Testicular cancer. Light micrograph of a section through part of a seminiferous tubule in a human testis (testicle), showing replacement of the normal germ cells with neoplastic germ cells. These large, pale-staining 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. Other cell type present is the Sertoli cell, with smaller nuclei of irregular shape. Magnification x500 when printed at 10 cm.