Deformed sperm. Coloured scanning electron micrograph (SEM) showing a deformed macrocephalic spermatozoon (sperm). This sperm's large heavy head makes it less motile and even if it did reach the egg it is unlikely to be able to fertilize it. While all men have some deformed sperm, large numbers may cause male infertility. Sperm defects may be due to a blockage of the epididymis in the testes or to damage to the spermatic ducts, usually caused by a sexually transmitted disease. Varicose veins in the scrotum may also be a factor. Other causes of male infertility include low sperm production and male impotence. Magnification unknown.