Ovarian Follicular Dynamics During the Estrous Cycle in the Mare

The mare is polyestrous, seasonal long-day breeder. The endocrinology of the estrous cycle involves a balance between hormones produced by the pineal gland, hypothalamus, pituitary gland, ovaries, and endometrium. Growth of antral follicles in the ovary occurs in wave-like patterns, and is influenced by several factors such as stage of the estrous cycle, season, pregnancy, age, breed and the individual. Variations among mares in regards to the time of emergence and the type of follicular waves (minor or major, ovudatory or anovulatory) can be significant; however, most commonly, a major ovulatory wave begins at mid-cycle and one follicle (occasionally two, rarely three) becomes dominant and ovulates while the other subordinate follicles in the wave regress. During follicular growth, approximately 7-11 follicles per wave emerge at diameters of 5-6 mm and enter a common-growth phase of about 6 days, in which follicles grow at an approximately similar rate and each follicle has the capacity for future dominance. Subsequently, the deviation process occurs, in which several intra-follicular factors increase in the largest follicle of the ovulatory wave but not in the subordinate follicles; and only the dominant follicle continues to grow. To date, the intra-follicular insulin-like growth factor (IGF) system is the main known mechanism that initiates follicular deviation. The intra-follicular changes in the future dominant follicle apparently increase the responsiveness of this follicle to decreasing follicle stimulation hormone (FSH) and increasing luteinizing hormone (LH) concentrations which occur at that time. The other follicles of the wave have the same capacity for dominance, but do not reach a similar preparatory stage before being negatively affected by the changing gonadotropin concentrations. Thus, the largest follicle alone continues to grow, becomes dominant, and eventually ovulates. This paper reviews the current basic understanding of the endocrine regulation of the estrous cycle, growth patterns of ovarian antral follicles, and the hormonal and the intra-follicular aspects of follicular deviation in mares.