Thecal vasculature and oocyte maturation during follicular atresia in the sheep and pig.

During follicular atresia in the sheep and pig there is a marked reduction in the vasculature of the theca interna underlying the membrana granulosa. In the sheep there is no reduction of thecal blood vessels adjacent to the cumulus which remains viable. By contrast, in the pig the cumulus degenerates during atresia and the underlying thecal vasculature is greatly diminished. However, despite loss of contact between the follicle cells and the oocyte, the germinal vesicle remains intact.