The terms "testis" and "testicle" are often used interchangeably, though technically the testicle includes not only the testis but also the vas deferens and the epididymis, which are two tubular structures that serve to store and transport sperm. The testes themselves are made up of hundreds of tiny coiled tubules called "seminiferous tubules," which are the site of sperm cell generation.
Each testis in covered with a fibrous capsule called the "tunica albuginea." Within this capsule, the testis is divided into 200 to 400 lobes, or sections. Each lobe contains between three and 10 seminiferous tubules. Each tubule is lined with germ cells. During their progress down the length of the tubule, the germ cells pass through several stages of development before finally entering the epididymis as mature sperm cells. The testis also contain Sertoli cells that support the lining of the tubules and contribute to sperm cell maturation, and Leydig cells that are the primary site for the production of the hormone testosterone in males. Optimal sperm cell production occurs at a temperature just slightly below normal core body temperature, which is why the testes are suspended away from the body in the scrotal sacs. Muscles in the scrotum contract and expand as needed to maintain the optimal testicular temperature.