF1 and AF12. That is, the developing dominant follicle assumed first position in the hierarchy without delay. There was a recovery delay (mean decrease rather then increase) between Hours 0 and 12 for the future dominant follicle in group AF123. The delay may have reflected the sometimes static condition of these small follicles at Hour 0. In two earlier studies, the largest follicle was ablated at an equivalent to 0.5 or 1 day after the expected beginning of deviation (mean for largest follicle: 9.0 mm [10] and 10.3 mm [11]). Combined for the two reported studies, the second largest follicle continued to grow uninterrupted to become a dominant follicle in 4 of 14 (28%) heifers, remained approximately static for 2–5 days, and then grew to become dominant (44%), or regressed without becoming dominant (28%). An apparent average growth plateau (no significant changes) or static condition occurred during Hours 0–16 for F2, F3, and F4 in experiment 2. Hours 24 and 32 were not included in the analysis because of missing data. However, inspection of the available observations suggested that the growth plateau continued until Hour 24 and the means for F2, F3, and F4 began to decrease between Hours 24 and 32. Apparently, after the rapid establishment of deviation, the subordinate follicles on average remain viable but under the continued suppression of the dominant follicle for about a day before beginning to regress. It is not known whether the change from suppression to regression involves solely a continuation of inadequate FSH. The previous and present results are consistent with the proposal [7] that deviation becomes established in <8 h. Although not a planned comparison, a significant reduction in growth rate of F2 in experiment 2 occurred within 8 h (between Hours 0 and 8).
The hour of expected and observed deviation in the heifers that developed one dominant follicle during wave 1 increased progressively and significantly as the number of ablated follicles increased. The mean diameter of the largest follicle at the hour of the beginning of observed deviation did not change significantly. These results are attributable to the increasing time required for the progressively smaller retained follicles to grow to a diameter characteristic of deviation and indicate that deviation is closely tied to the diameter attained by the largest follicle.
Following ablation of all wave-1 follicles 4.0 mm or larger (group AF-all), a 4.0-mm follicle of wave 2 was first detected at Hours 12–48 in all heifers. These results are consistent with the literature [3, 4]. The small follicles that emerged at Hours 12–48 in groups AF12 and AF123 were defined as wave-2 follicles, but they intermingled and grew similarly to the retained small follicles from wave 1. In 2 of 10 heifers, the dominant follicle originated from these wave-2 follicles.
The continued decrease in FSH concentrations after the expected beginning of deviation in the controls agrees with the results of recent studies [3, 4]. Following ablation, the FSH concentrations increased between Hours 0 and 12. This agrees with a study in which FSH concentrations were determined every hour [7]; concentrations increased between Hours 5 and 12. In the present study, FSH increased to an equivalent extent (no significant difference) by Hour 12 in groups AF1, AF12, and AF123. This result suggested that the FSH-suppressing activity at Hour 0 was a function of F1, the only ablated follicle common to the three groups. In groups AF1 and AF12, the retained follicles apparently developed FSH-depressing capacity by 12 h after ablation of the larger follicles. However, FSH continued to increase after Hour 12 in groups AF123 and AF-all. The FSH increase in group AF123 likely reflects the time required for the small retained wave-1 follicles to develop FSH-depressing capacity. Similarly, the new follicles of wave 2 in group AF-all would have needed to grow to about 5 mm before causing an FSH decline. This conclusion is consistent with previous studies in which multiple follicles developed FSH-depressing ability when they reached 5 mm, and two or more follicles were more effective than one [3, 4]. In group AF12, the three heifers that formed new follicles at Hours 12–48 had higher FSH concentrations than the four that did not. In group AF123, the two heifers that formed a dominant follicle during wave 2 rather than wave l had fewer growing wave-1 follicles (mean, 0.5) and higher FSH concentrations at Hour 24 (comparable to the concentrations for group AF-all) than in any other heifer in the group. The high concentrations of FSH in these two heifers accounted for the postablation FSH peak occurring 12 h later in this group than in group AF1 and AF12. Thus, decreased diameter and number of wave-1 retained follicles within groups in the postablation period likely had a role in increasing the FSH response to ablation. In turn, the increased magnitude of the FSH surge stimulated the emergence of new follicles.
In conclusion, during the last 24 h of the common growth phase, smaller follicles (<7.0 mm at expected beginning of deviation; Hour 0) grew more slowly than large follicles (>7.0 mm), and the two or three largest follicles grew in parallel. Following ablation of 1, 2, or 3 of the largest follicles at Hour 0, the largest retained follicle became dominant but only when the retained follicle was >7.0 mm. When the largest retained follicle was <7.0 mm, diameter ranking at Hour 0 was not involved in selection of the dominant follicle; the first follicle to reach 7.0 mm became dominant in 88% of waves. On average, the three largest future subordinate follicles began to deviate in diameter simultaneously. The hour of observed deviation after ablation of the largest follicles, increased progressively as the number of ablated large follicles increased, but the mean diameter of the largest follicle at the beginning of deviation did not change.
ACKNOWLEDGMENTS
The authors thank Pharmacia & Upjohn, Kalamazoo, MI, for a gift of lutalyse, the U.S. Department of Agriculture Animal Health Program for antigens and the primary antisera for the FSH assay, and Susan Jensen for technical assistance.
FOOTNOTES
1 Research supported by the University of Wisconsin-Madison, Madison, WI; U.S. Department of Agriculture grant 99-35203-7669; and The Eutherian Foundation, Cross Plains, WI. 
2 Correspondence: O.J. Ginther, Department of Animal Health and Biomedical
Sciences, 1656 Linden Drive, University of Wisconsin-Madison,
Madison, WI 53706. FAX: 608 262 7420; ojg{at}ahabs.wisc.edu
Accepted: March 12, 2001.
Received: February 5, 2001.
REFERENCES
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