Changes in the Vasculature of Bovine Corpus Luteum During the Estrous Cycle and Prostaglandin F2alpha-Induced Luteolysis.

To determine whether the corpus luteum (CL) vasculature has a role in the local regulation of CL function, we monitored the number and the density of blood vessels in the center of bovine CL during the estrous cycle and during PGF2α-induced luteolysis. CLs were collected at five stages: early stage (Days 2-3 post-ovulation), developing stage (Days 5-7), mid stage (Days 8-12), late stage (Days 15-17), and regressed stage (Days 19-21). To examine the effect of PGF2α on luteal vasculature, cows were injected with a luteolytic dose of PGF2α analogue on Day 10 post-ovulation and then CLs were collected by transvaginal ovariectomy at 0.5 (n=3), 2 (n=3) and 12 h (n=3) after injection. For a control, CLs were collected from uninjected cows on Day 10 post-ovulation. The CL sections were immunohistochemically stained with von Willebrand Factor (specific for endothelial cells) and α-smooth muscle actin (specific for smooth muscle). To investigate the density of blood vessels, after staining with von Willebrand Factor antibody, an eyepiece grid was used to cover an area on each luteal section. Each point of the grid superimposed on positive immunostaining or on capillary lumina was counted and the total number of points was divided by the total number superimposed on luteal cells and the remainder of luteal tissue ×100%. Immunohistochemical staining with von Willebrand antibody revealed that the density of luteal blood vessels was significantly higher at the developing and late luteal stages (P<0.05) than in the other stages, whereas the number of blood vessels stained with α-smooth muscle actin, was higher at the late and regressed luteal stages (P<0.05) than at the other stages. Furthermore, both density of blood vessels and number of blood vessels with smooth muscle were significantly higher at 2 h and 12 h after PGF2α administration (P<0.05) than without PGF2α injection (control). These results demonstrated that the number of blood vessels with smooth muscle per unit area increased as the result of losing steroidogenic cells and capillary endothelial cells in the regressing CL, whereas the density of total endothelial cells decreased. The overall results demonstrate that during structural luteolysis, the capillaries lacking in smooth muscle disappeared earlier than blood vessels with smooth muscle layer, suggesting that the loss of capillaries results in a reduced supply of nutrients to luteal cells which is followed by cell death. This research was supported by Grants-in-Aid for Scientific Research (No. 18380166 and 19580326) of the Japan Society for the Promotion of Science (JSPS). (poster)