The Male Reproductive System

The male reproductive system consists of the testes, a series of ducts, accessory glands and supporting structures. Sperm is produced in the testes. It then flows into the epididymis where it is stored, before it is propelled through a series of ducts to the exterior. The sperm matures within the ducts. Accessory sex glands secrete substances necessary to protect the gametes and to facilitate their movement along different portions of the ducts. The supporting structures play several roles including the delivery of sperm into the female reproductive tract and the support of the testes.

The scrotum

The scrotum refers to an external sac of skin which contains the testicles and epididymis. The scrotum looks like a single sac externally, extending from the root of the penis, with ridge down the centre called the raphe. The left and right testes is separated internally by the scrotal septum. The testes is made up of a outer layer of skin and a subcutaneous layer of smooth muscles called the dartos muscle. A series of small bands of skeletal muscle collectively named the cremaster muscle (cremaster=suspender) extends from the internal oblique muscle through the spermatic cord and surrounds each testes.

The production of sperm is optimum at temperatures 2-3 °C below body temperature. This is maintained as the scrotum are outside the pelvic cavity. The cremaster and dartos muscles contract in response to cold temperatures and relaxes in warmer temperatures. As the cremaster muscle contracts, the testes are moved closer to the body allowing them to absorb more body heat. The dartos muscle causes the scrotum to become tight, reducing heat loss.

The testes

The testes are where sperm are produced. They are a pair of oval glands which are approximately 5 cm long and 2.5 cm in diameter. Each testes are partially covered by serous membrane called the tunica vaginalis (tunica=sheath). The tunica vaginalis is thought to be derived from the peritoneum during foetal development. Beneath the tunica vaginalis is a white fibrous capsule of dense irregular connective tissues called the tunica albuginea (albu = white). The capsule extends inwards splitting the testes into a series of compartments called lobules.

There are approximately 200-300 lobules, each containing 2-3 tightly coiled seminiferous tubules. This is where sperm are produced. Between the seminiferous tubules are clusters of leydig (interstitial) cells. These produce and secrete testosterone.

Seminiferous tubules

Seminiferous tubules of made up of spermatogenic cells (sperm generating) and sertoli cells which support spermatogenesis. Stem cells called spermatogonia are found towards the outer side of the tubules, with cells at later stages of meiosis towards the lumen. Around the tubule are sertoli cells which stretch from the basement membrane to the lumen. Sertoli cells nourish developing spermatocytes, spermatids and sperm, phagocytize excessive cytoplasm as the sperm develops, controls the release of sperm into the lumen, and secrete fluids which help transport the sperm. It also produces inhibin which regulates the amount of testosterone and FSH. Sertoli cells are joined to each other by tight junction internal to the basement membrane and the spermatogonia forming a blood-testis barrier. This prevents the immune response from acting upon the cells.

Ducts of the Testis

Sperm is pushed by fluids secreted by the sertoli cells along the seminiferous tubules into straight tubules which flow into a network of tubules called the rete testis (rete=network) The sperm then move into coiled ducts called efferent ducts in the epididymis which join to form a single tube called the ductus epididymis.

Epididymis

The epididymis is around 4 cm long and found on the posterior border of each testes. The epididymis is made up of 3 sections. The efferent ducts join to form the ductus epididymis in the head section, which leads to a slightly thiner body and tail sections.

The ductus epididymis is made up of a epithelial layer with branched microvilli, and a layer of smooth muscle. Loops of the duct are joined by connective tissue.

The epididymis is the site of sperm maturation which usually takes around 14 days. The epididymis is also the site where sperm is stored and helps propel sperm into the ductus deferens. Sperm can last around 7 months and unejaculated sperm is reabsorbed.

Ductus deferens

At the tail of the epididymis becomes wider and becomes the ductus deferens. This ascends along the posterior border of the epidiymis through the spermatic cord into the pelvic cavity. Here it loops over the anterior side of the pubic bone and bladder passes medial to the ureters and down the posterior side of the bladder. The end section of the duct is wider and called the ampulla. The ductus deferens has 3 layers of smooth muscle which helps convey sperm during sexual arousal.

Ejaculatory ducts

The ductus deferens joins the ejaculatory ducts which start superior to the base of the prostate gland and passes through it joining the two ductus deferens to the prostatic urethra.

The urethra

The urethra is divided into 3 sections, the prostatic urethra passing through the prostate, the membranous urethra passing through the deep muscles of the perineum, and the spongy or penile urethra as it passes through the penis. The urethra ends at the external urethral orifice.

Spermatic Cord

The spermatic cord refers to a supportive structure containing the ductus deferens, testicular artery and veins (pampiniform plexus) , nerves, lymphatic vessels and the cremaster muscle. The spermatic cord extends from the inguinal canal to the testes.

Accessory Sex Glands

Seminal vesicles

The seminal vesicles are two pouch like glands about 5cm in length. It lies at the base of the bladder between the bladder and the rectum. The seminal vesicles secrete an alkaline viscous fluid containing fructose, prostaglandins, and clotting proteins. The alkalinity of the fluid neutralizes the acidity of the urethra and female reproductive tract, which would otherwise kill sperm. Fructose provides energy to the sperm, the prostaglandins increase sperm motility and viability and is thought to stimulate contractions of smooth muscle in the female reproductive tract, and the clotting factors cause the semen to coagulate after ejaculation. Fluid from the seminal vesicles make up 60% of semen volume.

Prostate

The prostate gland is a doughnut shaped gland about 4cm wide, 3 cm high and 2cm deep. It is inferior to the urinary bladder. The prostate gland secretes a milky and slightly acidic (pH 6.5) fluid which contains citric acid, proteolytic enzymes, acid phosphatase, and seminalplasmin. Citric acid is involved in ATP production (energy) via the krebs cycle, proteolytic enzymes break down the clotting proteins in semen, and seminalplasmin is an antibiotic which destroys bacteria naturally occurring in semen and the lower female reproductive tract. The role of acid phosphatase is not known. Prostatic secretions make up about 25% of the volume of semen.

Bulbourethral glands

The bulbourethral glands are pea sized glands found inferior to the prostate on either side of the membranous urethra. The bulbourethral glands secrete an alkaline fluid and mucus into the spongy urethra. The alkaline neutralizes the acid in the urethra and the mucus lubricates the lining of the urethra and the tip of the penis reducing sperm damage during ejaculation.

The penis

The penis is subdivided into 3 parts, the root, the body and the glands penis. Within the body of the penis are 3 masses of erectile tissue, each surrounded by a covering of fibrous tissue called the tunica albuginea. These are ensheathed in 2 layers of fasciae (connective tissue). The 2 dorsolateral bodies are called the corpora cavernosa penis (copora= main body, cavernosa = hollow). The midventeral mass surrounds the urethra and are called the corpus spongiosum penis. The three bodies are held together by the deep perineal fascia which is surrounded by a layer of superficial fascia.

At the root of the penis, the corpus spongiosum is expanded and known as the bulb of the penis. The corpora cavernosa penis becomes tapered and spilits into the crura of the penis (crus= resembling a leg). The bulb is attatched to the deep muscles of the perineum and the crus are attatched to the iscial and inferior pubic rami.

At the distal end of the penis, the corpus spongiosum is enlarged forming an acorn shaped region called the glans penis. The urethra widens within the glans penis forming an opening called the external urethral orifice (orificium= making a mouth) A loose layer of skin known as the prepuce or foreskin covers the glans.

The penis is suspended by two ligaments. The fundiform ligament arises from the linea alba (connective tissue sheet at the mid line of the abdomen) and the suspensory ligament of the penis arises from the pubic symphysis. The fundiform ligament is superficial to the suspensory ligament.

The erectile tissue of the penis contain many sinuses (spaces) filled with blood supplied by arteries. During sexual arousal, a reflex reaction causes nitric oxide to be released. This causes muscles in the erectile tissues to relax allowing blood vessels to dialate. This allows more blood to flow into the sinuses. The increased pressure from the blood causes the stiffening the penis causing an erection.