Ovulation.

Three events lead to the ovulation of a normal oocyte: cytological and biochemical changes in the follicle wall, disintegration of the follicle apex and oocyte maturation. The remodelling of the follicle wall results from plasmin and collagenase activities. The thinning of the follicular apex, in addition to these enzymes, involves hydrolases liberated by dying ovarian epithelial cells. PGF2 alpha and histamine are also involved but it is not known precisely how they contribute to the apical dissociation. The nuclear and cytoplasmic maturation of the oocyte is highly dependent on the synthetic activities of granulosa cells which are regulated by LH and FSH. The pulsatile secretion of these gonadotrophins is not necessary for the final phase of Graafian follicle growth and rupture. Why high levels of gonadotrophins, normally reached during the preovulatory surge, completely change the structure and the biochemical activities of all follicular compartments remains unknown and in fact has never been studied. Moreover, there is very little information concerning the mechanisms involved both in the increase of blood flow during the LH surge and later in the blood stasis at the follicular apex. Steroids, whatever their levels and ratio, are of little if any concern in follicle rupture and nuclear maturation. However, their importance has been clearly demonstrated in the cytoplasmic maturation of the oocyte of some species.