Structural changes in gonadal basement membranes during sex differentiation in the frog Rana rugosa

Here we report that structural changes in gonadal basement membranes during sex differentiation in the frog Rana rugosa are revealed using an antibody to its laminin component. Immunohistochemical staining indicated that the first sexual dimorphism appeared in testicular cords and ovarian cavities in differentiating gonads of tadpoles at St. 25-3W, three weeks after they reached St. 25. During development, as the testis enlarged, testicular cord partitions appeared to form by invagination of the testicular epithelium. Ovarian cavities also increased in volume. Laminin-positive basement membranes initially surrounded a partial surface of oocytes close to the ovarian cavity, fully covering growing oocytes by St. X. Laminin-reactive signals were present in somatic cells outside seminiferous tubules in the testis and outside oocytes in one-year-old frogs. BrdU-labeling showed that the number of dividing germ cells increased continuously in male gonads but increased in females only up to St. V, declining at St. X and thereafter. The number of dividing germ cells declined when the basement membranes had fully covered the oocytes. Together, these findings suggest that the first sexual dimorphism in the gonad of R. rugosa first appears as a structural change in the basement membranes. Finally, we speculate that the basement membrane on the surface of oocytes may affect their proliferation in this species.