The cellular biology and changes in the extracellular matrix of ovarian follicles during their development are reviewed. During growth of the bovine ovarian follicle the follicular basal lamina doubles 19 times in surface area. It changes in composition, having collagen IV alpha 1-26 and laminin alpha 1, beta 2 and gamma 1 at the primordial stage, and collagen IV alpha 1 and alpha 2, reduced amounts of alpha 3-alpha 5, and a higher content of laminin alpha 1, beta 2 and gamma 1 at the antral stage. In atretic antral follicles laminin alpha 2 was also detected. The follicular epithelium also changes from one layer to many layers during follicular growth. It is clear that not all granulosal cells have equal potential to divide, and we have evidence that the granulosal cells arise from a population of stem cells. This finding has important ramifications and supports the concept that different follicular growth factors can act on different subsets of granulosal cells. In antral follicles, the replication of cells occurs in the middle layers of the membrana granulosa, with older granulosal cells towards the antrum and towards the basal lamina. The basal cells in the membrana granulosa have also been observed to vary in shape between follicies. In smaller antral follicles, they were either columnar or rounded, and in follicles > 5 mm the cells were all rounded. The reasons for these changes in matrix and cell shapes are discussed in relation to follicular development.