Cross sections of the medullary region of control testes at stage 26 showed interconnected Sox9þ cords surrounded by stromal tissue consisting of blood vessels, mesenchymal cells and fibro- blasts (Fig. 2D). As in bipotential gonads, a single layer of cuboidal cells formed the surface epithelium (Fig. 2A). Immunofluorescent staining of cytokeratin indicated the epithelial nature of cells in both the medullary cords and the surface epithelium. While cell nuclei in cords gave a strong fluorescent signal for Sox9, surface epithelium cell nuclei remained Sox9 negative (Fig. 2D).