Testicle

In animals that have separate sexes, including humans, the testicle (or testis; plural: testes) is the male reproductive gland. It is the equivalent of the female ovary. Its two main jobs are to make sperm and to produce androgens, especially testosterone. Testosterone release is controlled by luteinizing hormone (LH) from the anterior pituitary gland. Sperm production is directed by follicle-stimulating hormone (FSH) from the same gland, along with testosterone made inside the gonads.

Male humans typically have two testicles of similar size, housed in the scrotum—a pouch that extends from the abdominal wall. It is normal for one testicle to hang lower than the other due to differences in blood vessel layout; in about 85% of men, the right testicle hangs lower. To estimate testicle volume, a doctor may feel it and compare it to ellipsoids of known size (an orchidometer), or use calipers, a ruler, or ultrasound to measure length, width, and depth. Volume is then calculated using the ellipsoid formula (length × width × depth × 0.52), though a more accurate formula multiplies those three measurements by 0.71. An average adult testicle is about 5 cm by 2 cm by 3 cm. The Tanner scale, which tracks male genital maturity, assigns a stage based on volume: stage I is under 1.5 cm³, and stage V is over 20 cm³. Normal volume ranges from 15 to 25 cm³, with an average of 18 cm³ per testis (range 12–30 cm³). The number of sperm a man produces is directly linked to testicular volume, since larger testicles have more seminiferous tubules and Sertoli cells. Men with bigger testicles tend to release more sperm per ejaculate.

Inside, each testicle is wrapped in a tough, fibrous layer called the tunica albuginea. Beneath this are tightly coiled tubes known as seminiferous tubules. These tubules are lined with germ cells, which, starting at puberty and continuing into old age, develop into sperm cells (spermatozoa). The developing sperm travel through the seminiferous tubules to the rete testis in the mediastinum testis, then through the efferent ducts to the epididymis, where they mature. From there, they move into the vas deferens and are eventually expelled through the urethra via muscular contractions.

Within the seminiferous tubules, germ cells go through spermatogenesis, turning into spermatogonia, spermatocytes, spermatids, and finally spermatozoa. These gametes carry DNA for fertilizing an egg. Sertoli cells form the true lining of the seminiferous epithelium and are essential for supporting germ cell development. They also secrete inhibin, a hormone that helps regulate sperm production by suppressing FSH release from the pituitary. Surrounding the seminiferous tubules are peritubular myoid cells. Between the tubules lie Leydig cells, which produce and secrete testosterone and other androgens important for puberty (including traits like facial hair), sexual behavior, and libido. Testosterone itself controls testicular volume. Immature Leydig cells, interstitial macrophages, and epithelial cells are also present.

The testis gets its blood from three arteries: the testicular artery, the cremasteric artery, and the artery to the ductus deferens. The blood supply and lymphatic drainage of the testes and scrotum are separate. The paired testicular arteries come directly from the abdominal aorta and pass through the inguinal canal, while the scrotum and external genitalia are supplied by the internal pudendal artery (a branch of the internal iliac artery). The testis also has backup blood flow from the cremasteric artery, which branches from the inferior epigastric artery.