Testis

Where are the testes?

They hang in the scrotum on either side of the scrotal septum, and the left is usually a little lower than the right. The spermatic cord suspends each testis from above, and the scrotal ligament, a remnant of the gubernaculum, ties its lower end to the scrotum. (StatPearls, Abdomen and Pelvis: Testes)

The scrotum holds the testes a few degrees below core body temperature, which sperm production needs. The cremaster muscles raise or lower the testes from day to day as the temperature changes. (OpenStax A&P 2e, 27.1; StatPearls, Abdomen and Pelvis, Scrotum and Abdomen and Pelvis: Testes)

Each testis forms in the abdomen and descends through the abdominal wall and inguinal canal into the scrotum, in the seventh month of fetal life. (OpenStax A&P 2e, 27.1; StatPearls, Abdomen and Pelvis: Testes)

What is a testis made of?

Its outer coverings are the tunica vaginalis, a sac with parietal and visceral layers, and beneath it the tough white tunica albuginea; Gray’s 1918 text adds a third, innermost tunic, the tunica vasculosa. The tunica albuginea turns inward along the back of the testis, and septa radiate from there to split the testis into cone-shaped lobules. OpenStax counts 300 to 400 lobules, while Gray’s 1918 text cites older estimates of 250 and 400. (OpenStax A&P 2e, 27.1; StatPearls, Abdomen and Pelvis: Testes; Gray 1918, 3c. The Male Genital Organs)

Tightly coiled seminiferous tubules fill each lobule. Sperm pass from them into the straight tubules and the rete testis, then leave through the efferent ductules, 15 to 20 of them in OpenStax and 12 to 20 in Gray’s 1918 text. (OpenStax A&P 2e, 27.1; Gray 1918, 3c. The Male Genital Organs)

What does the testis do?

Sperm develop in the seminiferous tubules from puberty on, starting from germ cells near the tubule wall. New cycles start at intervals, and the tubules do not all keep in step. (OpenStax A&P 2e, 27.1; StatPearls, Physiology, Male Reproductive System)

Sertoli cells reach from the base of the tubule wall to its lumen and nurse the developing sperm, controlling the ions and amino acids around them. Tight junctions between neighboring Sertoli cells form the blood-testis barrier, which hides the later germ cells from the immune system. (OpenStax A&P 2e, 27.1; StatPearls, Histology, Sertoli Cell)

Leydig cells, between the tubules, make testosterone, about 6 to 7 mg a day. Luteinizing hormone from the anterior pituitary gland controls testosterone output, and follicle-stimulating hormone controls sperm production. (OpenStax A&P 2e, 27.1; StatPearls, Abdomen and Pelvis: Testes)

What lies next to the testis?

The epididymis runs along the back of each testis, with its head at the upper pole. The tunica vaginalis covers the testis except at the back and top, where the epididymis and the spermatic cord attach. (StatPearls, Abdomen and Pelvis: Testes)

The testicular arteries leave the abdominal aorta just below the renal arteries, cross over the ureter and reach the testis in the spermatic cord. The artery of the deferent duct and the cremasteric artery add to the supply. (StatPearls, Abdomen and Pelvis: Testes)

Its veins form the pampiniform plexus, which joins into the testicular vein. The right testicular vein drains into the inferior vena cava and the left into the left renal vein, and lymph follows the arteries to the preaortic nodes. (StatPearls, Abdomen and Pelvis: Testes)

Where the sources differ

OpenStax and Holstein and colleagues (2003) say a new cycle of sperm production starts about every 16 days; StatPearls says about every 13 days. OpenStax and StatPearls both say the cycles are not in step across the tubules. (OpenStax A&P 2e, 27.1; Holstein et al. 2003, Reprod Biol Endocrinol; StatPearls, Physiology, Male Reproductive System)

The sources give different times for making a sperm: OpenStax gives about 64 days from spermatogonium to formed sperm, while Holstein and colleagues (2003) give 4.6 cycles, or 74 days, in the testis. (OpenStax A&P 2e, 27.1; Holstein et al. 2003, Reprod Biol Endocrinol)

OpenStax gives each adult testis as about 4 to 5 cm long. StatPearls gives 3 to 5 cm long and 2 to 3 cm wide. (OpenStax A&P 2e, 27.1; StatPearls, Abdomen and Pelvis: Testes)