10.1 Male Reproductive Organs

Structures and Functions of the Male Reproductive System

The male reproductive system produces, nourishes, and delivers sperm while also secreting hormones that drive male development. Each organ has a specific job, and understanding how they connect is essential for topics like fertility, sexual function, and hormonal regulation.

Primary Reproductive Organs

The testes are the primary reproductive organs because they handle both sperm production and hormone secretion.

• Located in the scrotum, which hangs outside the body cavity to keep testicular temperature about 2–3°C below core body temperature. This cooler environment is required for normal spermatogenesis.
• Inside each testis, tightly coiled seminiferous tubules are where sperm cells are actually produced. If uncoiled, these tubules would stretch roughly 250 meters per testis.
• Between the tubules, Leydig cells (interstitial cells) produce testosterone.

The epididymis is a tightly coiled tube sitting on the posterior surface of each testis. Sperm entering the epididymis are immature and immotile. During their 2–3 week transit through the epididymis, they gain motility and the ability to fertilize an egg. The epididymis also stores mature sperm until ejaculation.

Sperm Transport and Accessory Glands

Once sperm leave the epididymis, they travel through a series of ducts and mix with secretions from three accessory glands. Each gland adds a different component to the final semen.

• Vas deferens (ductus deferens): A muscular tube roughly 30 cm long that carries sperm from the epididymis up and over the bladder to the ejaculatory duct. Its thick smooth muscle wall contracts during ejaculation to propel sperm forward. This is the tube that gets cut during a vasectomy.
• Seminal vesicles: A pair of glands posterior to the bladder that contribute about 60% of total seminal fluid volume. Their secretion is rich in fructose (the primary energy source for sperm) and prostaglandins.
• Prostate gland: Sits inferior to the bladder and surrounds the prostatic urethra. It contributes roughly 25–30% of seminal fluid, which is slightly alkaline and contains enzymes, citric acid, and zinc. The prostate also produces prostate-specific antigen (PSA), a clinically important marker.
• Bulbourethral glands (Cowper's glands): Two pea-sized glands inferior to the prostate. They release a small amount of clear, mucus-like pre-ejaculatory fluid that lubricates the urethra and neutralizes any residual acidic urine before ejaculation.

Male Copulatory Organ

The penis delivers sperm into the female reproductive tract and also serves as the urinary outlet.

• It contains three columns of erectile tissue: two corpora cavernosa (dorsal) and one corpus spongiosum (ventral), which surrounds the spongy urethra.
• The glans penis, the expanded distal tip, is densely packed with sensory nerve endings and is covered by the prepuce (foreskin) in uncircumcised males.
• The urethra running through the corpus spongiosum serves a dual role, carrying both urine and semen (though never at the same time, thanks to the internal urethral sphincter).

Spermatogenesis and Hormonal Regulation

Stages of Sperm Production

Spermatogenesis is the process of producing mature sperm from stem cells. It takes place in the seminiferous tubules and requires approximately 64 days from start to finish.

1. Spermatogonia (diploid stem cells, $2n$) line the basement membrane of the seminiferous tubules. Some divide by mitosis to maintain the stem cell population; others commit to becoming sperm.
2. Committed spermatogonia undergo mitosis to become primary spermatocytes ($2n$).
3. Primary spermatocytes complete meiosis I, producing two secondary spermatocytes ($n$).
4. Secondary spermatocytes complete meiosis II, yielding four spermatids ($n$), each with 23 chromosomes.
5. During spermiogenesis, spermatids transform into functional spermatozoa. This involves forming the acrosome cap, condensing the nucleus, developing the flagellum, and shedding excess cytoplasm.

Sertoli cells (sustentacular cells) within the tubule walls physically support and nourish developing sperm throughout this entire process. They also form the blood-testis barrier, which protects developing sperm from the immune system.

Hormonal Control Mechanisms

Spermatogenesis is regulated by the hypothalamic-pituitary-gonadal (HPG) axis:

1. The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses.
2. GnRH stimulates the anterior pituitary to secrete two gonadotropins: FSH and LH.
3. LH acts on Leydig cells, stimulating them to produce testosterone.
4. FSH acts on Sertoli cells, promoting their support of spermatogenesis. FSH also makes Sertoli cells more responsive to testosterone.
5. Testosterone itself is required locally within the tubules for sperm development to proceed.

Two negative feedback loops keep the system in balance:

• Rising testosterone levels inhibit GnRH release from the hypothalamus and LH release from the anterior pituitary.
• Inhibin, produced by Sertoli cells when sperm count is high, selectively suppresses FSH secretion.

Sperm Cell Structure and Function

Sperm Cell Anatomy

A mature spermatozoon is one of the smallest cells in the body, and every part of its structure serves a specific function.

• Head: Contains a highly condensed haploid nucleus (23 chromosomes). The anterior portion is capped by the acrosome, a specialized lysosome packed with digestive enzymes (including hyaluronidase and acrosin) needed to penetrate the egg.
• Midpiece: Packed with mitochondria arranged in a tight spiral. These mitochondria generate the ATP that powers the flagellum. The midpiece also contains a centriole that plays a role in cell division after fertilization.
• Tail (flagellum): A long whip-like structure built on a microtubule core arranged in the classic 9+2 pattern (nine outer doublets surrounding two central singlets). Its beating motion propels the sperm at roughly 1–3 mm per minute through the female reproductive tract.

Sperm Maturation and Fertilization

Sperm that have left the male body still aren't fully capable of fertilizing an egg. Two more steps must occur in the female reproductive tract:

1. Capacitation: Over approximately 5–6 hours, the sperm's plasma membrane undergoes chemical changes that remove cholesterol and glycoproteins from its surface. This destabilizes the acrosomal membrane and increases flagellar activity, producing a hyperactivated swimming pattern.
2. Acrosome reaction: When a capacitated sperm contacts the zona pellucida (the glycoprotein shell around the egg), the acrosome releases its enzymes. These enzymes digest a path through the zona pellucida so the sperm can reach the egg's plasma membrane.
3. Sperm-egg fusion: The sperm membrane binds to the egg membrane through specific receptor-ligand interactions. This triggers the cortical reaction in the egg, which hardens the zona pellucida and blocks additional sperm from entering (preventing polyspermy).

Semen Composition and Function

Components of Seminal Fluid

Semen is far more than just sperm. Sperm cells make up only about 1% of total ejaculate volume. The rest is seminal plasma, a mixture of secretions from the accessory glands.

A normal sperm concentration ranges from 15 to over 200 million sperm per mL. Both motility (percentage of sperm that are swimming) and morphology (percentage with normal shape) are clinically important for assessing fertility.

Physiological Roles of Semen

• Nutrition and energy: Fructose from the seminal vesicles is the primary fuel source for sperm. Citric acid from the prostate also contributes to ATP production.
• pH buffering: Semen has an alkaline pH of about 7.2–8.0. This is critical because the vaginal environment is acidic (pH ~3.8–4.5), and sperm cannot survive long in acidic conditions.
• Transport and protection: After ejaculation, semen initially coagulates (thanks to clotting factors from the seminal vesicles), which helps it adhere near the cervix. Within 15–30 minutes, PSA and other prostatic enzymes liquefy the coagulum, freeing the sperm to swim into the uterus.

Erection and Ejaculation Processes

Mechanism of Penile Erection

Erection is primarily a vascular event controlled by the parasympathetic nervous system.

1. Sexual arousal (physical or psychological) activates parasympathetic neurons that release nitric oxide (NO) at the vascular smooth muscle of the penile arteries.
2. Nitric oxide triggers smooth muscle relaxation, which dilates the deep arteries of the penis and increases blood flow into the sinusoidal spaces of the corpora cavernosa and corpus spongiosum.
3. As these spaces fill with blood, the expanding erectile tissue compresses the veins against the surrounding tunica albuginea. This veno-occlusive mechanism traps blood in the penis and maintains rigidity. Intracavernous pressure can exceed 100 mmHg during full erection.

Phases of Ejaculation

Ejaculation is a sympathetic reflex with two distinct phases:

1. Emission phase: Sympathetic stimulation causes the smooth muscle of the vas deferens, seminal vesicles, and prostate to contract. Sperm and glandular secretions are deposited into the prostatic and membranous urethra, where they mix. The internal urethral sphincter closes to prevent retrograde flow into the bladder.
2. Expulsion phase: Rhythmic contractions of the bulbospongiosus and ischiocavernosus muscles (somatic motor neurons) forcefully propel semen out through the external urethral orifice. Average ejaculate volume is 2–5 mL.

After ejaculation, a refractory period occurs during which another erection and ejaculation are temporarily impossible. This period varies widely between individuals and tends to lengthen with age.

Endocrine Function of the Testes

Testosterone Production and Effects

Leydig cells in the interstitial tissue between seminiferous tubules are the primary source of testosterone. They are stimulated by LH, and an adult male typically produces about 5–7 mg of testosterone per day.

Testosterone has widespread effects throughout the body:

• Reproductive: Maintains spermatogenesis, supports libido, and promotes growth of the reproductive organs
• Secondary sexual characteristics: Deepening of the voice, growth of facial and body hair, male-pattern fat distribution
• Musculoskeletal: Increases muscle mass and strength; promotes bone density and epiphyseal closure during puberty
• Behavioral/cognitive: Influences mood, aggression, and cognitive function

Testosterone circulates in the blood mostly bound to sex hormone-binding globulin (SHBG) and albumin. Only the small free (unbound) fraction is biologically active.

Other Testicular Hormones

• Inhibin: Produced by Sertoli cells, this hormone selectively suppresses FSH from the anterior pituitary. It acts as a direct signal of sperm production status: when sperm count is adequate, inhibin rises and dials back FSH.
• Estradiol: Small amounts are produced when the enzyme aromatase converts testosterone to estradiol. Despite being thought of as a "female hormone," estradiol in males is important for maintaining bone mineral density and modulating libido.
• Andropause: Unlike the abrupt hormonal shift of menopause in females, males experience a gradual decline in testosterone production beginning around age 30, dropping roughly 1% per year. Over decades, this can contribute to decreased muscle mass, reduced bone density, lower libido, and changes in mood or energy. The term "andropause" is somewhat informal; clinically, significant decline is referred to as late-onset hypogonadism.