Transmission electron micrograph (TEM) of the ultrastructure of a human testis showing a Sertoli cell-only seminiferous epithelium in which there is an absence of germ cells. The epithelium shows four Sertoli cell nuclei and abundant Sertoli cell cytoplasm. This relatively rare condition of germ cell aplasia is often of unknown aetiology; may be familial or acquired due to radiation, drugs, cryptorchidism or toxins. The lack of germ cells equates to infertility. Magnification: x2,000 when printed at a width of 10cm.