Interactions of Apoptosis and Extracellular Matrices in Granulosa Cells of Atretic Follicles in Porcine Ovaries

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The process of apoptosis of follicular granulosa cells in porcine ovaries during follicular atresia was investigated by in situ DNA 3'end-labeling at the level of individual cells. Histochemical changes in the follicular basement membrane (BM) were visualized by immunofluorescent staining of BM extracellular matrix (ECM) components, i.e. type IV collagen, laminin and fibronectin. At the first stage, granulosa cells located on the inner surface of the granulosa layer appeared to undergo apoptosis, followed by neighboring granulosa cells. No apoptotic granulosa cells making tight contact with intact BM were observed. Detachment and degeneration of the granulosa cell layer and fragmentation of BM occurred in follicles at the advanced stage of atresia. Finally, intermittent structures of BM and subsequent invasion of macrophages and fibroblasts were observed. Therefore we concluded that granulosa cell apoptosis is an initial symptom of follicular atresia in the porcine ovaries, and the degradation of BM follows granulosa cell apoptosis in the pig. Our results suggest that ECM components of follicular BM act as survival factors on follicular granulosa cells in porcine ovaries.