CHANGES IN EXTRACELLULAR-MATRIX COMPONENTS AND STEROIDOGENIC ENZYMES DURING GROWTH AND ATRESIA OF ANTRAL OVARIAN FOLLICLES IN THE SHEEP

To investigate the involvement of extracellular matrix (ECM) in follic ulogenesis in the sheep, parallel changes in ECM components and key st eroidogenic enzymes were studied by quantitative immunohistochemistry and immunoblotting during follicular growth and atresia. Growth of ova rian follicles from 1 to 5 mm in diameter was characterized by a progr essive increase in P450 cholesterol sidechain cleavage levels in both thecal (p < 0.001) and granulosa cells (p < 0.001), an increase in P45 0 aromatase levels in granulosa cells of follicles larger than 3.5 mm (p < 0.001), and an increase in levels of P450 17 alpha-hydroxylase C- 17,C-20 lyase (P450(17 alpha)) in the theca interna. In addition, duri ng follicular growth, a change in localization of cells expressing P45 0(17 alpha) within the theca interna was observed, positive cells bein g sparse within the theca interna of small follicles and specifically located close to the basal laminae in large follicles. In parallel, fo llicular growth was associated with an increase in levels of type I co llagen in granulosa cell layers (p < 0.01) and an increase in levels o f fibronectin (p < 0.05), particularly the specific ED-A alternatively spliced variant of fibronectin, in the theca externa. Follicular atre sia was characterized by a loss of P450 aromatase in granulosa cells ( p < 0.001) and a decrease in levels of P450(17 alpha) in the theca int erna (p < 0.05). Simultaneously, levels of fibronectin (p < 0.05), par ticularly the ED-A variant of fibronectin, decreased in the theca exte rna of atretic follicles. Within the wall of granulosa cells, levels o f fibronectin (p < 0.05), laminin, type IV collagen, and heparan sulfa te proteoglycans strongly increased during follicular atresia. Overall , these results show that follicular growth and atresia were associate d with distinct changes in levels of ECM components, suggesting that E CM components may play a role in the regulation of proliferation, diff erentiation, and apoptosis of follicular cells.