FOLLICULAR DYNAMICS DURING THE BOVINE ESTROUS-CYCLE - A LIMITING FACTOR IN IMPROVEMENT OF FERTILITY

The use of ultrasonographic techniques for visualizing bovine ovaries has elucidated patterns of follicular development in cattle and provided some insight into the regulation of follicular growth and development. In cattle the development of large antral follicles occurs in waves that consist of the contemporaneous appearance of around three to six follicles 5 mm or more in diameter. Over several days one follicle grows larger than the others and is selected to become the dominant follicle, while the smaller subordinate follicles regress. The dominant follicle appears to regulate the growth of subordinate follicles, since the appearance of the next wave is accelerated if the dominant follicle is destroyed and delayed if the lifespan of the dominant follicle is prolonged experimentally. Follicular waves occur regularly when plasma gonadotropin concentrations are basal and the number of waves per cycle is correlated with cycle length. There is evidence that subtle changes in hormonal milieu regulate the pattern of waves. Waves are preceded by small increases in plasma follicle stimulating hormone (FSH) and natural or experimentally induced decreases in basal FSH are associated with the demise of the dominant follicle. Increases in luteinizing hormone (LH) pulse frequency are observed during prolonged development of dominant follicles. Although the mechanisms that promote the selection of a dominant follicle have not been elucidated, the results reviewed in this paper make it clear that a great deal of progress has been made in understanding patterns of follicular development and their regulation. These advances have provided insight into some of the limitations of current methods for regulating bovine fertility, interesting experimental models for future studies, and information that may be used for improving reproductive efficiency in cattle.