Testicular cancer. Light micrograph of a section through an undifferentiated teratoma in a testis (testicle). Teratomas are encapsulated malignant (cancerous) tumours that contain tissue or organ components, such as hair or teeth. Undifferentiated teratomas have poorly differentiated cells that resemble those of a carcinoma. They are also known as embryonal carcinomas and have a bad prognosis. Testicular teratomas tend to occur most often in men between the age of 20 to 35 years.