Localization of laminin proteins during early follicular development in pig and rabbit ovaries

Laminin, a major component of the basal lamina, is known to be important in the differentiation of epithelial cells. The outer granulosa cell layer of ovarian follicles is attached to a basal lamina surrounding the follicle and it has been demonstrated that proteins of the basal lamina can alter the steroidogenic capacity and cytoskeletal composition of mature granulosa cells. The present studies were carried out to evaluate the developmental expression of laminin proteins in the basal lamina of ovarian follicles during early stages of granulosa cell differentiation. Ovaries from sexually immature pigs (0-20 weeks of age) and rabbits (0-12 weeks of age) were used for this study because formation of primordial follicles and initiation of follicular growth occur partially or completely postnatally. Specific antibodies were made against laminin beta 1/gamma 1 chains using protein purified by high resolution two-dimensional polyacrylmide gel electrophoresis. These antibodies were used in immunohistochemical localization studies to demonstrate that laminin is associated with the basal lamina surrounding egg clusters and their connections to the ovarian surface epithelium and ovarian rete during early ovarian development. In addition, laminin was observed to associate with a continuous matrix that surrounds forming primordial follicles as they are isolated from the egg clusters. Laminin is localized in the basal lamina of primordial, primary, secondary, and tertiary follicles of both pig and rabbit ovaries. Laminin proteins are not only expressed throughout early follicular development in pigs and rabbits, but are also found surrounding the germ cell compartments prior to initiation of meiosis and formation of primordial follicles. These results demonstrate that laminin proteins are deposited in the basal lamina well before granulosa cells undergo morphological differentiation. This pattern of expression suggests that laminin proteins alone do not control changes in granulosa cell morphology during early development but may be required to maintain cell lineage commitment.