Ovarian follicular dynamics: comparative study among cattle and equine species

Abstract

The bovine estrous cycle is characterized by two or three waves of follicular development and each wave results in a follicle of pre-ovulatory diameter. The types of follicular waves in mares are major waves (characterized by dominant and subordinate follicles) and minor waves (largest follicle does not attain the diameter of a dominant follicle). In both species, the emergence of a follicular wave is stimulated by a FSH surge. This surge reaches a peak when follicles attain 4 mm in diameter in cattle and 13 mm in mare. Some days after the peak of the FSH surge and the emergence of the wave the follicles begin to undergo deviation. In this moment, the two largest follicles attain about 8.5 and 7.7 mm in diameter in cattle and 22 and 19mm in mare. The deviation is characterized by continued growth of the largest follicle to become the dominant follicle and reduced or terminated growth of the remaining follicles to become subordinate follicles. The two largest follicles grow in parallel until deviation and the future dominant follicle emerges approximately 6 h in cattle and 24 h in mares before the second largest follicle. Probably, this is enough time for the largest follicle to establish the deviation process before the second largest follicle reaches a similar diameter. The largest follicle suppresses circulating FSH concentrations to below the requirements of the smaller follicles, which causes their regression. The follicle produced FSH suppressants appear to be estradiol and inhibin. Besides FSH suppressing ability the largest follicle also develops the ability to utilize the reduced concentrations of FSH for its continued growth. There are elevated concentrations of circulating LH at the moment the deviation in both species and the activity of IGF-1 increases rapidly in the dominant follicle.