Histomorphometry of dominant and subordinate bovine ovarian follicles

The study was designed to quantitatively characterize the histomorphological attributes of dominant and subordinate follicles in relation to follicular wave dynamics. Heifers (n = 27) were examined daily using ultrasonography to record the growth of individual follicles from 2 days before ovulation until the day of ovariectomy to obtain growing (n = 7), early static (n = 6), late static (n = 6) and regressing (n = 5) phase anovulatory dominant follicles of Wave 1, as well as preovulatory (n = 6) and subordinate (n = 42) follicles. The wall thickness of Wave 1 dominant follicles decreased dramatically (P < 0.01) during the late-static (60.2 +/- 4. 3 microm) and regressing (41.8 +/- 4.3 microm) phases compared to earlier phases. Cells of the granulosa layer of the dominant follicle of Wave 1 became loose during the late-static phase, with an increase (P < 0.001) in number of degenerating cells. Dominant follicles of Wave 1 were lined by fibroblast-like flattened cells during the regressing phase. One day after wave emergence (i.e., before selection), the three largest follicles of the wave were histomorphologically indistinguishable. The wall of the preovulatory follicle close to the medulla of the ovary was thicker (P < 0.01) than the wall facing the ovarian surface. The wall of subordinate follicles was thinner (P < 0.01) and had a lower mitotic index (P < 0.01) than that of their dominant counterparts 3 days and 6 days after wave emergence. In summary, follicular status, ascribed by ultrasonography, was associated with quantitatively distinct histomorphological characteristics. Morphometric changes in the dominant follicle during immature, mature, and post-mature phases were similar to, but occurred later than, those of subordinate follicle. The dominant follicles of Wave 1 entered histological atresia at the time of emergence of Wave 2.