Male reproductive organs

The male reproductive system consists of the penis, testes, epididymis, ejaculatory ducts, prostate, and accessory glands. These organs function together to produce sperm and deliver semen from the testes via ejaculation. For more information on the embryological development of the male reproductive system, see “Development of the reproductive system”.

Function

Urination and release of semen

Gross anatomy ^[1][2]

Primarily composed of erectile tissue, blood vessels, nerves, and connective tissue

Structure

• Root: composed of the bulb of the penis and two crura
  • Two crura anchor the corpora cavernosa to the pubic arch.
  • The bulb is a continuation of the corpus spongiosum.
• Body (shaft): contains the erectile tissue
  • Corpus spongiosum
    • Location: lies ventrally between the corpora cavernosa
    • Function: keeps the urethra open during erection
  • Corpora cavernosa (paired)
    • Location: dorsal and proximal to the corpus spongiosum
    • Function: enable erection
• Glans penis: the distal end of the corpus spongiosum
  • Attached to the frenulum and covered by foreskin
    • Runs from the deep surface of the body to the prepuce
    • Medial fold located ventrally
    • Corona: margin of the glans penis
  • Contains the external urethral meatus
    • Fossa navicularis: opening of the external urethra
    • Urethral glands (glands of Littre): present along the length of the penile urethra
• Muscles
  • Bulbospongiosus muscle (paired)
    • Cover the bulb of the penis
    • Empty the urethra of residual semen/urine and contribute to erection
  • Ischiocavernosus muscle (paired)
    • Cover the crura of the penis
    • Compress the crura to move blood into the body of the penis and maintain erections

Arteries ^[1][2]

Veins

Lymphatics

Innervation

Microscopic anatomy ^[1][2]

• Tunica albuginea: thick fibrous layer of connective tissue surrounding the corpora cavernosa and the corpus spongiosum
• Corpora cavernosa: composed of connective tissue and smooth muscle
• Corpus spongiosum: composed of connective tissue (primarily elastic fibers) and smooth muscle

Embryology ^[1]

• Genital tubercle → glans penis, corpora cavernosa, corpus spongiosum
• Urogenital folds → ventral shaft of the penis
• See “Development of the reproductive system”.

• The testes are paired organs composed of seminiferous tubules, the site of spermatogenesis. They are also responsible for the secretion of male sex hormones.
• The scrotum encases the testes and is connected to the abdominal wall via the spermatic cord.

Structure ^[1][2][3]

• Testes
  • Ellipsoid-shaped
  • Composed of ∼ 600 seminiferous tubules, which connect to the rete testis
  • Surrounded by a thick capsule (tunica albuginea)
  • Connect to the abdominal wall via the spermatic cord
• Testicular lobules
  • Interlobular septa divide the testis into testicular lobules.
  • Each testicular lobule contains 1–4 seminiferous tubules.
• Spermatic cord
  • Extension of the abdominal wall into the scrotum
  • Contents of the spermatic cord
    • Three arteries: testicular artery, ductus deferens artery, cremasteric artery
    • Three nerves: genital branch of genitofemoral, cremasteric nerve, sympathetic nerve fibers
    • Three other structures: ductus deferens, pampiniform plexus, lymphatic vessels
• Scrotum
  • Distal sac-like end of the spermatic cord that is continuous with the anterior abdominal wall
  • Layers of the scrotum (from internal to external)
    • Tunica albuginea: highly vascular, fibrous layer of connective tissue
    • Tunica vaginalis: visceral layer; derived from peritoneum
    • Internal spermatic fascia: derived from the transversalis fascia
    • Cremasteric fascia and cremaster muscle: originates from the internal oblique muscle
    • External spermatic fascia: derived from the aponeurosis of the external oblique muscle
    • Dartos fascia and dartos muscle: contraction causes skin wrinkles → regulates temperature
    • Scrotal skin: wrinkles → ↑ skin thickness → ↓ heat loss

“TIE turns into ICE”:
Transversalis fascia → Internal spermatic fascia
Internal oblique → Cremasteric muscle and fascia
External oblique → External spermatic fascia

Vasculature, lymphatics, and innervation of the testes and scrotum ^[3]

The Left Gonadal vein is the LonGest.

Microscopic anatomy ^[1][2]

Spermatogonia

• Description: undifferentiated germ cells lining the seminiferous tubules
  • Type A (dark): do not undergo mitosis
  • Type A (pale): form type B spermatogonia
  • Type B: undergo mitosis → mature into primary spermatocytes
• Function: differentiate into primary spermatocytes

Sertoli cells

• Description
  • Columnar epithelial cells lining seminiferous tubules (non-germ cells)
  • Homologous to granulosa cells in females
  • Connected to one another via tight junctions
  • Form the blood-testis barrier: separation of gametes in seminiferous tubules from the immune system
  • Sensitive to temperature: ↑ temperature (e.g., due to cryptorchidism or varicocele) → ↓ inhibin B secretion and ↓ production of spermatozoa
• Function
  • Produce inhibin B
    • Stimulated by FSH
    • Increased inhibin B levels lead to decreased FSH secretion (negative feedback loop).
  • Contain aromatase, which is responsible for the conversion of testosterone and androstenedione to estrogen ^[4]
  • Secrete androgen-binding protein
  • Produce Mullerian inhibitory factor
  • Regulate spermatogenesis
  • Provide protection and nourishment for developing spermatocytes

The S-rule for Sertoli cells: line the Seminiferous tubes, Sustain Sperm Synthesis, and Suppress FSH.

Leydig cells

• Description
  • Interstitial endocrine cells with cytoplasm rich in cholesterol (used for testosterone production)
  • Homologous to theca interna cells in females
  • Not affected by temperature
• Function
  • Produce testosterone
    • Stimulated by LH
    • Increased serum testosterone levels lead to decreased LH secretion (negative feedback loop).
    • Testosterone also diffuses to Sertoli cells, where it stimulates spermatogenesis.
  • Secrete androgens during the embryonic period
  • During puberty, testosterone production increases, which allows for the start/maintenance of full spermatogenesis.
  • Conversion of testosterone to estrogen via aromatase ^[4]

“Leydis Love Testosterone”: Leydig cells are LH-stimulated and secrete Testosterone.

Sperm

• Head: formed by the acrosome and the nucleus
• Tail: formed by the midpiece (mitochondria and microtubules), principal piece (flagellum), and the end piece

Blood-testis barrier

• Formed by tight junctions between Sertoli cells
• Separates gametes in seminiferous tubules from the immune system
• Divides seminiferous tubules into:
  • Basal compartment
    • Contains spermatogonia and immature primary spermatocytes
    • In contact with blood and lymph
  • Adluminal compartment
    • Contains mature spermatocytes and spermatids
    • Not in contact with blood and lymph

“Sertoli fence for sperm defense”: Sertoli cells form the blood-testes barrier.

Spermatogenesis and spermiogenesis ^[1][2]

Spermatogenesis

• The process by which spermatogonia (see “Microscopic anatomy” above) become spermatids
• Begins in puberty
• Occurs in the seminiferous tubules
• As spermatogonia mature, they move from the basal layer of the seminiferous tubule to its lumen
• Spermatogonia (2n, diploid) cross the blood-testis barrier → primary spermatocytes (2n, diploid) → secondary spermatocytes (1n, haploid) → spermatids (1n, haploid)
• Duration: ∼ 2 months

Spermiogenesis

• The process by which spermatids become spermatozoa
• Requires testosterone
• Formation of the acrosome
  • Derived from the Golgi apparatus
  • Contains acid phosphatase, neuraminidase, and hyaluronidase
• Loss of cytoplasmic content
• Condensation of genetic material
• Spermatid elongation
• Development of the flagellum (dysfunctional or absent in primary ciliary dyskinesia/Kartagener syndrome)

GONIUM is GOING to be sperm: Spermatogonia becomes a spermatid.
ZOON ZOOMS to the egg: Mature spermatozoon travel to the egg.

• Testes: SRY gene on Y chromosome encodes for testis determining factor → primitive sex cords development → testes formation ^[1]
  • Testes develop in the retroperitoneum, then descend into the scrotum:
    • Transabdominal migration of both testes begins during week 7 and is usually complete by 32 weeks' gestation.
    • Testes descend through the inguinal canal, along with their associated vasculature, nerves, and ductus deferens.
    • The descending testes are supported by a layer of the peritoneum (tunica vaginalis), which is normally secured to the superior and inferior posterior pole of the testis.
    • With the assistance of the gubernaculum at the lower pole, the testes are pulled into the scrotum.
    • The spermatic cord, which holds the vas deferens and testicular vasculature, meets the testis within the scrotum.
  • Play a role in further sexual development
    • Mullerian inhibitory factor (Sertoli cells) suppresses the development of the paramesonephric ducts.
    • Androgens (Leydig cells) stimulate the development of the mesonephric ducts.
• Scrotum ^[1]
  • Derived from the labioscrotal swelling
  • Development driven by dihydrotestosterone (DHT)
  • See “Development of the reproductive system” for more information.

Epididymis ^[1]

• Gross anatomy: long, coiled duct along the posterior aspect of the testis → distal end is continuous with ductus deferens
• Function
  • Storage and maturation of spermatozoa
  • Propulsion of spermatozoa into the ductus deferens
• Embryology: derived from the mesonephric duct (differentiation requires testosterone)

Ductus deferens ^[1]

• Gross anatomy
  • Long, muscular duct that enters the pelvis at the deep inguinal ring (lateral to inferior epigastric artery)
  • Crosses the umbilical artery and obturator nerve → passes superior to the ureter → expands at the distal end to form the ampulla → joins the ejaculatory duct
• Function: transports spermatozoa from the epididymis to the ejaculatory duct and provides fructose to the spermatozoa
• Embryology: derived from the mesonephric duct (differentiation requires testosterone)

Ejaculatory ducts ^[1]

• Gross anatomy: formed by the ductus deferens and seminal vesicle ducts → open into the prostatic urethra
• Function: propel spermatozoa with seminal fluid into the urethra
• Embryology: derived from the mesonephric duct (differentiation requires testosterone)

Accessory glands ^[2]

Prostate (see below)

Seminal vesicles

• Gross anatomy
  • Paired glands located on the inferoposterior surface of the bladder
  • Connect with the ductus deferens to form the ejaculatory ducts → empty into the prostatic urethra
• Microscopic anatomy
  • Lobulated glandular tissue
  • Lined with pseudostratified columnar epithelium
  • Contain secretory granules
• Function: secrete an alkaline fluid that makes up the greater part of seminal fluid
  • Contains sorbitol dehydrogenase which converts sorbitol into fructose and thus provides energy for spermatozoa motility
  • Contains citrate, fructose, prostaglandins, and other proteins
• Embryology: derived from the mesonephric duct (differentiation requires testosterone)

Bulbourethral gland (Cowper gland)

• Gross anatomy: located in the deep perineal pouch within the urogenital diaphragm
• Function: secretes mucus into the bulbar urethra for lubrication during ejaculation
• Embryology: : derived from the pelvic portion of the urogenital sinus (differentiation requires DHT)

An accessory gland of reproduction located at the base of the bladder and composed primarily of glandular, fibrous, and smooth muscle tissue.

Function ^[2]

Secretion of:

• Components of semen
  • Proteolytic enzymes (regulated by DHT) to maintain the fluidity of semen
  • Acid phosphatase, citric acid, prostaglandins, fibrinolysins, lipids, amylase
• Prostate-specific antigen (PSA)

Gross anatomy ^[1][2]

• Location
  • Posterior to the pubic symphysis, inferior to the bladder, anterior to the rectum, superior to the perineal membrane
  • The base wraps around the neck of the urinary bladder.
  • The apex surrounds the proximal urethra (prostatic urethra).

Structure

• Capsules
  • External: continuation of the pelvic fascia (false capsule)
  • Internal: true capsule
• Five anatomic lobes: Lobe location is described relative to the urethra (e.g., anterior = anterior to the urethra).
  • Anterior (isthmus)
    • Anterior to the urethra
    • Does not contain glandular tissue
  • Posterior
    • Below the ejaculatory ducts and behind the urethra
    • Contains glandular tissue
    • Most common anatomic location of malignant transformation → prostate cancer ^[2]
  • Middle (median): between the ejaculatory ducts and the urethra
  • Lateral (paired; right and left lobe)
    • On both sides of the urethra
    • Most common anatomic site of benign prostatic hyperplasia (BPH) → causes obstruction of urinary flow ^[5]

Vasculature, lymphatics, and innervation of the prostate gland ^[5]

Microscopic anatomy ^[7]

General

• Composed of glandular, fibrous, and smooth muscle tissue
• Glandular epithelium contains foamy cytoplasm and abundant secretory granules, RER, and lysosomes
• The fibrous capsule forms septae that penetrate the gland and divide it into lobes.

Functional zones

• Peripheral zone
  • Largest area of glandular tissue with some smooth muscle
  • Palpable on digital rectal examination (DRE)
  • Most common site of prostate cancer
• Central zone (periurethral zone)
  • Fibromuscular tissue
  • Surrounds the ejaculatory ducts
  • Accounts for 20–25% of the prostate's tissue
  • Site of origin for 1–5% of prostate cancer
  • Location: from the base of the prostate to the seminal colliculus (verumontanum)
• Transitional zone
  • Smallest area of glandular tissue: tubuloalveolar glands with columnar epithelium
  • Surrounds the urethra
  • Most common site of benign prostatic hypertrophy
  • Accounts for 10% of the prostate's tissue
  • Site of origin for 20% of prostate cancer
  • Location: proximal to the seminal colliculus (verumontanum)
  • Contains two lateral lobes and a median lobe
  • Urethral crest → on the posterior midline
  • Surrounds the distal end of the preprostatic urethra
  • Prostatic ducts: drain into the prostatic sinus, a groove on the sides of the urethral crest
  • Ejaculatory ducts: open into the prostatic urethra, on the seminal colliculus

Glands

• There are ∼ 30–50 branched tubuloalveolar glands
• Main prostatic glands are located peripherally
• Periurethral submucosal glands

Embryology ^[8]

• Prostate derives from the urogenital sinus (differentiation depends upon the presence of DHT)
• See “Development of the reproductive system.”

There are three parts of the male sexual response: erection, emission, and expulsion.

Erection ^[2][9]

• Description: The penis becomes rigid as its corpora cavernosa fill with blood due to sensory and/or mental stimulation of primarily parasympathetic nerves.
• Autonomic control
  • Parasympathetic: pelvic splanchnic nerves (S2–S4) → cavernous nerves → nitric oxide (NO) release → ↑cGMP → cavernous muscle relaxation → vasodilation → erection ^[10]
  • Sympathetic: superior hypogastric plexus (T11–L2) → fibers travel along pelvic plexus → norepinephrine release → ↑ calcium influx → cavernous muscle contraction → vasoconstriction → antagonizes erection (detumescence)
• Types of erection
  • Reflex erection (mostly sensory stimulation)
    • Genital stimulation → pudendal nerve afferents → sacral erection center S2–S4 (parasympathetic nervous system) → erection
    • Injuries to the cervical or thoracic spinal cord may result in an erection.
  • Psychogenic erection (mostly mental stimulation)
    • Cortical stimulation via visual, auditory, and sensory stimuli or fantasy → impact on the thoracolumbar erection center T11–L2 (sympathetic nervous system) → inferior mesenteric plexus and superior hypogastric plexus → some sympathetic neurons act synergistically with sacral parasympathetic signals (S2–S4) to increase blood flow to corpora cavernosa → erection
    • Sympathetic innervation to the smooth muscle of vas deferens, seminal vesicle, and internal sphincter of the bladder
  • Nocturnal penile tumescence
    • Spontaneous erection while sleeping or upon waking up
    • The cause is not entirely understood.
    • Nocturnal tumescence monitoring serves as a means of differentiating psychological from physiological erectile dysfunction.

PDE-5 inhibitors (e.g., sildenafil) inhibit hydrolysis of cGMP → ↑ NO → vasodilation → prolonged erection

S2, S3, and S4 keep the penis off the floor.

Ejaculation ^[2][9][11]

Emission

• During the first phase of ejaculation, sperm and secretions from the reproductive glands are transported through the reproductive ducts (epididymis, vas deferens, and ejaculatory duct) into the internal urethra.
• Ejaculate consists of prostatic secretion, seminal vesicle fluid, and spermatozoa
• Secretion of fluid from accessory glands promotes sperm survival.
• Stimulated primarily by the sympathetic nervous system (hypogastric nerve; T11–L2) → closure of the bladder neck and contraction of reproductive glands (seminal vesicle, bulbourethral, and prostate glands)

Expulsion

• During the second phase of ejaculation, the semen is transported through the urethra and expelled from the body.
• Stimulated by:
  • Sympathetic nervous system: hypogastric nerve (T11–L2)
  • Somatic nervous system: pudendal nerve → rhythmic contraction of the bulbocavernosus muscle → expulsion of semen from the urethra

Remember the ejaculatory pathway of sperm with SEVEn UP: Seminiferous tubules → Epididymis → Vas (ductus) deferens → Ejaculatory duct → Urethra → Penis

Innervation of male sexual response: Point, Squeeze, and Shoot.
1. Parasympathetic → Point (erection)
2. Sympathetic → Squeeze (emission)
3. Somatic (pudendal nerve) → Shoot (expulsion)

Penis

• Priapism
• Erectile dysfunction
• Phimosis
• Paraphimosis
• Balanitis
• Carcinoma of the penis
• Penile fracture
• Sexually transmitted infections
• Peyronie disease

Testes and scrotum

• Testicular tumors
• Testicular torsion
• Hydrocele
• Varicocele
• Cryptorchidism

Prostate

• Benign prostatic hyperplasia
• Prostate cancer
• Prostatitis

Other

• Epididymitis
• Anterior urethral injury
• Posterior urethral injury