Overview of the Male Reproductive System

The male reproductive system is comprised of two testes, a series of ducts and a number of accessory glands.

Testes and scrotum

The testes are suspended within the scrotum outside the body. They originally develop within the abdomen, and then descend through the inguinal canal around the 7th month of fetal development. The testes are located externally because optimum sperm production requires temperatures that are 2 – 3ºC below core body temperature.

The scrotum has two muscle tissues associated with it – the dartos muscle and the cremaster muscle. These muscles help to raise or lower the scrotum in response to changes in temperature, and help to maintain the optimum temperature for sperm production.

The testes hold tightly coiled tubes called seminiferous tubules, which contain spermatogenic cells, which will go on to form the sperm, and Sertoli cells, which provide support for the immature spermatogenic cells. In between the seminiferous tubules lie Leydig cells, which secrete testosterone.

Epididymis

The epididymis is a highly coiled tube that lies along the posterior border of the testis. It is the site of sperm maturation, where they develop the ability to swim and to fertilise the egg. It is also involved in the reabsorption of defunct sperm and helps propel sperm into the vas deferens via smooth muscle contractions.

Vas deferens

The vas deferens transports sperm from the epididymis to the urethra in preparation for ejaculation. At the prostate gland, the vas deferens are joined by the seminal vesicles, resulting in the formation of the ejaculatory ducts.

Penis

The penis contains the urethra, the passage through which semen and urine exit the body. The penis can be divided into three regions, the body, the glans penis and the root. The body contains the erectile tissue – the two corpora cavernosa and the corpus spongiosum. The corpora cavernosa contains many sinuses and the deep arteries of the penis that dilate and fill with blood during arousal and cause an erection. The urethra runs through the corpus spongiosum, which helps keep the urethra open during ejaculation. The glans penis is the distal part of the penis, and the root is the proximal part of the penis where it attaches to the body.

Sperm production

Sperm are haploid cells, which means they have half the number of chromosomes compared to a diploid somatic cell, for example a skin cell. In humans, haploid cells have 23 chromosomes, and diploid cells have 46 chromosomes.

Approximately 300 million sperm are produced in the testes every day through a process known as spermatogenesis. Human spermatogenesis takes 65 – 75 days and results in the production of four spermatozoa. 

First, a diploid spermatogonium cell undergoes mitosis, producing two diploid spermatogonia. The spermatogonia undergo differentiation to become primary spermatocytes, which then undergo the process of meiosis.

Meiosis has two steps – the first step produces two secondary spermatocytes, and the second step produces four spermatids.These spermatids then mature, develop tails, and become spermatozoa.Spermatogenesis requires the coordinated release of hormones such as gonadotropin-release hormone from the hypothalamus and LH and FSH from the pituitary gland. Problems with hormone release can result in infertility.

Semen production

Semen (also known as seminal fluid) is composed of secretions from the seminal vesicles, the prostate and the bulbourethral gland as well as sperm cells.

The seminal vesicles secrete a viscous alkaline fluid that helps neutralise the acidic environment of the urethra and the female tract. It also contains fructose, prostaglandins and clotting proteins – fructose is used for energy, prostaglands help with motility and transport of sperm within the female tract by facilitating smooth muscle contraction, and clotting proteins help semen to coagulate and stick better in the female tract.

The prostate gland secretes a milky fluid that makes up ~25% of the total semen volume. It contains citric acid, proteolytic enzymes and acid phosphatase. The secretion enters into the prostatic urethra and aids with sperm motility and viability.

The bulbourethral glands (also known as Cowper’s glands) secrete alkaline fluid and mucous. As with the seminal vesicles, the alkaline secretion helps neutralise the acidic environments of the male urethra and female reproductive tract. The mucous lubricates the end of the penis and the urethra and helps prevent sperm damage.