Cystic ovarian follicles in cattle

A review

Abstract

Cystic ovarian follicles are considered as important causes of reproductive failure in cattle. The economic loss it causes in the dairy industry is mainly due to increases in days open in the postpartum period, and subsequent increases in culling rate. The disease process is a consequence of a mature ovarian follicle that fails to ovulate at the appointed time in the oestrous cycle. Recent studies suggest that this anovulatory ovarian follicular structure may persist as a dominant structure, preventing further follicular growth. Alternatively, it may be replaced by another cystic follicle or it may regress, allowing initiation of a follicular wave followed by the development of a dominant follicle and ovulation. In certain cases, these cystic follicular structures are luteinized and secrete progesterone. Our understanding of the aetiology and treatment for cystic ovarian follicle is incomplete at this time. However, it is critical to consider the hypotheses developed in recent years. Past findings suggested that perturbation of the hypothalamo-hypophyseal-ovarian (HHO) axis, due to many exogenous and endogenous factors, caused anovulation. One such major perturbation may be aberrant changes in the receptor expression in the HHO axis for the hormones involved in maturation, deviation, dominance, and ovulation of the follicle. Besides the receptors, conditions such as insensitivity of the hypothalamus to the positive feedback effect of estradiol may have a role in the aetiology. Based on studies with experimentally induced and naturally occurring cystic ovarian follicles, insufficient, inadequate, and inappropriate timing of luteinizing hormone (LH) release and levels around the time of ovulation have been suggested as the major causes for ovulation failure. However, not all studies support this hypothesis. Hitherto, hormonal preparations that release LH from the anterior pituitary, or that have LH-like action, are used to treat cysts. Gonadotropin releasing hormone (GnRH) fits the former category, and human chorionic hormone (hCG) the latter group. However, recent studies suggest that correcting LH deficiency may not be an efficient protocol based on the observation that there has been abundance of LH in cows after formation of cystic ovarian follicles. Cystic ovarian follicles that are heavily luteinized respond to prostaglandin F2? (PGF2?).