Control of ovarian function in cattle

In cattle, ovarian function is controlled by complex local and systemic feedback mechanisms involving gonadotrophins from the pituitary gland and steroids and proteins from the ovaries. This control system ensures that in more than 96% of females, only one follicle will ovulate per oestrous cycle. Follicular growth and development in cattle occurs in a wave-like pattern, with two or three waves of follicles growing and regressing per oestrous cycle. Each wave is characterized by the emergence, from a pool of growing follicles, of a large dominant follicle which reaches a mature stage of development and may be induced to ovulate and form a functional corpus luteum with a single treatment of either human chorionic gonadotrophin (hCG) or gonadotrophin-releasing hormone (GnRH). The growth of this dominant follicle is associated with a marked reduction in both the number and growth of subordinate follicles in both ipsi- and contralateral ovaries, suggesting a systemic exertion of 'dominance'. The dominance concept is further supported by the observation that the subsequent wave of follicular growth cannot be detected until after the start of the regression of the previous dominant follicle. Furthermore, the dominance mechanism does not involve an action of inhibin, as previously proposed. In addition, in cattle, inhibin may not have a primary role in the control of follicle-stimulating hormone (FSH) release, unlike its role in other species such as sheep. Gonadotrophins provide the primary endocrine drive for the growth of follicles > 2 mm in diameter, but it is now becoming evident that other systemic and locally produced factors profoundly influence ovarian function. For example, short-term treatment of cattle with recombinant bovine somatotropin (BST) can double the number of small (< 5 mm) antral follicles without altering the pattern of circulating gonadotrophin concentrations. These follicles appear to be fully functional since they can be stimulated to develop further and ovulate with superovulatory treatment regimens. Dominance also appears to be exerted via a different pathway from that involved in the BST-induced recruitment of small follicles. The identification of the factor responsible for dominance, together with the elucidation of the mechanisms controlling follicular recruitment and growth should ensure that the full benefits ensuing from the precise control of ovarian function in cattle are achieved.