Comparative study of the dynamics of follicular waves in mares and women

Deviation in growth rates of the follicles of the ovulatory wave begins at the end of a common growth phase and is characterized by continued growth of the developing dominant follicle (F1) and regression of the largest subordinate follicle (F2). Follicle diameters during an interovulatory interval were compared between 30 mares and 30 women, using similar methods for collecting and analyzing data. Follicles were tracked and measured daily by ultrasonography. Diameter at follicle emergence (mares, 13 mm; women, 6 mm) and the required minimal attained diameter for assessment of follicles (mares, 17 mm; women, 8 mm) were chosen to simulate the reported ratio between the two species in mean diameter of F1 at the beginning of deviation (mares, 22.5 mm; women, 10.5 mm). F1 emerged before F2 (P < 0.02) in each species, and the interval between emergence of the two follicles was similar (not significantly different) between species. Growth rate for F1 and F2 during the common growth phase was similar within species, and the percentage of diameter increase was similar between species. Proportionality between species in diameter of F1 at deviation (2.2 times larger for mares than for women) and at maximum preovulatory diameter (2.1 times larger) indicated that relative growth of F1 after deviation was similar between species. A predeviation follicle was identified in 33% of mares and 40% of women and was characterized by growth to a diameter similar to F1 at deviation but with regression beginning an average of 1 day before the beginning of deviation. The incidence of a major anovulatory wave preceding the ovulatory wave was not different between species (combined, 25%). Results indicated that mares and women have comparable follicle interrelationships during the ovulatory wave, including 1) emergence of F1 before F2, 2) similar length of intervals between sequential emergence of follicles within a wave, 3) similar percentage growth of follicles during the common growth phase, and 4) similar relative diameter of F1 from the beginning of deviation to ovulation. Similar follicle dynamics between mares and women indicate the mare may be a useful experimental model for study of folliculogenesis in women, with the advantage of larger follicle size.     

Follicular and hormonal dynamics during the first follicular wave in heifers

A few days after the first follicular wave emerges as 4-mm follicles, follicular deviation occurs wherein 1 follicle of the wave continues to grow (dominant follicle) while the others regress. The objectives of this study were to characterize follicle growth and associated changes in systemic concentrations of gonadotropins and estradiol at 8-h intervals encompassing the time of follicle deviation. Blood samples from heifers (n = 11) were collected and the ovaries scanned by ultrasound every 8 h from 48 h before to 112 h after the maximal value for the preovulatory LH surge. The follicular wave emerged at 5.8 +/- 5.5 h (mean +/- SEM) after the LH surge, and at this time the future dominant follicle (4.2 +/- 0.8 mm) was larger (P < 0.001) than the future largest subordinate follicle (3.6 +/- 0.1 mm). There was no difference in growth rates between the 2 follicles from emergence to the beginning of the deviation (0.5 mm/8 h for each follicle), indicating that, on average, the future dominant follicle maintained a size advantage over the future subordinate follicle. Deviation occurred when the 2 largest follicles were 8.3 +/- 0.2 and 7.8 +/- 0.2 mm in diameter, at 61.0 +/- 3.7 h after wave emergence. Diameter deviation was manifested between 2 adjacent examinations at 8-h intervals. Mean concentrations of FSH decreased, while mean concentrations of LH increased 24 and 32 h before deviation, respectively, and remained constant (no significant differences) for several 8-h intervals encompassing deviation. In addition to the increase and decrease in circulating estradiol concentrations associated with the preovulatory LH surge, an increase (P < 0.05) occurred between the beginning of deviation and 32 h after deviation. The results supported the hypotheses that deviation occurs rapidly (within 8 h), that elevated systemic LH concentrations are present during deviation, and that deviation is not preceded by an increase in systemic estradiol.     

Relationship between days of the luteolytic period and locations of the preovulatory follicle and CL in interovulatory intervals with two or three follicular waves in heifers

Diameter of follicles was determined every 12 hours and progesterone (P4), FSH, and LH concentrations were determined every 6 hours from Day 12 (Day 0 = ovulation) to the ovulation at the end of the interovulatory interval (IOI). Groups were assigned on the basis of an ipsilateral (Ipsi) versus contralateral (Contra) relationship between the preovulatory follicle and CL and two follicular waves (2W) versus three waves (3W) per IOI. Numbers of IOIs were Ipsi-2W (n = 6), Ipsi-3W (n = 6), and Contra-3W (n = 8). Normalization to the end of luteolysis (day that P4 was closest to 1.0 ng/mL) indicated for the first time that concentrations of P4 and FSH were greater (P < 0.05) in 3W IOIs than in 2W IOIs for the 3 days before the beginning of a P4 decrease. The beginning of a P4 decrease occurred about 5 days and 6 hours after emergence of the preovulatory wave at 6 mm in 2W and 3W IOIs, respectively. On the day of diameter deviation between the future dominant and largest subordinate follicles in wave 3 of 3W IOIs, the future dominant follicle had the following characteristics: (1) distribution of diameters differed (P < 0.01) from unimodality; (2) diameter was greater (P < 0.05) in the Contra-3W group (9.8 ± 0.4 mm) than in the Ipsi-3W group (8.8 ± 0.3 mm); (3) diameter was similar to the diameter at the beginning of the P4 decrease (9.6 ± 0.9 mm); and (4) diameter was as small or smaller than diameter of the largest subordinate in seven of 14 heifers compared with zero of seven heifers in wave 2 of 2W IOIs. The differences involving deviation may be related to a reported greater frequency of the Contra-3W group than Ipsi-3W group. Results supported the hypothesis that emergence of the ovulatory wave occurs well before the beginning of luteolysis in 2W IOIs and near the beginning of luteolysis in 3W IOIs.     

Relationships between FSH surges and follicular waves during the estrous cycle in mares

Individual follicles >/=15 mm were monitored daily by ultrasonography in 12 mares during the estrous cycle. Follicular waves were designated as major waves (primary and secondary) and minor waves based on maximum diameter of the largest follicle of a wave (major waves, 34 to 47 mm; minor waves, 18 to 25 mm). Dominance of the largest follicle of major waves was indicated by a wide difference (mean, 18 mm) in maximum diameter relative to the second largest follicle. Dominant follicles of primary waves (n=12) emerged (attained 15 mm) at a mean of Day 12 and resulted in the ovulations associated with estrus (ovulation=Day 0). The dominant follicle of a secondary wave (n=1) emerged on Day 2 and subsequently ovulated in synchrony with the dominant follicle of the primary wave, which emerged on Day 9. The largest follicles of minor waves (n=4) emerged at a mean of Day 5, reached a mean maximum diameter 3 days later, and subsequently regressed. There was a significant increase in mean daily FSH concentrations either 6 days (primary wave) or 4 days (minor waves) before the emergence of a wave. Mean concentrations of FSH decreased significantly 2 days after emergence of the primary wave. Divergence between diameter of the dominant and largest subordinate follicle of the primary wave was indicated by a significantly greater mean diameter of the dominant follicle than of the largest subordinate follicle 3 days after wave emergence and by the cessation of growth of the largest subordinate follicle beginning 4 days after the emergence of a wave. Surges of FSH were identified in individual mares by a cycle-detection program; surges occurred every 3 to 7 days. Elevated mean FSH concentrations over the 6 days prior to emergence of the primary wave was attributable to a significantly greater frequency of individual FSH surges before wave emergence than after emergence and to an increase in magnitude of peak concentrations of FSH associated with individual surges.     

Differential blood flow changes between the future dominant and subordinate follicles precede diameter changes during follicle selection in mares

Diameter deviation during a follicular wave is characterized by the continued growth of the developing dominant follicle and reduced growth and regression of the subordinate follicles. This study considered the hypothesis that reduced blood flow in the future largest subordinate follicle precedes the beginning of diameter deviation. The hypothesis was tested by quantifying the daily changes in blood-flow velocities and blood-flow area within the wall of follicles before and during diameter deviation in mares (n = 7). The blood-flow end points were quantified daily by transrectal color Doppler ultrasonography. Follicles were identified retrospectively by rank as F1 (largest) and F2 according to the maximum attained diameter. Follicles were grouped into nine F1 diameter ranges of 3.0 mm each (equivalent to 1 day's growth) centered on 6.5, 9.5, 12.5, 15.5, 18.5, 21.5, 24.5, 27.5, and 30.5 mm. Diameter deviation began in the 24.5-mm group, as indicated by a smaller (P < 0.05) difference between F1 and F2 in the 24.5-mm group than in the 27.5-mm group. Based on a similar approach, peak systolic velocity and time-averaged maximum velocity of blood flow began to deviate between F1 and F2 in the 18.5-mm group (P < 0.04) and blood flow area began to deviate in the 21.5-mm group (P < 0.009). Thus, differential blood flow area between F1 and F2 began an average of 3.0 mm (equivalent to 1 day) and differential blood-flow velocities began an average of 6.0 mm before the beginning of diameter deviation. The results demonstrated that deviation between F1 and F2 in the blood flow of the follicle walls occurred 1 or 2 days before deviation in follicle diameter during follicle selection in mares.     

Mechanism of follicle deviation in monovular farm species

Diameter deviation is a distinctive change in growth rates among the follicles of a wave, heralding the formation of a dominant follicle and subordinate follicles. When the follicles are about 5mm in cattle and 13 mm in horses, the wave-stimulating FSH surge reaches peak concentrations. Follicle and FSH manipulation studies in both species have shown that the declining portion of the surge before the beginning of deviation is a function of multiple growing follicles that require the decreasing FSH. During this time, all follicles of the wave have the potential for future dominance. Deviation begins when the two largest follicles on average are 8.5 and 7.7 mm in cattle and 22.5 and 19.0 mm in horses or about 3 days after the FSH peak in both species. The FSH/follicle relationship is close so that a change in one event soon causes a detectable change in the other. Thus, the difference in diameter between the two largest follicles at the beginning of deviation is compatible with rapid establishment of the destiny of the two follicles before the second-largest follicle can also show dominance. The deviation mechanism is initiated when FSH concentrations are low and the most advanced follicle reaches a specific developmental stage. In cattle, the future dominant follicle develops greater LH-receptor expression than the other follicles about 8 h before the beginning of diameter deviation. Estradiol and free IGF-1 begin to establish higher concentrations in the future dominant follicle than in other follicles and activin-A is transiently elevated in both follicles a few hours before the beginning of diameter deviation. In horses, estradiol, free IGF-1, activin-A, and inhibin-A begin to increase differentially in the future dominant follicle about 1 day before deviation. These changes underlie a greater responsiveness to LH and FSH by the developing dominant follicle than for other follicles, thereby accounting for deviation. Results of in vitro studies, although frequently done in other species, support this conclusion.     

Follicular and FSH dynamics in ewes with a history of high and low ovulation rates

Daily transrectal ultrasound scanning and twice-daily blood sampling were used to monitor the temporal relationships between FSH concentrations and follicle development during complete interovulatory intervals for ewes in which the ovulation rate in each of the 2 previous years was high or low (> or = 3 and < or = 2 ovulations, respectively). Follicles that reached > or = 5 mm were used to define a follicular wave and were tracked retrospectively to 3 mm (emergence). The hypothesis that FSH surges (identified with a computer program) and follicular waves (retrospectively determined based on ultrasound scanning) are temporally associated was supported in both groups by the emergence of an anovulatory or ovulatory follicular wave near the peak of an FSH surge. Further support for the hypothesis was a significant increase in FSH concentrations before and a significant decrease after follicular-wave emergence in both groups independent of the identification of FSH surges. Ewes with a history of high ovulation rates had smaller follicles (anovulatory and ovulatory) and more ovulations, but the 2 groups were similar in the number of ovulatory follicular waves and associated FSH surges, number and characteristics of the FSH surges, and mean FSH concentrations per interovulatory interval. Surges of FSH were periodic (every 3 or 4 d) regardless of the ovulation-rate group or follicle response. In ewes with a low ovulation rate, the nonovulatory FSH surges were most frequently associated with emergence of detected anovulatory follicular waves. In ewes with a high ovulation rate, more FSH surges were not associated with a detected follicular wave, as defined, presumably because the largest follicle did not reach 5 mm. The results indicated that the factors resulting in a high ovulation rate were not exerted through circulatory patterns or concentrations of FSH but involved a shorter growth phase and smaller maximal diameter of follicles.     

Ovarian antral follicular dynamics and their relationships with endocrine variables throughout the oestrous cycle in breeds of sheep differing in prolificacy.

Transrectal ultrasonography of ovaries was performed each day in non-prolific Western white-faced (n = 12) and prolific Finn ewes (n = 7), during one oestrous cycle in the middle portion of the breeding season (October-December), to record the number and size of all follicles > or = 3 mm in diameter. Blood samples collected once a day were analysed by radioimmunoassay for concentrations of LH, FSH and oestradiol. A cycle-detection computer program was used to identify transient increases in concentrations of FSH and oestradiol in individual ewes. Follicular and hormonal data were then analysed for associations between different stages of the lifespan of the largest follicles of follicular waves, and detected fluctuations in serum concentrations of FSH and oestradiol. A follicular wave was defined as a follicle or a group of follicles that began to grow from 3 to > or = 5 mm in diameter within a 48 h period. An average of four follicular waves per ewe emerged during the interovulatory interval in both breeds of sheep studied. The last follicular wave of the oestrous cycle contained ovulatory follicles in all ewes, and the penultimate wave contained ovulatory follicles in 10% of white-faced ewes but in 57% of Finn ewes. Transient increases in serum concentrations of FSH were detected in all animals and concentrations reached peak values on days that approximated to follicle wave emergence. Follicular wave emergence was associated with the onset of transient increases in serum concentrations of oestradiol, and the end of the growth phase of the largest follicles (> or = 5 mm in diameter) was associated with peak serum concentrations of oestradiol. Serum FSH concentrations were higher in Finn than in Western white-faced ewes during the follicular phase of the cycle (P < 0.05). There were no significant differences in serum concentrations of LH between Western white-faced and Finn ewes (P > 0.05). Mean serum concentrations of oestradiol were higher in Finn compared with Western white-faced ewes (P < 0.01). It was concluded that follicular waves (follicles growing from 3 to > or = 5 mm in diameter) occurred in both prolific and non-prolific genotypes of ewes and were closely associated with increased secretion of FSH and oestradiol. The increased ovulation rate in prolific Finn ewes appeared to be due primarily to an extended period of ovulatory follicle recruitment.     

Selection of a dominant follicle and suppression of follicular growth in heifers

The following aspects of follicle-stimulating hormone (FSH)-follicular relationships were studied in heifers: (1) the role of the decline in circulating levels of FSH in selection of a dominant follicle of a follicular wave; (2) the relationship of an FSH nadir (low levels between surges) to the absence of development of new follicles of a detectable diameter during the interim between the emergence of successive waves. A recombinant DNA-derived bovine FSH was used. Administration of bovine follicle-stimulating hormone (bFSH) for two days before the time of selection of the dominant follicle of the first post-ovulatory follicular wave delayed the time of divergence of the follicles into dominant and subordinates (first significant divergence: bFSH treatment before selection, Day 4.0; bFSH treatment after selection, Day 2.5; controls, Day 2.5: ovulation, Day 0). Significantly greater growth of the first and second largest subordinates occurred in the pre-selection group. A superovulatory dose of bFSH for 4 days with PGF₂-induction of luteolysis resulted in multiple ovulations when begun on Day 1 (before the expected time of follicle divergence; mean 2.8 ovulations per heifer) than when begun on Day 5 (after divergence; mean 1.0 ovulation per heifer). Administration of bFSH during the expected time (Days 5 and 6) of an FSH nadir did not alter the day of detectable emergence of the next follicular wave. Results supported the following hypotheses: (1) a decline in the wave-stimulating FSH surge is an integral component of the selection mechanism that results in the divergence into dominant and subordinate follicles; (2) the nadir between FSH surges does not account directly for the absence of the development of new follicles between the emergence of waves.     

Follicular deviation and acquisition of ovulatory capacity in bovine follicles.

Selection of dominant follicles in cattle is associated with a deviation in growth rate between the dominant and largest subordinate follicle of a wave (diameter deviation). To determine whether acquisition of ovulatory capacity is temporally associated with diameter deviation, cows were challenged with purified LH at known times after a GnRH-induced LH surge (experiment 1) or at known follicular diameters (experiments 2 and 3). A 4-mg dose of LH induced ovulation in all cows when the largest follicle was > or =12 mm (16 of 16), in 17% (1 of 6) when it was 11 mm, and no ovulation when it was < or =10 mm (0 of 19). To determine the effect of LH dose on ovulatory capacity, follicular dynamics were monitored every 12 h, and cows received either 4 or 24 mg of LH when the largest follicle first achieved 10 mm in diameter (experiment 2). The proportion of cows ovulating was greater (P < 0.05) for the 24-mg (9 of 13; 69.2%) compared with the 4-mg (1 of 13; 7.7%) LH dose. To determine the effect of a higher LH dose on follicles near diameter deviation, follicular dynamics were monitored every 8 h, and cows received 40 mg of LH when the largest follicle first achieved 7.0, 8.5, or 10.0 mm (experiment 3). No cows with a follicle of 7 mm (0 of 9) or 8.5 mm (0 of 9) ovulated, compared with 80% (8 of 10) of cows with 10-mm follicles. Thus, follicles acquired ovulatory capacity at about 10 mm, corresponding to about 1 day after the start of follicular deviation, but they required a greater LH dose to induce ovulation compared with larger follicles. We speculate that acquisition of ovulatory capacity may involve an increased expression of LH receptors on granulosa cells of the dominant follicle and that this change may also be important for further growth of the dominant follicle.     

Timing of emergence of ovulatory follicles in polyovulatory goats

The current study characterized the timing of emergence of ovulatory follicles during the follicular phase of the estrous cycle in polyovulatory does and assessed whether selection may influence ovulation rate through differences in ovarian follicular dynamics, by characterizing preovulatory follicular emergence and growth in two ecotypes of Neuquen-Criollo Argentinean goats (Short-Hair, n=11 and Long-Hair, n=9). During the breeding season, the time of estrus was synchronized in all does with two doses of a prostaglandin analogue. Ovarian laparoscopies were performed on days 17 and 19 after the induced estrus (day 0) and 7-15 h after the beginning of the subsequent estrus. Results indicate that both ecotypes of goats have common features in the ovarian follicular population and in the patterns of preovulatory follicular enlargement. In all the goats, most of the preovulatory follicles arose from the pool of follicles present in the ovary between days 17 and 19 of the estrous cycle. These follicles were all larger than 2mm at emergence, being the largest growing follicle present in the ovaries on days 17 and 19 in 56.5 and 78.6% of the does, respectively. The appearance of new follicles remained unaffected, while the mean number of small growing follicles decreased (P<0.05) during the follicular phase, indicating that preovulatory follicles do not suppress the emergence of new follicles but inhibit the growth of small follicles. A separate analysis of single and double ovulating does showed that 75% of the second ovulatory follicles in polyovulatory goats was present on the ovarian surface between days 17 and 19 of the estrous cycle, but appeared later in the other 25% of the estrous cycles. These findings support the hypothesis that follicular dominance effects are exerted during the preovulatory period, when the growth of follicles other than the ovulatory is inhibited, and that increases in ovulation rate in small ruminants are related to a reduced incidence of follicular atresia and an extended period of ovulatory follicle recruitment.     

Pattern of follicular development in high fecundity Olkuska ewes during the estrous cycle

Folliculogenesis was studied daily in the 18 oestrous cycles in six prolific Olkuska ewes from October to December using transrectal ultrasonography to record the number and size of all ovarian follicles > or =2 mm in diameter. Blood samples were taken once a day and were analyzed for concentrations of FSH, LH, estradiol and progesterone. Follicular and hormonal data were analyzed for associations between different stages of development of the follicular waves and concentrations of FSH and estradiol. The first wave during which at least one follicle reached maximum diameter of > or =4 mm after ovulation, was defined as a wave 1, and the following waves were numbered sequentially. Waves 1, 2, 3, 4 and the ovulatory one emerged on days: -2 to 4, 4 to 8, 6 to 11, 10 to 12 and 11 to 15, respectively. The mean number of follicles per wave that reached diameter of > or =4 mm was 4.15 +/- 1.1 and 16.62 +/- 8.6 follicles per estrous cycle of a total 299 follicles were observed. Significantly more follicles (p> or =0.05) emerged on days 2, 8 and 13 than in other days. Serum FSH concentrations fluctuated from 0.11 ngml(-1) on day 2 to preovulatory maximum 1.81 ngml(-1) on day 17 of the estrous cycle. The emergence of follicular waves was associated with elevations of FSH concentrations in blood serum. The mean increase in FSH concentration was followed by the recruitment of follicles of the next wave. The mean daily FSH concentration and the mean number of follicles emerging each day were negatively correlated. The length of the interwave interval (4.4 +/- 1.6 days) did not differ significantly from the interval between pulses of FSH (4.8 +/- 0.3 days). The mean serum estradiol concentrations showed fluctuations until day 14 and then gradually increased from 5.47 +/- 0.3 pgml(-1) to reach a peak 13.14 +/- 0.2 pgml(-1) on the day before ovulation. To summarize, the growth of ovarian follicles during the estrous cycle in high fecundity Olkuska sheep exhibited a distinct wave-like pattern. Ovarian follicles emerged from the pool of 2 mm follicles. The preovulatory follicles originated from the large follicle population were present in the ovary at the time of luteal regression. The initial stages of the growth of the largest follicles appears to be controlled primarily by increases in FSH secretion.     

Selection of the dominant follicle in cattle: role of two-way functional coupling between follicle-stimulating hormone and the follicles

The functional coupling between the declining portion of the FSH surge and the growing follicles of a wave was studied by treating heifers with a minimal dose of estradiol to decrease FSH concentrations without an associated change in LH concentrations. Estradiol treatment when the largest follicle reached >/= 6.0 mm (Hour 0) resulted in depression of both FSH concentrations and diameter of the largest follicle by Hour 8. The smaller follicles were also inhibited. These results supported the hypothesis that FSH continues to be needed by the growing follicles even when the FSH concentrations are decreasing during the declining portion of the FSH surge. Estradiol treatment when the largest follicle was >/= 8.5 mm (expected time of follicular deviation) also resulted in a transient decrease in both FSH concentrations and diameter of the largest follicle, but the diameters of the smaller follicles were not affected. These results supported the hypothesis that the low concentrations of FSH at the expected time of deviation, although inadequate for the smaller follicles, were required for continued growth of the largest follicle. In another study, ablation (Hour 0) of the largest follicle was done at >/= 7.5 mm vs. >/= 8.5 mm. The mean FSH concentrations for the 8.5-mm groups were greater for the ablation group than for the control group at Hours 8 and 12, but there was no difference between the 7.5-mm groups at any hour. These results supported the hypothesis that by the time the largest follicle reaches the expected beginning of deviation it has developed a greater capacity for suppressing FSH. It is postulated that the essence of the selection of a dominant follicle is a close two-way functional coupling between changing FSH concentrations and follicular growth.     

Follicle selection in monovular species

Follicle deviation is proposed to be the eminent event in follicle selection in monovular species. At deviation, the largest follicle establishes dominance apparently before the second-largest follicle can reach a similar diameter. In cattle, based on diameters of the two follicles at the beginning of deviation, the mechanism becomes established in <8 h. An FSH:follicle-coupling hypothesis has been supported as the essence of follicle selection. According to the hypothesis, the growing follicles cause the FSH decline from the peak of the wave-stimulating FSH surge until deviation, even though the follicles continue to require FSH (two-way functional coupling involving multiple follicles). During multiple-follicle coupling, inhibin is the primary FSH suppressant. Near the beginning of deviation, the largest follicle secretes increased estradiol, and apparently both estradiol and inhibin contribute to the continuing FSH decline; only the more-developed largest follicle is able to utilize the low FSH concentrations (single-follicle coupling). Deviation is encompassed by a transient elevation in LH in heifers and by a component, often distinct, of the long ovulatory LH surge in mares. In heifers, receptors for LH appear in the granulosa cells of the future dominant follicle about 8 h before the beginning of deviation. The LH stimulates the production of estradiol and insulin-like growth factor-1. These intrafollicular factors and perhaps others account for the responsiveness of the largest follicle to the low concentrations of FSH. The smaller follicles have not reached a similar developmental stage and because of their continued and close dependency on FSH become susceptible to the low concentrations. Thereby, follicle selection is established.     

Ovarian follicular and luteal dynamics in wapiti during the estrous cycle

The reproductive tracts of 13 mature hinds were examined daily by transrectal ultrasonography and blood samples were taken daily from October to January to characterize follicular, luteal, and endocrine dynamics in wapiti during the estrous season. Follicle development occurred in waves characterized by regular, synchronous development of a group of follicles in temporal succession to a surge in serum FSH concentration. The mean interovulatory interval was 21.3 +/- 0.1 d, but was shorter in hinds exhibiting two follicular waves than in hinds exhibiting three and four waves (P < 0.05). The interwave interval was similar among waves in two-wave cycles and the first wave of three-wave cycles. All other interwave intervals in three- and four-wave cycles were shorter (P < 0.05). The maximum diameter of the dominant follicle of the first wave was similar among two-, three-, and four-wave cycles. For all other waves in three- and four-wave cycles, the maximum diameter was smaller (P < 0.05). Corpus luteum diameter and plasma progesterone concentrations were similar between two- and three-wave cycles, but the luteal phase was longer (P < 0.05) in four-wave cycles. The dominant follicle emerged at a diameter of 4 mm at 0.4 +/- 0.1 and 0.8 +/- 0.1 d before the largest and second largest subordinate follicles, respectively. The follicle destined to become dominant was larger (P < 0.05) than the largest subordinate follicle one day after emergence, which coincided with the first significant decrease in serum FSH concentration. We concluded that the estrous cycle in wapiti is characterized by two, three, or four waves of follicular development (each preceded by a surge in circulating FSH), that there is a positive relationship between the number of waves and the duration of the cycle, and an inverse relationship between the number of waves and the magnitude of follicular dominance (diameter and duration of the dominant follicle).     

Associations between FSH concentrations and major and minor follicular waves in pregnant mares

Follicular waves were detected in 19 pregnant mares (Days 11 to 40) by a significant increase followed by a significant decrease in diameters of follicles after removing large (>/=25 mm) follicles from the data sets. The waves were defined as major (largest follicle, >/=35 mm; n=18) or minor (largest follicle, <35 mm; n=17). Six mares (32%) had 2 successive major waves beginning on mean Days 15.2 and 26.8; 6 had a solitary major wave beginning on Days 11 to 20; and 6 had only minor waves occurring at irregular intervals. The mean interval between minor waves (7.8 days) was less (P<0.05) than for major waves (11.7 days). Mean divergence in diameters of the largest and second largest follicles of a wave began 4 days after the detected emergence of consecutive major waves, and was taken as the beginning of the expression of dominance by the largest follicle. The interval from emergence to divergence was several days longer (P<0.05) for solitary major waves than for consecutive waves. Dominance was not detected for the minor waves, using mean diameters of the 2 largest follicles, but was apparent on inspection of individual wave profiles in 5 of 17 (29%) minor waves. Minor waves, compared with major waves, had larger diameter of follicles on the day of wave emergence (15.0 versus 12.1 mm), and significantly greater variation in the day of attainment of maximal diameter of largest follicle and small follicles. A mean increase in FSH was temporally associated with the emergence of both major and minor waves. In mares with minor waves, concentrations of FSH were higher, on average, over Days 11 to 40, which seemed consistent with the origin of follicular waves from larger follicles in the basal populations. The lower overall FSH levels in mares with major waves seemed at least partly due to depression of FSH levels beginning at the time of divergence between the 2 largest follicles.     

Follicular and hormonal response to experimental suppression of FSH during follicle deviation in cattle

A near steroid-free fraction of bovine follicular fluid was used to suppress FSH concentrations at the expected time of follicle deviation or when the largest follicle of Wave 1 reached > or = 8.0 mm (actual mean diameter, 8.4 mm; Hour 0). It was hypothesized that the low concentrations of FSH associated with deviation are inadequate for the smaller follicles but are needed for continued growth of the largest follicle. Control heifers (n=8) received 10 mL of saline, and treated heifers (n=16) received either 8.8 mL or 13.3 mL of the follicular-fluid fraction at Hours 0, 12, and 24. Between Hours -48 and 0, FSH concentrations decreased (P<0.05) and diameters of the 4 largest follicles increased (Hour effect, P<0.0001) similarly between groups. Concentrations of LH in the controls increased (P<0.05) between Hours -24 and -12 and decreased (P<0.05) between Hours 8 and 36, demonstrating a transient LH surge encompassing the expected beginning of deviation. In the treated group, a comparable increase in LH occurred before deviation but a decrease did not occur until after Hour 48. By Hour 4.5, the FSH concentrations in the treated group decreased (P<0.05) to below the concentrations in the controls. Suppressed diameter (P<0.001) of the largest follicle was detected at the first post-treatment examination (Hour 12; 7.5 h after FSH suppression) and was accompanied by reduced (P<0.04) systemic estradiol concentrations. The mean growth rates of the 3 smaller follicles in both the treated and control groups began to decrease at Hours -12 to 24 and were not different between groups during Hours 0 to 36. Concentrations of FSH in the treated group returned to control concentrations by Hour 24 (hour of last treatment). A rebound (P<0.05) in concentrations of FSH to >100% above control concentrations occurred by Hour 48 and was accompanied by resumed growth of the largest follicle in 75% of the heifers between Hours 48 and 72. The results demonstrated that the low concentrations of FSH associated with deviation can be further reduced by treatment with a nonsteroidal factor of follicular origin. Transient reduction of FSH concentrations to below the already low control concentrations inhibited the largest follicle but did not further inhibit the smaller follicles. These results support the hypothesis that the low FSH concentrations associated with follicle deviation are below the minimal requirements of the smaller or subordinate follicles but are needed for continued growth of the largest or dominant follicle in cattle.     

Developmental pattern of small antral follicles in the bovine ovary

The study was designed to characterize the developmental pattern of 1- to 3-mm follicles and to determine the stage at which the future dominant follicle first attains a size advantage among its cohorts. In experiment 1, heifers (n = 18) were examined every 24 h by transrectal ultrasonography for one interovulatory interval (IOI). In experiment 2, cows (n = 9) were examined every 6 h from 5 to 13 days after ovulation to monitor precisely the diameter changes of individual follicles >/=1 mm during emergence of wave 2. Results revealed a change over days (P < 0.05) in the number of 1- to 3-mm follicles, with a maximum (P < 0.05) 1 or 2 days before wave emergence (conventionally defined as the time when the dominant follicle is first detected at 4 mm), followed 3-4 days later by a maximum (P < 0.05) in the number of >/=4-mm follicles. The profiles of small (1-3 mm) and large (>/=4-mm) follicles were inversely proportional (r = -0.79; P = 0.01). The profile of the number of 1- to 3-mm follicles during wave emergence was similar (P = 0.63) between waves in two-wave IOI, but differed (P < 0.01) among waves in three-wave IOI as a result of a greater number of follicles in the ovulatory wave (P < 0.04). As well, the number of follicles in the ovulatory wave tended to be greater (P < 0.06) in three-wave IOI than in two-wave IOI. The future dominant follicle was first identified at a diameter of 1 mm and emerged 6-12 h earlier than the first subordinate follicle (P < 0.01). After detection of the dominant follicle at 1 mm (0 h), its diameter differed from that of the first and second subordinate follicles at 24 h (P = 0.04) and 12 h (P = 0.01), when the dominant follicle was 2.4 +/- 0.17 mm and 1.7 +/- 0.14 mm, respectively. The growth rate of the dominant follicle differed from that of the first and second subordinate follicles at 120 h (P = 0.03) and 108 h (P = 0.02), when the dominant follicle was 9.5 +/- 0.30 mm and 8.8 +/- 0.49 mm, respectively. Emergence of the future dominant (r = 0.71), first (r = 0.73), and second (r = 0.76) subordinate follicles was temporally associated (P < 0.01) with a rise in circulating concentrations of FSH. Transient, nocturnal elevations in plasma FSH concentration were followed within 6 h by an increase in the growth rate of 1- to 3-mm follicles. We conclude that 1) 1- to 3-mm follicles develop in a wave-like manner in association with surges in plasma concentrations of FSH, 2) 1- to 3-mm follicles are exquisitely responsive to transient elevations in FSH, and 3) selection of the dominant follicle is manifest earlier than previously documented and is characterized by a hierarchical progression over a period encompassing the entire FSH surge (5 days).     

Emergence and deviation of follicles during the development of follicular waves in cattle

The nature of emergence and deviation of follicles during follicular waves in cattle was studied in 3 experiments by re-examining data from previous projects. Wave emergence was defined as the day or examination (when more than 1 examination per day) the future dominant follicle was 4 mm (Day 0 or Examination 0). Deviation was defined as the beginning of the greatest difference in growth rates between the 2 largest follicles and between 2 consecutive examinations. The search for deviation in an individual wave was done retrospectively from the examination with the maximum diameter of the second largest follicle. In Experiment 1, follicles were assessed ultrasonically for 28 waves every 8 h. The number of examinations that encompassed the emergence of all growing 3-mm follicles was 10.0 +/-0.5 (mean +/-SEM; equivalent to 3.3 d) and extended from mean Examination -3.1 +/-0.3 to mean Examination 6.0 +/-0.6. A mean of 24 growing 3-mm follicles was found, and the maximal attained diameters were 4 mm (46%), 5 mm (25%), and >/=6 mm (29%). More (P<0.05) 3-mm follicles at Examinations -2 and -1 grew to >/=6 mm than to 4 or 5 mm, whereas more 3-mm follicles at Examinations 2 to 6 grew to only 4 mm. On average, the future dominant follicle appeared as a 3-mm follicle (Examination -2.1 +/-0.2) 6 and 10 h earlier (P<0.03) than for the largest (Examination -1.4 +/-0.3) and second-largest (Examination -0.8 +/-0.4) future subordinates, respectively. This result supported the hypothesis that the future dominant follicle has, on the average, an early developmental advantage. In Experiment 2 (n=33 waves), data were normalized to the day at the beginning of deviation (Day 2.8 +/-0.2) when the mean diameters of the dominant and largest subordinate follicle were 8.5 +/-0.2 mm and 7.2 +/-0.2 mm, respectively. This result suggests that the follicle selected to become dominant, as manifested by deviation, is the first follicle to develop to a decisive stage. In Experiment 3 (n=19 waves), FSH concentrations were lower (P<0.05) on the day at the beginning of deviation (8.5 +/-0.5 ng/ml) than on the day before (10.1 +/-0.8 ng/ml), with no continuing decrease after deviation. This temporal result suggests that the attainment of approximate basal levels of FSH is a component of the deviation mechanism.     

Folliculogenesis during the transitional period and early ovulatory season in mares

Individual follicles were monitored by ultrasonography in 15 mares during the transitional period preceding the first ovulation of the year and in 9 mares during the first interovulatory interval. During the transitional period, 7 mares developed 1-3 anovulatory follicular waves characterized by a dominant follicle (maximum diameter greater than or equal to 38 mm) that had growing, static, and regressing phases. The emergence of a subsequent wave (anovulatory or ovulatory) did not occur until the dominant follicle of the previous wave was in the static phase. After the emergence of the subsequent wave, the previous dominant follicle regressed. The mean (+/- s.d.) length of the interval between successive waves was 10.8 +/- 2.2 days. Before the emergence of waves (identified by a dominant follicle), follicular activity seemed erratic and follicles did not reach greater than 35 mm. During the interovulatory interval, 6 mares developed 2 waves (an anovulatory wave and a subsequent ovulatory wave) and 3 mares developed only 1 detected wave (the ovulatory wave). The ovulatory follicle at the end of the transitional period reached 20 mm earlier (Day - 15), grew slower (2.6 +/- 0.1 mm/day; mean +/- s.e.m.) but reached a larger diameter on Day - 1 (50.5 +/- 1.1 mm) than for the ovulatory follicle at the end of the interovulatory interval (Day - 10, 3.6 +/- 0.2 mm/day, 44.4 +/- 1.0 mm, respectively; P less than 0.05 for each end point). The interval from cessation of growth of the largest subordinate follicle to the occurrence of ovulation was longer (P less than 0.05) for end of the transitional period (9.5 +/- 0.7 days) than for the end of the interovulatory interval (6.8 +/- 0.6 days). Results demonstrated the occurrence of rhythmic follicular waves during some transitional periods and the occurrence of 2 waves during some of the first oestrous cycles of the year.