Ovarian follicular cysts in dairy cows.

Ovarian follicular cysts are anovulatory follicular structures that occur in 10 to 13% of dairy cows. This review focuses upon the dynamics of cyst growth, development, and persistence as well as on associated endocrine and cellular mechanisms. During the estrous cycle of cows, two to four waves of follicular growth occur. From a cohort of recruited follicles, one is selected for continued growth and dominance while the other undergo atresia and regress. In contrast, cysts have long been thought to be static structures that persist for extended periods. Although cysts can persist for extended periods, most regress over time and are replaced during subsequent follicular waves. The next dominant follicle either ovulates or develops into a new cyst. The recruitment of a cohort of follicles from which a cyst develops and the growth rate of cysts to ovulatory size are similar to ovulatory follicular waves, but the cyst continues to grow for a longer period. The interval between waves of follicular growth is longer for cows with cysts than for cows with normal estrous cycles. Each wave is preceded by a transient increase in circulating FSH. Near the time of cyst development and persistence, the concentration of FSH is not different from that during normal estrous cycles. Serum concentrations of LH and estradiol-17 beta are higher in cows that develop cysts than in cows that do not. Conversely, hypothalamic content of GnRH is lower in cows with cysts. Thus, cysts are dynamic structures, and their development and lifespan are likely associated with altered hypothalamic-hypophysial-ovarian function.