Monitoring follicular development in cattle by real-time ultrasonography: a review

The application of real-time ultrasonography to monitoring ovarian function in mammals has advanced the understanding of follicular dynamics and its regulation. Follicular development is a wave-like sequence of organised events. The waves consist of the synchronous growth of small (4 to 5 mm) antral follicles, followed by the selection and growth of one dominant follicle which achieves the largest diameter and suppresses the growth of the subordinate follicles. In the absence of luteal regression, the dominant follicle eventually regresses (becomes atretic) and a new follicular wave begins. The dominant follicle regulates the growth of the subordinate follicles, because the appearance of the next wave is accelerated if the dominant follicle is ablated, and delayed if the lifespan of the dominant follicle is prolonged. During bovine oestrous cycles, two or three successive waves emerge, on average, on the day of ovulation (day 0) and day 10 for two-wave cycles, and on days 0, 9 and 16 for three-wave cycles. During the oestrous cycle there are thus two or three successive dominant follicles, and the last of these ovulates. Ovarian folliculogenesis is a complex process involving interactions between pituitary gonadotrophins, ovarian steroids and non-steroidal factors. Subtle changes in the hormonal milieu regulate folliculogenesis and the emergence of a follicular wave is preceded by a small increase in the concentration of plasma follicle-stimulating hormone. The mechanisms that promote the selection of a dominant follicle have not been elucidated, but considerable progress has been made in understanding follicular development and its regulation. Most treatments designed to control the development of follicular waves have been based on the physical or hormonal removal of the suppressive effect of the dominant follicle, and the consequent controlled induction of the emergence of a new follicular wave. The studies reviewed here describe current methods for regulating the bovine ovarian cycle, interesting models for future studies, and information that may be used for improving reproductive efficiency.