Mechanism controlling ovulation rate in ewes in relation to seasonal anoestrus

Regulation of folliculogenesis and the determination of ovulation rate in ruminants

The paper presents an update of our 1993 model of ovarian follicular development in ruminants, based on knowledge gained from the past 15 years of research. The model addresses the sequence of events from follicular formation in fetal life, through the successive waves of follicular growth and atresia, culminating with the emergence of ovulatory follicles during reproductive cycles. The original concept of five developmental classes of follicles, defined primarily by their responses to gonadotrophins, is retained: primordial, committed, gonadotrophin-responsive, gonadotrophin-dependent and ovulatory follicles. The updated model has more extensive integration of the morphological, molecular and cellular events during folliculogenesis with systemic events in the whole animal. It also incorporates knowledge on factors that influence oocyte quality and the critical roles of the oocyte in regulating follicular development and ovulation rate. The original hypothetical mechanisms determining ovulation rate are retained but with some refinements; the enhanced viability of gonadotrophin-dependent follicles and increases in the number of gonadotrophin-responsive follicles by increases in the throughput of follicles to this stage of growth. Finally, we reexamine how these two mechanisms, which are thought not to be mutually exclusive, appear to account for most of the known genetic and environmental effects on ovulation rate.

The conflict between hierarchical ovarian follicular development and superovulation treatment

In mammals with a low ovulation rate phenotype, ovarian follicular development is thought to be hierarchical with few, if any, antral follicles at similar stages of development. The hypothesis being tested herein was that if most follicles are in a functionally different state, then the application of exogenous hormones to increase ovulation rate will not overcome the hierarchical nature of follicular development. Using sheep as the experimental model, the functional states of all non-atretic antral follicles > or =2 mm diameter were assessed in individual ewes (N=10/group) during anoestrus with or without pregnant mare's serum gonadotrophin (PMSG) treatment, or after a standard superovulation regimen, or during the follicular phase of the oestrous cycle. The functional states of these follicles were assessed by measuring the FSH- or human chorionic gonadotrophin (hCG)-induced cAMP responses of granulosa cells in vitro. There were significant overall effects across the treatment groups on the responses of granulosa cells to either FSH or LH (both P<0.001). It was concluded that for anoestrous ewes with or without PMSG treatment, and ewes during the follicular phase, granulosa cell populations of many follicles (> or =2 mm diameter) did not share a similar cAMP response to FSH ( approximately 50% of follicles) or hCG (>90% of follicles) either on a per cell or total cell basis. After superovulation, < or =30 and 10% respectively of the granulosa cell populations shared similar responses to FSH and LH with regard to follicular diameter and cAMP output. Thus, exogenous hormone treatments used routinely for increasing oocyte yield do not effectively override the hierarchical pattern of ovarian follicular development during the follicular phase.

Comparative expression of luteinizing hormone and follicle-stimulating hormone receptors in ovarian follicles from high and low prolific sheep breeds

Expression of gonadotropin receptors and granulosa cell sensitivity to gonadotropin hormones by small (1-3 mm) and large (3.5-7 mm) follicles were compared in Romanov (ROM, ovulation rate = 3) and Ile-de-France (IF, ovulation rate = 1) ewes in the early and late follicular phase. In healthy follicles, LH receptor levels in granulosa cells increased with increasing follicular size (p < 0. 001) while FSH receptor levels decreased (p < 0.05). In granulosa cells of large follicles, LH receptor (LHR) mRNA levels were greater in the late than in the early follicular phase (p < 0.001, p < 0.05, for ROM and IF, respectively). In the early follicular phase, LHR levels in granulosa (p < 0.001) and theca cells (p < 0.05) of small follicles were greater in ROM than in IF ewes. FSH receptor mRNA levels in granulosa cells of small and large ROM follicles were greater than in the corresponding IF follicles (p < 0.05). Finally, a greater responsiveness (increase in cAMP secretion) to both FSH and hCG was observed by granulosa cells collected during the early follicular phase from ROM vs. IF ewes. Data provide evidence that the greater ovulation rate in the ROM as compared to the IF breed is associated with a greater gonadotropin responsiveness during the early follicular phase.

Factors affecting folliculogenesis in ruminants

This review addresses the reasons for the lack of progress in the control of superovulation and highlights the importance of understanding the mechanisms underlying follicular development. The present inability to provide large numbers of viable embryos from selected females still restricts genetic improvement, whilst variability in ovarian response to hormones limit the present capacity for increasing reproductive efficiency.

Females are born with a large store of eggs which rapidly declines as puberty approaches. If these oocytes are normal then there is scope for increasing the reproductive potential of selected females. Oocytes must reach a certain size before they can complete all stages of development and the final changes that occur late in follicular development. It is likely that oocytes that do not produce specific factors at precise stages of development will not be viable. Hence, it is important to characterize oocyte secreted factors since there are potential indicators of oocyte quality.

The mechanisms that determine ovulation rate have still not been fully elucidated. Indeed follicular atresia, the process whereby follicles regress, is still not known. A better understanding of these processes should prove pivotal for the synchronization of follicular growth, for more precise oestrous synchronization and improved superovulatory response.

Nutrition can influence a whole range of reproductive parameters however, the pathways through which nutrition acts have not been fully elucidated. Metabolic hormones, particularly insulin and IGFs, appear to interact with gonadotrophins at the level of the gonads. Certainly gonadotropins provide the primary drive for the growth of follicles in the later stages of development and both insulin and IGF-1, possibly IGF-2, synergize with gonadotrophins to stimulate cell proliferation and hormone production. More research is required to determine the effects of other growth factors and their interaction with gonadotropins.

There is evidence, particularly from studies with rodents, that steroids can also modulate follicular growth and development, although information is very limited for ruminants. There may be a rôle for oestrogens in synchronizing follicular waves, to aid in oestrous synchronization regimes and for removing the dominant follicle to achieve improved superovulatory responses. However more information is required to determine whether these are feasible approaches.

Heritability for litter size is higher in sheep than in cattle. Exogenous gonadotropins are a commercially ineffective means of inducing twinning in sheep and cattle. Although there are differences in circulating gonadotropin concentrations, the mechanism(s) responsible for the high ovulation appear to reside essentially within the ovaries. The locus of the Booroola gene, a major gene for ovulation rate, has been established but not specifically identified. However sheep possessing major genes do provide extremely valuable models for investigating the mechanisms controlling ovulation rate, including a direct contrast to mono-ovulatory species such as cattle.

In conclusion, the relationship between oocyte quality, in both healthy follicles and those follicles destined for atresia, must be resolved before the future potential for increasing embryo yield can be predicted. In addition, a greater understanding of the factors affecting folliculogenesis in ruminants should ensure that the full benefits ensuing from the precise control of ovarian function are achieved. The improved use of artificial insemination and embryo transfer that would ensue from a greater understanding of the processes of folliculo genesis, coupled with the new technologies of genome and linkage mapping, should ensure a more rapid rate of genetic gain.

Effects of season and phase of the estrous cycle on steroidogenesis and LH-FSH sensitivity of large ovine follicles perfused in vitro

The aims of this study were to compare steroïdogenesis (progesterone, androstenedione and estradiol production) and response to LH and FSH challenge by whole perifused follicles 4 to 5.5 mm in diameter, obtained at different periods of the breeding season (onset, middle, end), during anestrus and the luteal phase. We have observed that all follicles do not have the same steroïdogenetic potential and do not respond with the same intensity to LH and FSH. At the middle of the breeding season, LH and FSH supplementation was ineffective in increasing progesterone secretion by follicles (0.19+/-0.05 vs. 0.20+/-0.03 ng/mL). In contrast, gonadotrophin challenge elicited significant (P<0.05) increases in androstenedione (0.94+/-0.34 vs. 0.35+/-0.09 ng/mL) and estradiol (120+/-11 vs. 49+/-10 pg/mL) production immediately after its administration. At the onset of the breeding season, steroidogenesis was identical under both basal and gonadotrophin-stimulated conditions unlike that in middle of the breeding season. However follicles were more sensitive to the gonadotrophin challenge in terms of estradiol production than those collected at the middle of the breeding season (220+/-45 vs. 120+/-11 pg/mL). Follicles obtained at the end of the breeding season featured higher progesterone (2.61+/-0.81 vs. 0.19+/-0.05 ng/mL; P<0.05) and lower estradiol production (10+/-3 vs. 49+/-10 pg/mL; P<0.05) that was not influenced by LH and FSH. Basal androstenedione secretion was comparable to that observed at the middle of the breeding season (0.42+/-0.10 vs. 0.35+/-0.09 ng/mL), but the response to stimulation was significantly higher (1.82+/-0.61 vs. 0.94+/-0.34 ng/mL; P<0.05). In anoestrus and the luteal phase, follicles presented higher progesterone and androstenedione and lower estradiol concentrations (P<0.05) compared with those obtained during the follicular phase at the middle of the breeding season. In the luteal phase, follicles remained capable of responding to LH-FSH challenge by increasing estradiol secretion (9+/-1 before and 21+/-6 pg/mL after LH-FSH; P<0.05). In contrast, in the luteal phase, estradiol production was not increased by LH-FSH challenge (7+/-2 vs. 12+/-4 pg/mL).

Melatonin treatment of embryo donor and recipient ewes during anestrus affects their endocrine status, but not ovulation rate, embryo survival or pregnancy

Thirty-two Border Leicester x Scottish Blackface ewes that lambed in March were individually penned with their lambs from April 16th and given daily an oral dose of 3 mg melatonin at 1500 h (Group M). A further 32 acted as controls (Group C). Within each group half were used as embryo donors (Group D) following superovulation and half received embryos (Group R) following an induced estrus. Prior to weaning on 21 May ewes received ad libitum a complete diet providing 9 megajoules (MJ) of metabolizable energy and 125 g/kg crude protein. Thereafter each received 1.6 kg of the diet daily. In early June each ewe received an intravaginal device (300 mg progesterone) inserted for 12 d. Donors were superovulated with 4 i.m. injections of porcine FSH 12 h apart, commencing 24 h before progesterone withdrawal. Ovulation in recipients was induced with 800 IU PMSG injected i.m. at progesterone removal. Donor ewes were inseminated 52 h after progesterone withdrawal. Embryos were collected 4 d later and transferred to recipients. Melatonin suppressed plasma prolactin (P < 0.001) and advanced estrus (P < 0.05) and timing of the LH peak (P < 0.05). These events also occurred earlier in donors than in recipients (P < 0.01). Mean (+/- SEM) ovulation rates for melatonin-treated and control donors were 5.5 +/- 0.71 and 4.7 +/- 0.66, respectively (NS). Corresponding recipient values were 3.3 +/- 0.40 and 3.4 +/- 0.39 (NS). Mean (+/- SEM) embryo yields were 2.9 +/- 0.64 and 2.6 +/- 0.73 for melatonin-treated (n = 15) and control (n = 16) donors, respectively, and for the 12 ewes per treatment that supplied embryos, corresponding numbers classified as viable were 2.7 +/- 0.47 and 2.3 +/- 0.61 (NS). Following transfer, 57% of embryos developed to lambs when both donor and recipient received melatonin, 86% when only the donor received melatonin, 91% when only the recipient received melatonin, and 67% when neither received melatonin (NS). Thus, embryo survival following transfer was not improved by treating recipients with melatonin. Gestation length and lamb birthweights were unaffected by melatonin. Unlike nonpregnant control ewes, melatonin-treated recipients that failed to remain pregnant sustained estrous cyclicity following embryo transfer.

Alterations in follicular maturation associated with within-breed variation in ovulation rate in Chios sheep

Within-breed comparisons may be helpful to identify, in a given genetic background (Chios sheep), ovarian strategies and control mechanisms associated with altered ovulation rate. High and low ewes were identified from two large groups (n = 27 and n = 33 in Exp. 1 and Exp. 2 respectively) of Chios ewes submitted to repeated laparoscopies (24 times in Exp. 1 and six times in Exp. 2). High ovulatory ewes (n = 6 and n = 7 in Exp. 1 and Exp. 2 respectively) had mean ovulation rates of 4.3 (Exp. 1) and 4.2 (Exp. 2) while low ovulatory ewes (n = 6 and n = 7 in Exp. 1 and Exp. 2 respectively) had mean ovulation rates of 2.5 (Exp. 1) and 1.9 (Exp. 2). In Exp. 1, follicular function was compared in these two groups of ewes using follicles obtained at 30 h following luteolysis in the same ewes before and after unilateral ovariectomy (ULO). In Exp. 2, circulating follicle stimulating hormone (FSH) concentrations were measured from the end of the luteal phase up to the preovulatory surge in high and low ewes. Thereafter, to demonstrate a causal link between high FSH and high ovulation rate, pituitary downregulation was achieved by a 17-day gonadotrophin releasing hormone (GnRH) agonist treatment and the ovarian response to similar amounts of exogenous gonadotrophins compared between high and low ewes. Numbers of oestrogenic (in vitro oestradiol > 250 pg ml-1 h-1) follicles on the first ovary removed (Exp. 1) were 2.16 +/- 0.5 vs. 1.33 +/- 0.17 in high and low ewes (P = 0.1). Following ULO, these numbers were 3.33 +/- 0.33 and 2.5 +/- 0.18 (P < 0.05 between high and low ewes). There were no significant differences between the first and second ovaries for any of the parameters studied. Follicles from high ovulatory ewes (n = 33) differed from those of low ovulatory ewes (n = 23) by a smaller size (P < 0.01), a reduced number of granulosa cells (P < 0.01) together with decreased oestradiol (P < 0.05) and testosterone (P < 0.01) production in vitro. However, steroid production per cell (oestradiol per granulosa cell, testosterone per thecal cell) was similar in the two groups of sheep. FSH concentrations (Exp. 2) in high ovulatory ewes were significantly higher than those of low ovulatory ewes during the late luteal phase, and the decrease in FSH concentrations was steeper (1.4 ng) during the early follicular phase for high ovulatory ewes than low ovulatory ewes. Chemical hypophysectomy achieved by a 17-day treatment with a GnRH agonist demonstrated that these high FSH concentrations may be important to generate the high ovulation rate of the 'high' ewes as ovulation rate of high and low ewes was similar following chemical hypophysectomy followed by administration of similar amounts of exogenous gonadotropins to both groups of ewes. It is concluded that, despite different genetic control of their high ovulation rate (Chios-polygenic; Booroola-major gene), alterations in follicular function and its control are very similar in high ovulatory Chios and in FecB carriers.