Formation of epithelial cords from ovarian TA mesenchymal precursors - see also Ref . A) Panoramic view of ovarian surface and adjacent cortex. Dashed line indicates interface between TA and stroma of the ovarian cortex. Dotted box indicates an early appearance of SE cells adjacent to the ovarian cortical stroma. Inset, epithelial channel (se-ch); se and arrow, surface epithelium cells; taf and arrowhead, TA flap; white arrowhead – a lack of SE cells above the TA; white arrow -bilaminar epithelial cord. B) Detail from (A) shows association of CK+ (brown color) fibroblasts (+fb,) with the TA flap surface (arrowhead), and transition from mesenchymal to epithelial morphology (fb/se) and surface epithelium cells (se, arched arrow). C) Detail from (A) shows CK+ epithelial cord consisting of two layers of epithelial cells and lying between the ovarian cortex (ovc) and TA. Note diminution of CK immunoexpression in TA fibroblasts (+/-fb). D) Parallel (black arrows) and perpendicular (arrowheads) view of epithelial cords in the upper ovarian cortex (ovc). Dashed line indicates a segment of CK+ TA with flap and differentiating SE cover (left white arrow). Dotted line indicates CK- segment lacking SE cower (right white arrowhead). F29 indicates female age in years; #28 = case No. Bar in (C), for (B and C). Cytokeratin 18 immunostaining (brown color) and hematoxylin counterstain.