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.

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.

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.

Role of diameter differences among follicles in selection of a future dominant follicle in mares

Follicles > or = 5 mm were ablated in pony mares by a transvaginal ultrasound-guided technique on Day 10 (ovulation = Day 0). Follicle emergence (at 15 mm, experiment 1; at 6 mm, experiment 2) and development of the new wave was monitored by transrectal ultrasound. Deviation was defined as the beginning of a marked difference in growth rates between the two largest follicles. In experiment 1, mares were grouped (n = 4 per group) into controls, ablation-controls (ablations at Day 10 only), and a two-follicle model (periodic ablation sessions so that only the two largest follicles developed). There were no significant indications that the two-follicle model altered follicle diameters, growth rates, or time intervals of the two retained follicles at or between events (follicle emergence, deviation, and ovulation). In experiment 2, the two-follicle model (n = 14) was used for follicle and hormonal characterization and hypothesis testing, without the tedious and error-prone necessity for tracking many (e.g., 20) individual follicles. The future dominant follicle emerged a mean of 1 day earlier (p < 0.008) than the future subordinate follicle, the growth rates for the two follicles between emergence and deviation (6 days later) did not differ, and the dominant follicle was larger at the beginning of deviation (23.1 +/- 0.8 mm versus 19.6 +/- 0.9 mm; p < 0.0001). Mean FSH and LH concentrations increased (p < 0.05) concomitantly from emergence of the future dominant follicle and peaked 3 days later when the follicle was a mean of 13 mm. Thereafter, the two hormones disassociated until ovulation: FSH decreased and LH increased. Results supported the hypothesis that the future dominant follicle has an early size advantage over future subordinate follicles and indicated that the advantage was present as early as 6 days before deviation.

Effect of LH or GnRH on the dominant follicle of the first follicular wave in beef heifers

A study was designed to characterise ovarian follicular dynamics in heifers treated with porcine luteinizing hormone (pLH) or gonadotropin releasing hormone (GnRH) on days 3, 6 or 9 (ovulation = day 0), corresponding to the growing, early-static, and late-static phases of the first follicular wave. Following ovulation, 65 beef heifers were assigned, by replicate, to the following seven treatment groups: 25 mg im of pLH on days 3, 6 or 9 (n = 9 per group); 100 microg im of GnRH on days 3, 6 or 9 (n = 9 per group); or controls (no treatment; n = 11). Ovulation occurred within 36 h in 67%, 100% and 67% of heifers treated with pLH and in 89%, 56% and 22% of heifers treated with GnRH on days 3, 6 or 9, respectively (treatment-by-day interaction, P < 0.09). Combined for all treatment days, ovulation rates were 78% and 56% in pLH- and GnRH-treated groups, respectively (P < 0.09). Overall, mean day (+/- SD) of emergence of the second follicular wave in heifers that ovulated was different from that in controls or in heifers that did not ovulate (P < 0.05). Mean (+/- SD) day of emergence of the second wave occurred earlier (day 5.6+/-1.2; P < 0.05) in heifers that ovulated after treatment on day 3 (n = 14) than in controls (day 8.7+/-1.6; n = 11); however, wave emergence in all heifers treated on day 6 (day 8.1+/-0.5; n = 18) did not differ from controls, regardless of whether or not ovulation occurred. In the heifers that ovulated in response to treatment on day 9 (n = 8), the emergence of the second follicular wave was delayed (day 10.9+/-0.4; P < 0.05). The day of emergence of the second wave in the 14 treated heifers that failed to ovulate, irrespective of the day of treatment (day 8.9+/-1.4) did not differ from control heifers. The emergence of the second wave was more synchronous in day 6 heifers (regardless of whether they ovulated) and in day 9 heifers that ovulated compared to control heifers (P < 0.05). Results did not support the hypothesis that the administration of pLH or GnRH at known stages of the follicular wave in cycling heifers would consistently induce ovulation or atresia and, thereby, induce emergence of a new follicular wave at a predictable interval. New wave emergence was induced consistently (1.3 days post-treatment) only in those animals that ovulated in response to treatment. However, 22% of LH-treated heifers and 44% of GnRH-treated heifers failed to ovulate. Treatments did not induce atresia of the dominant follicle or alter the interval to new wave emergence in animals that did not ovulate in response to treatment.

Follicle selection in cattle: follicle deviation and codominance within sequential waves

Follicle deviation during bovine follicular waves is characterized by continued growth of a developing dominant follicle and reduction or cessation of growth of subordinate follicles. Characteristics of follicle deviation for waves with a single dominant follicle were compared between wave 1 (begins near ovulation; n = 15) and wave 2 (n = 15). Follicles were defined as F1 (largest), F2, and F3, according to maximum diameter. No mean differences were found between waves for follicle diameters at expected deviation (F1, > or =8.5 mm; Hour 0) or observed deviation or in the interval from follicle emergence at 4.0 mm to deviation. For both waves, circulating FSH continued to decrease (P < 0.05) after Hour 0, estradiol began to increase (P < 0.05) at Hour 0, and immunoreactive inhibin began to decrease (P < 0.05) before Hour 0. A transient elevation in circulating LH reached maximum concentration at Hour 0 (P < 0.01) in both waves and was more prominent (P < 0.0001) for wave 1. Waves with codominant follicles (both follicles >10 mm) were more common (P < 0.02) for wave 1 (35%) than for wave 2 (4%). Codominants (n = 6) were associated with more (P < 0.05) follicles > or=4 mm and a greater concentration (P < 0.04) of circulating estradiol at Hours -48 to -8 than were single dominant follicles (n = 15). A mean transient increase in FSH and LH occurred in the codominant group at Hour -24 and may have interfered with deviation of F2. In codominant waves, deviation of F3 occurred near Hour 0 (F1, approximately 8.5 mm). A second deviation involving F2 occurred in four of six waves a mean of 50 h after the F3 deviation and may have resulted from a greater suppression (P < 0.05) of FSH in the codominant group after Hour 0. In conclusion, follicle or hormone differences were similar for waves 1 and 2, indicating that the deviation mechanisms were the same for both waves. Waves that developed codominant follicles differed in hormone as well as follicle dynamics.

Follicular-fluid factors and granulosa-cell gene expression associated with follicle deviation in cattle

Intrafollicular changes in the largest follicle (F1) and second-largest (F2) follicle were examined in relation to follicle diameter deviation. Deviation is characterized by continued growth of the largest follicle and the cessation of growth of the smaller follicles. Granulosa cells and follicular fluid were obtained from slaughterhouse ovaries (n = 95 pairs, experiment 1), and follicular fluid was collected in vivo (n = 28 heifers, experiment 2). Several ranges in the diameter of F1 were used to represent the progressive growth of the follicle. The diameter range with the first significant increase in the difference between F1 and F2 was determined for each end point and was used as an indicator of the sequence of events associated with diameter deviation. An increased difference for diameter and for estradiol concentration occurred (P: < 0.05) simultaneously at the 8.5- to 8.9-mm range in both experiments. In experiment 1, the increased difference between F1 and F2 in LH receptor (LHr) mRNA expression occurred (P: < 0.05) at the 8.0- and 8.4-mm range. In F2 of experiment 2, there was a progressive decrease (P: < 0.05) in free insulin-like growth factor (IGF)-1 and a progressive increase (P: < 0.05) in IGF binding protein (BP)-2 across the follicle-diameter ranges (7.5-11.2 mm). No differences were detected between F1 and F2 for 3beta-hydroxysteroid dehydrogenase mRNA expression in experiment 1 and testosterone, total inhibin, and dimeric inhibin-A concentrations in experiment 2. The results indicated that the acquisition of granulosa cell LHrs by F1, as indicated by increased LHr mRNA expression, occurred one diameter range before an increased difference between F1 and F2 for diameter or estradiol concentrations. On a temporal basis, it is concluded that LHr acquisition plays a role in the establishment of diameter deviation. In addition, the reduced growth of F2 may have involved the reduced bioavailability of IGF-1 in association with elevated IGFBPs.

Prostaglandin F2 alpha receptors in the early bovine corpus luteum

Since the early CL (< or = 4 days after ovulation) does not regress after injection of PGF2 alpha, this study was designed to determine whether number or affinity of PGF2 alpha receptors was lower in the early as compared with the midstage CL. Heifers were randomly assigned to have ovaries removed on Day 2, 4, 6, or 10 (n = 4 heifers per day; Day 0 = day of ovulation). Plasma progesterone concentrations and the weight and size of the CL increased from Day 2 to 6, indicating normal CL development. Plasma membranes from individual CL were evaluated for PGF2 alpha receptor concentration and affinity by Scatchard analysis. CL from each of the 4 days of the estrous cycle were not different with respect to PGF2 alpha receptor concentration (number per microgram of plasma membrane protein) and affinity. To examine tissue specificity, PGF2 alpha binding was evaluated in 12 organs or tissues. High-affinity PGF2 alpha receptors were found in the CL and adrenal medulla but not in granulosa cells or other tissues. In conclusion, a single class of high-affinity PGF2 alpha receptors was present within the bovine CL by 2 days after ovulation; therefore the reported lack of responsiveness to PGF2 alpha in the early CL was not attributable to a deficiency of high-affinity PGF2 alpha receptors.

Selection of the dominant follicle in cattle: establishment of follicle deviation in less than 8 hours through depression of FSH concentrations

Deviation in follicle diameter in cattle is characterized by continued growth of the largest follicle of a follicular wave and a reduction or cessation of growth of the smaller follicles. Deviation begins when the largest follicle reaches about 8.5 mm. Two experiments were done to test the hypothesis that the deviation mechanism is established in < 8 h, as indicated by the temporal relationships between follicle removal and an increase in FSH concentrations (Experiment 1) and between a decrease in FSH concentrations and follicle inhibition (Experiment 2). In Experiment 1, the role of the first follicle to reach 8.5 mm was studied by follicle ablation (Hour 0). The combined mean FSH concentrations for the control group (n = 8) and ablation group before ablation (n = 7) progressively decreased (P < 0.02) over two 8-h intervals before the largest follicle reached > or = 8.5 mm (Hour-16, 1.77 +/- 0.11 ng/mL; Hour 0, 1.49 +/- 0.08 ng/mL). In controls, the concentrations continued to decrease (P < 0.02) until Hour 10 (1.21 +/- 0.09 ng/mL). Ablation of the largest follicle at > or = 8.5 mm resulted in increased (P < 0.02) circulating FSH concentrations between Hours 5 (1.34 +/- 0.04 ng/mL) and 8 (1.61 +/- 0.09 ng/mL). Growth rate of the second-largest follicle between Hours 0 and 8 was greater (P < 0.05) in the ablation group than in the controls, and the second largest follicle became dominant in 7 of 7 heifers following ablation of the largest follicle. In Experiment 2, a minimal single injection of a depressant of FSH concentrations (4.4 mL of steroid-reduced follicular fluid) was given when the largest follicle was a mean of 8.4 mm (Hour 0; controls, n = 4; treated, n = 4). An interaction of group and hour (P < 0.005) for FSH concentrations was attributable to an FSH decrease (P < 0.002) by Hour 6 and an increase (P < 0.002) between Hours 9 and 12 in the treated group. The growth rate of the largest follicle between Hours 0 and 12 was less (P < 0.05) in the treated group (0.2 +/- 0.2 mm/12 h) than in the control group (1.2 +/- 0.4 mm/12 h). The reduced diameter was recorded within 6 h after suppression of FSH concentrations, supporting the hypothesis. Our preferred interpretation is that when the largest follicle reaches a critical diameter of about > or = 8.5 mm, FSH concentrations continue to decrease and become lower than required by the smaller follicles but not the largest follicle. The results further indicate that a close temporal coupling between a change in FSH concentrations and the follicular response could establish the deviation mechanism in < 8 h or before the second largest follicle reaches a similar critical diameter.

Effects of time of insemination relative to ovulation on pregnancy rate and embryonic-loss rate in mares

The effects of pre-ovulatory and post ovulatory insemination on pregnancy rate and embryonic-loss rate were studied in 268 mares in two experiments. Within each experiment mares were randomised within replicates as follows: to be inseminated on the day the pre-ovulatory follicle reached 35 mm (pre-ovulatory group), to be inseminated on the day of ovulation (Day 0 group), and to be inseminated on the day after ovulation (Day 1 group). Ultrasonic pregnancy diagnoses were performed on Days 11, 12, 13 and 14 (Experiment 1) and Days 11, 12, 13, 14, 15, 20 and 40 (Experiment 2). Combined for the two experiments, pregnancy rates were different (P less than 0.01) among the three groups. For Experiment 2, pregnancy rate within the pre-ovulatory group was lower (P less than 0.05) for insemination 4 days or more before ovulation than for up to 3 days before ovulation. Pregnancy rate was lower (P less than 0.05) for the Day 0 group than for the pre-ovulatory group inseminated up to 3 days before ovulation. In Experiment 2, ovulation was detected by examinations every 6 h; pregnancy rate was greater (P less than 0.05) for mares inseminated 0 to 6 h after ovulation than for mares inseminated at 18 to 24 h. No pregnancies occurred when mares were inseminated 30 h or more after ovulation. The mean day of first detection of the embryonic vesicle was different (P less than 0.0001) among the three groups. Diameter of embryonic vesicle averaged over Days 11 to 15 also differed (P less than 0.0001) among groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Follicle selection in cattle: role of luteinizing hormone

The circulating concentrations of LH were reduced by administration of 50 mg of progesterone every 8 h for 72 h, beginning when the largest follicle was 6.0 mm (experiment 1; n = 10). Progesterone treatment prevented the transient increase in LH that accompanied deviation (partitioning into dominant and subordinate categories) in control heifers (n = 10). The reduced LH concentrations were associated with reduced growth of the largest follicle, beginning a mean of 31 h after deviation, but did not alter the time of deviation or the growth and regression of the second-largest follicle. In experiment 2, 0 mg (controls) or 50 mg of progesterone was given every 8 h for three injections, beginning when the largest follicle was 7.0 mm (predeviation group) or 9.0 mm (postdeviation group; n = 8 for each of the four groups). Blood samples from the jugular vein and follicular-fluid samples from the two largest follicles were taken 8 h after the last treatment when the largest follicle was a mean of 8.7 mm in the predeviation group and 10.8 mm in the postdeviation group. In the controls, follicular-fluid concentrations of estradiol and free insulin-like growth factor (IGF)-1 in the largest follicle and IGF binding protein (IGFBP)-2 in the second-largest follicle were higher (P: < 0.05) in the postdeviation group than in the predeviation group. Progesterone treatment lowered (P: < 0.006) the circulating LH concentrations to a similar extent in both groups. In the predeviation group, progesterone treatment did not have a significant effect on any of the characteristics of the largest follicle. In the postdeviation group, the largest follicle of the progesterone-treated heifers had significant reductions in diameter and in follicular-fluid concentrations of estradiol and free IGF-1. Follicular-fluid concentrations of immunoreactive inhibin were not different for any of the comparisons. The results supported the hypothesis that LH has a positive effect on diameter of the largest follicle but not until after the beginning of diameter deviation. In addition, the results indicated that LH is involved in the production of estradiol by the largest follicle and that free IGF-1 concentrations increase in the largest follicle during deviation.

Embryonic loss in mares: Incidence and ultrasonic morphology

Pregnancy was determined by ultrasound on Days 11, 15, 20, 25, 30, 35, 40, 45, and 50 in 154 ponies and 27 horses. In ponies, the embryonic loss rate for Days 11 to 15 (28 154 , 18.2%) was greater (P<0.01) than for any of the subsequent five-day intervals (0% to 3.3%). There were no losses during Days 11 to 15 in horses (0 27 ), and the difference between ponies and horses was significant. The loss rates for the seven periods encompassing Days 15 to 50 were not significantly different among periods. Pseudopregnancy occurred more frequently (P<0.01) following embryonic loss after Day 20 (Days 11 to 15, 26%; Days 15 to 20, 33%; after Day 20, 100%). Embryonic vesicles that were lost during Days 11 to 15 were smaller on the average than control vesicles. However, most of the vesicles grew at an apparently normal rate. Two of five vesicles that were lost between Days 15 and 20 and three of four that were lost between Days 20 and 25 were undersized during preceding examinations. Undersized vesicles were found during 13 415 (3%) examinations during Days 11 to 20 in mares that maintained the embryo and in 21 106 (20%) in mares that lost the embryo. Embryonic vesicles that were lost during Days 11 to 15 usually disappeared without previous indications, except in three mares in which the vesicle was floating in a small collection of fluid. Ultrasonic indications of impending loss at later stages included failure of fixation, an echogenic ring (vesicle) or mass floating in a collection of fluid, an echogenic area in the dead embryo, absence of heart beat, and a gradual decrease in volume of placental fluids with disorganization of the placental membranes. The solid remnants and at least some of the fluids resulting from late embryonic and early fetal death were retained sometimes for weeks or months until the debris was apparently expelled through an open cervix.

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 cattle: dynamics of follicular fluid factors during development of follicle dominance

Follicle diameter deviation during follicular waves in cattle begins with a reduction in growth rates of developing subordinate follicles, in contrast to the maintenance of a constant growth rate by a developing dominant follicle. In experiment 1, the temporal changes encompassing deviation in concentrations of follicular fluid factors relative to one another in the three largest follicles (F1, F2, and F3) were studied. Follicular fluid samples were collected when F1 reached diameter ranges of 7.0-7.9, 8.0-8.9, 9.0-9.9, and 10.0-10.9 mm (n = 12 per range). The first increase (P < 0.05) in the difference between F1 and F2 for estradiol occurred at the 8.0- to 8.9-mm range, which was one range earlier than for diameter (P < 0.05). Free insulin-like growth factor (IGF)-1 concentrations in F1 were similar among diameter ranges, but concentrations in F1 were higher (P < 0.05) than in F2 for each range except 7.0-7.9 mm. Concentrations of free IGF-1 in F2 decreased (P < 0.05). No significant differences were detected in concentrations of progesterone, androstenedione, total inhibin, and inhibin-A. Averaged over follicles, inhibin-B decreased (P < 0.05) between the 8.0- to 8.9- and 10.0- to 10.9-mm ranges, and activin-A increased (P < 0.05) between the 7.0- to 7.9- and 9.0- to 9.9-mm ranges. However, no differences were found among follicles. In experiment 2, changes associated with the development of dominance by F2 were studied using ablation of F1 at the beginning of expected deviation (F1, 8.5 mm; Hour 0) as the reference point. Follicular fluid factors were compared at Hour 12 between F2 of a control group (F1 intact; n = 10) and an ablated group (F1 ablated; n = 10). Diameter (P < 0.02), estradiol (P < 0.001), free IGF-1 (P < 0.002), and progesterone (P < 0.003) were greater and IGF-binding protein-2 was lower (P < 0.01) in F2 of the ablated group at Hour 12. No differences were detected in concentrations of androstenedione, total inhibin, and inhibin-A. The results of the two experiments indicated, on a temporal basis, that intrafollicular changes in estradiol and the IGF system, but not in the inhibin/activin system, could account for a reported greater FSH responsiveness by the future dominant follicle than by the future subordinate follicles by the beginning of diameter deviation in cattle.

Alterations in follicular estradiol and gonadotropin receptors during development of bovine antral follicles

It was hypothesized that growth divergence of dominant and subordinate follicles during Wave 1 and growth termination of the dominant follicle would be associated with changes in the number of gonadotropin receptors on granulosa cells and estradiol in follicular fluid. To test this hypothesis, follicular development of 16 Holstein heifers was monitored by ultrasound, and follicles were collected on Days 2,4,6 and 10 (Day 0 = ovulation). Dominant follicles were compared across days, whereas dominant and largest subordinate follicles were compared on Days 2 and 4 only. The numbers of LH and FSH receptors on the granulosa cells of dominant follicles did not differ significantly over Days 2, 4, 6 and 10. In contrast, concentrations of estradiol in follicular fluid decreased (P < 0.05) from Days 2 to 10 (373 +/- 150 to 42 +/- 12 ng/ml) and concentrations of progesterone in follicular fluid increased (P < 0.05) from Days 2 to 10 (12.2 +/- 2.3 to 24.4 +/- 4.8 ng/ml). Correspondingly, the ratio of estradiol:progesterone in the dominant follicles decreased (P < 0.05) from Days 2 to 10. Comparisons between dominant and subordinate follicles indicated greater (P < 0.05) estradiol concentrations in the dominant follicle on Day 2, but the number of gonadotropin receptors was not different until Day 4. Thus, differences in concentrations of follicular fluid estradiol, but not numbers of granulosa cell gonadotropin receptors, were associated with the early growth divergence of dominant and subordinate follicles (Day 2) and the eventual growth termination of the dominant follicle (Day 10). Late divergence (Day 4) was associated with higher gonadotropin receptor numbers and follicular estradiol concentrations in the dominant than in the subordinate follicles. These results indicate that an increase in estradiol productivity of the selected dominant follicle occurred before an increase in the number of gonadotropin receptors.

Selection of the dominant follicle in cattle: role of estradiol

Involvement of estradiol in the deviation in growth rates between the two largest follicles of a wave was studied in 39 heifers. In experiment 1, the largest follicle remained intact in a control group and was ablated in five estradiol-treated groups when the largest follicle reached 8.5 mm or larger (expected beginning of deviation; Hour 0). The ablation groups were given a single injection of 0, 0.004, 0.02, 0.1, or 0.5 mg of estradiol. Blood samples were taken from a jugular vein every hour at Hours 0 to 16. By Hour 8, FSH concentrations were greater (P < 0.05) in the ablation group that received 0 mg of estradiol than in the controls. Among the estradiol groups, that receiving 0.02 mg had the lowest detectable increase in estradiol. In this group, FSH concentrations were not suppressed below the control concentrations, but the increase in FSH concentrations following ablation of the largest follicle was delayed for 2 or 3 h. This delay in the increase of FSH concentrations corresponded to the hours that estradiol was maximal. In experiment 2, blood samples were taken every 4 h from the caudal vena cava cranial to the junction with the ovarian veins in heifers with the largest follicle intact (controls) or ablated at 8.5 mm or larger (Hour 0). Averaged over Hours 4 to 48, estradiol concentrations were higher (P < 0.04) in the controls than in the ablation group. During Hours 0 to 12, estradiol concentrations increased (P < 0.05) in the controls, whereas FSH concentrations decreased (P < 0.05). In the ablation group, estradiol concentrations were lower than in the controls by Hour 4, and FSH concentrations increased (P < 0.05) between Hours 4 and 12. These results support the hypothesis that the largest follicle releases increased estradiol into the blood at the beginning of follicular deviation, and that the released estradiol is involved in the continuing depression of FSH concentrations to below the requirement of the smaller follicles.

Role of luteinizing hormone in follicle deviation based on manipulating progesterone concentrations in mares

The effects of several doses of progesterone on FSH and LH concentrations were used to study the role of the gonadotropins on deviation in growth rates of the two largest follicles during the establishment of follicle dominance. Progesterone was given to pony mares at a daily dose rate of 0 mg (controls), 30 mg (low dose), 100 mg (intermediate dose), and 300 mg (high dose). All follicles > or = 6 mm were ablated at Day 10 (Day 0 = ovulation) to initiate a new follicular wave; prostaglandin F(2alpha) was given to induce luteolysis, and progesterone was given from Days 10 to 24. The low dose did not significantly alter any of the ovarian or gonadotropin end points. The high dose reduced (P < 0.05) the ablation-induced FSH concentrations on Day 11. Maximum diameter of the largest follicle (17.2 +/- 0.6 mm) and the second-largest follicle (15.5 +/- 0.9 mm) in the high-dose group was less (P < 0.04) than the diameter of the second-largest follicle in the controls (20.0 +/- 1.0 mm) at the beginning of deviation (Day 16.7 +/- 0.4). Thus, the growth of the two largest follicles was reduced by the high dose, presumably through depression of FSH, so that the follicles did not attain a diameter characteristic of deviation in the controls. The intermediate dose did not affect FSH concentrations. However, the LH concentrations increased in the control, low, and intermediate groups, but then decreased (P < 0.05) in the intermediate group to pretreatment levels. The LH decrease in the intermediate group occurred 2 days before deviation in the controls. The maximum diameter of the largest follicle was less (P < 0.0001) in the intermediate group (27.3 +/- 1.8 mm) than in the controls (38.9 +/- 1.5 mm), but the maximum diameter of the second-largest follicle was not different between the two groups (19.0 +/- 1.1 vs. 20.3 +/- 1.0 mm). Thus, the onset of deviation, as assessed by the second-largest follicle, was not delayed by the decrease in LH. Diameter of the largest follicle by Day 18 in the intermediate group (23.1 +/- 1.6 mm) was less (P < 0.05) than in the controls (28.0 +/- 1.0 mm). These results suggest that circulating LH was not involved in the initiation of dominance (inhibition of other follicles by the largest follicle) but was required for the continued growth of the largest follicle after or concurrently with its initial expression of dominance.

Morphological characterization of follicle deviation in Nelore (Bos indicus) heifers and cows

Follicle diameter deviation is defined as the beginning of the differential change in growth rates between the largest and next largest follicles subsequent to wave emergence and is considered a key component of follicle selection. Follicle selection has been extensively studied in European breeds of cattle (Bos taurus) but has not been critically studied in Zebu breeds (Bos indicus). The objectives of the present study were to determine and compare the morphological characteristics of deviation associated with the first post-ovulatory wave (Wave 1) of the estrous cycle in Nelore heifers (n=8) and nonlactating cows (n=11). Beginning on the day of ovulation (day 0), the three largest follicles (F1-F3, respectively) were individually tracked every 12 h for 6d using transrectal ultrasonography. In individual animals, deviation was determined graphically using visual inspection of the diameter profiles of F1, F2 and sometimes F3 (observed deviation) and mathematically using segmented regression analysis of the diameter differences between F1 and F2 or sometimes F3 (calculated deviation). Mean day of emergence of Wave 1 when F1 reached >3 mm (approximately 1 d after ovulation) and growth rate of F1 during deviation (approximately 1.4 mm/d) were not significantly different between heifers and cows. The results of determining the beginning of deviation within heifers and cows using the observed and calculated methods were not significantly different. Averaged over both methods, diameter deviation occurred 2.8 d after ovulation when F1 reached 5.7 mm in heifers, and 2.4 d after ovulation when F1 reached 6.1 mm in cows. In conclusion, the emergence of Wave 1 and growth rates and diameters of the future dominant follicles at the beginning of deviation were similar in Nelore heifers and nonlactating cows, regardless of the methods used to determine deviation. Relative to Holstein cattle, emergence of Wave 1 appeared to occur about 1 d later and diameter of the future dominant follicle at the beginning of deviation was about 2 mm smaller in Nelore.

Pulsatility of systemic FSH and LH concentrations during follicular-wave development in cattle

Changes in systemic FSH and LH pulsatility in temporal association with follicular-wave emergence and follicle deviation were studied in cattle. Wave emergence was defined as occurring when the future dominant follicle first reached 4 mm, as established retrospectively. Follicle deviation was defined as the beginning of a change in growth rates between the 2 largest follicles and occurred 60.4 +/- 4.2 h after wave emergence. Follicles were tracked by transrectal ultrasound scanning every 8 h, and blood samples for pulse characterization were collected every 20 min from before emergence until after deviation. Pulses were characterized by the Pulsar program applied to each 8-h increment, centered on the hour of follicle scanning in each heifer (n = 6). Pulsatility of FSH was not detected for any of the 8-h increments. The mean FSH concentrations for the 24 samples per 8 h increased (P < 0.05) between 8 h before and 8 h after wave emergence, followed by a decrease 40 to 16 h before deviation. The low mean values continued for 24 h after deviation. Pulses of LH were detected for all 8-h increments. The LH mean of all concentrations per 8 h and pulse frequency increased (P < 0.05) between the hour of wave emergence and 32 h after emergence and then pulse frequency plateaued at a mean interpeak interval of 1.3 h. Increased LH means for all concentrations per 8 h and basal concentration were reached 32 h before deviation. The results indicated that elevated concentrations of LH and reduced concentrations of FSH were present 32 to 16 h before to at least 24 h after the beginning of follicle deviation. However, an abrupt, short-term change in FSH concentrations or in LH pulsatility in close temporal association with follicle deviation that could act as an acute stimulus to initiate deviation was not found.

Follicle Selection in Cattle: Relationships among Growth Rate, Diameter Ranking, and Capacity for Dominance

Follicles of wave 1 were designated F1, F2, and so forth, according to descending diameter at the expected (F1, > or =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.

Superovulatory response following ablation-induced follicular wave emergence at random stages of the oestrous cycle in cattle

Based on the premise that superovulation in cattle is optimal when superstimulation is initiated at the time of follicular wave emergence, the present study was done in beef heifers to determine if the superovulatory response following a single bolus of gonadotrophin treatment after follicle ablation (induced wave) at random stages of the oestrous cycle is comparable to the same gonadotrophin treatment at mid-dioestrus (spontaneous wave). In Experiment 1, heifers were assigned to nonablation (n = 18) and ablation (n = 20) groups. In nonablated heifers, superstimulatory treatment was given as a single subcutaneous injection (Folltropin-V, 400 mg) at mid-di-oestrus to coincide with emergence of the spontaneous follicular wave 8 to 12 days after oestrus. In ablated heifers, the same superstimulatory treatment was given 1 day after ablation of all follicles > or = 5 mm at random stages of the oestrous cycle to coincide with emergence of the ablation-induced wave. In both the nonablation and ablation groups, PGF2 alpha (Estrumate, 500 micrograms) was given 48 h after the superstimulatory treatment and artificial insemination was done 60 and 72 h later. Reproductive tracts were collected at the time of slaughter 6 or 7 days after insemination. Observations made in Experiment 1, indicated that some ablated heifers had only partial luteal regression at the time of insemination, while some others exhibited behavioral oestrus as early as 24 h after PGF2 alpha treatment. The design was amended in Experiment 2 to address these problems. Heifers were assigned to nonablation (n = 17), ablation-alone (n = 20) or ablation plus progestogen (n = 20) groups. Follicle ablation, superstimulatory treatment, artificial insemination and collection of reproductive tracts were done as in Experiment 1. However, all heifers were given two doses of PGF2 alpha (500 micrograms/dose) 48 and 60 h after superstimulatory treatment to ensure complete luteal regression, and heifers in the ablation plus progestogen group received a norgestomet ear implant at the time of follicle ablation to prevent early ovulations. The implant was removed at the time of the second PGF2 alpha treatment. In Experiments 1 and 2, the means for the ovarian and superovulatory responses were not significantly different between groups. Averaged over the nonablation and all ablation groups for Experiments 1 and 2, the mean number. of corpora lutea, fertilized ova and transferable embryos were 22.9 vs 18.6, 7.3 vs 7.8 and 5.4 vs 5.6, respectively. In summary, follicle ablation at random stages of the oestrous cycle followed by a single bolus of gonadotrophin treatment 1 day later resulted in a superovulatory response that was comparable to the same superstimulatory treatment administered around the time of spontaneous wave emergence at mid-dioestrus. The ablation/superstimulation method described herein offers the advantage of initiating superstimulatory treatment forthwith and assuring that treatment is concomitant with wave emergence to achieve an optimal superovulatory response. Moreover, the full extent of the oestrous cycle is available for superstimulation and the need for detecting oestrus or ovulation and waiting 8 to 12 days to initiate treatment is eliminated.

In vivo effects of an intrafollicular injection of insulin-like growth factor 1 on the mechanism of follicle deviation in heifers and mares

In cattle and mares, free insulin-like growth factor 1 (IGF-1) is higher in the future dominant follicle (F1) than in the future largest subordinate follicle (F2) before deviation in diameter or selection is manifested between the two follicles. The effect of IGF-1 on other follicular-fluid factors and on the destiny of F2 were studied in two experiments in each species, using a total of 40 heifers and 42 mares. An injection of IGF-1 was made into F2 at the expected beginning of deviation (heifers, F1 >or= 8.5 mm; mares, F1 >or= 20.0 mm; Hour 0). In heifers, follicular fluid was taken from F2 at Hours 3, 6, 12, or 24; each heifer was sampled only once. In mares, sequential F2 samples were taken from each mare at Hours 0, 6, and 24 or at Hours 12 and 24. Transvaginal ultrasound guidance was used for treatment and sample collection. In heifers, IGF-1 treatment of F2 stimulated the secretion of estradiol (P < 0.05) between Hours 3 and 6 and androstenedione (P < 0.05) between Hours 3 and 12. In F2 of control heifers, estradiol decreased (P < 0.05) and androstenedione did not change significantly. In mares, IGF-1 treatment of F2 did not affect the concentrations of estradiol during the 24-h posttreatment period; androstenedione decreased (P < 0.04) in the IGF-1 group and increased (P < 0.006) in the controls. Compared with control mares, the IGF-1 group had higher (P < 0.04) activin-A at Hours 12 and 24 and higher (P < 0.0006) inhibin-A at Hour 24. After ablating F1 at Hour 24 in mares, F2 became dominant and ovulated in more mares (P < 0.0002) in the IGF-1 group (12/14) than in the control group (2/14). These results are consistent with reported temporal relationships among follicular factors during deviation in both species and indicate that IGF-1 plays a key role in controlling the temporal relationships; however, no indication was found that IGF-1 stimulated estradiol production in mares during the 24 h after treatment.

Controlling interrelationships of progesterone/LH and estradiol/LH in mares

The interrelationships of progesterone, estradiol, and LH were studied in mares (n=9), beginning at the first ovulation (Day 0) of an interovulatory interval. An increase in mean progesterone concentrations began on Day 0 and reached maximum on Day 6, with luteolysis beginning on Day 14. A common progesterone threshold concentration of about 2 ng/ml for a negative effect on LH occurred at the beginning and end of the luteal phase. Progesterone and LH concentrations decreased at a similar rate from Day 6 until the onset of luteolysis on Day 14, consistent with a decreasing positive effect of LH on progesterone. Concentrations of LH during the increase in the ovulatory surge consisted of two linear regression segments involving a rate of 0.4 ng/ml/day for Days 14-22 and 1.8 ng/ml/day for Day 22 to 1 day after the second ovulation. The end of the first segment and beginning of the second segment was 2 days before ovulation and was the day the ovulatory estradiol surge was at a peak.

Effect of GnRH treatment during the anovulatory season on multiple ovulation rate and on follicular development during the ensuing pregnancy in mares

Seasonally anovulatory mares were injected, i.m., twice daily with a GnRH analogue (GnRH-A), and hCG was given when the largest follicle reached 35 mm in diameter. In Exp. 1, treatment was initiated on 23 December when the largest follicle per mare was less than or equal to 17 mm. An ovulatory response (ovulation within 21 days) occurred in 17 of 30 (57%) GnRH-A-treated mares on a mean of 15.8 days. The shortest interval to ovulation in control mares (N = 10) was 57 days. The diameter of the largest follicle first increased significantly 6 days after start of treatment. In Exp. 2, treatment was begun on 15 January and mares were categorized according to the largest follicle at start of treatment. The proportion of mares ovulating within 21 days increased significantly according to initial diameter of largest follicle (less than or equal to 15 mm, 9/25 mares ovulated; 15-19 mm, 13/21; 20-24 mm, 20/24; greater than 25 mm, 3/3). The multiple ovulation rate was greater (P less than 0.01) for treated mares (27/86 mares had multiple ovulations) than for control mares (2/35). Treated mares in which the largest follicle at start of treatment was greater than or equal to 25 mm had a higher (P less than 0.01) multiple ovulation rate (9/14) than did mares in which the largest follicle was less than 25 mm (18/72). The pregnancy rate for single ovulators was not different between control mares (26/30 pregnant mares) and treated mares (43/54).(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the embryonic vesicle and progesterone in embryonic loss in mares

Characteristics of spontaneous embryonic loss in 21 mares were compared with those of 52 contemporary mares that maintained pregnancy. Embryonic losses were, in retrospect, grouped according to day of loss and length of the interovulatory interval, respectively, as follows: group 1, less than or equal to day 20 and less than or equal to 30 days (n = 10); group 2, less than or equal to day 20 and greater than 30 days (n = 3); and group 3, greater than day 20 and greater than 30 days (n = 8); ovulation was day 0. Mean diameter of the embryonic vesicle in group 1 was smaller (P less than 0.05) on days 12-18 than in the pregnancy-maintained group, but among the pregnancy-maintained group and the embryonic-loss groups, the mean individual growth rates of vesicles was similar (no significant difference). A more frequent (P less than 0.05) location of the vesicles in the uterine body on day 13 in group 1 was due to a greater proportion of small vesicles and for day 18 was due to a greater incidence of fixation failure. Luteal regression occurred at the expected time in 77% of the mares with loss sooner than day 20. Low concentration of progesterone on days 12, 15 and 18, a detected decrease in diameter of the corpus luteum on days 15 and 18, and an interovulatory interval of less than or equal to 30 days indicated that luteolysis was not prevented by the embryonic vesicle in group 1.(ABSTRACT TRUNCATED AT 250 WORDS)