Testicular cancer. Coloured transmission electron micrograph (TEM) of a seminoma cancer cell in tissue from a testis (testicle). Seminomas are malignant (cancerous) tumours that tend to spread to lymph nodes via the lymphatic system. They are the most common form of testicular cancer and occur most often in men between the age of 30 to 45 years. Here the characteristic round cell can be seen with a cytoplasm (orange) with few organelles and round central nucleus (pink) with two nucleoli. Magnification: x1500 ; when printed at 10 centimetres wide.