| To evaluate the regulation of ovarian follicular growth and synchronization of ovulation, the following experiments were conducted. In Experiment 1, lactating cows were assigned to Ovsynch or Presynch treatments. The conception rate for the Ovsynch was greater than the Presynch treatment. In Experiment 2, ovaries were collected from beef heifers on day 2 or 3 of the estrous cycle. Follicles were classified as estrogen-active or estrogen-inactive follicles. Vascular density was greater in the estrogen-active follicles as well as thecal vascular endothelial growth factor (VEGF), endothelial nitric oxide synthase (eNOS), and follicular VEGF. In Experiment 3.1, ewes were treated with FSH to induce follicular growth or saline to allow atresia, and ovaries were collected on day 16. In Experiment 3.2, ewes were treated with FSH and hCG on day 15 to stimulate ovulation, and ovaries were collected at 0, 2, 4, 8, 12, 24, and 48 h after hCG. In Experiment 3.3, ovaries from superovulated ewes were collected at slaughter on days 2, 4, 10, and 15. In healthy antral follicles, eNOS was found exclusively in the endothelium of blood vessels but not in early and advanced atretic follicles. At 24 and 48 h, eNOS staining became intense in theca externa (TE) and theca interna (TI) of healthy antral follicles. eNOS staining was greatest on day 4 and least on day 15 of luteal development. In Experiment 4, VEGF, fibroblast growth factor (FGF), eNOS, and related receptor expression during the periovulatory period were quantified using a real-time reverse transcription polymerase chain reaction (real-time RT-PCR) assay. Ovaries were collected from FSH-hCG stimulated ewes. The mRNA levels of VEGF in theca tissues remained unchanged at 0, 2, 4, 8, 12, and 24 h. However, the mRNA levels of VEGF in theca tissues increased at 48 h. The linear relationship among VEGF, Flt-1, and eNOS mRNA was found in thecal tissue, and KDR was found in granulosa tissue during the periovulatory period. The exponential relationship between Flt-1 and time was found in granulosa tissue. Results from the studies herein highlight the importance of angiogenic factors during the final stages of follicular growth. |
