Light micrograph of a human testis showing numerous neoplastic germ cells in a seminiferous tubule. These pre-invasive cells have round nuclei and pale cytoplasm. They are dysfunctional germ cells derived from gonocytes and/or primordial germ cells in the foetal testis. Other cells in the tubule are Sertoli cells with irregular nuclei often with a central nucleolus. Classified as neoplastic, these cells have the potential to escape from the tubules and become metastatic cancer cells. The finding of neoplastic germ cells in the tubules is termed germ cell neoplasia in situ (GCNIS). Epoxy resin section, Toluidine blue stain. Magnification: x 500 when width printed at 10cm.