Testicular cancer. Light micrograph of a section through part of a human seminiferous tubule in a testis (testicle), showing the replacement of the normal germ cells with neoplastic germ cells. These large rounded cells have malignant (cancerous) potential and are believed to be abnormal gonocytes that have persisted from fetal life. Also referred to as carcinoma-in-situ, these cells are the source of most testicular germ cell tumours. Magnification: x150 when printed 10 centimetres wide.