Control of antral follicle development and selection in sheep and cattle

Abstract

The development of antral follicles in sheep and cattle is dependent on FSH, but large antral follicles can shift their gonadotrophic dependence from FSH to LH. The mechanisms that result in the selection of at least one ovulatory follicle from identical follicular cohorts, exposed to the same endocrine environment, still remain to be elucidated. The aim of this research was to extend in vitro results from the rodent to sheep and cattle and, using both in vivo and in vitro models, to identify factors that can enhance or attenuate the action of gonadotrophins in stimulating follicle development. Using sheep with ovarian autotransplants, we have obtained evidence to show that a number of factors inhibit ovarian function in vivo, whereas only insulin-like growth factor I (IGF-I) has a stimulatory effect. Further study of the mechanism of action of these factors at a cellular level has been made possible by the development of a serum-free granulosa cell culture system for both sheep and cattle that allows induction and maintenance of oestradiol production. Using this model system, we have confirmed many of the results from our studies in vivo and have shown that IGF-I and insulin interact at physiological concentrations to influence both cellular proliferation and oestradiol production. Overall, these data support the hypothesis that the physiological basis of follicle selection is the differential expression of factors that modulate the action of gonadotrophins on follicular cells at key points during the process of follicle development.

Developmental competence of oocytes recovered from postpartum cows using ultrasound-guided follicular aspiration

“…In ruminants, the cycle of follicular growth and development may be independent of the estrous cycle (Campbell, 1995). However, Sirois and Fortune (1988) disagreed with the view that follicular waves and follicular tumover are continuous and independent of the phase of the estrous cycle, and they supported the hypothesis that follicular growth occurs at specific times of the estrous cycle in the cow.…”

“…Folliculogenesis is controlled by complex relationships between intrafollicular steroids, growth factors, extraovarian factors and the hypothalamo-pituitary-ovarian feedback system (Campbell et al 1995). Folliculogenesis in the cow is a lengthy process that it takes about 180 days for a follicle to grow from its primordial stage to ovulatory size (Cahill and Mauleon, 1981).…”

Postpartum ovarian activity and response to progestagen treatment in rustic beef cows (Spanish Retinta) underfed to simulate their typical ranging conditions

“…In contrast to dominant preovulat02:Y follicles which mostly develop during the follicular phase under conditions of decreasing FSH and increasing LH release, the dominant follicles that grow within diestrus are subjected to relatively high FSH levels and low frecpiency pulses of LH. Under these circiomstances, the lifespan of these luteal phase follicles is probably shortened by the low LH availability and the decrease in FSH release caused by the E2 and the inhibin that they are themselves secreting (as reviewed by Campbell et al, 1995). Indeed, the lifespan of luteal phase follicles can be prolonged if an increased LH pulse frequency is induced during this phase.…”

Effects of initial GnRH removal from 5-d CO-Synch + CIDR protocol on ovarian parameters and fixed timed artificial insemination pregnancy rates in beef cattle

“…In the antral follicle, the theca cells are dependent upon LH to be able to further convert the steroid hormone progesterone into androstenedione (Smitz and Cortvrindt, 2002). As the follicle progresses and becomes selected for dominance the granulosa cells develop receptors for LH as well (Campbell et al, 1995).…”