Testicular cancer. Light micrograph of a section through a malignant teratoma in a testicle. At upper left, epithelial cells have formed a cyst (dark ring) into which haemorrhage (bleeding, red) has occurred. The cyst is surrounded by pale mesenchyme-like tissue. Teratomas are tumours derived from germ cells, the precursors to sperm cells. They most commonly occur in the testes or ovaries.