Testicular cancer. Light micrograph of a section through an undifferentiated teratoma (dark, right) with seminoma (light, left) 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. Seminomas are another form of malignant tumour and are the most common form of testicular cancer. Here the two forms are coexisting in one tumour.