# Lecture 21: The Male Reproductive System

## Anatomy and Physiology II

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## Learning Objectives

By the end of this lecture, students will be able to:

1. Identify the major organs of the male reproductive system and their functions
2. Describe the anatomy and histology of the testis, including seminiferous tubules and interstitial cells
3. Outline the process of spermatogenesis and the stages of sperm maturation
4. Describe the structure of a mature spermatozoon
5. Trace the pathway of sperm from the testis to the exterior
6. Describe the composition of semen and the contributions of accessory glands
7. Explain the hormonal regulation of male reproductive function (hypothalamic-pituitary-gonadal axis)

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## Lecture Content

### I. Overview of the Male Reproductive System

The male reproductive system serves three primary functions: production of sperm (spermatogenesis), production of testosterone, and delivery of sperm to the female reproductive tract. Its organs include the **testes** (gonads) that produce sperm and testosterone; the **duct system** consisting of the epididymis, ductus (vas) deferens, ejaculatory duct, and urethra for transporting and maturing sperm; the **accessory glands** including the seminal vesicles, prostate gland, and bulbourethral glands that produce seminal fluid; and the **external genitalia** consisting of the penis and scrotum.

### II. The Scrotum and Testes

#### Scrotum

The scrotum is a sac of skin and superficial fascia hanging from the perineum, divided into two compartments by the scrotal septum, each containing one testis. Its primary function is **temperature regulation**, as the testes must be maintained at approximately 2-3 degrees Celsius below core body temperature for proper spermatogenesis. Three mechanisms accomplish this. The **cremaster muscle**, a skeletal muscle, elevates the testes toward the body in cold temperatures. The **dartos muscle**, smooth muscle in the scrotal wall, wrinkles the scrotal skin to reduce surface area in cold. The **pampiniform venous plexus** acts as a countercurrent heat exchanger, with venous blood from the testis cooling arterial blood entering the testis.

#### Testes

The testes are paired, oval organs measuring approximately 4 cm in length and 2.5 cm in diameter, each weighing about 10-15 g. They are surrounded by the **tunica albuginea**, a dense white fibrous capsule that extends inward as **septa**, dividing each testis into approximately 250-300 **lobules**. Each lobule contains 1-4 **seminiferous tubules**, the sites of sperm production, with a total tubule length of approximately 250 m per testis. Between the tubules lie the **interstitial (Leydig) cells**, which produce testosterone.

#### Seminiferous Tubule Histology

The wall of each seminiferous tubule contains two types of cells. The **spermatogenic cells** are germ cells at various stages of development arranged from the periphery to the lumen: spermatogonia at the basal lamina, then primary spermatocytes, secondary spermatocytes, spermatids, and finally spermatozoa at the lumen. The **sustentacular (Sertoli) cells** are large supporting cells extending from the basal lamina to the lumen. They form the **blood-testis barrier** through tight junctions between adjacent Sertoli cells, protecting developing sperm from immune attack. Sertoli cells also nourish developing sperm, phagocytize defective cells, secrete testicular fluid, secrete androgen-binding protein (ABP), secrete inhibin, and mediate the effects of FSH and testosterone on spermatogenesis.

<image>A multi-panel figure of testicular anatomy. Panel A: Sagittal view of the male reproductive system showing the testis within the scrotum, epididymis on the posterior surface, ductus deferens ascending through the inguinal canal, seminal vesicle, prostate gland, bulbourethral glands, ejaculatory duct, and urethra passing through the penis. Panel B: A cross-section of the testis showing the tunica albuginea, septa dividing the testis into lobules, coiled seminiferous tubules within each lobule, the rete testis (network of channels collecting from tubules), efferent ductules, and the epididymis. Panel C: A magnified cross-section of a single seminiferous tubule showing the layered arrangement of spermatogenic cells from the periphery (spermatogonia at the basal lamina) through primary and secondary spermatocytes to spermatids and mature spermatozoa near the lumen. Tall Sertoli cells span the full thickness of the epithelium with tight junctions forming the blood-testis barrier. Interstitial (Leydig) cells are shown in the connective tissue between adjacent tubules.</image>

### III. Spermatogenesis

Spermatogenesis is the process of producing mature spermatozoa from spermatogonia. It occurs in the seminiferous tubules, begins at puberty, and continues throughout life, taking approximately **64-74 days** from spermatogonium to mature sperm.

#### Stages

The first stage is **spermatocytogenesis** (mitotic division). **Spermatogonia** (type A) are stem cells at the basal lamina that undergo mitosis to self-renew and produce type B spermatogonia. Type B spermatogonia divide by mitosis to form **primary spermatocytes**. All cells at this stage are diploid (2n = 46 chromosomes).

The second stage is **meiosis**. The primary spermatocyte (2n) undergoes meiosis I to produce two **secondary spermatocytes** (n = 23, each chromosome still having 2 chromatids). Meiosis I includes crossing over and independent assortment, generating genetic diversity. Each secondary spermatocyte (n) undergoes meiosis II to produce two **spermatids** (n = 23, single chromatids). The net result per primary spermatocyte is **four haploid spermatids**.

The third stage is **spermiogenesis**, a differentiation process involving no cell division. Spermatids undergo extensive remodeling to become spermatozoa: the nucleus condenses, excess cytoplasm is shed and phagocytized by Sertoli cells, the acrosome forms from the Golgi apparatus (containing enzymes for penetrating the oocyte), a centriole forms the flagellum (tail), and mitochondria spiral around the proximal portion of the tail (midpiece) to provide ATP for motility.

The final step is **spermiation**, the release of mature spermatozoa from Sertoli cells into the tubular lumen.

### IV. Structure of a Mature Spermatozoon

A mature spermatozoon is approximately 60 micrometers in total length and consists of three main regions. The **head** is flattened and oval, containing the highly condensed haploid nucleus. Covering the anterior two-thirds of the nucleus is the **acrosome**, a cap-like structure containing hydrolytic enzymes (hyaluronidase, acrosin) needed for penetration of the oocyte. The **midpiece (neck)** contains a core of microtubules (the axoneme) surrounded by a mitochondrial sheath consisting of spirally arranged mitochondria that provide ATP for flagellar movement. The **tail (flagellum)** consists of the principal piece (the longest portion, with the 9+2 microtubule arrangement of the axoneme surrounded by a fibrous sheath) and the end piece (the narrowest portion). The flagellum propels the sperm at approximately 1-4 mm per minute in the female tract.

### V. Sperm Transport — Duct System

From their site of production, sperm travel through the **seminiferous tubules** to the **straight tubules** to the **rete testis** (a network in the mediastinum testis) to the **efferent ductules** (12-20) and then into the **epididymis**. The epididymis is a comma-shaped organ on the posterior surface of the testis consisting of approximately 6 m of coiled duct organized into three regions: the head (which receives efferent ductules), body, and tail (which is continuous with the ductus deferens). The epididymis performs three key functions: sperm maturation (during which sperm gain motility and fertilization capacity over approximately 20 days), sperm storage, and propulsion via peristalsis.

The **ductus (vas) deferens** is an approximately 45 cm long muscular tube that ascends from the epididymis through the inguinal canal into the pelvic cavity. Its thick smooth muscle wall generates strong peristaltic contractions during ejaculation. **Vasectomy** involves surgical cutting and sealing of the ductus deferens for male sterilization. The ductus deferens expands at the posterior bladder to form the **ampulla**, which joins with the duct of the seminal vesicle to form the **ejaculatory duct** that passes through the prostate and opens into the prostatic urethra.

The **urethra** serves as the shared pathway for urine and semen (though not simultaneously) and consists of the prostatic urethra, membranous urethra, and spongy (penile) urethra, terminating at the external urethral orifice.

### VI. Accessory Glands and Semen

#### Seminal Vesicles

The seminal vesicles are paired glands on the posterior surface of the bladder that produce approximately 60-70% of semen volume. Their secretion is an alkaline, viscous fluid rich in fructose (providing energy for sperm), prostaglandins (stimulating smooth muscle contractions in the female tract), and clotting factors (temporarily coagulating semen after ejaculation).

#### Prostate Gland

The prostate is a single, walnut-sized gland inferior to the bladder, surrounding the prostatic urethra. It produces approximately 25-30% of semen volume as a slightly acidic, milky fluid containing citrate (a nutrient), proteolytic enzymes (PSA and pepsinogen, which liquefy coagulated semen after 15-30 minutes), and acid phosphatase. The prostate is clinically significant because benign prostatic hyperplasia (BPH) and prostate cancer are common in older males.

#### Bulbourethral (Cowper's) Glands

The bulbourethral glands are paired, pea-sized glands inferior to the prostate that produce approximately 5% of semen volume. Their secretion is a clear, alkaline mucus released before ejaculation to neutralize acidic urine residues in the urethra and provide lubrication.

#### Semen

Semen is a mixture of spermatozoa and secretions from the accessory glands. The volume per ejaculation is 2-5 mL, containing 20-150 million sperm per mL (counts below 15 million/mL constitute oligospermia and are associated with fertility issues). Semen has a slightly alkaline pH of 7.2-7.6 that protects sperm from the acidic vaginal environment. It also contains fructose, prostaglandins, clotting and fibrinolytic enzymes, buffers, and zinc.

<image>A diagram showing the anatomy and contributions of the male accessory reproductive glands. Panel A: A posterior view of the male reproductive tract showing the seminal vesicles (paired, on the posterior bladder surface), the prostate gland (surrounding the prostatic urethra), and the bulbourethral glands (inferior to the prostate). Ducts from each gland are shown converging on the urethra. Panel B: A pie chart showing the relative contributions to semen volume — seminal vesicles (60–70%), prostate gland (25–30%), bulbourethral glands (~5%), and spermatozoa with testicular fluid (< 5%). Each segment lists the key components of that gland's secretion. Panel C: A labeled diagram of a single mature spermatozoon at high magnification showing the acrosome, nucleus (head), centriole, mitochondrial sheath (midpiece), axoneme, fibrous sheath (principal piece of the tail), and end piece, with approximate dimensions.</image>

### VII. Hormonal Regulation — Hypothalamic-Pituitary-Gonadal (HPG) Axis

The HPG axis controls male reproductive function through a hierarchical cascade. The **hypothalamus** releases **GnRH (gonadotropin-releasing hormone)** in a pulsatile manner, which travels via the hypothalamic-hypophyseal portal system to the **anterior pituitary**. In response to GnRH, the anterior pituitary releases two gonadotropins: **FSH (follicle-stimulating hormone)**, which acts on Sertoli cells to stimulate spermatogenesis, promote sperm production, and stimulate secretion of ABP and inhibin; and **LH (luteinizing hormone)**, which acts on Leydig (interstitial) cells to stimulate testosterone synthesis and secretion.

**Testosterone** is the primary androgen produced by Leydig cells. It is required for spermatogenesis (acting on Sertoli cells via ABP locally), development and maintenance of male secondary sex characteristics (facial and body hair, deep voice, increased muscle mass, male-pattern fat distribution), growth of male reproductive organs, libido, and anabolic effects including protein synthesis stimulation, bone growth (with epiphyseal closure at puberty), and erythropoiesis. In the blood, testosterone circulates bound to sex hormone-binding globulin (SHBG, approximately 60%) and albumin (approximately 38%), with only approximately 2% free and active. In target tissues, testosterone is converted to **dihydrotestosterone (DHT)** by 5-alpha-reductase (in prostate, skin, and hair follicles), with DHT being more potent than testosterone. Some testosterone is also converted to **estradiol** by aromatase, which is important for bone health and epiphyseal closure.

**Negative feedback** maintains the system in balance. Testosterone inhibits GnRH at the hypothalamus and LH at the anterior pituitary. **Inhibin** from Sertoli cells selectively inhibits FSH release from the anterior pituitary.

### VIII. Clinical Correlations

**Cryptorchidism** is an undescended testis that, if uncorrected, leads to impaired spermatogenesis and increased risk of testicular cancer. **Benign prostatic hyperplasia (BPH)** is age-related prostate enlargement that compresses the urethra and causes difficulty urinating. **Prostate cancer** is the most common cancer in males, detected through PSA screening and digital rectal examination. **Erectile dysfunction** is the inability to achieve or maintain an erection, with causes including vascular disease, neuropathy, medications, and psychological factors, and is treated with PDE5 inhibitors such as sildenafil. **Hypogonadism** involves insufficient testosterone production, leading to decreased libido, muscle loss, osteoporosis, and infertility. **Testicular torsion** is twisting of the spermatic cord causing ischemia of the testis and constituting a surgical emergency.

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