The testicle is the male gonad in animals. Like the ovaries to which they are homologous, testes are components of both the reproductive system and the endocrine system. The primary functions of the testes are to produce sperm (spermatogenesis) and androgens, primarily testosterone. Both functions of the testicle are influenced by gonadotropic hormones produced by the anterior pituitary gland. Luteinizing hormone (LH) results in testosterone release. The presence of both testosterone and follicle-stimulating hormone (FSH) is needed to support spermatogenesis.
Almost all healthy male vertebrates have two testes. In mammals, the testes are often contained within an extension of the abdomen called the scrotum. In mammals with external testes, it is most common for one testicle to hang lower than the other. While the size of the testicle varies, it is estimated that 21.9% of men have a higher left testicle, while 27.3% of men have reported equally-positioned testicles. Human testicles are smaller than chimpanzee testicles, but larger than gorilla testicles.
Under a tough membranous shell, the tunica albuginea contains very fine coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells (germ cells) that develop into sperm cells (also known as spermatozoa or male gametes) from puberty into old age. The developing sperm travels through the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly-created sperm cells mature. The sperm moves into the vas deferens, and are eventually expelled through the urethra, via the urethral orifice, through muscular contractions. Leydig cells, localized between seminiferous tubules, produce and secrete testosterone and other androgens important for sexual development and puberty including secondary sexual characteristics like facial hair, sexual behavior. They also support libido, spermatogenesis, and erectile function. In addition, testosterone also controls testicular volume.