Review of Follicular Growth and the Bovine Estrous Cycle

Introduction Figure 1 summarizes some key aspects of the bovine estrous cycle. Changes in the 2 key structures on the ovary, the preovulatory follicle and the corpus luteum, are emphasized in this figure. The function of 4 important hormones is also summarized in this figure. Estradiol-17ß from the preovulatory follicle causes the cow to manifest estrus behaviour and have an LH surge. The LH surge causes ovulation of the preovulatory follicle about 28 h later. The cells that remain from the preovulatory follicle develop into the corpus luteum. The corpus luteum grows in size during the first part of the estrus cycle and then reaches a plateau phase in which it maintains a large size (20-25 mm diameter). The major hormone coming from the corpus luteum is progesterone and the increase in size of the corpus luteum is reflected in increased concentrations of progesterone in the blood. If the cow becomes pregnant the corpus luteum maintains a large size and progesterone concentrations remain elevated. These high progesterone concentrations prevent the cow from coming into estrus or having a subsequent ovulation. If the cow does not become pregnant then the corpus luteum will decrease at about day 17-20 of the estrous cycle (day of estrus = 0). The reason the corpus luteum regresses is because of secretion of prostaglandin F2α (PGF2α) from the non-pregnant uterus. After exposure to PGF2α there is a decrease in circulating progesterone concentrations as well as a subsequent decrease in size of the corpus luteum. Thus, a major feature of the estrous cycle is the development and regression of the corpus luteum.