Foetal testis, light micrograph. Seminiferous cords contain pale-staining round spermatogonia and numerous densely packed immature Sertoli cells of irregular shape. Cords are bordered by several layers of elongated mesenchymal cells and fibroblasts. In the interstitial tissue between the cords are clusters of Leydig cells (deep blue) secreting high levels of androgens that stimulate testis development. Epoxy resin section, Toluidine blue stain. Magnification: x900 when width printed at 10cm.