Ultrastructural features of the follicular wall in developing follicles of the sexually immature ostrich (<i>Struthio camelus</i>)

The ultrastructure of the follicular wall in primordial, previtellogenic and vitellogenic follicles of the sexually immature ostrich is described in the present study. The follicular wall consists of a zona radiata, granulosa cell layer, basal lamina and thecal layer. Cytoplasmic processes from the plasma membranes of the granulosa cell layer and the ovocyte form the zona radiata in previtellogenic and vitellogenic follicles. The granulosa cell layer transforms from simple cuboidal epithelium in primordial follicles to simple columnar or pseudostratified columnar epithelium in previtellogenic and vitellogenic follicles. Transosomes were observed along the apical and lateral plasma membranes of granulosa cells. The thecal layer in previtellogenic and vitellogenic follicles consists of interna and externa components. The fibroblasts in the theca externa contain microfilaments, which are thought to be actin filaments. The study revealed ultrastructural features, which are associated with the transportation of yolk precursors and nutrients into the ovoplasm. In addition, the study indicates that, although the cells in the theca externa contain microfilaments, they do not possess the ultrastructural characteristics of smooth muscle cells.