Increased ovarian follicular angiogenesis and dynamic changes of follicular vascular plexuses induced by equine chorionic gonadotropin in the gilt.

Follicular angiogenesis and capillary degeneration are crucial ovarian processes in folliculogenesis. The present study was conducted to assess the changes in population of follicular vascular plexuses with different capillary status in prepubertal gilts 72 h after equine chorionic gonadotropin (eCG) (1,250 IU) treatment, using combined vascular corrosion casting and scanning electron microscopy. Follicular fluid concentrations of estradiol, progesterone and vascular endothelial growth factor (VEGF) were determined to confirm the follicular status. Based on the proliferative or degenerative characteristics of their capillaries, follicles were classified into three categories: active angiogenesis, low angiogenesis and degeneration. Irrespective of exogenous gonadotropin treatment in vivo, small follicular vascular plexuses (<4 mm in diameter) exhibited all three conditions in casted ovaries, while medium (4-5 mm) and large (>5 mm) plexuses showed only active angiogenesis or degeneration. eCG treatment significantly increased the population of large, but decreased that of small follicular plexuses. Most large follicular vascular plexuses showed active angiogenesis with higher follicular fluid estradiol: progesterone ratios and VEGF concentration. eCG also increased the percentage of medium follicular plexuses with active angiogenesis. The populations of small follicular plexuses with active angiogenesis were higher in controls, but decreased after eCG treatment. However, treatment of gilts with the gonadotropin increased the percentage of small plexuses (<1.0 mm) with low angiogenesis and those (1-3.9 mm) with extensive capillary degeneration. These findings are consistent with the hypothesis that angiogenesis is involved in selection and growth of small follicles in gilts under the regulation of gonadotropin.