Human Anatomy: The Scrotum and Testes

The Scrotum

• A sac of skin and superficial fascia located outside the abdominopelvic cavity at the root of the penis.
• Contains the paired testes.
• Sperm production is temperature sensitive.
• The scrotal location allows the testes to maintain a temperature ~3°C lower than core body temperature.
• Regulated by temperature changes.
• Cold temperature raises the scrotum to the body and wrinkles the scrotal skin, reducing surface area and heat loss.
• Warm temperature lowers the scrotum and relaxes the skin, increasing surface area and distance from the body for cooling.

The Testes

• Lie within the scrotum.
• Approximately 4 cm long and 2.5 cm wide.
• Primary function is to produce sperm and androgens (such as testosterone).
• Each testis is divided into lobules, each of which contains 1-4 tightly coiled seminiferous tubules (site of spermatogenesis).
• Sperm pass from the seminiferous tubules through the rete testis & into the efferent ducts of the epididymis.
• The epididymis is where sperm undergo further maturation (acquire motility & fertilization ability) and storage until ejaculation.
• Interstitial (Leydig) cells produce androgens.
• Sustentacular (Sertoli) cells support spermatogenesis.
• Spermatogonia, spermatocyte, and spermatid all form sperm.
• Myoid cells contract to move sperm and fluid through tubules.

The Penis

• The male copulatory organ. Responsible for delivering sperm into the female reproductive tract.
• Attached root and free shaft that ends in an enlarged tip known as the glans penis.
• Loose fold of skin that covers the glans penis is called the prepuce or foreskin.
• Internal anatomy includes: spongy urethra, corpus spongiosum, and corpora cavernosa.
• Erection occurs when vascular spaces fill with blood.

Male Duct System

• Epididymis, ductus deferens (vas deferens), ejaculatory duct, and urethra.

Epididymis

• Regions: caput (head), corpus (body), and cauda (tail).
• Structure: Highly coiled duct ~6 m long. Pseudo-stratified epithelia with stereocilia.
• Function: Sperm maturation (acquisition of motility and fertilizing ability). Sperm storage in cauda possible for several months. Sperm is ejaculated from the caudal epididymis, not the testes.

Ductus Deferens

• Also called vas deferens.
• ~45 cm long.
• Runs upwards from the epididymis through the inguinal canal into the pelvic cavity.
• Descends over the posterior bladder wall.
• Terminal end expands to form the ampulla of the ductus deferens.
• Finally joins with the duct of the seminal vesicle to form the ejaculatory duct.
• Structure: Mucosa: Pseudo-stratified epithelium. Muscular layer: Thick layers of smooth muscle.
• Ejaculation: Peristaltic waves of smooth muscle contraction expel sperm into the urethra.
• Vasectomy is a method of contraception that involves severing the ductus deferens.

Urethra

• Terminal portion of the male duct system.
• Conveys both urine and semen.
• Three regions: Prostatic urethra, membranous urethra, and spongy urethra.
• Mucosa contains scattered urethral glands that secrete mucus into the lumen just before ejaculation.

Accessory Glands

• Seminal Vesicles, Prostate, Bulbourethral glands.

Seminal Vesicles

• Large hollow glands (~15 cm uncoiled).
• Outer fibrous capsule with a thick inner layer of smooth muscle (contracts during ejaculation).
• Secretes yellow viscous alkaline fluid containing fructose, ascorbic acid, coagulating enzymes, and prostaglandins (enhance sperm motility and fertilization).
• Accounts for 70% of semen volume.

Prostate

• Single doughnut-shaped gland that encircles the urethra just inferior to the bladder.
• Outer thick capsule containing 20-30 compound tubulo-alveolar glands embedded in smooth muscle and connective tissue.
• Secretes milky acidic fluid containing citrate, several enzymes, and prostate-specific antigen (activates sperm).
• Accounts for up to 30% of semen volume.
• Contraction of smooth muscle causes prostatic secretions to enter the urethra.

Bulbourethral Glands

• Inferior to the prostate.
• Secretes a thick clear alkaline mucus (neutralizes traces of acidic urine in the urethra prior to ejaculation).

Semen

• A mixture of sperm, testicular fluid, and accessory gland secretions.
• Function:
  • Transport medium for sperm.
  • Provides nutrients and chemicals that protect and activate sperm.
  • Facilitates sperm movement.
• Components:
  • Prostaglandins: Stimulate contraction in the female tract, facilitating sperm uptake.
  • Relaxin: Enhances sperm motility.
  • Clotting factors: Promote semen coagulation.
• Typical ejaculate is 2-5 ml of semen containing 15-150 x 106 sperm/ml (pH 7.2).