Ovarian follicular development and ovulation in mammals

Abstract

This review is a state of the art on folliculogenesis and ovulation in mammals. Basal folliculogenesis is a phase of slow follicular growth, during which the oocyte acquires meiotic competence. Terminal folliculogenesis is a phase of rapid development, during which the ovulatory follicle is selected and completes its maturation while the oocyte acquires developmental competence. This review describes the different functional changes occurring within follicles and oocytes throughout these two phases, the underlying regulatory mechanisms and the mechanisms that determine natural ovulation rate in different species and breeds. Known key factors for folliculogenesis are BMP, KITLG and AMH for the initiation of follicular growth, IGF and insulin for transition between basal and terminal folliculogenesis, FSH for the outset of terminal folliculogenesis and the selection of ovulatory follicles, and LH for final follicular development, oocyte maturation and ovulation. Ovulation is triggered by a preovulatory LH surge that induces maturation of the cumulus oocyte complex within the ovulatory follicle, rupture of the follicular wall and corpus luteum formation. Ovarian folliculogenesis is controlled by external factors such as photoperiod and nutrition, and the hypothalamo-pituitary system integrates endocrine and environmental signals. Nutritional supply, relayed by hormonal and metabolic signals, can modulate both the hypothalamo-pituitary and ovarian activity.