Gonadotrophic control of follicular development and function during the oestrous cycle of the ewe

Abstract

In the adult non-pregnant ewe the secretion of FSH is sufficient to ensure a continuous growth and development of antral follicles to 3-5 mm size at all times. Further development and increased secretion of oestradiol through the final 72 h to ovulation depends on adequate stimulation by LH. During anoestrus and the luteal phase of the cycle LH pulses occur too infrequently to stimulate sufficient oestradiol to evoke an LH surge. Moreover, during the luteal phase progesterone secreted by the corpus luteum not only reduces the frequency of LH pulses but also inhibits the ability of oestrogen to evoke an LH surge. At the time of luteal regression the frequency of LH pulses increases to at least one per hour due to the fall in progesterone secretion. This change in pulse frequency of LH is associated with a decrease in the secretion of FSH, probably because of a direct inhibitory action of oestrogen on the anterior pituitary gland. The dominant follicle is probably relatively independent of circulating levels of FSH due to the high concentration of oestradiol and FSH within the microenvironment of the follicular cavity. Once the oestrogen secretion achieves a certain level a preovulatory surge of LH (and FSH) occurs. Increased sensitivity of the anterior pituitary to LH-RH and increased secretion of LH-RH from the hypothalamus both play a part in producing the LH surge. The rise in prolactin at this time probably reflects a decrease in hypothalamic dopamine turnover which is necessary for maximum release of LH-RH. The preovulatory LH surge initially stimulates and then totally inhibits further secretion of oestrogen and androgen from the ovulatory follicle. This suppression of steroid secretion is accompanied by a second peak of FSH at about the time of ovulation. The function of this second peak of FSH remains unknown although it may be responsible for the development of the large antral follicles which occur on Days 3 and 4. It is probably more important in those mammals like the rat and hamster which only form a functional corpus luteum if pregnancy occurs and in which oestrogen is necessary for implantation.

Control of antral follicle development and selection in sheep and cattle

“…Recent studies have shown that cultured granulosa cells from small sheep follicles do develop the ability to respond to LH in parallel with an ability to secrete oestradiol and that this responsiveness depends on FSH (B. K. Campbell, unpublished). Furthermore, while low doses of LH stimulated oestradiol synthesis, higher doses were inhibitory, which again closely parallels responses observed in vivo (Baird and McNeilly, 1981). From the evidence presented above, we conclude that the culture system for sheep and cattle granulosa cells that we have developed is physiological and that physiologically relevant inferences can be drawn from it.…”

“…Furthermore, complete removal of LH by passive immunization inhibits FSH-stimulated follicular development (McNeilly et al, 1991). Using a similar model in conjunction with the ovarian autotransplant model (Baird and McNeilly, 1981), we have confirmed and extended these results to show that LH pulses of low amplitude and frequency have little effect on the ability of FSH to stimulate the development of large antral follicles ( Fig. 1) and, in the absence of LH, these follicles remain critically dependent on FSH for gonadotrophic support ( Fig.…”

“…In animals exhibiting oestrous cycles, these regular waves of ovulatory follicle development are interrupted by the process of ovulation. Within 10-12 h of luteal regression a preovulatory follicle(s), which is derived from either gonadotrophic-dependent or ovulatory follicles, emerges and under the influence of increased LH stimulation this follicle(s) will secrete sufficient oestradiol to induce the preovulatory LH surge within 48 -60 h (Baird and McNeilly, 1981). The endocrine changes that occur during this follicular phase result in removal, through ovulation or atresia, of the antral follicle hierarchy, such that follicle development is re-initiated in the period after ovulation from the recruitable pool of gonadotrophin-responsive follicles in response to the peak of FSH concentrations on day 1 (Campbell et al, 1990).…”

Impact of endotoxemia on ovarian signaling and function

“…This pulse pattern during the follicular phase is key to control of the reproductive cycle and facilitates the gonadotropin environment necessary to stimulate the pre-ovulatory E2 rise (Baird and McNeilly, 1981;Wallace et al, 1988;Campbell et al, 1990). The GnRH receptor works via a cascade starting with activation of phospholipase C hydrolyzing phosphatidylinositol 4,5-biphosphate to inositol 1,4,5-triphosphate and diacylgycerol (Limor et al, 1989;Perrin et al, 1989;Anderson et al, 1992).…”

Effects of induced acute phase response in ewes on early embryo survival

“…The surge in GnRH secretion subsequently stimulates a surge in LH from the anterior pituitary that results in ovulation (Baird and McNeilly, 1981).…”