Localization of apoptotic cells in the cystic ovarian follicles of cows: a DNA-end labeling histochemical study.

We examined the frequency of apoptosis in cystic follicular cells to investigate the cause of the delay in regression of cystic follicles. Paraffin sections of healthy antral follicles, early and late atretic ones, and early and late cystic ones were stained using the terminal deoxynucleotidyl transferase (Tdt)-mediated biotinylated deoxyuridine triphosphates (dUTP) nick end-labeling (TUNEL) method to detect apoptotic cells. In the granulosa layer of early cystic and atretic follicles, TUNEL-positive cells were evident. In the theca interna of both early and late atresia, high frequencies of TUNEL-positive cells were observed. In the theca interna, a high frequency of TUNEL-positive cells was noted in the early cystic follicles, whereas their frequency decreased in late cystic follicles. These results suggest that apoptosis occurs in the granulosa and theca interna cells of cystic as well as atretic follicles, but the frequency of apoptosis in theca interna cells decreases in late cystic follicles, which may be responsible for the delay of follicular regression.