The male reproductive system
The reproductive systems produce gametes and sex hormones. In males, the system makes and delivers sperm.
Overview
Both reproductive systems have primary sex organs (gonads) that make gametes and hormones, and accessory structures that store, transport, and support gametes. Male gonads are the testes; female gonads are the ovaries. Gametes are haploid, made by meiosis, so fertilisation restores the diploid number.
The testes
The testes sit in the scrotum, outside the body cavity, where they are about 2-3 degrees C cooler than core temperature, which sperm production requires. Inside, coiled seminiferous tubules produce sperm. Between the tubules, interstitial (Leydig) cells make testosterone. Sustentacular (Sertoli) cells nourish developing sperm.
Spermatogenesis
Sperm production begins at puberty and continues through life, producing millions of sperm per day. Stem cells divide by mitosis, then undergo meiosis to form haploid spermatids, which mature into sperm. The whole process takes about 64-74 days.
Sperm structure
A sperm has a head containing the nucleus with 23 chromosomes, capped by an acrosome holding enzymes that help penetrate the egg; a midpiece packed with mitochondria to power movement; and a tail (flagellum) that propels it.
The duct system
Sperm mature and are stored in the epididymis, on the back of each testis. During ejaculation, they travel through the vas deferens, which joins the duct from the seminal vesicle to form the ejaculatory duct, then pass through the urethra, which runs through the penis. A vasectomy cuts the vas deferens as a contraceptive method.
Accessory glands and semen
Semen is sperm plus fluids from accessory glands. The seminal vesicles provide fructose for energy; the prostate adds a slightly alkaline fluid that helps sperm survive the acidic vagina; the bulbourethral glands secrete a clear lubricating fluid. An ejaculate typically contains tens to hundreds of millions of sperm.
Hormonal control
The hypothalamus releases GnRH, which triggers the anterior pituitary to release FSH and LH. LH stimulates Leydig cells to make testosterone; FSH, with testosterone, stimulates sperm production. Testosterone drives puberty changes, muscle and bone growth, and sex drive, and feeds back to reduce GnRH, LH, and FSH.
Male health topics
The prostate often enlarges with age (benign prostatic hyperplasia), squeezing the urethra and causing frequent urination. Prostate cancer is one of the most common cancers in men. Testicular cancer is most common in young men and is highly treatable when found early. Anabolic steroid misuse suppresses natural testosterone and sperm production through negative feedback.
Key takeaways
Testes in the scrotum make sperm in seminiferous tubules and testosterone in Leydig cells. Spermatogenesis uses meiosis and takes about two months. Sperm have an acrosome, mitochondria-rich midpiece, and tail. Sperm travel from epididymis through vas deferens and urethra. Accessory glands make most of semen. GnRH, LH, FSH, and testosterone control the system by negative feedback.
Flashcard
Organs making gametes and hormones
Check your understanding
Why are the testes located outside the body cavity?