Male Reproductive System: Anatomy and Physiology (Chapter 26 Part B)

Male Reproductive System

Anatomy of the Male Reproductive System

The male reproductive system is specialized for the production, maturation, and delivery of sperm. It consists of primary sex organs (gonads), accessory ducts, and accessory glands.

• Testes: Sperm-producing male gonads located within the scrotum.

• Sperm transport occurs through a series of ducts:

  1. Epididymis

  2. Ductus deferens (vas deferens)

  3. Ejaculatory duct

  4. Urethra

• Accessory sex glands: Seminal glands (seminal vesicles), prostate, and bulbo-urethral glands secrete fluids that mix with sperm to form semen.

Scrotum and Testes

The scrotum is a sac of skin and superficial fascia that houses the testes outside the abdominopelvic cavity, maintaining a temperature about 3°C lower than core body temperature, which is essential for sperm production.

• Temperature regulation: The scrotum responds to temperature changes by moving the testes closer to or further from the body. The dartos muscle (smooth muscle) wrinkles the skin to reduce heat loss.

• Testes are divided into lobules by septa, each containing 1–4 seminiferous tubules (site of sperm production).

• Each testis is surrounded by two tunics:

  • Tunica vaginalis: Outer layer derived from peritoneum.

  • Tunica albuginea: Inner fibrous capsule.

• Blood supply:

  • Testicular arteries arise from the abdominal aorta.

  • Testicular veins arise from the pampiniform venous plexus, which helps cool arterial blood before it reaches the testes.

• Spermatic cord contains blood vessels, nerves, lymphatics, ductus deferens, and cremaster muscle.

• Interstitial (Leydig) cells outside seminiferous tubules produce androgens (mainly testosterone).

Clinical Note: Testicular Cancer

• Most common in men ages 15–35, though rare overall.

• Risk factors: history of mumps with orchitis, cryptorchidism (undescended testes).

• Sign: painless, solid mass in testis.

• 90% cure rate with surgery and often radiation or chemotherapy.

Male Duct System

The duct system transports sperm from the testes to the exterior of the body.

• Epididymis: Stores and matures sperm. The duct is about 6 m (20 ft) long. Sperm gain motility here and can be stored for several months.

• Ductus deferens: ~45 cm long, passes through the inguinal canal, expands to form the ampulla, and joins the duct of the seminal vesicle to form the ejaculatory duct. Smooth muscle propels sperm during ejaculation.

• Vasectomy: Surgical cutting and ligating of the ductus deferens; nearly 100% effective for birth control.

• Urethra: Conveys both urine and semen (at different times). Has three regions:

  • Prostatic urethra: Surrounded by prostate.

  • Intermediate (membranous) urethra: In urogenital diaphragm.

  • Spongy urethra: Runs through penis; opens at external urethral orifice.

Penis and Male Perineum

The penis is the male copulatory organ, consisting of the root, shaft, and glans penis. The prepuce (foreskin) covers the glans and may be removed by circumcision.

• Erectile tissue:

  • Corpus spongiosum: Surrounds urethra, forms glans and bulb.

  • Corpora cavernosa: Paired dorsal erectile bodies.

• Erection: Erectile tissue fills with blood, causing the penis to become rigid.

• Male perineum: Diamond-shaped region between pubic symphysis, coccyx, and ischial tuberosities; contains root of penis and anus.

Male Accessory Glands

Accessory glands produce the bulk of semen and support sperm function.

• Seminal glands (seminal vesicles):

  • Located on posterior bladder surface.

  • Produce viscous, alkaline seminal fluid (fructose, citric acid, coagulating enzyme, prostaglandins).

  • Comprise 70% of semen volume.

  • Enhance sperm motility.

• Prostate:

  • Encircles urethra inferior to bladder.

  • Secretes milky, slightly acidic fluid (citrate, enzymes, prostate-specific antigen [PSA]).

  • Plays a role in sperm activation; makes up one-third of semen volume.

• Bulbo-urethral glands:

  • Pea-sized, inferior to prostate.

  • Produce thick, clear mucus during sexual arousal to lubricate glans penis and neutralize acidic urine in urethra.

Clinical Note: Prostate Disorders

• Benign prostatic hyperplasia: Noncancerous enlargement of the prostate, common in older men; can cause urinary retention and infections.

• Prostate cancer: Third most common cause of cancer death in males; detected by digital exam and PSA levels; treated with surgery and radiation.

Semen

Semen is a mixture of sperm and accessory gland secretions, providing a medium for sperm transport and survival.

• 2–5 ml semen ejaculated, containing 20–150 million sperm/ml.

• Contains fructose for ATP production, protects and activates sperm, and facilitates movement (relaxin hormone).

• Alkaline fluid neutralizes acidity of male urethra and female vagina.

• Prostaglandins decrease viscosity of cervical mucus and stimulate reverse peristalsis in uterus.

• Antibiotic chemicals destroy some bacteria.

Male Reproductive Physiology

Male Sexual Response

• Erection:

  • Engorgement of erectile tissue with blood, initiated by sexual stimuli (touch, sight, sounds, smells).

  • Parasympathetic reflex releases nitric oxide (NO), relaxing smooth muscle and allowing blood to fill erectile tissue.

  • Expansion of corpora cavernosa compresses venous drainage, maintaining erection.

  • Corpus spongiosum keeps urethra open.

  • Impotence: Inability to attain erection.

• Ejaculation:

  • Propulsion of semen from duct system via sympathetic spinal reflex.

  • Ducts and glands contract, bladder sphincter constricts, and bulbospongiosus muscles contract to expel semen.

  • Followed by resolution: period of relaxation.

Clinical Note: Erectile Dysfunction

• Caused by insufficient NO release, vascular or nervous system issues, diabetes, or psychological factors.

• Affects ~50% of men over 40.

• Treated with drugs like sildenafil (Viagra) that enhance NO effect.

Spermatogenesis

Spermatogenesis is the process of forming male gametes (sperm) in the seminiferous tubules, beginning at puberty and continuing throughout life.

• Adult males produce ~90 million sperm daily.

• Involves three main phases:

  1. Mitosis of spermatogonia: Stem cells divide to produce primary spermatocytes.

  2. Meiosis: Primary spermatocytes (2n) undergo meiosis I to form secondary spermatocytes (n), which undergo meiosis II to form spermatids (n).

  3. Spermiogenesis: Spermatids mature into spermatozoa (sperm), developing a head (with acrosome), midpiece (with mitochondria), and tail (flagellum).

Key Cell Types in Seminiferous Tubules

• Sustentocytes (Sertoli cells): Support and nourish developing sperm, form blood-testis barrier, secrete testicular fluid, and regulate spermatogenesis via androgen-binding protein (ABP) and inhibin.

• Spermatogenic cells: Give rise to sperm.

• Myoid cells: Contract to move sperm and fluid through tubules.

• Interstitial (Leydig) cells: Produce testosterone.

Comparison Table: Spermatogenesis vs. Oogenesis

Feature	Spermatogenesis	Oogenesis
Location	Testes	Ovaries
Timing	Puberty to old age	Begins in fetal life, ends at menopause
Gametes per cycle	4 sperm	1 ovum + 2-3 polar bodies
Error rate	3-4%	~20%

Clinical Note: Infertility

• Affects ~1 in 7 couples; often due to sperm quality or quantity.

• Environmental toxins, heat, and lifestyle factors may contribute.

Hormonal Regulation of Male Reproductive System

The hypothalamic-pituitary-gonadal (HPG) axis regulates sperm and testosterone production.

1. Hypothalamus releases gonadotropin-releasing hormone (GnRH).

2. GnRH stimulates anterior pituitary to secrete FSH and LH.

3. FSH stimulates sustentocytes to release androgen-binding protein (ABP), making spermatogenic cells receptive to testosterone.

4. LH stimulates Leydig cells to release testosterone.

5. Testosterone triggers spermatogenesis.

6. Feedback inhibition by rising testosterone and inhibin (from sustentocytes) suppresses GnRH, FSH, and LH release.

Testosterone Activity and Effects

• Synthesized from cholesterol; converted to dihydrotestosterone (DHT) in prostate and estradiol in brain.

• Promotes spermatogenesis and development of male accessory organs.

• Induces male secondary sex characteristics:

  • Pubic, axillary, and facial hair growth

  • Deepening of voice (larynx enlargement)

  • Increased bone and muscle mass

  • Increased basal metabolic rate

  • Sex drive (libido)

• Deficiency leads to atrophy of accessory organs and impaired sexual function.

Summary Table: Key Structures and Functions

Structure	Function
Testes	Produce sperm and testosterone
Epididymis	Stores and matures sperm
Ductus deferens	Transports sperm during ejaculation
Seminal vesicles	Produce seminal fluid (fructose-rich)
Prostate	Secretes fluid for sperm activation
Bulbo-urethral glands	Lubricate and neutralize urethra
Penis	Delivers sperm to female tract