Formation of ovarian SE from TA mesenchymal precursors. A) Tunica albuginea (ta) fibroblasts (fb) showing strong CK immunoexpression (brown color). One cell in mesenchymal-epithelial transition is apparent at the surface (fb/se). B) Ovarian stroma (os) shows no CK staining of fibroblasts (fb). C) Fibroblasts associated with the TA surface exhibit mesenchymal-epithelial transitory forms (D). E) Appearance of SE cells (se) is associated with a diminution of CK immunoexpression (-fb). F) Differentiation of the SE is associated with a diminution of cellularity in adjacent TA. Bar in (B), for (A-E). Cytokeratin 18 immunostaining, hematoxylin counterstain.