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Leydig cells, also known as interstitial cells of Leydig, are found adjacent to the seminiferous tubules in the testicle and their function is to produce testosterone in the presence of luteinizing hormone (LH).
Testes size in primates such as humans, chimpanzees, and gorillas appears to be related to sexual competition among males. Males with the largest testes (chimpanzees) need to compete with all the other males in their group for access to fertile females. Therefore, the more sperm they produce, the better their chances are. Gorillas have a harem mating system where the silver-back male is the only male allowed access to the females. Since he has no competition, sperm quantity is not necessary for successful reproduction.
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.
human testes are small in comparison to chimps and gorillas due to little male-male competition, testes produce androgens, primarily testosterone, testes are homologous to ovaries, producing sperm rather than ova, or human testes function best when cooler than other body parts, hence their external location