Relationships between follicular fluid composition and follicular/oocyte quality in the mare

Abstract

The ovarian physiology of the mare is an interesting model. Actually, we could highlight that in contrast with other domestic mammals, no preovulatory LH surge occurs in this species, but LH rises gradually during oestrus with a maximum 1–2 days after ovulation. Moreover, in vitro maturation rate of oocytes remains low in the mare, and in vitro fertilisation methods give only limited success. Thus, follicular development and maturation could involve, at least in part, some species-specific mechanisms and studies on follicular fluid composition may contribute to understanding these events, as well as to alleviate some of the problems encountered. The studies reported here are on the evolution of intrafollicular content of steroids, protein patterns and insulin-like growth factor binding proteins (IGFBPs) profiles during follicular growth, maturation and atresia in the mare, in relation to follicular and oocyte quality. We demonstrated that (1) follicular growth of the dominant follicle is associated with an increase in intrafollicular oestradiol-17β and progesterone levels and a decrease in IGFBP-2 and IGFBP-5, (2) follicular maturation is associated with a decrease in intrafollicular estradiol-17β level, a further increase in progesterone level, a slight increase in IGFBP-2 and finally, the induction of the follicular expression of an Mr 200 000 protein, whose identity is still unknown, and (3) follicular preovulatory regression and atresia are characterised by alterations in steroidogenesis, as well as an increase in intrafollicular levels of IGFBP-2, IGFBP-4 and IGFBP-5. Interestingly, we obtained data which suggest that the expression of the Mr 200 000 protein relates to oocyte resumption of meiosis and cumulus expansion. Finally, relationships between follicular content and follicular quality needs further work in the mare, in order to predict fertility and to increase the embryo production in vivo and in vitro in this species.