10.1 Male reproductive anatomy and physiology

The male reproductive system is a complex network of organs and glands working together to produce, nourish, and deliver sperm. From the testes in the scrotum to the accessory glands, each component plays a crucial role in fertility and sexual function.

Hormonal regulation orchestrates the intricate process of spermatogenesis, while the composition of semen ensures sperm survival. Understanding these processes is key to grasping male reproductive health and potential issues that may arise.

Male reproductive system anatomy

Testes and scrotum

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• Testes are the male gonads that produce sperm and male sex hormones, primarily testosterone
• Suspended in the scrotum, which maintains the optimal temperature for spermatogenesis
  • Scrotum is a skin-covered sac that hangs outside the body to keep the testes cooler than core body temperature
  • Cremaster muscle in the scrotum can contract to pull the testes closer to the body for warmth or relax to allow them to hang lower for cooling

Epididymis, vas deferens, and ejaculatory ducts

• Epididymis is a highly coiled tube that sits atop each testis and stores and matures sperm
  • Connects the testis to the vas deferens
  • Sperm mature and gain motility as they pass through the epididymis
• Vas deferens is a muscular tube that transports mature sperm from the epididymis to the ejaculatory ducts during ejaculation
  • Also known as the ductus deferens
  • Passes through the inguinal canal and into the pelvic cavity
• Ejaculatory ducts are formed by the union of the vas deferens and the duct from the seminal vesicle
  • Empty into the urethra
  • Carry sperm and seminal fluid during ejaculation

Urethra and penis

• Urethra is a tube that runs through the penis and carries both urine and semen, though not simultaneously
  • Sphincter muscle contracts during ejaculation to prevent urine from mixing with semen
  • Divided into prostatic, membranous, and spongy portions
• Penis is the male copulatory organ
  • Consists of erectile tissue (corpora cavernosa and corpus spongiosum) that fills with blood during sexual arousal, causing an erection
  • Glans penis at the tip contains a high concentration of nerve endings
  • Foreskin (prepuce) is a fold of skin that covers the glans penis in uncircumcised males

Spermatogenesis and hormonal regulation

Stages of spermatogenesis

• Spermatogenesis is the process of sperm production that occurs in the seminiferous tubules of the testes
  • Begins at puberty and continues throughout a man's life
  • Takes approximately 64-74 days to complete
• Spermatogonia, the immature germ cells, line the seminiferous tubules
  • Undergo mitosis to maintain their population and produce primary spermatocytes
• Primary spermatocytes undergo meiosis I to produce secondary spermatocytes
  • Secondary spermatocytes then quickly undergo meiosis II to produce spermatids
• Spermatids differentiate into mature spermatozoa (sperm) through the process of spermiogenesis
  • Involves the formation of the acrosome, condensation of the nucleus, and development of the tail
  • Sertoli cells support and nourish the developing sperm throughout the process

Hormonal regulation of spermatogenesis

• Testosterone, produced by the Leydig cells in the testes, is essential for spermatogenesis
  • Acts on the Sertoli cells, which nourish and support the developing sperm
  • Maintains libido and secondary sexual characteristics
• Hypothalamus and anterior pituitary gland regulate testosterone production and spermatogenesis through a negative feedback loop
  • Hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary
  • Anterior pituitary releases follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
  • FSH stimulates Sertoli cells to support spermatogenesis, while LH stimulates Leydig cells to produce testosterone
  • High levels of testosterone provide negative feedback to the hypothalamus and anterior pituitary, reducing GnRH, FSH, and LH secretion

Semen composition and ejaculation

Semen components and their functions

• Semen, also known as seminal fluid, is the fluid that is ejaculated during sexual activity
  • Contains sperm and secretions from the seminal vesicles, prostate gland, and bulbourethral glands
  • Provides a protective and nutritive medium for sperm
• Seminal vesicles contribute about 60% of the semen volume
  • Their alkaline secretions contain fructose, which provides energy for the sperm
  • Also contain prostaglandins, which stimulate uterine contractions to aid in sperm movement
• Prostate gland contributes about 30% of the semen volume
  • Its slightly acidic secretions contain enzymes, such as prostate-specific antigen (PSA), which help to liquefy the semen and enhance sperm motility
  • Also contains zinc, citric acid, and other substances that support sperm function
• Bulbourethral glands secrete a clear, alkaline fluid
  • Neutralizes any residual acidity in the urethra from urine
  • Lubricates the tip of the penis during sexual arousal

Ejaculation process

• Ejaculation is the expulsion of semen from the urethra
  • Occurs in two phases: emission and expulsion
• Emission phase involves the smooth muscles of the vas deferens, seminal vesicles, and prostate contracting to push semen into the urethra
  • Bladder neck sphincter contracts to prevent retrograde ejaculation into the bladder
• Expulsion phase involves the rhythmic contraction of the bulbocavernosus muscle
  • Forcefully expels semen from the urethra
  • Accompanied by a feeling of orgasm due to the release of oxytocin and endorphins

Prostate and accessory glands

Prostate gland

• Prostate gland is a walnut-sized gland located inferior to the urinary bladder
  • Surrounds the urethra
  • Contributes secretions to the semen
• Prostatic fluid is slightly acidic and contains enzymes, such as prostate-specific antigen (PSA)
  • Helps to liquefy the semen after ejaculation, allowing sperm to swim more freely
  • Also contains zinc, citric acid, and other substances that support sperm function
• Disorders of the prostate gland can affect male reproductive and urinary function
  • Benign prostatic hyperplasia (BPH) is a non-cancerous enlargement of the prostate that can cause urinary difficulties
  • Prostate cancer is a malignant growth in the prostate that can spread to other parts of the body
  • Regular prostate screenings are important for early detection and treatment of these conditions

Seminal vesicles and bulbourethral glands

• Seminal vesicles are a pair of glands located posterior to the urinary bladder
  • Produce a large portion of the seminal fluid
  • Secretions contain fructose for sperm nutrition, prostaglandins to stimulate uterine contractions, and other substances that enhance sperm survival and motility
• Bulbourethral glands, also known as Cowper's glands, are pea-sized glands located inferior to the prostate on either side of the urethra
  • Secrete a clear, alkaline fluid that neutralizes residual urine acidity in the urethra
  • Provide lubrication for the tip of the penis during sexual arousal
  • Secretions may appear as pre-ejaculate (pre-cum) during sexual stimulation