Male Reproductive System Notes

Big Picture

The male reproductive system has one job: produce sperm, deliver it to the female reproductive tract, and secrete the hormones that make all of that possible. Every structure on the next pages does some part of that work.

Foundation

The Four Components of the Male Reproductive System

Testes (gonads). Produce sperm and secrete hormones.
System of ducts: epididymis, ductus deferens, ejaculatory ducts, urethra. Transport and store sperm. Assist in sperm maturation. Convey sperm to the exterior.
Accessory glands: seminal vesicles, prostate, bulbourethral glands. Produce the fluid portion of semen.
Supporting structures: scrotum and penis. Protect the testes and deliver sperm into the female reproductive tract.

Anchor point: when you're learning a new structure, ask yourself which of these four jobs it serves. That's how the parts connect.

Scrotum & Testes

Sperm production needs a temperature 2 to 3 degrees Celsius below core body temp. Everything about the scrotum's design serves that single requirement.

External Anatomy

Scrotum

The supporting structure for the testes. Loose skin and subcutaneous layer hanging from the root of the penis.

Externally

A single pouch separated into lateral portions by a median ridge called the raphe.

Internally

The scrotal septum divides the scrotum into two sacs, each containing one testis. The septum is made of subcutaneous layer plus the dartos muscle.

Temperature Regulation

Cremaster & Dartos Muscles

These two muscles work together to keep testicular temperature 2 to 3 degrees C below core body temp.

Cremaster Muscle

Series of small skeletal muscle bands that descend as an extension of the internal oblique through the spermatic cord and surround the testes.

Dartos Muscle

Bundles of smooth muscle within the subcutaneous layer of the scrotal skin.

How They Work Together

Cold: Both contract. Cremaster pulls testes toward the abdominal cavity to conserve heat by bringing them closer to core body temperature. Dartos tightens scrotal skin to reduce heat loss.

Warm: Both relax. Testes drop away from the body. Scrotal skin loosens and dissipates heat.

Gonad

Testes

Paired oval glands in the scrotum. Also called testicles.

Development

Develop near the kidneys in the posterior abdomen. Begin descent in the last half of the 7th month of gestation, passing through the inguinal canals into the scrotum.

Function

Produce sperm via spermatogenesis
Secrete testosterone (the primary male sex hormone)

Histology

Tissue Layers Covering the Testes

From superficial to deep:

Tunica vaginalis. Serous membrane derived from the peritoneum. Forms during testicular descent. Provides a partial cover.
Tunica albuginea. White fibrous capsule of dense irregular CT. Extends inward to form septa, dividing each testis into 200 to 300 lobules.
Seminiferous tubules. Tightly coiled tubes (1 to 3 per lobule). The actual site of spermatogenesis.

Two Cell Types in the Seminiferous Tubules

Spermatogenic cells: sperm-forming cells
Sertoli (sustentacular) cells: support spermatogenesis

Support Cell

Sertoli (Sustentacular) Cells

Large cells embedded among the spermatogenic cells, extending from the basement membrane to the lumen of the seminiferous tubule.

Joined to each other by tight junctions, which form the blood-testis barrier.

Functions

Nourish spermatocytes, spermatids, and sperm
Phagocytize excess spermatid cytoplasm as development proceeds
Control movement of spermatogenic cells through the tubule
Control the release of sperm into the lumen
Produce fluid for sperm transport
Secrete the hormone inhibin
Regulate the effects of testosterone and FSH

Hormone-Producing Cell

Leydig (Interstitial) Cells

Clusters of cells found in the spaces between adjacent seminiferous tubules.

Function

Secrete testosterone, the most prevalent androgen.

What Testosterone Does in Males

Promotes masculine characteristics
Promotes libido (sex drive)

Androgen: a hormone that promotes masculine characteristics.

Immune Protection

The Blood-Testis Barrier

Tight junctions between Sertoli cells form a barrier internal to the basement membrane and spermatogonia.

Why It Matters

Substances must first pass through Sertoli cells before they can reach developing sperm.
It isolates developing gametes from the blood, preventing the immune system from mounting an attack against sperm cell surface antigens (which it would otherwise recognize as foreign).
Spermatogonia are not behind the barrier. Only the more mature stages are protected.

Sperm have unique surface antigens that the immune system would attack as foreign. The barrier is the only thing standing between your sperm and your immune cells.

Clinical Correlation: Hydrocele

Definition: Collection of serous fluid in the tunica vaginalis.

Cause: Injury to the testes or inflammation of the epididymis.

Treatment: Usually none required.

Clinical Correlation: Cryptorchidism

Definition: Testes fail to descend into the scrotum.

Prevalence: 3% of full-term infants, 30% of premature infants.

Treatment: 80% resolve spontaneously in the first year. Surgical correction if not descended by 18 months.

If untreated:

Bilateral cryptorchidism leads to sterility because the higher temperature destroys cells in the early stages of spermatogenesis.
30 to 50 times greater risk of testicular cancer.

Spermatogenesis & Hormonal Control

Spermatogenesis takes about two months from start to finish, and 300 million sperm complete the journey every day. None of this happens without a tight hormonal feedback loop running between the hypothalamus, anterior pituitary, and testes.

Exam Scope: Anatomy vs. Physiology

For this anatomy exam, focus on the structure of the spermatozoon (head, acrosome, neck, middle piece, principal piece, end piece). The spermatogenesis process itself is physiology and will not be tested here.

For hormones, you only need to know what cell or gland produces each hormone and what each hormone does. Specific feedback mechanisms (negative feedback loops, full hormonal axis details) are physiology content and will be covered there.

Sperm Formation

Spermatogenesis: Step by Step

Definition: The sequence of events by which spermatogonia transform into mature sperm.

Dormancy. Spermatogonia have been dormant in the seminiferous tubules since fetal development. At puberty, they begin to increase in number.
Mitotic divisions. Spermatogonia undergo several mitotic divisions to form primary spermatocytes, the largest germ cells in the seminiferous tubules.
Meiosis I (reduction division). Each primary spermatocyte divides into two haploid secondary spermatocytes, half the size of primary spermatocytes.
Meiosis II. Each secondary spermatocyte divides into two haploid spermatids (so 4 spermatids per primary spermatocyte).
Spermiogenesis. Spermatids gradually transform into mature sperm.
Release. Mature sperm enter the lumen of the seminiferous tubule.

The most mature sperm cells are found progressively nearest the lumen. The progression is: primary spermatocytes → secondary spermatocytes → spermatids → spermatozoa → lumen.

Two Important Numbers

Total time: about two months
Daily production: about 300 million sperm

Sperm Anatomy

Structure of a Spermatozoon

Highly adapted for reaching and penetrating an oocyte.

Head

Flattened and pointed. Contains a nucleus with 23 highly condensed chromosomes.

Acrosome: cap-like vesicle covering the anterior 2/3 of the nucleus. Filled with enzymes that help the sperm penetrate a secondary oocyte for fertilization.

Tail (4 Subdivisions)

Neck. Constricted region just behind the head. Contains centrioles that form the microtubules of the rest of the tail.
Middle piece. Mitochondria arranged in a spiral. Provide ATP for locomotion and metabolism.
Principal piece. The longest part of the tail.
End piece. Terminal tapering portion.

Survival

Within the female reproductive tract: no more than 48 hours.

Hypothalamic-Pituitary-Gonadal Axis

Hormonal Control of Testicular Function

At puberty, hypothalamic neurosecretory cells increase secretion of GnRH (gonadotropin-releasing hormone). GnRH stimulates gonadotrophs in the anterior pituitary to release two gonadotropins: LH and FSH.

Luteinizing Hormone (LH)

Stimulates Leydig (interstitial) cells to secrete testosterone.
When testosterone levels are high enough, testosterone inhibits both LH and GnRH (negative feedback).
In some target cells (external genitalia, prostate), 5-alpha reductase converts testosterone into dihydrotestosterone (DHT), another androgen.

Follicle-Stimulating Hormone (FSH)

Indirectly stimulates spermatogenesis.
FSH plus testosterone stimulate Sertoli cells to secrete androgen binding protein (ABP) into the lumen of the seminiferous tubules. ABP binds testosterone and keeps the local concentration high.
Testosterone stimulates the final steps of spermatogenesis.
When spermatogenesis reaches sufficient levels, Sertoli cells release inhibin, a protein hormone that inhibits FSH secretion by the anterior pituitary.

Negative Feedback Loop (Testosterone)

Testosterone high → inhibits GnRH from hypothalamus → less LH from anterior pituitary → Leydig cells secrete less testosterone → testosterone falls → GnRH released again → cycle restarts.

Hormone Effects

Effects of Testosterone & DHT Across the Lifespan

Both testosterone (T) and DHT bind to the same androgen receptors at target cells throughout the body. The hormone-receptor complex regulates gene expression.

1. Prenatal Development

T stimulates male pattern of reproductive system ducts and descent of testes.
DHT stimulates development of external genitals.
T converts in the brain to estrogen, which may play a role in development of certain brain regions.

2. Puberty (Sexual Characteristics)

Development and enlargement of male sex organs.
Secondary sex characteristics: muscular and skeletal growth, wide shoulders, narrow hips, facial and chest hair, thickening of skin, increased sebaceous secretion, enlargement of larynx (deeper voice).

3. Sexual Function

Male sex behavior, spermatogenesis, libido in both sexes.
Note: in females, the adrenal cortex is the main source of androgens.

4. Anabolic Effects

Stimulate protein synthesis. Heavier muscle and bone mass.

The Duct System: Path of Sperm

Every duct exists to do one of three things: transport sperm, store sperm, or mature sperm. Trace the path from the seminiferous tubules to the external urethral orifice and you have the entire male duct system.

The Pathway

Sperm Path Through the Male Duct System

Sertoli cells produce fluid. The fluid pressure pushes sperm and fluid along the path:

Seminiferous tubules Straight tubules Rete testis Efferent ducts Epididymis (ductus epididymis) Ductus (vas) deferens Ejaculatory duct Prostatic urethra Membranous urethra Spongy urethra External urethral orifice

Memorize this flow. Every other detail in this section attaches to one stop on this path.

Sperm Maturation

Epididymis

Comma-shaped organ along the posterior border of each testis.

Function

Sperm maturation: sperm acquire motility and ability to fertilize over about 14 days.
Propels sperm by peristaltic contraction of smooth muscle.
Stores sperm. Viable up to several months.

Three Parts

Head: superior part. Tightly coiled ductus epididymis joined by efferent ducts from the testis.
Body: narrow mid-portion.
Tail: smaller inferior portion. Continues as the ductus (vas) deferens.

Histology

Lined with pseudostratified columnar epithelium.
Encircled with smooth muscle.
Stereocilia line the columnar cells. Long branching microvilli that increase surface area for reabsorption of degenerated sperm.

Sperm Transport

Ductus (Vas) Deferens

Continuation of the tail of the epididymis with increased diameter and less coiling.

Path

Ascends along the posterior border of the epididymis through the spermatic cord, enters the pelvic cavity, loops over the ureter, passes down the posterior surface of the bladder.

Function

Conveys sperm from epididymis toward urethra by peristaltic contractions during sexual arousal.
Can store sperm for several months.

Special Features

Ampulla: dilated terminal portion.
Mucosa: pseudostratified columnar epithelium and lamina propria.
Muscularis: three layers of smooth muscle. Inner and outer longitudinal, middle circular.

Supporting Structure

Spermatic Cord

Supporting structure that ascends out of the scrotum.

Ductus deferens
Pampiniform plexus (testicular artery and veins drain the testes and carry testosterone into circulation)
Autonomic nerves
Lymphatic vessels
Cremaster muscle

Anatomical Passageway

Inguinal Canal

Passageway in the anterior abdominal wall, superior and parallel to the inguinal ligament.

Structures Through the Inguinal Canal (Male)

Spermatic cord
Ilioinguinal nerve

Two Parts

Origin (deep ring): slit-like opening in the aponeurosis of the transversus abdominis.
Termination (superficial ring): triangular opening in the aponeurosis of the external oblique.

In females, the round ligament of the uterus and the ilioinguinal nerve pass through the inguinal canal.

Convergence Point

Ejaculatory Ducts

Formed by the duct from the seminal vesicle joining the ampulla of the ductus deferens.

Location

Just superior to the base of the prostate. Pass inferiorly and anteriorly through the prostate. Terminate in the prostatic urethra.

Function

Eject sperm and seminal vesicle secretions into the prostatic urethra just before semen is released through the urethra.

Shared Pathway

Urethra (3 Subdivisions)

Shared terminal duct of the reproductive and urinary systems. Passes through the prostate, deep muscles of the perineum, and the penis.

Function

Passageway for both semen and urine.

Three Subdivisions

Prostatic urethra: passes through the prostate.
Intermediate (membranous) urethra: passes through the deep muscles of the perineum.
Spongy urethra: passes through the corpus spongiosum of the penis.

The external urethral orifice is where the spongy urethra meets the external surface.

Clinical Correlation: Varicocele ("bag of worms")

Definition: Swelling in the scrotum due to dilation of the veins draining the testes (pampiniform plexus).

Symptoms: More obvious when standing.

Treatment: No treatment required unless it impairs sperm production or contributes to infertility.

Accessory Glands & Semen

The accessory glands (seminal vesicles, prostate, bulbourethral glands) make up the bulk of the volume of semen. Sperm are only a tiny fraction of the final fluid. Each gland contributes something specific.

Largest Volume Contributor (60%)

Seminal Vesicles

Paired pouch-like glands.

Location

Posterior to the base of the urinary bladder, anterior to the rectum.

Secretes

An alkaline, viscous fluid containing:

Fructose: used by sperm for ATP production.
Prostaglandins: support motility and viability. May stimulate smooth muscle contraction in the female reproductive tract.
Clotting proteins: help semen coagulate after ejaculation.

Why Alkaline Matters

Neutralizes the acidic environment of the male and female reproductive tracts. Acid kills sperm.

Volume

About 60% of total semen volume.

Donut-Shaped Gland (25%)

Prostate

One donut-shaped gland the size of a golf ball.

Location

Inferior to the urinary bladder. Surrounds the prostatic urethra.

Secretes

A milky, slightly acidic fluid (pH 6.5) entering the prostatic urethra through many prostatic ducts.

Citric acid: used by sperm for ATP production via Krebs cycle.
Proteolytic enzymes (break down clotting proteins from seminal vesicles): PSA (prostate-specific antigen), pepsinogen, lysozyme, amylase, hyaluronidase.
Acid phosphatase: unknown function.
Seminalplasmin: antibiotic that destroys bacteria.

Lifespan Changes

Slow growth from birth to puberty. Rapid expansion until age 30. Stable until 45. May enlarge after 45.

Volume

About 25% of total semen volume.

Pre-Ejaculate Glands

Bulbourethral Glands (Cowper's Glands)

Pea-sized paired glands.

Location

Inferior to the prostate, on either side of the membranous urethra within the deep muscles of the perineum. Ducts open into the spongy urethra.

Functions

During sexual arousal, secrete an alkaline fluid that protects sperm by neutralizing acids from urine remaining in the urethra.
Secrete mucus for lubrication of the end of the penis. Decreases the number of sperm damaged during ejaculation.
Provide sperm with transport medium, nutrients, and protection from the hostile acidic environment.

The Final Product

Semen

Definition: "Seed." A mixture of sperm and seminal fluid (combined secretions of seminiferous tubules, seminal vesicles, prostate, and bulbourethral glands).

Volume & Sperm Count

2.5 to 5 mL per ejaculation
50 to 150 million sperm per mL
Fewer than 20 million per mL is likely infertile

pH

Slightly alkaline (7.2 to 7.7). Necessary because sperm move through both an acidic male urethra and an acidic female reproductive tract.

What Each Component Looks Like

Prostatic secretion: milky appearance.
Seminal vesicle and bulbourethral fluid: sticky.

Coagulation & Liquefaction

Liquid semen coagulates within 5 minutes (clotting proteins from seminal vesicles).
After 10 to 15 minutes, semen reliquifies (PSA and other enzymes from the prostate).
Failure or delay of liquefaction immobilizes sperm and inhibits movement through the cervix.

Capacitation

Sperm pass through the uterus and uterine tubes, where secretions trigger functional changes that prepare sperm to fertilize an oocyte. Capacitation cannot occur until sperm are inside the female reproductive tract.

Clinical Correlation: Hemospermia

Definition: Presence of blood in semen.

Most common cause: Inflammation of the blood vessels lining the seminal vesicles.

Treatment: Antibiotics.

Penis

The penis serves two functions: ejaculation of semen and excretion of urine. Its structure is built around erectile tissue that fills with blood under the right conditions.

External Anatomy

Penis: Three Components

Definition: Passageway for ejaculation of semen and excretion of urine. Cylindrical shape.

1. Body

Three cylindrical masses of tissue, each surrounded by fibrous tissue (tunica albuginea), all enclosed by skin and subcutaneous tissue:

Two dorsolateral masses (corpora cavernosa penis)
One smaller mid-central mass (corpus spongiosum penis): contains the spongy urethra. Keeps it open during ejaculation.

Erectile tissue: numerous blood sinuses (vascular spaces) lined by epithelial cells, surrounded by smooth muscle and elastic CT.

2. Glans Penis

The distal end of the corpus spongiosum. Slightly enlarged acorn-shaped region.

Corona: margin of the glans.
Prepuce (foreskin): loose-fitting cover over the glans in an uncircumcised penis.

3. Root

Attached, proximal portion.

Bulb of the penis: expanded posterior continuation of the base of the corpus spongiosum. Attaches to the inferior surface of deep perineal muscles. Enclosed by the bulbospongiosus muscle (aids ejaculation).
Crura of the penis: two tapered portions of the corpora cavernosa that bend laterally and attach to the ischial and inferior pubic rami. Surrounded by the ischiocavernosus muscle.

Supporting Ligaments

Fundiform ligament: arises from the inferior part of the linea alba.
Suspensory ligament: arises from the pubic symphysis.

Physiologic Process

Erection & Emission

Erection

Enlargement of the penis. Caused by blood filling the erectile tissue (vascular sinuses) of the corpora cavernosa.

Emission

The discharge of a small volume of semen before ejaculation.

Clinical Correlation: Priapism

Definition: Persistent and painful erection that does not involve sexual desire or excitement.

Cause: Often due to blood disorders, medications, or trauma.

Treatment: Medical emergency requiring treatment to prevent permanent tissue damage.

Big Picture

The Four Components of the Male Reproductive System

Scrotum & Testes

Scrotum

Externally

Internally

Cremaster & Dartos Muscles

Cremaster Muscle

Dartos Muscle

How They Work Together

Testes

Development

Function

Tissue Layers Covering the Testes

Two Cell Types in the Seminiferous Tubules

Sertoli (Sustentacular) Cells

Functions

Leydig (Interstitial) Cells

Function

What Testosterone Does in Males

The Blood-Testis Barrier

Why It Matters

Spermatogenesis & Hormonal Control

Spermatogenesis: Step by Step

Two Important Numbers

Structure of a Spermatozoon

Head

Tail (4 Subdivisions)

Survival

Hormonal Control of Testicular Function

Luteinizing Hormone (LH)

Follicle-Stimulating Hormone (FSH)

Negative Feedback Loop (Testosterone)

Effects of Testosterone & DHT Across the Lifespan

1. Prenatal Development

2. Puberty (Sexual Characteristics)

3. Sexual Function

4. Anabolic Effects

The Duct System: Path of Sperm

Sperm Path Through the Male Duct System

Epididymis

Function

Three Parts

Histology

Ductus (Vas) Deferens

Path

Function

Special Features

Spermatic Cord

Contents

Inguinal Canal

Structures Through the Inguinal Canal (Male)

Two Parts

Ejaculatory Ducts

Location

Function

Urethra (3 Subdivisions)

Function

Three Subdivisions

Accessory Glands & Semen

Seminal Vesicles

Location

Secretes

Why Alkaline Matters

Volume

Prostate

Location

Secretes

Contents

Lifespan Changes

Volume

Bulbourethral Glands (Cowper's Glands)

Location

Functions

Semen

Volume & Sperm Count

pH

What Each Component Looks Like

Coagulation & Liquefaction

Capacitation