Ovarian follicles, ovulations and progesterone concentrations in aged versus young mares

The objectives of this study were: 1) to document age-related ovulation failure in mares and 2) to contrast the number of ovarian follicles, occurrence of ovulations, and postovulatory concentrations of progesterone in aged versus young mares. In Experiment 1, 4 of 10 aged (25- to 33-years-old) mares were anovulatory between July 1 and September 1, 1989. In Experiment 2, two of 25 aged (20- to 30-years-old) and none of 21 young (3- to 12-years-old) mares were anovulatory between February 1 and June 30, 1990. The average (+/- SEM) day of the first ovulation was later (P<0.05) for aged versus young mares (May 9 +/- 7.1 vs April 25 +/- 7.4 days, respectively). There tended (P<0.10) to be fewer 11- to 20-mm ovarian follicles in aged versus young mares (2.8 +/- 0.2 vs 5.3 +/- 0.1, respectively), but there was no difference (P>0.10) in the total number of ovarian follicles in aged versus young mares (21.0 +/- 0.3 vs 26.1 +/- 0.2, respectively) during the pooled periovulatory period of the first and second (single) ovulations. The number of ovulatory cycles during the study period was less (P=0.01) for aged versus young mares (2.2 +/- 0.3 vs 3.2 +/- 0.3). Plasma progesterone concentrations on Days 10 and 15 of the first ovulatory cycle were higher (P<0.05) in aged versus young mares.     

Regulation of circulating gonadotropins by the negative effects of ovarian hormones in mares

The functional and temporal relationships between circulating gonadotropins and ovarian hormones in mares during Days 7-27 (ovulation = Day 0) was studied using control, follicle ablation, and ovariectomy groups (n = 6 mares/group). In the follicle-ablation group, all follicles > or = 6 mm were ablated on Day 7, and every 2 days thereafter, newly emerging follicles were also ablated. Estradiol concentrations decreased (P < 0.01) similarly in the controls and the follicle-ablation group between Days 7 and 11 and by Day 15 began to increase in the controls and continued to decrease in the follicle-ablation group. Concentrations of progesterone were not affected by follicle ablation, but diameter of the corpus luteum was greater (P < 0.05) by Day 21 in the follicle-ablation group; these results indicated that the follicles were involved in morphologic luteolysis, but not in functional luteolysis. Concentrations of LH were higher (P < 0.05) on Days 15 and 16 in the follicle-ablation group than in the controls, indicating an initial negative effect of follicles on LH. Immunoreactive inhibin and estradiol decreased (P < 0.0001) and FSH and LH increased (P < 0.05) within 1 or 2 days after ovariectomy; these changes occurred more slowly in the follicle-ablation group. The maximum value for an FSH surge in each control mare was below the lower 95% confidence limit in the ovariectomy group. Maximum concentration for the periovulatory LH surge in the controls was not different from the mean maximum LH concentrations in the ovariectomy group. Our interpretation is that the gonadotropin surges resulted from changes in the magnitude of the negative effects of ovarian hormones on the positive effects of extraovarian control. There was no indication of a positive ovarian effect on either FSH or LH.     

Effect of treatment with progesterone and oestradiol when starting treatment with an intravaginal progesterone releasing insert on ovarian follicular development and hormonal concentrations in Holstein cows

Ovarian follicular development and concentrations of gonadotrophin and steroid hormones were studied in non-lactating Holstein cows following administration of progesterone (P(4)) or oestradiol benzoate (ODB) at the start of treatment with an intravaginal progesterone releasing insert (IVP(4)) in a 2 by 2 factorial experiment. Cows were treated at random stages of the oestrous cycle with an IVP(4) device (Day 0) and either no other treatment (n=8), 200 mg of P(4) IM (n=9), 2.0 mg of ODB IM (n=8) or both P(4) and ODB (n=9). Seven days later devices were removed and PGF(2alpha) was administered. Twenty-four hours later 1.0mg of ODB was administered IM. Oestrus was detected in 97.1% and ovulation in 64.7% (effect of treatment, P=0.41) of cows within 96 h of removing inserts. In the cows that ovulated, day of emergence of the ovulatory follicle was delayed (P<0.01) and more precise (P<0.05) in cows treated with ODB compared to the cows treated with P(4). Interval from wave emergence to ovulation and the diameter of the ovulatory follicle was less (P<0.05) in cows treated with ODB compared to cows treated with P(4). Combined treatment with P(4) and ODB at the time of starting treatment with an IVP(4) device did not significantly change the pattern of ovarian follicular development compared to treatment with ODB alone. Concentrations of LH and FSH in plasma were less in cows treated with ODB between Days 0 and 4 (P<0.05) while treatment with P(4) increased concentrations of FSH in plasma between Days 0 and 4 (P<0.05). When anovulatory cows were compared to ovulatory cows, diameters of follicles (P<0.001) and growth rate of follicles (P<0.01) were less in anovulatory cows between Days 7 and 9, while concentrations of FSH in plasma were greater (P<0.01), concentrations of LH similar (P>0.90) and concentrations of oestradiol were less (P=0.01) in the anovulatory cows between Days 4 and 10. Our findings support a hypothesis that ovarian follicular development following administration of P(4) or ODB at the start of treatment with an IVP(4) device differs. Anovulatory oestrus may have been associated with reduced maturity and/or later emergence of ovarian follicles.     

Experimental assumption of dominance by a smaller follicle and associated hormonal changes in mares.

A two-follicle model was used to study the nature of selection of the dominant follicle in mares by ablating neither or one of the two follicles on the day the larger follicle reached >/= 20 mm (Day 0). The larger follicle became the dominant follicle in all mares in which both follicles (n = 8) or only the larger follicle (n = 10) was retained. When only the smaller follicle (n = 9) was retained, it became dominant and ovulated in six mares and became atretic in three mares; the difference in diameter between the two follicles on Day 0 was less (p < 0.01) in mares in which the retained smaller follicle grew and ovulated (2.2 +/- 0.6 mm) than in the mares in which the follicle became atretic (5.9 +/- 1.2 mm). A decline (p < 0. 0001) in FSH concentrations occurred over Days -4 (8.4 +/- 0.7 ng/ml) to 0 (5.9 +/- 0.3 ng/ml), averaged over all groups, and the decline continued for several more days in the groups with both follicles or with only the larger follicle retained. In the group with only the smaller follicle retained, compared to the group with both follicles retained, FSH concentrations and diameter of the smaller follicle increased between Days 0 and 1 (significant interaction for each end point). After Day 1, FSH concentrations continued to increase when the smaller retained follicle became atretic; concentrations decreased when the smaller retained follicle became dominant. An increase (p < 0.0001) in LH concentrations occurred over Days -4 (12.2 +/- 1.1 pg/ml) to 0 (21.1 +/- 2.0 pg/ml), averaged over the three groups. In 23 of 27 mares, a transient peak in LH concentrations occurred within 2 days of Day 0. In the groups with both follicles or with only the larger follicle retained, an increase (p < 0.0001) in systemic estradiol concentrations occurred between Day 0 (5.3 +/- 0.6 pg/ml) and Day 2 (7.5 +/- 0.4 pg/ml). When only the smaller follicle was retained, estradiol did not begin to increase until Day 2, and it increased only when the retained follicle grew and became dominant. The beginning of an increase in estradiol and continued decrease in FSH at the expected beginning of deviation were attributable to the future dominant follicle; there was no indication that the smaller follicle was involved.     

Critical role of insulin-like growth factor system in follicle selection and dominance in mares

The role of the insulin-like growth factor (IGF) system in the deviation in growth rates among follicles (follicle selection) was studied in mares using an IGF binding protein (BP) to reduce the follicular-fluid concentrations of IGFs. The future dominant follicle (F1) was treated by intrafollicular injection at the expected beginning of deviation (F1 > or = 20 mm; Day 0). The experimental groups were control (no injection, n = 8), vehicle (injection of vehicle; n = 6), and BP (injection of 250 microg of recombinant human IGFBP-3; n = 6). A sample of follicular fluid was taken from F1 on Day 1 in all groups. Compared with the control group, IGFBP-3 reduced (P < 0.05) the follicular-fluid concentration of free IGF-1 by 90%; lowered (P < 0.05) the concentrations of estradiol, activin-A, inhibin-A, and vascular endothelial growth factor; and increased (P < 0.05) the concentration of androstenedione. The diameter of F1 decreased and the diameter of F2 increased after Day 0 in the BP group, compared with the control and vehicle groups. A greater (P < 0.05) increase in circulating concentrations of FSH between Days 0 and 1 occurred in the BP group than in the other groups and accounted for the increased growth of F2. Dominance and ovulation from F1 occurred from fewer (P < 0.03) mares in the BP group (1 of 6) than from the control and vehicle groups combined (11 of 14); the remaining mares in the BP group ovulated from F2. Results indicated that the IGF system has a critical intrafollicular role in the differential changes in concentrations of follicular-fluid factors between the future dominant and subordinate follicles, leading to the development of follicle dominance (selection) and ovulation in mares.     

Interrelationships of estradiol, inhibin, and gonadotropins during follicle deviation in pony mares

The changes in circulating concentrations of FSH, LH, estradiol, and total inhibin associated with the beginning of follicle diameter deviation were compared among the last anovulatory follicular wave of the year and the first and second ovulatory waves in pony mares ( n=7 ). Follicle diameters and circulating hormone concentrations for each wave were normalized to the observed beginning of deviation (Day 0). Follicle deviation was demonstrated during the anovulatory wave as well as during the ovulatory waves, and the diameter of the future dominant follicle at the beginning of deviation was similar for the three waves (overall mean: 23.7+/-0.6 mm). Circulating estradiol concentrations did not increase during the last anovulatory wave but increased similarly for the two ovulatory waves, beginning near the onset of deviation. There were no differences among waves in concentrations of inhibin encompassing deviation. The FSH concentrations for the wave-stimulating FSH surge did not differ significantly among the three waves; combined for the three waves, concentrations decreased between Days -3 and 7. Circulating LH did not increase during the last anovulatory wave but increased during the first and second ovulatory waves beginning on Days 6 and -2, respectively. Results indicated that the increase in circulating estradiol at the beginning of deviation was not required for suppression of the wave-stimulating FSH surge and the initiation of deviation, based on an estradiol increase in association with deviation during the ovulatory waves but not during the anovulatory wave. Concentrations of inhibin were similar among waves and, therefore on a temporal basis, the similar suppression of FSH was attributable to inhibin. The later increase in LH before the first ovulation was not attributable to estradiol, based on the similarity between the two ovulatory waves in the increasing estradiol concentrations.     

Disruption of periovulatory FSH and LH surges during induced anovulation by an inhibitor of prostaglandin synthesis in mares

The role of passage of follicular fluid into the peritoneal cavity during ovulation in the transient disruption in the periovulatory FSH and LH surges was studied in ovulatory mares (n=7) and in mares with blockage of ovulation by treatment with an inhibitor of prostaglandin synthesis (n=8). Mares were pretreated with hCG when the largest follicle was ≥32 mm (Hour 0). Ultrasonic scanning was done at Hours 24 and 30 and every 2h thereafter until ovulation or ultrasonic signs of anovulation. Blood samples were collected at Hours 24, 30, 32, 34, 36, 38, 48, and 60. Ovulation in the ovulatory group occurred at Hours 38 (five mares), 40, and 44. Until Hour 36, diameter of the follicle and concentrations of FSH, LH, and estradiol-17β (estradiol) were similar between groups. Between Hours 34 and 36, a novel transient increase in estradiol occurred in each group, and color-Doppler signals of blood flow in the follicular wall decreased in the ovulatory group and increased in the anovulatory group. In each group, FSH and LH periovulatory surges were disrupted by a decrease or plateau between Hours 38 and 48 and an increase between Hours 48 and 60. The discharge of hormone-laden follicular fluid into the peritoneal cavity at ovulation was not an adequate sole explanation for the temporally associated transient depression in FSH and LH. Other routes from follicle to circulation for gonadotropin inhibitors played a role, based on similar depression in the ovulatory and anovulatory groups.     

Follicle suppression of circulating follicle-stimulating hormone and luteinizing hormone before versus after emergence of the ovulatory wave in mares

The effect of the ovarian follicles on plasma concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) before versus after the expected emergence of the ovulatory follicular wave was studied on Days 0 to 18 (Day 0 = ovulation) in four groups of mares (n = 6/group). In addition to a control group, all follicles ≥6 mm in diameter were ablated on Days 0.5, 6.5, or 12.5 in a herd of mares with reported emergence at 6 mm of the future ovulatory follicle on mean Day 10.5. Concentrations of FSH were not different between the Day-0.5 or Day-6.5 ablation groups and the corresponding controls. However, ablation on Day 12.5 resulted in an immediate FSH increase (group-by-day interaction, P < 0.003). For LH, ablation on Day 0.5 resulted in an interaction (P < 0.02), partially from lower (P < 0.05) concentrations on each of Days 15.5 to 18.0 than that in the controls, whereas ablation on Days 6.5 or 12.5 did not result in a significant group effect or interaction. Testosterone concentration, but not progesterone or estradiol concentration, was lower (P < 0.04) on Day 2 in the Day-0.5 ablation group than that in the controls. We inferred that follicles did not contain adequate FSH suppressors on Days 0.5 and 6.5 and that they were present only in the Day-12.5 ablation group or after the expected emergence of the ovulatory wave. The hypothesis of an association between low postovulatory concentrations of an ovarian steroid and low concentrations of LH after Day 15 was supported.     

Influence of ovaries and photoperiod on reproductive function in the mare.

A 16 h daily photoperiod hastened the onset of the ovulatory season (first ovulation); gonadotrophin and follicular changes prior to the onset were similar in intact light-treated and control mares. A preovulatory decline in FSH concentrations before the onset of the ovulatory season preceded the decrease in number of follicles (15--25 mm) and the rise in LH concentrations which was temporally associated with the growth of an ovulatory follicle. Seasonal changes of FSH and LH concentrations were found in ovariectomized mares and were influenced by photoperiod. During the anovulatory season, there was no ovarian influence on gonadotrophin concentrations. However, during the ovulatory season the ovaries exerted a positive influence on seasonally elevated LH concentrations during oestrus and a negative influence during dioestrus. The ovaries exerted a negative influence on seasonally elevated FSH concentrations throughout the oestrous cycle. The onset of the ovulatory season occurred at the time of the first sustained increase in LH concentrations resulting from positive seasonal (increasing photoperiod) and ovarian influences.     

Supplementation of equine early spring transitional follicles with luteinizing hormone stimulates follicle growth but does not restore steroidogenic activity

This study was conducted to test the hypothesis that supplementation of growing follicles with LH during the early spring transitional period would promote the development of steroidogenically active, dominant follicles with the ability to respond to an ovulatory dose of hCG. Mares during early transition were randomly assigned to receive a subovulatory dose of equine LH (in the form of a purified equine pituitary fraction) or saline (transitional control; n = 7 mares per group) following ablation of all follicles >15 mm. Treatments were administered intravenously every 12 h from the day the largest follicle of the post-ablation wave reached 20 mm until a follicle reached >32 mm, when an ovulatory dose of hCG (3000 IU) was given. Saline-treated mares during June and July were used as ovulatory controls. In a preliminary study, injection of this pituitary fraction (eLH) to anestrus mares was followed by an increase in circulating levels of LH (P < 0.01) but not FSH (P > 0.6). Administration of eLH during early transition stimulated the growth of the dominant follicle (Group x Day, P < 0.00001), which attained diameters similar to the dominant follicle in ovulatory controls (P > 0.1). In contrast, eLH had no effect on the diameter of the largest subordinate follicle or the number of follicles >10 mm during treatment (P > 0.3). The numbers of mares that ovulated in response to hCG in transitional control, transitional eLH and ovulatory control groups (2 of 2, 3 of 5 and 7 of 7, respectively) were not significantly different (P > 0.1). However, after hCG-induced ovulation, all transitional mares returned to an anovulatory state. Circulating estradiol levels increased during the experimental period in ovulatory controls but not in transitional eLH or transitional control groups (Group x Day, P = 0.013). In addition, although progesterone levels increased after ovulation in transitional control and transitional eLH groups, levels in these two groups were lower than in the ovulatory control group after ovulation (Group, P = 0.045). In conclusion, although LH supplementation of early transitional waves beginning after the largest follicle reached 20 mm promoted growth of ovulatory-size follicles, these follicles were developmentally deficient as indicated by their reduced steroidogenic activity.     

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.     

Treatment with recombinant equine follicle stimulating hormone (reFSH) followed by recombinant equine luteinizing hormone (reLH) increases embryo recovery in superovulated mares

The dynamics of ovarian follicular development depend on a timely interaction of gonadotropins and gonadal feedback in the mare. The development and efficacy of genetically cloned recombinant equine gonadotropins (reFSH and reLH) increase follicular activity and induce ovulation, respectively, but an optimum embryo recovery regimen in superovulated mares has not been established. The objective of this study was to determine if treatment with reFSH followed by reLH would increase the embryo per ovulation ratio and the number of embryos recovered after superovulation in mares. Sixteen estrous cycling mares of light horse breeds (4-12 years) were randomly assigned to one of two groups: Group 1; reFSH (0.65mg)/PBS (n=8) and Group 2; reFSH (0.65mg)/reLH (1.5mg) (n=8). On the day of a 22-25mm follicle post-ovulation mares were injected IV twice daily with reFSH for 3 days (PGF(2α) given IM on the second day of treatment) and once per day thereafter until a follicle or cohort of follicles reached 29mm after which either PBS or reLH was added and both groups injected IV twice daily until the presence of a 32mm follicles, when reFSH was discontinued. Thereafter, mares were injected three times daily IV with only PBS or reLH until a majority of follicles reached 35-38mm when treatment was discontinued. Mares were given hCG IV (2500IU) to induce ovulation and bred. Embryo recovery was performed on day 8 day post-treatment ovulation. Daily jugular blood samples were collected from the time of first ovulation until 8 days post-treatment ovulation. Blood samples were analyzed for LH, FSH, estradiol, progesterone and inhibin by validated RIA. Duration of treatment to a ≥35mm follicle(s) and number of ovulatory size follicles were similar between reFSH/reLH and reFSH/PBS treated mares. The number of ovulations was greater (P<0.01) in the reFSH/reLH group, while the number of anovulatory follicles was less (P<0.05) compared to the reFSH/PBS group. Number of total embryos recovered were greater in reFSH/reLH mares than in the reFSH/PBS mares (P≤0.01). The embryo per ovulation ratio tended to be greater (P=0.07) in the reFSH/reLH mares. Circulating concentrations of estradiol, inhibin, LH and progesterone were not statistically different between groups. Plasma concentrations of FSH were less (P<0.01) in the reFSH/reLH treated mares on days 0, 1, 4, 6, 7 and 8 post-treatment ovulation. In summary, reFSH with the addition of reLH, which is critical for final follicular and oocyte maturation, was effective in increasing the number of ovulations and embryos recovered, as well as reduce the number of anovulatory follicles, making this a more viable option than treatment with reFSH alone. Further evaluation is needed to determine the dose and regimen of reFSH/reLH to significantly increase the embryo per ovulation ratio.     

Continuous administration of low-dose GnRH in mares II. Pituitary and ovarian responses to uninterrupted treatment beginning near the autumnal equinox and continuing throughout the anovulatory season

We tested the hypothesis that continuous subcutaneous treatment with low-dose GnRH, administered to mares from late September/early October through March, would prevent the development of seasonal anovulation. Quarter Horse mares (n=20) were stratified by age and body condition score and assigned randomly to either a saline control (n=9) or a GnRH (n=11) treatment group. Gonadotropin-releasing hormone was delivered continuously via osmotic minipumps, with sham pumps placed in control mares. Initial pumps were inserted on Day 3 following ovulation or during the follicular phase if the next anticipated ovulation did not occur by 9 October. Delivery rate of GnRH was 2.5 microg/h (60 microg/day) for the first 60 days, followed by 5.0 microg/h (120 microg/day) thereafter. Pumps were replaced every 30 days. Eighty and 100% of all mares had become anovulatory by 1 November and 1 December, respectively, and remained anovulatory through the end of February. Neither serum concentrations of LH throughout the study nor total releasable pools of LH in March differed between groups. Although control mares that exhibited ovulatory cycles after study onset had greater (P<0.05) mean concentrations of LH during the follicular phase and metestrus compared to GnRH-treated mares, neither size of ovulatory follicles nor interovulatory intervals differed between groups. Serum concentrations of FSH were not affected by treatment, but were lowest (P<0.05) from November through January. Continuous infusion of low-dose GnRH, beginning soon after autumnal equinox and continuing until just after vernal equinox, failed to prevent the occurrence of or to hasten transition from seasonal anovulation.     

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.     

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.     

Circulating concentrations of inhibin-related proteins during the ovulatory cycle of the domestic fowl (Gallus domesticus) and after induced cessation of egg laying

Circulating inhibin A, inhibin B, activin A, total immunoreactive inhibin alpha-subunit (ir-alpha inhibin), LH, FSH and progesterone concentrations were measured throughout the normal ovulatory cycle and after cessation of egg laying induced by feed restriction to investigate the potential involvement of inhibins and activins in the ovulatory cycle of the domestic hen. Plasma inhibin A varied significantly (P < 0.05) during the ovulatory cycle; the concentration was highest at the preovulatory LH surge and reached a nadir 10 h later, at about the time the F(2) follicle makes the transition to become the new F(1) follicle. Plasma FSH concentrations did not change significantly throughout the cycle and showed no correlation with inhibin A. Total ir-alpha inhibin concentrations were much higher than those of inhibin A at all stages of the ovulatory cycle and showed no correlation with inhibin A or FSH. Plasma concentrations of inhibin B and of activin A were below the detection limit of the assays in all plasma samples analysed. In the feed restriction study, plasma inhibin A and total ir-alpha inhibin showed little change until the last day of oviposition (day 0) after which they fell significantly (P < 0.05) and remained low to the end of the experiment (approximately 70-78% decrease relative to day -4). Conversely, plasma FSH increased after cessation of laying and was significantly higher (P < 0.05) from day 3 to the end of the study (approximately 50% increase on day 6 relative to day -4). Plasma FSH values were negatively correlated with inhibin A (r = -0.39; P < 0.005) and total ir-alpha inhibin (r = -0.36; P < 0.005). Plasma LH and progesterone also decreased (P < 0.05) during feed restriction. The decrease in LH preceded the terminal oviposition and the associated fall in inhibin A by 2 days; there was a positive correlation between LH and inhibin A (r = 0.35; P < 0.005). Taken together these findings support (i) a role for LH in promoting inhibin A secretion by preovulatory follicles and (ii) an endocrine role for inhibin A secreted by preovulatory follicles in the maintenance of tonic FSH secretion in laying hens.     

Exercise affects both ovarian follicular dynamics and hormone concentrations in mares

The objectives were to evaluate the effects of exercise on ovarian folliculogenesis and related hormones in mares. Mares (n = 11) were randomly assigned into a control (non-exercised) or treatment (exercised) group. Treatment mares (n = 5) were moderately exercised for 30 min, 6 d/wk. All mares underwent daily transrectal ultrasonographic examinations and ovarian follicles > 6 mm were measured. Blood samples were collected during the first (Cycle 1) and last (Cycle 4) cycle, and serum concentrations of cortisol, LH, and FSH were determined. Mean cortisol concentrations were elevated (P < 0.05) in exercised mares, 6.29 ± 0.22 compared with 5.62 ± 0.16 ng/dL (mean ± SEM), 30 min post exercise. There were no significant differences between groups in mean FSH concentrations; however, exercised mares had lower (17.3 ± 6.4 vs 41.1 ± 5.5 ng/mL; P < 0.05) peak LH concentrations. Furthermore, exercised mares experienced a longer (24.7 ± 0.8 vs 22.2 ± 0.8 d; P < 0.05) mean interovulatory interval for all cycles combined, fewer (P < 0.05) follicles 6 to 20 mm in diameter, and an increased (P < 0.05) number of follicles >20 mm following deviation. The dominant and largest subordinate follicle in exercised mares had a greater (P < 0.05) mean diameter on the day of deviation, suggesting delayed deviation. Exercised mares also tended (P = 0.06) to have an increased number of cycles with at least two dominant follicles compared to control (62 vs 36%, respectively), indicating a decreased ability of the largest follicle to assert dominance. Under the conditions of this study, moderately exercising mares induced higher cortisol concentrations, lowered peak LH concentrations, and altered ovarian follicular dynamics.     

Follicle and systemic hormone interrelationships during spontaneous and ablation-induced ovulatory waves in mares

The characteristics of ovulatory follicular waves were studied for spontaneous waves and waves induced during the next estrous cycle by ovarian follicle ablations and administration of PGF2alpha 10 days after ovulation in 21 mares. In the induced group, both the days of the FSH surge and day of deviation were more synchronized, LH concentrations were greater before and after deviation, estradiol concentrations were greater after deviation, and the ovulatory follicle grew at a faster rate (3.4+/-0.2 compared with 2.7+/-0.1 mm/day). The frequency of two dominant follicles/wave was not different between induced waves (7 of 21) and spontaneous waves (9 of 21), but both dominant follicles ovulated more frequently in induced waves (6 of 7 waves compared with 0 of 9).     

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).     

The mare: a 1000-pound guinea pig for study of the ovulatory follicular wave in women

The mare is a good comparative model for study of ovarian follicles in women, owing to striking similarities in follicular waves and the mechanism for selection of a dominant follicle. Commonality in follicle dynamics between mares and women include: (1) a ratio of 2.2:1 (mare:woman) in diameter of the largest follicle at wave emergence when the wave-stimulating FSH surge reaches maximum, in diameter increase of the two largest follicles between emergence and the beginning of deviation between the future dominant and subordinate follicles, in diameter of each of the two largest follicles at the beginning of deviation, and in maximum diameter of the preovulatory follicle; (2) emergence of the future ovulatory follicle before the largest subordinate follicle; (3) a mean interval of 1 day between emergence of individual follicles of the wave; (4) percentage increase in diameter of follicles for the 3 days before deviation; (5) deviation 3 or 4 days after emergence; (6) 25% incidence of a major anovulatory follicular wave emerging before the ovulatory wave; (7) 40% incidence of a predeviation follicle preceding the ovulatory wave; (8) small but significant increase in estradiol and LH before deviation; (9) cooperative roles of FSH and insulin-like growth factor 1 and its proteases in the deviation process; (10) age-related effects on the follicles and oocytes; (11) approximate 37-hour interval between administration of hCG and ovulation; and (12) similar gray-scale and color-Doppler ultrasound changes in the preovulatory follicle. In conclusion, the mare may be the premier nonprimate model for study of follicle dynamics in women.