Morphological evidence of apoptosis and the prevalence of apoptotic versus mitotic cells in the membrana granulosa of ovarian follicles during spontaneous and induced atresia in ewes.

Apoptosis is a process by which granulosa cells are thought to be deleted during ovarian follicular atresia. The aims of the present studies, using sheep as the experimental model, were to determine 1) whether morphological changes in cells composing the membrana granulosa during the process of atresia conformed with the general criteria of apoptotic cell death as assessed using tissue sections stained with hematoxylin and eosin; 2) whether cells classified as apoptotic on the basis of their morphology contained fragmented DNA using an in situ 3' end-labeling technique; and 3) the degree of apoptosis and mitosis within the granulosa cell populations of large antral follicles (> or = 3 mm in diameter) during both spontaneous and experimentally induced atresia using stereological methods. The results showed that most degenerate granulosa cells in follicles undergoing atresia display the morphological characteristics of apoptosis, suggesting that this is the most common pathway of cell deletion. Typical features were cells containing nuclei with marginated chromatin; cells with a single small densely staining nucleus (pyknotic appearance); cells with multiple smaller, densely staining nuclear fragments; and densely staining membrane-bound bodies (apoptotic bodies) either singly or in clusters. Cells with morphological features more typical of oncosis or necrosis were sometimes observed, but mainly during the later stages of atresia. All cells classified as apoptotic on the basis of morphological criteria contained fragmented DNA as measured by 3' end-labeling. Apoptotic bodies and/or cells were found in all follicles examined, including those classified as healthy. The overall prevalence of apoptotic cells plus apoptotic bodies expressed as a percentage of the total granulosa cell number per follicle varied from 0.02% to 0.20% in healthy follicles, varied from 0.21% to 2.00% in follicles in early (primary) atresia, and was > 2.0% in follicles in later (secondary) atresia. Percentages of mitotic cells in healthy follicles were > 0.5% in all but one of those examined and were < 1.0% in all follicles classified as atretic. Both morphological and 3' end-labeling results indicated that apoptotic cells were widely disseminated throughout the membrana granulosa, including the cell layer adjacent to the basement membrane. Collectively, these observations indicate that during early atresia, apoptosis occurs randomly and is not limited to specific areas within follicles. Our finding that apoptotic cell death and mitosis occur simultaneously within the same follicle is consistent with the notion that atresia is determined by a dynamic equilibrium between cell division, differentiation, and death.