Transmission electron micrograph (TEM) of the ultrastructure of neoplastic germ cells in the adult human testis showing the dysfunctional germ cells with circular nuclei and three Sertoli cell nuclei of irregular shapes. Formerly described as carcinoma-in-situ (CIS) these pre-invasive neoplastic germ cells are now classified as germ cell neoplasia in situ (GCNIS) and are thought to be derived from arrested or dormant primordial germ cells or gonocytes in early foetal life. GCNIS cells can proliferate in the seminiferous tubules but not necessarily migrate into the intertubular tissue. In about 50% of cases the cells may exhibit uncontrolled proliferation and malignant transformation, becoming invasive and leading to testicular germ cancer. Magnification: x5,000 when width printed at 10cm.