Temporal associations among ovarian events in cattle during oestrous cycles with two and three follicular waves

For 18 two-wave interovulatory intervals in heifers, the follicular waves were first detected on Days -0.2 +/- 0.1 and 9.6 +/- 0.2, and for 4 three-wave intervals on Days -0.5 +/- 0.3, 9.0 +/- 0.0 and 16.0 +/- 1.1 (ovulation is Day 0). The day-to-day mean diameter profile of the dominant follicle of the 1st wave and the day of emergence of the 2nd wave were not significantly different between 2-wave and 3-wave intervals. There were no indications, therefore, that events occurring during the first half of the interovulatory interval were associated with the later emergence of a 3rd wave. The dominant ovulatory follicle differed significantly (P less than 0.05 at least) between 2-wave and 3-wave intervals in day of emergence (Day 9.6 +/- 0.2 and 16.0 +/- 1.1), length of interval from emergence of follicle to ovulation (10.9 +/- 0.4 and 6.8 +/- 0.6 days), and diameter on day before ovulation (16.5 +/- 0.4 and 13.9 +/- 0.4 mm). The mean length of 2-wave interovulatory intervals (20.4 +/- 0.3 days) was shorter (P less than 0.01) than for 3-wave intervals (22.8 +/- 0.6 days). The mean day of luteal regression for 2-wave and 3-wave intervals was 16.5 +/- 0.4 and 19.2 +/- 0.5 (P less than 0.01). For all intervals, luteal regression occurred after emergence of the ovulatory wave, and the next wave did not emerge until near the day of ovulation at the onset of the subsequent interovulatory interval. In conclusion, the emergence of a 3rd wave was associated with a longer luteal phase, and the viable dominant follicle present at the time of luteolysis became the ovulatory follicle.     

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.     

Suppression of dominant and subordinate ovarian follicles by a proteinaceous fraction of follicular fluid in heifers

Nulliparous Holstein heifers were examined ultrasonically once daily during an interovulatory interval (ovulation = Day 0). Follicles with a diameter >/=4 mm were sequentially identified. Heifers were randomized into four groups (n = 4 heifers per group): untreated control heifers and those treated on Days 0 to 3, Days 3 to 6, or Days 6 to 11. Heifers designated for treatment were given an intravenous injection, twice daily, of a proteinaceous fraction of follicular fluid (PFFF; 16 ml) prepared by extracting bovine follicular fluid with activated charcoal. Mean cessation of growth of the dominant follicle of Wave 1 was later (P<0.005) in control heifers (Day 5.5) than in heifers treated on Days 0 to 3 (Day 1.5) or Days 3 to 6 (Day 3.5). Mean onset of regression of the dominant follicle of Wave 1 was later (P<0.005) in control heifers (Day 12.0) than in heifers treated on Days 0 to 3 (Day 5.0) or Days 3 to 6 (Day 7.5). Mean cessation of growth of the largest subordinate follicle of Wave 1 was later (P<0.05) in control heifers (Day 3.0) than in heifers treated on Days 0 to 3 (Day 1.2). Mean onset of regression of the largest subordinate follicle of Wave 1 was later (P<0.05) in control heifers (Day 7.0) than in heifers treated on Days 0 to 3 (Day 4.8). In heifers treated on Days 6 to 11, cessation of growth and onset of regression of the dominant follicle (means, Days 5.2 and 12.0, respectively) were not significantly different from those of the controls. The hypothesis that PFFF treatment on Days 0 to 3 would cause suppression of all follicles of Wave 1 was supported. The hypothesis that PFFF treatment on Days 3 to 6 would not alter growth of the dominant follicle of Wave 1 was not supported. The mean day of detection of the dominant follicle of Wave 2 was different (P<0.005) in control heifers (Day 8.5) than in heifers treated on Day 0 to 3 (Day 5.5) or Days 6 to 11 (Day 14.2). The mean length of the interovulatory interval was shorter (P<0.05) in control heifers (20.5 d) than in heifers treated on Days 6 to 11 (23.2 d). The hypothesis that PFFF treatment on Days 6 to 11 would delay the emergence of Wave 2 was supported. The proportion of heifers with 2-wave interovulatory intervals was 3 4 for control heifers and 0 4 , 1 4 , and 4 4 for heifers treated on Days 0 to 3, Days 3 to 6, and Days 6 to 11, respectively (3 4 vs 0 4 , P<0.05); the remaining heifers had 3-wave interovulatory intervals. On average, in PFFF-treated heifers, follicles stopped growing 1 d after treatment was started, and Wave 2 was detected 3 d after treatment was stopped.     

Effects of a dominant follicle on ovarian follicular dynamics during the oestrous cycle in heifers

Two hypotheses were tested: (1) a dominant follicle causes regression of its subordinate follicles, and (2) a dominant follicle during its growing phase suppresses the emergence of the next wave. Cyclic heifers were randomly assigned to one of four groups (6 heifers/group): cauterization of the dominant follicle of Wave 1 or sham surgery (control) on Day 3 or Day 5 (day of ovulation = Day 0). Ultrasonic monitoring of individually identified follicles was done once daily throughout the interovulatory interval. The onset of regression (decreasing diameter) of the largest subordinate follicle of Wave 1 was delayed (P less than 0.01) by cauterization of the dominant follicle of Wave 1 on Day 3 compared to controls (mean onset of regression, Days 10.8 +/- 2.1 vs 4.3 +/- 0.4). Cauterization of the dominant follicle of Wave 1 on Days 3 or 5 caused early emergence (P less than 0.01) of Wave 2 when compared to controls (Day-3 groups: Days 5.5 +/- 0.4 vs 9.6 +/- 0.7; Day-5 groups: Days 7.0 +/- 0.3 vs 9.1 +/- 0.4). The results supported the two hypotheses. In addition, cauterization of the dominant follicle of Wave 1 on Days 3 or 5 increased the incidence of 3-wave interovulatory intervals.     

Effect of estradiol valerate on ovarian follicles, emergence of follicular waves and circulating gonadotropins in heifers

An experiment was designed to examine the effect of estradiol valerate (EV) on the growth and regression of follicles of a wave and on the emergence of the next follicular wave. Twenty-six beef heifers were xamined daily by ultrasonography and randomly allocated to 1 of 4 treatment groups at the time of ovulation (Day 0): unterated control heifers and those that received 5 mg EV intramuscularly on Day 1, Day 3 or Day 6. Maximum diameter of the dominant follicle was greater (P<0.05) in control heifers than in heifers treated on Day 1 or Day 3. Mean day of onset of regression of the dominant follicle was later (P<0.05) in control heifers than in heifers treated on Day 1 but was not different from heifers treated on Day 3. In heifers treated on Day 6, cessation of growth, maximum diameter and onset of regression were not different from that of control heifers. The emergence of the next follicular wave was earlier (P<0.05) in heifers treated on Day 1 than in control heifers, whereas wave emergence was delayed (P<0.05) in heifers treated on Day 3 or Day 6. The mean day of maximum concentration of FSH prior to the emergence of the next wave was earlier in heifers treated with EV on Day 1 and later in heifers treated on Day 3 or Day 6 compared with that of the controls (P<0.05). Treatment on Day 1 or Day 3 resulted in a significant LH surge in 8 13 heifers, whereas no LH surges were detected in control heifers or in heifers treated on Day 6. The hypothesis that EV suppresses the growth of the dominant follicle, was supported. Estradiol valerate treatment resulted in early emergence of the next follicular wave in heifers treated on Day 1, but treatment on Day 3 or Day 6 resulted in delayed emergence of the next follicular wave.     

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.     

Minor FSH surge,minor follicular wave,and resurgence of preovulatory follicle several days before ovulation in heifers

Blood samples were collected and follicle diameters were determined daily beginning on Day 12 (Day 0 = ovulation) in 35 interovulatory intervals (IOIs) in heifers. A minor follicular wave with maximal diameter (6.0 ± 0.3 mm) on Day −4 was detected in six of seven IOIs that were scanned for follicles 4 mm or greater. The number of IOIs with a CV-identified minor FSH surge toward the end of the IOI was greater (P < 0.03) in two-wave IOIs (10/17) than in three-wave IOIs (4/18). The 17 two-wave IOIs were used for study of the temporal relationships among preovulatory follicle, FSH, LH, and estradiol. Daily growth rate of the preovulatory follicle was maximum on Days −11 to −7, minimum (P < 0.05) on Days −7 to −4, and increased (resurged, P < 0.05) on Days −4 to −3. A transient increase in FSH was maximum on mean Day −4, and the peak of a minor FSH surge occurred on Day −4.5 ± 0.2. Concentration of LH and estradiol increased between Days −5 and −4. Results demonstrated resurgence of the preovulatory follicle apparently for the first time in any species. Resurgence seemed more related temporally to the minor FSH surge than to the LH increase, but further study is needed. Results supported the novel hypotheses that a minor FSH surge near the end of the IOI is temporally associated with (1) the emergence of a minor follicular wave and (2) the resurgence in growth rate of the preovulatory follicle.     

Changes in the cumulus-oocyte complex of subordinate follicles relative to follicular wave status in cattle

We investigated factors that affect cumulus-oocyte complex (COC) morphology and oocyte developmental competence in subordinate follicles on different days after follicular wave emergence in beef heifers. In Experiment 1, heifers (n = 13) were assigned at random to COC aspiration during the growing/static (Days 1 to 3) or regressing (Day 5) phase of subordinate follicle development (follicular wave emergence = Day 0). Follicular wave emergence was induced by transvaginal ultrasound-guided follicular ablation, ovaries were collected at slaughter, all follicles > or = 2 mm except the dominant follicle were aspirated, and COC were microscopically evaluated for morphology. There was a greater percentage of COC with expanded cumulus layers on Day 5 (42.4%) than on Days 1 to 3 (2.2%). In Experiment 2, heifers (n = 64) at random stages of the estrous cycle had all follicles > or = 5 mm ablated and 4 d later, 2 doses of PGF were injected 12 h apart; heifers were monitored daily by ultrasonography for ovulation (Day 0 = follicular wave emergence). Heifers were assigned to the following time periods for oocyte collection from subordinate follicles: Days 0 and 1 (growing phase), Days 2, 3 and 4 (static phase), and Days 5 and 6 (regressing phase). Ovaries were individually collected at slaughter, and all follicles > or 2 mm except for the dominant follicle were aspirated. The COC were morphologically evaluated and then matured, fertilized and cultured in vitro. Expanded COC were more frequent during the regressing phase (53.4%) than the growing or static phase (14.4 and 17.8%, respectively; P < 0.05). While the proportions of COC with > or = 4 layers of cumulus cells and denuded oocytes were higher (P < 0.05) in the growing and static phases, the production of morulae was highest (P < 0.05) with COC collected from subordinate follicles during the regressing phase. In Experiment 3, heifers (n = 18) were assigned at random to oocyte collection from subordinate follicles 3 and 4 d (static phase) or 5 and 6 d (regressing phase) after follicular wave emergence. The heifers were monitored ultrasonically for ovulation (Day 0 = follicular wave emergence); COC were collected from all follicles (> or = 5 mm) except for the dominant follicle by transvaginal ultrasound-guided follicle aspiration 3 to 6 d later. Recovered oocytes were stained and examined microscopically to evaluate nuclear maturation. A higher proportion of oocytes collected on Days 5 and 6 showed evidence of nuclear maturation (50%) than on Days 3 and 4 (8.3%; P < 0.05). Results support the hypothesis that COC morphology and oocyte developmental competence change during the growing, static and regressing phases of subordinate follicle development.     

Ovarian dynamics and their associations with peripheral concentrations of gonadotropins,ovarian steroids,and inhibin during the estrous cycle in goats

Ovarian changes determined by daily transrectal ultrasound and its relationship with FSH, LH, estradiol-17beta, progesterone, and inhibin were investigated in six goats for three consecutive interovulatory intervals. Estrous cycles were synchronized using two injections of prostaglandin F2alpha analogue 11 days apart. All follicles 3 mm or greater in diameter and corpora lutea were measured daily. A follicular wave was defined as one or more follicles growing to 5 mm or greater in diameter. The day that the follicles reached 3 mm in diameter was defined as the day of wave emergence, and the first wave after ovulation was defined as wave 1. During the interovulatory interval (mean +/- SEM, 21.3 +/- 0.4 days; n = 18), follicular waves emerged at 0.3 +/- 0.5, 6.5 +/- 0.2, and 12.1 +/- 0.4 days for wave 1, wave 2, and wave 3, respectively, in goats with three waves of follicular development and at -0.6 +/- 0.3, 4.7 +/- 0.2, 9.4 +/- 0.5, and 13.4 +/- 0.5 days for wave 1, wave 2, wave 3, and wave 4, respectively, in goats with four waves of follicular development (Day 0 = the day of ovulation). The mean diameter of the largest follicle of the ovulatory wave was significantly larger than those of the largest follicles of the other waves. Corpora lutea could be identified ultrasonically at Day 3 postovulation and attained 12.1 +/- 0.3 mm in diameter on Day 8. Transient increases in plasma concentrations of FSH were detected around the day of follicular wave emergence. The level of FSH was negatively correlated with that of inhibin. These results demonstrated that follicular waves occurred in goats and that the predominant follicular wave pattern was four waves with ovulation from wave 4. These results also suggested that the emergence of follicular waves was closely associated with increased secretion of FSH.     

Follicular dynamics during the ovulatory season in goats

Growth and regression of ovarian follicles>or=3 mm were studied by transrectal ultrasonography for 4 interovulatory intervals in each of 5 Saanen goats. The observed number of growing identified 4-mm follicles per day differed (P<0.05) from randomness, indicating that follicles, on the average, emerged in groups (waves). Averaged over all interovulatory intervals, the number of 3-mm follicles on each day that later reached >or=6 mm followed a pattern of significant peaks on Days 0 (ovulation), 4,8 and 14. A follicular wave was defined by consecutive days of entry of follicles>or=6 mm into the wave, and the day of emergence was defined as the first day that the >or=6 mm follicles were 3 mm. In 15 of 20 (75%) interovulatory intervals, 1 wave emerged during each of Day -2 to Day 1 (Wave 1); Days 2 to 5 (Wave 2); Days 6 to 9 (Wave 3); and Days 10 to 15 (Wave 4). Ovulation occurred during Wave 4. The mean days of emergence of Waves 1 to 4 were Days -1, 4, 8 and 13, respectively. However, in 5 of these 15 interovulatory intervals, 50% of the apparent waves merged or were continuous so that a distinction could not be made between 2 waves. The largest follicle grew to a larger (P<0.05) maximum diameter for Waves 1 (8.7+/-0.3 mm) and 4 (9.7+/-0.3 mm) than for Waves 2 (7.2+/-0.2 mm) and 3 (7.3+/-0.2 mm). The following observations suggested that the phenomenon of follicular dominance was more common during Waves 1 and 4 than during Waves 2 and 3: 1) the interwave intervals (days) were longer (P<0.05) for Waves 1 (3.4+/-0.2) and 4 (4.3+/-0.6) than for Waves 2 and 3 (2.5+/-0.2 for each wave) and 2) the correlation between maximum diameter of largest follicle and the subsequent interwave interval was significant for Waves 1 and 4 but not for Waves 2 and 3. The 5 remaining interovulatory intervals were irregular and involved more than 4 waves, including 2 interovulatory intervals with prolonged follicular phases (14 and 21) and failures of ovulation. In conclusion, the predominant follicular-wave pattern was 4 waves with ovulation from Wave 4, and apparent follicular dominance was expressed during some follicular waves, especially during Waves 1 and 4.     

Effect of purified llama ovulation-inducing factor (OIF) on ovarian function in cattle

Two experiments were designed to determine the effect of purified ovulation inducing factor (OIF) on ovarian function in cattle. In Experiment 1, prepubertal heifers (n = 11 per group) were treated on Day 5 (Day 0 = day of follicular wave emergence) of the follicular wave with an intramuscular dose of saline (1 mL), GnRH (100 μg), or purified OIF (1 mg/100 kg body weight). Ovulation occurred in 9/11 heifers treated with GnRH, and 1/11 heifers in each of the OIF- and saline-treated groups (P < 0.05). Compared to saline-treated controls, OIF treatment was associated with a smaller dominant follicle diameter (P < 0.01), a rise in plasma FSH concentration (P < 0.1), and earlier emergence of the next follicular wave (P < 0.05). In Experiment 2, sexually mature heifers were given either GnRH or purified OIF on Days 3, 6 or 9 of the first follicular wave (i.e., early growing, early static, or late static phase of the dominant follicle; n = 5 per group per day), or were untreated (n = 10). In heifers treated with OIF on Day 6, the dominant follicle diameter profile tended to be smaller than in controls, and was associated with a rise (P < 0.05) in plasma FSH concentrations. A similar rise in FSH was detected after OIF treatment on Day 9. Compared to untreated controls, treatment with OIF and GnRH was associated with a larger CL diameter (Days 3 and 6 groups; P < 0.05) and a greater concentration of plasma progesterone (Days 6 and 9 groups; P < 0.05). Treatment with purified OIF did not induce ovulation in heifers, but hastened new follicular wave emergence in prepubertal heifers, influenced follicular dynamics in a phase-specific manner in mature heifers, and was luteotrophic.     

Temporal interrelationships among luteolysis, FSH and LH concentrations and follicle deviation in mares

The effect of altered LH concentrations on the deviation in growth rates between the 2 largest follicles was studied in pony mares. The progestational phase was shortened by administration of PGF2alpha on Day 10 (Day 0=ovulation; n=9) or lengthened by daily administration of 100 mg of progesterone on Days 10 to 30 (n=11; controls, n=10). All follicles > or = 5 mm were ablated on Day 10 in all groups to initiate a new follicular wave. The interovulatory interval was not altered by the PGF2alpha treatment despite a 4-day earlier decrease in progesterone concentrations. Time required for growth of the follicles of the new wave apparently delayed the interval to ovulation after luteolysis. The FSH concentrations of the first post-ablation FSH surge were not different among groups. A second FSH surge with an associated follicular wave began by Day 22 in 7 of 11 mares in the progesterone group and in 0 of 19 mares in the other groups, indicating reduced functional competence of the largest follicle. A prolonged elevation in LH concentrations began on the mean day of wave emergence (Day 11) in the prostaglandin group (19.2 +/- 2.2 vs 9.0 +/- 0.7 ng/mL in controls; P<0.05), an average of 4 d before an increase in the controls. Concentrations of LH in the progesterone group initially increased until Day 14 and then decreased so that by Day 18 the concentrations were lower (P<0.05) than in the control group (12.9 +/- 1.6 vs 20.2 +/- 2.6 ng/mL). Neither the early and prolonged increase nor the early decrease in LH concentrations altered the growth profile of the second-largest follicle, suggesting that LH was not involved in the initiation of deviation. However, the early decrease in LH concentrations in the progesterone group was followed by a smaller (P<0.05) diameter of the largest follicle by Day 20 (26.9 +/- 1.7 mm) than the controls (30.3 +/- 1.7 mm), suggesting that LH was necessary for continued growth of the largest follicle after deviation.     

Associations between emergence of follicular waves and fluctuations in FSH concentrations during the estrous cycle in ewes

Folliculogenesis was studied daily in 16 interovulatory intervals in 5 Polypay ewes from mid February through April using transrectal ultrasonic imaging. The 3-mm follicles attaining > or = 5 mm on Days--1 (ovulation=Day 0) to 11 showed significant peak numbers on Days 0, 5 and 10. The number of 3- and 4-mm follicles that did not reach > 4 mm was not significant, indicating that these follicles did not manifest a wave pattern. A follicular wave was defined as one or more follicles growing to > or = 5 mm; the day the follicles were 3 mm was the day of wave emergence, and the first wave after ovulation was Wave 1. Waves 1, 2 and 3 emerged on Days -1 to 2,4 to 7 and 8 to 10, respectively. Four interovulatory intervals in April were short (9 to 14 d), indicating the end of the ovulatory season. In the remaining 12 intervals, the ovulatory wave was Wave 3 in one interval, Wave 4 in 8 intervals, and Wave 5 or 6 in 3 intervals. The ovulatory wave followed the rhythmic pattern of Waves 1 to 3 by emerging on Days 11 to 14 in 50% of the intervals. In the remaining intervals, either the ovulatory wave was Wave 4 but did not emerge until Day 16 or other waves intervened between Wave 3 and the ovulatory wave. The longest intervals (22, 24 and 24 d) had >4 waves. Based on a cycle-detection program, peak values of FSH fluctuations were temporally associated with the emergence of waves as indicated by the following: 1) tendency (P < 0.08) for an increase in FSH concentrations between 3 and 2 days before emergence of a wave; 2) close agreement between mean number of waves per interval (mean +/- SEM, 4.1 +/- 0.3) and mean number of identified FSH fluctuations (4.5 +/- 0.3); 3) close agreement in length of interwave intervals (4.0 +/- 0.3) and interpeak (FSH) intervals (3.6 +/- 0.2); 4) positive correlation (r(2)=0.8) for number of the 2 events (follicular waves and FSH fluctuations) within intervals; and 5) a closer (P < 0.01) temporal relationship between the 2 events than would have been expected if the events were independent. The results support a relationship between transient increases in FSH concentrations and emergence of follicular waves throughout the interovulatory interval in Polypay ewes, with the 2 events occurring approximately every 4 d.     

Two-way coupling between FSH and the dominant follicle in heifers

Follicular Wave 1 and 2 and the associated FSH Surge 1 and 2 were used to designate the first two waves and surges of the interovulatory interval in two experiments in heifers. In experiment 1, a group with early (group E, N = 9) and late (group L, N = 5) development of the dominant follicle of Wave 1 were used as natural models to study FSH/follicle coupling. The day of wave emergence and the day of deviation in diameters between the two largest follicles were not different between groups. Emergence of Wave 2 and maximal FSH concentration in Surge 2 was approximately 1 day later (P < 0.03) in group L. Diameter of the dominant follicle of wave 1 (13.8 ± 0.3 mm vs. 12.0 ± 0.3 mm) and FSH concentrations in Surge 2 (0.29 ± 0.02 ng/mL vs. 0.21 ± 0.03 ng/mL) were first greater (P < 0.05) in group E than in group L at 4 and 5 days, respectively, after wave emergence. In experiment 2, treatment with estradiol (N = 8) when the dominant follicle of Wave 1 was ≥11 mm (Hour 0) resulted in a decrease (P < 0.02) in FSH and slower (P < 0.05) growth rate of the follicle between Hours 0 and 4. Results supported the following hypotheses: (1) the FSH surge that stimulates emergence of a follicular wave is associated with final growth of the dominant follicle of the previous anovulatory wave; and (2) suppression of FSH Surge 2 when the dominant follicle of Wave 1 is ≥11 mm is associated with a decrease in diameter. It is concluded for the first time that two-way FSH/follicle coupling in heifers continues during final growth of the dominant follicle of Wave 1 and that Surge 2 is the FSH source.     

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.     

Ovarian follicular activity and hormonal profile during estrous cycle in cows: the development of 2 versus 3 waves

Most estrous cycles in cows consist of 2 or 3 waves of follicular activity. Waves of ovarian follicular development comprise the growth of dominant follicles some of which become ovulatory and the others are anovulatory. Ovarian follicular activity in cows during estrous cycle was studied with a special reference to follicular waves and the circulating concentrations of estradiol and progesterone. Transrectal ultrasound examination was carried out during 14 interovulatory intervals in 7 cows. Ovarian follicular activity was recorded together with assessment of serum estradiol and progesterone concentrations. Three-wave versus two-wave interovulatory intervals was observed in 71.4% of cows. The 3-wave interovulatory intervals differed from 2-wave intervals in: 1) earlier emergence of the dominant follicles, 2) longer in length, and 3) shorter interval from emergence to ovulation. There was a progressive increase in follicular size and estradiol production during growth phase of each wave. A drop in estradiol concentration was observed during the static phase of dominant anovulatory follicles. The size of the ovulatory follicle was always greater and produced higher estradiol compared with the anovulatory follicle. In conclusion, there was a predominance of 3-wave follicular activity that was associated with an increase in length of interovulatory intervals. A dominant anovulatory follicle during its static phase may initiate the emergence of a subsequent wave. Follicular size and estradiol concentration may have an important role in controlling follicular development and in determining whether an estrous cycle will have 2 or 3-waves.     

Follicular populations during the estrous cycle in heifers. I. Influence of day

Ovarian follicles ⩾ 2 mm were studied in 22 Holstein heifers by daily ultrasound examinations. There were significant differences (P < 0.0001) among days of the estrous cycle for diameter of the largest and second largest follicles and in the numbers of follicles 2–3 mm, 4–6 mm, 7–10 mm, 11–13 mm, > 13 mm, and total number of follicles ⩾2 mm. Patterns of the mean profiles for all follicular endpoints except the number of follicles 4–6 mm and total number of follicles ⩾ 2 mm were bimodal. The days encompassed by the first and second portions, respectively, of the bimodal profiles were approximately: diameter of largest follicle, Days 0–14 and 15–21 (ovulation); diameter of second largest follicle, Days 0–7 and 8–20; number of follicles 2–3 mm, Days 1–11 and 12–20; number of follicles 7–10 mm, Days 0–6 and 7–18; number of follicles 11–13 mm, Days 0–8 and 9–20; and number of follicles > 13 mm, Days 2–14 and 16–21. Data for the various categories were recombined to demonstrate relationships between the numbers of follicles 2–3 mm and ⩾ 4 mm during the interovulatory interval. There were significant differences (P < 0.0001) among days in both 2–3 mm and ⩾ 4 mm follicular categories. Differences appeared due to periods of higher mean numbers of follicles 2–3 mm which began between Days 2 or 3 and Days 15 or 16 and reached maximum levels on Day 7 and Day 19, respectively. There was an inverse relationship between the number of follicles 2–3 mm vs ⩾ 4 mm and between the diameters of largest and second largest follicles. The process of selection of the follicle destined to ovulate appeared to become manifest as selective growth of the preovulatory follicle with concurrent decrease in diameter of the second largest follicle and regression of the other follicles in the various follicular categories. A similar process apparently occurred early in the interovulatory interval. There was apparently selective growth of a follicle to preovulatory size by Day 6, coincident decrease in diameter of the second largest follicle, and apparent regression of other follicles in the ultrasonically detectable pool. The only apparent difference was that the follicle which attained preovulatory diameter early in the interovulatory interval remained in the ovary for 5 or 6 days, then regressed, while the follicle which attained preovulatory diameter at approximately Day 18–20 ovulated.     

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.     

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).     

Synchronization of follicular wave emergence prior to superovulation in Bactrian camel (Camelus bactrianus)

This study was conducted to synchronize follicle wave emergence prior to superovulation using either GnRH or progestogen treatments, in Bactrian camels. GnRH group camels (n=5) received 20 microg of the GnRH analogue Buserelin on Days -18 and -4 of the experiment (initiation of superovulation=Day 0). Camels in the progestogen group (n=5) received two consecutive treatments of progestogens, 7 days apart, on Days -14 and -8 of the experiment. On each occasion, each female received three norgestomet implants and 200mg progesterone (i.m.) and all implants were removed 14 days after the first progestogen treatment coinciding with Day -1 of superovulation. A combination of eCG and FSH was used to induce superovulation and the growth of all subsequent follicles and CLs were monitored daily by ultrasonography. Following the first GnRH injection, mature follicles ovulated within 1-2 days, and a new follicle wave emerged after 3+/-0.77 days. At the time of the second GnRH injection, a mature follicle (15.6+/-0.97 mm) ovulated and a new follicular wave emerged between 1 and 2 days after GnRH injection. Growing follicles at the time of the first progestogen treatment became either atretic (n=1) or persistent (n=4) and a new follicle wave (n=3) emerged 3-6 days later. At the initiation of superovulation, the diameters of the largest follicle in GnRH and progestogen groups were 7.4+/-0.59 and 20.5+/-2.26 mm, respectively but after superovulation and mating there was no significant differences in the number of unovulated follicles or CLs between groups. In conclusion, two GnRH injections, 14 days apart, may be used to synchronize follicle wave emergence in Bactrian camel.