Production and Transport of Male Gametes
Objective 5
Characterize the process of spermatogenesis, the organs involved, the route sperm take through the duct system during ejaculation, the additional organs involved in semen production, and the composition of semen and its role in sperm function.
Let’s consider the whole process, briefly; then we’ll look at the specific steps and functions in more detail. Male gametes are spermatozoa. Spermatozoa are produced in the testes and then stored and undergo final maturation in the epididymis. In preparation for ejaculation, they travel from the epididymis through the ductus deferens to just outside the prostate, near the neck of the urinary bladder. There, seminal vesicles, the prostate, and bulbourethral glands add secretions and the mixture is released into the urethra for ejaculation, which occurs through the external urethral orifice at the tip of the glans penis.
Spermatogenesis
Let’s break this process down into several steps.
Step 1. The tissue lining the seminiferous tubules generates gametes from a population of stem cells. Each seminiferous tubule is about 60 cm in length and there are about 500 tubules tightly coiled in each testis for an estimated total length of about 600 m of seminiferous tubules in one male (60 cm/tubule x 500 tubules/testis x 2 testes/male = 60,000 cm/male = 600 m/male = ⅓ mile/male). About 500 spermatozoa are released per mm of tubule per day, or about 300 million sperm per day (500 sperm/mm/day x 10 mm/cm x 60,000 cm/male = 300,000,000 sperm/male/day).
The capillaries of the testes and the interstitial cells that make testosterone are separated from the lining of the seminiferous tubule by a basement membrane. This arrangement forms a blood-testis barrier. The barrier protects developing sperm from the immune system. Oxygen and other nutrients diffuse easily across the blood-testis barrier but immune cells and antibodies cannot.
Spermatogenic stem cells, called spermatogonia, are found lining the basement membrane of the seminiferous tubules (opposite side of the basement membrane from the interstitial cells and blood capillaries). These cells constantly divide, with one daughter cell remaining “attached” to the basement membrane and the other differentiating to become a primary spermatocyte. Primary spermatocytes become secondary spermatocytes, secondary spermatocytes become early spermatids, and early spermatids become late spermatids.
Finally, once the developing cell has become nothing but a “payload” (acrosome to penetrate the egg plus paternal DNA to contribute to fertilization) on top of a “rocket” (flagellum plus mitochondria for energy metabolism), it is called a spermatozoon (“living sperm“).
The seminiferous tubules empty into the rete testis (Latin: “network of the testicle”). From there, sperm travel through efferent ducts into the head of the epididymis, then through the body of the epididymis, and into the tail of the epididymis.
Travel through the three