Male Reproductive Organ Anatomy

The male reproductive system is a network of external and internal organs that function to produce, support, transport, and deliver viable sperm for reproduction. Prenatally, the male sex organs are formed under the influence of testosterone secreted from the fetal testes; by puberty, the secondary sex organs further develop and become functional.

Penis: The primary organ for sexual intercourse and urination. It consists of three parts: the root (anchored to the pelvic floor), the body (shaft), and the glans (tip). Internally, it contains three erectile columns, consisting of paired corpora cavernosa and a single corpus spongiosum, which engorge with blood during erection. The urethra runs through the corpus spongiosum, serving as a passage for semen and urine.

Scrotum: A fibromuscular sac that houses the testes and maintains an optimal temperature for sperm production through thermoregulation by muscles such as the dartos and cremaster.

Testes: Oval-shaped glands located within the scrotum. They are responsible for producing sperm through spermatogenesis in the seminiferous tubules and synthesizing testosterone in Leydig cells.

Epididymis: A coiled tube attached to each testicle, where sperm mature and are stored until ejaculation

Ductus (Vas) deferens: A muscular tube that transports mature sperm from the epididymis to the ejaculatory ducts during ejaculation

Seminal vesicles: Produce a fructose-rich fluid that nourishes the sperm and is the primary contribution to semen volume

Prostate gland: Adds enzymes and citric acid to semen, maintaining its fluidity and supporting sperm viability.

Bulbourethral (Cowper's) glands:  Secrete a clear mucus that lubricates the urethra and neutralizes residual acidity from urine ^[1]

Sperm is produced in the testes and is transported through the epididymis, ductus deferens, ejaculatory duct, and urethra. Concomitantly, the seminal vesicles, prostate gland, and bulbourethral gland produce seminal fluid that accompanies and nourishes the sperm as it is emitted from the penis during ejaculation and throughout the fertilization process (see image below).

Prenatally, the male sex organs are formed under the influence of testosterone secreted from the fetal testes; by puberty, the secondary sex organs further develop and become functional. Testosterone is secreted by the fetal testes and is essential for the differentiation of the male genitalia. During this period, the gonads develop into testes, and the Wolffian ducts differentiate into structures such as the epididymis, vas deferens, and seminal vesicles. The presence of the sex-determining region Y protein (SRY) gene on the Y chromosome triggers this cascade of events, leading to the development of male characteristics. ^[2]

Testosterone is central to male reproductive function. It can be converted into dihydrotestosterone (DHT) or estradiol in peripheral tissues, influencing secondary sexual characteristics, libido, and spermatogenesis. ^[3]

Scrotum

The scrotum is a fibromuscular pouch divided by a median septum (visible externally as the scrotal raphe) that forms two compartments, each of which contains a testis, epididymis, and part of the spermatic cord. Layers of the scrotum consist of the skin, dartos muscle, external spermatic fascia, cremasteric fascia, and internal spermatic fascia, which is in close contact with the parietal layer of the tunica vaginalis. ^[4]

The layers of the scrotum from superficial to deep include: ^{[1, 5]}

• Skin - The outermost layer, characterized by its thinness and pigmentation, which aids in thermoregulation
• Dartos fascia - A layer of smooth muscle (dartos muscle) continuous with Scarpa's fascia in the abdomen and Colles' fascia in the perineum. This layer helps regulate temperature by wrinkling the skin to reduce heat loss.
• External spermatic fascia - Derived from the external oblique aponeurosis
• Cremasteric fascia - Contains the cremaster muscle, which elevates the testes as part of thermoregulation
• Internal spermatic fascia - Originates from the transversalis fascia
• Tunica vaginalis - A mesothelial layer with parietal and visceral components surrounding the testes

The skin and dartos layers of the scrotum are supplied by the perineal branch of the internal pudendal artery in addition to the external pudendal branches of the femoral artery. The layers deep to the dartos muscle are supplied by the cremasteric branch of the inferior epigastric artery. The veins of the scrotum accompany the arteries, eventually draining into the external pudendal vein and subsequently the greater saphenous vein. Lymphatic drainage of the skin of the scrotum is by the external pudendal vessels to the medial superficial inguinal lymph nodes.

The scrotum has a rich sensory nerve supply that includes the genital branch of the genitofemoral nerve (anterior and lateral scrotal surfaces), the ilioinguinal nerve (anterior scrotal surface), posterior scrotal branches of the perineal nerve (posterior scrotal surface), and the perineal branch of the posterior femoral cutaneous nerve (inferior scrotal surface).

The primary function of the scrotum is maintaining an optimal temperature for spermatogenesis. This is achieved through thermoregulatory mechanisms involving: ^[6]

• Contraction or relaxation of the dartos muscle to adjust surface area
• Elevation or descent of the testes via the cremaster muscle activity

Testes

Testes are the primary male reproductive organ and are responsible for testosterone and sperm production. Each testis is 4-5 cm long, 2-3 cm wide, weighs 10-14 g, and is suspended in the scrotum by the dartos muscle and the spermatic cord. ^[4] Each testis is covered by the tunica vaginalis testis, tunica albuginea, and tunica vasculosa.

Tunica vaginalis: The tunica vaginalis testis is the lower portion of the processus vaginalis and is reflected from the testes on the inner surface of the scrotum, thus forming the visceral and parietal layers. The visceral layer closely adheres to the testes, and the parietal layer lines the inner surface of the scrotum. Between these layers is a small cavity containing fluid that facilitates friction-free movement of the testes within the scrotum.

Tunica albuginea:  A dense, fibrous capsule that lies beneath the tunica vaginalis. Extensions of the tunica albuginea form septa that partition the testis into lobules, each containing seminiferous tubules. ^{[1, 7]}

Tunica vasculosa: Internal to the tunica albuginea is the tunica vasculosa, containing a plexus of blood vessels and connective tissue.

Seminiferous tubules: These coiled structures within each lobule are lined with germ cells (spermatogonia) and Sertoli cells, which support spermatogenesis and produce inhibin B. The tubules converge into a network called the rete testis, which connects to the epididymis via efferent ducts.

Leydig cells: Found in the interstitial spaces between the seminiferous tubules, these cells produce testosterone under stimulation by luteinizing hormone.

Bilateral testicular arteries originating from the aorta, just inferior to the renal arteries, provide arterial supply to the testes. They enter the scrotum in the spermatic cord via the inguinal canal and split into two branches at the posterosuperior border of the testis.

Additionally, the testes receive blood from the cremasteric branch of the inferior epigastric artery and the artery to the ductus deferens. The pampiniform plexus drains both the testis and epididymis before coalescing to form the testicular vein, usually above the spermatic cord formation at the deep inguinal ring. Lymphatic drainage via the testicular vessels passes into the abdomen, ending in the lateral aortic and pre-aortic nodes. The tenth and eleventh thoracic spinal nerves supply the testes via the renal and aortic autonomic plexuses.

Epididymis

The epididymis is a C-shaped structure lying intimately along the posterior border of each testis and includes the caput (head), corpus (body), and cauda (tail). ^{[9, 10]}

• The caput epididymis is bulbous and located proximally near the testis. It is primarily involved in the initial stages of sperm maturation.
• The corpus epididymis is elongated and serves as a transitional zone where further maturation occurs.
• The cauda epididymis is swollen and distal, functioning as a storage site for mature spermatozoa prior to ejaculation.

The tunica vaginalis covers the epididymis except at the posterior border. Vasculature and innervation of the epididymis is the same as that for the testes (see image below). Arterial supply is derived from the branches of the testicular artery, while venous drainage occurs through the pampiniform plexus. Innervation includes autonomic fibers that regulate smooth muscle contractions essential for sperm transport. ^[1]

The epididymis consists of a single, convoluted tubule that connects the efferent ducts of the testis to the vas deferens. It is surrounded by connective tissue septa, which segment the epididymis into discrete subcompartments. These septa are oriented perpendicular to the tubular structure and contribute to its organization. ^{[9, 10]}

The smooth muscle layer surrounding the epididymal tubules varies in thickness: thinnest at the caput and thickest at the cauda. This gradient facilitates sperm transport via peristaltic contractions. ^{[9, 10]}

The epididymis is lined with pseudostratified epithelium, which is composed of six main cell types, including principal cells, which are responsible for absorption and secretion within the lumen. These cells exhibit high metabolic and secretory activity regulated by androgens. The stereocilia on the epithelial surface significantly enhance its absorptive and secretory functions, facilitating sperm maturation. This includes resorption of excess testicular fluid and secretion of substances critical for sperm viability such as carnitine and glycoproteins. Spermatozoa enter the epididymis immotile and gain progressive motility and fertilizing capacity during their transit through this structure, a process taking 2-6 days. ^{[9, 10]}

Functions: ^{[9, 10]} The epididymis plays a critical role in sperm maturation, storage, and transport.

Sperm maturation:

• Spermatozoa entering the epididymis from the testis are immature and nonmotile. During their transit through the epididymal regions, they acquire motility and fertilizing capacity due to interactions with region-specific luminal environments.

Unique proteins, ions, antioxidants, and exosomes (epididymosomes) secreted by epithelial cells contribute to this maturation process.

Sperm storage:

• Mature sperm are stored in the cauda epididymis until ejaculation. This region's thicker smooth muscle layer plays a role in propelling sperm into the vas deferens during ejaculation.

Protection:

• The epididymis provides a protective environment for the maturation of sperm by neutralizing reactive oxygen species through antioxidant enzymes secreted by epithelial cells.
• Additionally, a blood-epididymis barrier shields the sperm from immune responses and harmful substances in circulation.

Ductus (vas) deferens

The ductus deferens develops from the mesonephric (Wolffian) ducts during embryogenesis. The ductus deferens is a thick-walled, muscular tube that plays a crucial role in the male reproductive system by transporting sperm from the epididymis to the ejaculatory ducts. ^[1]

The ductus (vas) deferens is a continuation of the epididymis. It is 30-45 cm long and transports sperm to the ejaculatory ducts. ^{[4, 11]} The convoluted portion of the ductus deferens becomes straighter (diameter 2-3 mm) as it travels posterior to the testis and medial to the epididymis. Subsequently, the ductus ascends on the posterior aspect of the spermatic cord until it reaches the deep inguinal ring, where it participates in the formation of the spermatic cord and loops over the inferior epigastric artery.

At this point, the ductus travels along the lateral pelvic wall, medial to the distal ureter, along the posterior wall of the bladder until it reaches the seminal vesicles dorsal to the prostate. Each ductus deferens has an artery usually derived from the superior vesical artery (artery to the ductus), with venous drainage to the pelvic venous plexus. Lymphatic drainage of the ductus deferens is to the external and internal iliac nodes and innervation is mainly through the autonomic plexus of mainly postganglionic sympathetic fibers derived from the inferior hypogastric plexus. ^[12]

Spermatic cord

The spermatic cord extends from the deep inguinal ring through the inguinal canal to the testis. The layers of the spermatic cord include (from outward to inward):

• External spermatic fascia (derived from the deep fascia of the external abdominal oblique muscle)
• Cremasteric fascia (derived from the internal oblique muscle). This layer contains the cremaster muscle, which is responsible for the elevation of the testis in response to temperature changes or physical stimuli. ^[1]
• Internal spermatic fascia (derived from the transversalis fascia). This innermost layer envelops the contents of the spermatic cord, providing additional support and protection. ^[1]

The spermatic cord also contains an additional inner layer of areolar and adipose tissue derived from preperitoneal/retromuscular tissue, which forms a supportive matrix around its contents. ^[13]

The structures that form the spermatic cord include:

• The ductus deferens  and associated vasculature and nerves (posterior wall of the cord). This muscular tube transports sperm from the epididymis to the ejaculatory duct. It is accompanied by its own artery (the artery of the ductus deferens) and nerve fibers that regulate its function. ^[1]
• The testicular artery  arising from the abdominal aorta, supplies oxygenated blood to the testis. Its pulsations can sometimes be felt within the spermatic cord. ^[1]
• The pampiniform plexus . A network of veins that drains deoxygenated blood from the testis, the pampiniform plexus plays a critical role in thermoregulation by cooling arterial blood before it reaches the testis. This venous network eventually forms the testicular vein. ^[1]
• The genital branch  of the genitofemoral nerve innervates the cremaster muscle. Sympathetic and parasympathetic nerve fibers regulate vascular tone and smooth muscle activity in associated structures. Lymphatic vessels drain lymph from the testes to iliac and lumbar lymph nodes. ^[1]

Ejaculatory ducts

The ejaculatory ducts are 2 cm in length and derived from the union of the seminal vesicle and the ampulla of the vas deferens. Each duct starts at the base of the prostate and terminates at the seminal colliculus (verumontanum).

The ducts are primarily collagenous conduits with minimal muscular layers, serving as channels for semen rather than actively contracting during ejaculation. The luminal diameter decreases progressively as the ducts approach their termination. The ejaculatory ducts are divided into three segments: an initial segment with a well-defined muscular layer, a middle segment also muscular, and a distal segment that may lack significant musculature. The openings of the ejaculatory ducts are typically located 2-3 mm lateral to the neck of the prostatic utricle at 5 and 7 o'clock positions relative to the verumontanum. ^{[14, 15]}

The vasculature, innervation, and lymphatics of the ejaculatory ducts are the same as those for the ductus deferens. The inferior vesical artery, a branch of the internal iliac artery, provides blood to the ejaculatory ducts. Venous blood is drained via the prostatic and vesical venous plexuses into the internal iliac vein. Sympathetic fibers from the hypogastric nerve mediate contraction during ejaculation. Lymph from the ejaculatory ducts drains into the internal iliac lymph nodes. ^[1]

During ejaculation, these ducts propel sperm and seminal fluid from the seminal vesicles and ductus deferens into the prostatic urethra. The high-pressure environment within the ejaculatory ducts prevents the retrograde flow of urine into these structures. ^[15]

Seminal vesicles

The two seminal vesicles are located between the bladder and the rectum and measure approximately 5 cm in length. The anterior surface is in contact with the posterior wall of the bladder and the posterior surface is in contact with rectovesical (Denonvilliers) fascia. The ampulla of the ductus deferens lies medial to the seminal vesicles and the prostatic venous plexus lies laterally. Arterial blood supply to the seminal vesicles includes branches from the inferior vesical and middle rectal arteries, while venous and lymphatic drainage accompanies these arteries. The inferior division of the hypogastric plexus provides innervation to the seminal vesicles.

The seminal vesicles contribute 70-85% of the total volume of semen. Their secretions are alkaline, which helps to neutralize the acidic environment of the female vagina, thereby enhancing sperm viability and motility. These secretions include: ^{[16, 17, 18]}

• Fructose - A critical energy source for sperm motility
• Prostaglandins - Facilitate sperm transport by inducing contractions in the female reproductive tract
• Fibrinogen-like proteins - Aid in semen coagulation after ejaculation
• Other components include potassium, phosphorus, citric acid, and enzymes that support sperm viability.

Bulbourethral glands

The bulbourethral glands, also known as Cowper's glands, are paired, pea-shaped exocrine glands. They develop from the urogenital sinus during weeks 10-12 of gestation. ^[19]

The bilateral bulbourethral glands are 2 cm in diameter and lie lateral to the membranous urethra and are enclosed by the external urethral sphincter. The excretory duct of the gland penetrates the perineal membrane and opens within the bulbar urethra. Vasculature, lymphatic drainage, and innervation are generally the same as those for the seminal vesicles. Arterial supply is likely derived from branches of the internal pudendal artery. Lymphatic drainage is directed to the internal iliac nodes, and innervation is mediated through both autonomic and somatic nerves, primarily via the pudendal nerve and pelvic plexus. ^[1]

The primary function of the bulbourethral glands is to produce a lubricating fluid that is released prior to ejaculation. This pre-ejaculatory fluid serves multiple purposes: it helps to cleanse the urethra of acidic urine remnants, provides lubrication for sperm passage, and aids in neutralizing the acidic environment of the female vagina, thereby enhancing sperm viability. ^[19]

Prostate

The prostate gland is an ovoid structure encompassing the proximal portion of the urethra and is approximately 2.5-3.0 cm × 4.0-4.5 cm, normally weighing 20-25 g. ^[11] The base of the prostate is in contact with the bladder, the apex is superior to the perineal membrane, the anterior border is in contact with the vesicoprostatic plexus, the posterior border is separated from the anterior surface of the rectum by the rectovesical (Denonvilliers) fascia, and the lateral border is in contact with the levator ani and the prostatic venous plexus. Fibers of the external urethral sphincter surround the prostate (see the image below).

Arterial supply to the prostate gland is derived from the inferior vesical artery and branches of the middle rectal artery. Venous drainage of the prostate forms the prostatic plexus, which eventually drains into the internal iliac vein and lymphatic drainage flows to the internal iliac nodes. Innervation is derived from the inferior portion of the hypogastric plexus, primarily to the connective tissue surrounding the gland.

Urethra

The male urethra is a fibromuscular tube, approximately 17.5-20 cm in length, extending from the bladder to the external urethral meatus at the tip of the glans penis. ^[20] The urethra serves as a passage for urine and semen. It is divided into four segments based on anatomical location: pre-prostatic, prostatic, membranous, and spongy (penile) urethra: ^{[1, 20, 21]}

Pre-prostatic urethra: ^{[1, 20, 21]}

The pre-prostatic urethra is a short segment located between the bladder neck and the prostate. It is surrounded by the internal urethral sphincter, a smooth muscle under sympathetic control that prevents retrograde ejaculation into the bladder during emission.

Prostatic urethra:

The prostatic urethra traverses the prostate gland and measures approximately 3-4 cm in length. It contains several key anatomical features such as: ^{[1, 20, 21]}

• The urethral crest, a longitudinal ridge on its posterior wall
• The verumontanum, an elevation that houses the openings of the ejaculatory ducts and the prostatic utricle
• The prostatic sinuses, located on either side of the crest, where prostatic ducts empty their secretions into the urethra

The prostatic urethra extends vertically from the bladder neck through the prostate before becoming the membranous urethra and before penetrating the perineal membrane.

Membranous urethra:

The membranous urethra is the shortest (1-2 cm) but narrowest segment of the male urethra. It passes through the deep perineal pouch and is surrounded by the external urethral sphincter, a striated muscle under voluntary control via the pudendal nerve (S2-S4). This segment also houses the bulbourethral glands (Cowper's glands), which secrete lubricating mucus into the urethra during sexual arousal. ^{[1, 20, 21]}

As the membranous urethra enters the deep perineal space, the urethra is surrounded by fibers of the external urethral sphincter, eventually entering the bulb of the corpus spongiosum, providing the orifice for the bulbourethral glands and subsequently becoming the penile urethra.

Spongy (penile) urethra:

The spongy or penile urethra is encased within the corpus spongiosum of the penis and measures about 15-16 cm in length, making it the longest segment. It extends from the bulb of the penis to the external urethral meatus. Notable features include: ^{[1, 20, 21]}

• The intrabulbar fossa, a slight dilation near its proximal end
• The fossa navicularis, a terminal dilation within the glans penis

External urethral meatus: ^[1]

The external urethral meatus is located at the tip of the glans penis and represents the narrowest part of the male urethra. Its diameter limits dilatation, making it clinically significant in procedures such as catheterization.

Penis

The penis is a copulatory and urinary organ of the male external genitalia, composed of three main parts: the root, body (shaft), and glans. ^{[1, 21, 22]}

Root

The root is the fixed, proximal part of the penis located in the superficial perineal pouch of the pelvic floor. ^{[1, 21, 22]} The root consists of two crura and the bulb — three bodies of erectile tissue attached to the pubic arch (crura) and perineal membrane (bulb).

Near the border of the pubic symphysis, the bilateral crura continue as the corpora cavernosa throughout the body of the penis. The bulb lies between the two crura, narrows anteriorly and continues as the corpus spongiosum. The root is stabilized by two ligaments: suspensory ligament and fundiform ligament. ^{[1, 21, 22]}

Body (Shaft)

The body is the free, pendulous part of the penis. It contains ^[1] the bilateral corpora cavernosa and the median corpus spongiosum. During penile erection, all three erectile bodies become engorged with blood. The corpora cavernosa are enveloped in a thick fibrous tunica albuginea, which is composed of longitudinal running superficial fibers and a deep layer of circular oriented fibers. The corpus spongiosum is penetrated by the urethra as it traverses the body of the penis (see the images below).

The shaft is enveloped by three fascial layers: ^{[1, 21, 22]}

• Superficial fascia (dartos fascia): Contains loose connective tissue and smooth muscle fibers. The superficial penile fascia is intertwined with dartos muscle fibers.
• Deep fascia (Buck's fascia): The deep penile fascia, or Buck's fascia, is a tough fascial layer that encompasses both corpora cavernosa and the corpus spongiosum.
• Tunica albuginea: Encloses each erectile body individually

Glans: ^{[1, 21, 22]}

The glans is an expansion of the distal corpus spongiosum. It includes:

• The corona, which marks its base
• The external urethral meatus, located at its tip

The corona of the penis is where the skin folds to become the prepuce (foreskin), enveloping the glans penis (see the image below).

The vasculature of the penis is extensive. The perineal artery (a branch of the internal pudendal artery) together with the posterior scrotal artery and the inferior rectal artery supply tissues from the bulb of the penis to the anus. The artery of the bulb of the penis, from the internal pudendal, penetrates the penile bulb and subsequently supplies the corpus spongiosum. The deep artery of the penis is one of two terminal branches of the internal pudendal artery; it enters the crus of the penis and continues through the length of the bilateral corpus cavernosum. The other terminal branch of the internal pudendal artery is the dorsal artery of the penis running along the dorsal surface of the penis supplying the penile skin and the glans penis (see the image below).

Venous drainage of the penis includes the veins draining the corpora cavernosa, which subsequently drains into the circumflex veins. These veins receive venous blood from the corpus spongiosum on the ventral aspect of the penis and wrap around the penis to drain into the deep dorsal vein. The superficial dorsal vein drains the penile skin and prepuce before draining via the superficial external pudendal vein into the external pudendal veins. The deep dorsal vein further drains blood from the glans penis and corpora cavernosa before joining the prostatic venous plexus. The lymphatic drainage of the penis encompasses three locations: the superficial inguinal nodes (penile skin), deep inguinal and external iliac nodes (glans penis), and internal iliac nodes (erectile tissue and urethra; see image below).

Sensory innervation to the penile skin is through the dorsal nerve of the penis, one of the terminal branches of the pudendal nerve. Autonomic innervation includes both sympathetic and parasympathetic aspects to the corpora cavernosum via the cavernous nerves. Sympathetic fibers originate at the level of T11-T12 and the parasympathetic ones originate from the pelvic plexus at S2-S4.

Testes

The testes are encapsulated by a dense connective tissue layer called the tunica albuginea, which extends inward to form septa. ^[1] These septa divide the testes into approximately 400 segments called lobules each of which is occupied by 2-4 seminiferous tubules, which are responsible for producing spermatozoa. ^[11] Each testis has 600-1200 seminiferous tubules with a total length of 280-400 m. ^[23] At the mediastinum testis, on the posterior border of the testis, the seminiferous tubules empty spermatozoa into the tubuli recti and rete testis, eventually coalescing to form 6-8 efferent ductules. ^[23] The efferent ductules drain spermatozoa into the epididymis (see image below).

The seminiferous tubule epithelium consists of proliferating spermatogenic cells and the sustentacular Sertoli cells. Spermatogenic cells are at various stages of spermatogenesis and Sertoli cells are columnar cells that extend from the basement membrane to the lumen of the seminiferous tubule. Sertoli cells provide structural support, nourishment, and a regulated environment for spermatogenesis. They also contribute to forming the blood-testis barrier, protecting developing germ cells from immune attack. ^[24]

Interstitial cells in the testis, including the Leydig cells, constitute 20-30% of the tissue in the gland and are found in between seminiferous tubules. The washed-out cytoplasm of the Leydig cells is due to a high lipid content in the form of cholesterol for the synthesis of testosterone. ^[23] Leydig cells play a critical role in maintaining male secondary sexual characteristics and supporting spermatogenesis. ^[25]

Telocytes in the interstitial tissue of human testes form an extensive network through their long processes (telopodes) and interact with Leydig cells, peritubular myoid cells, macrophages, and blood vessels. Telocytes may contribute to testicular homeostasis, immune surveillance, and structural integrity. ^[26]

Epididymis

The main component of the epididymis is a tightly packed, tortuous duct approximately 6 m long and 400 µm in diameter. ^[23] This structure is divided into three main regions: the head (caput), body (corpus), and tail (cauda): ^{[1, 9]}

Head of the epididymis:

The head consists of the most densely packed coils of efferent ductules, which are lined with ciliated columnar epithelium for transport of spermatozoa through the epididymis.

Body and tail of the epididymis: ^{[1, 9]}

The body of the epididymis serves as a conduit for spermatozoa as they continue to mature. The tail acts as a storage site for mature sperm until ejaculation. Both regions are lined by pseudostratified columnar epithelium with stereocilia, which increase the surface area for absorption and secretion. Smooth muscle layers surrounding these regions contract rhythmically to propel sperm toward the vas deferens during ejaculation.

The epithelium of the epididymal duct is highly specialized: ^[27]

• Principal cells dominate and are involved in secreting proteins and ions essential for sperm maturation.
• Clear cells contribute to luminal acidification, creating an environment conducive to maintaining sperm in a quiescent state.
• Basal cells serve as progenitor cells and help regulate epithelial function.

Ductus (vas) deferens

The ductus deferens is composed of pseudostratified columnar epithelium, including columnar cells and basal cells. The basal cells serve as progenitor cells, maintaining the integrity of the epithelium by proliferating and differentiating into columnar cells as needed. ^[28]  The underlying lamina propria is dense with elastic fibers and the wall of the ductus contains three thick smooth muscle layers. The outermost layer of adventitia is rich in blood vessels and nerves

Seminal vesicles

The seminal vesicles are tubulosaccular glands consisting of connective tissue and secretory epithelium projecting into the lumen of the gland. ^[23] The epithelium is pseudostratified with basal and columnar cells, while the wall of the vesicle is consistent with a thick wall of smooth muscle that contracts during ejaculation.

Prostate

The prostate is traditionally divided into three concentric zones: peripheral, central, and transitional. The glandular epithelium of all zones consists predominantly of pseudostratified columnar cells with basal cells and secretory cells. ^[1] The peripheral zone normally constitutes 70% of the prostate and contains the tubuloalveolar glands of the organ; the central zone normally constitutes 25% of the prostate and contains submucosal glands; and the transitional zone normally constitutes 5% of the prostate. ^[23]  Benign prostatic hyperplasia (BPH), a normal aging process, can significantly alter these proportions, as the transitional zone typically enlarges to a far greater extent throughout life than the other two zones. Additionally, there is a nonglandular anterior fibromuscular stroma located anteriorly, composed primarily of fibrous and smooth muscle tissue. ^[29]  The tubuloalveolar glands are embedded in a fibrous stroma and open through branching ducts in the prostatic urethra. The secretory nature of the epithelium is evident as it consists of pseudostratified epithelium containing basal and secretory cells.

Urethra

The prostatic urethra is lined by transitional epithelium, the membranous urethra is lined by stratified columnar epithelium, and the penile urethra is initially stratified columnar epithelium but becomes stratified squamous epithelium at the fossa navicularis.

Penis

The erectile bodies of the penis are composed of fibroelastic connective tissue, smooth muscle, and a network of vascular sinuses lined with endothelium. ^[23] The sinuses are continuous with the arteries that supply them and the veins that drain them.

Scrotum

In the relaxed state (noncontracted dartos and cremasteric muscles), the scrotum is smooth and pendulous. However, when the dartos and cremasteric muscles contract secondary to cold or emotional stimuli, the scrotum becomes smaller, rounder, and more wrinkled.

Testes

Although the shape of the testis varies little, the variation in size may be considerable. Contributing factors may include overall body habitus and race; however, certain conditions (i.e., Fragile X syndrome) are associated with larger (macro-orchidism) and smaller (micro-orchidism — Klinefelter syndrome) testicles. The testicular volume can range from 12-30 mL in healthy adult males. ^[30]

Epididymis

The two most common natural variations for the epididymis are the size and the rigidity with which the structure is attached to the testicle.

Seminal vesicles

The seminal vesicles show considerable variation in size, most notably when the vesicle is in a full or empty state.

Penis

In the flaccid state, the penis varies in length from 8-12 cm and in width from 3-4.5 cm. ^[11]  When the temperature is cold, contraction of the dartos muscle causes the penis to decrease in size. In the erectile state, the penis varies in length from 12-18 cm and in width from 4-5 cm. ^[11]

Scrotum

A developmental defect may occur that allows the tunica vaginalis to retain direct communication with the peritoneum and the possibility of subsequent bowel herniation. A congenital inguinal hernia can be differentiated from an acquired inguinal hernia by the intraoperative finding of bowel in contact with the testis. Bifid scrotum occurs when the two halves of the scrotum are separated by a cleft due to failed fusion of the paired genital swellings and is commonly associated with severe degrees of hypospadias or ambiguous genitalia. ^[11]

Testes

Numerical anomalies

Anorchidism is a rare condition consisting of bilateral absence of the testis. Monorchidism is the presence of one testis and polyorchidism is the presence of a supernumerary testis.

Migrational anomalies

Cryptorchidism (undescended testes) is one of the most frequent anomalies of the congenital organs and occurs in 1 in 500 male births. ^[11]  It can occur unilaterally or bilaterally. Undescended testes are usually small, atrophic, and often nonfunctional. Arrested migration may occur at any point during testis descent. Abdominal cryptorchidism occurs when the testes arrest superior to the inguinal canal, inguinal cryptorchidism occurs within the inguinal canal, and subinguinal cryptorchidism occurs between the superficial inguinal ring and the scrotum.

Patients with undescended testes are at an increased risk for testicular malignancy, most commonly testicular seminoma. The incidence of a testicular tumor in the general population is 1 in 100,000 and in males with a cryptorchid testis is 1 in 2550, resulting in an overall relative risk of greater than 40. ^[31]

Ductus (vas) deferens

The ductus deferens may be congenitally absent in a condition known as vasal aplasia. This condition has been linked to cystic fibrosis, resulting in azoospermia even though the process of spermatogenesis is often normal.

Spermatic cord

A short spermatic cord occurs when the growth of the spermatic cord does not keep pace with that of the rest of the body's. In such an instance, the testis may be higher in the scrotum, often referred to as an ascending or retractile testis.

Ejaculatory ducts

Ejaculatory duct obstruction is a rare condition and may lead to azoospermia or oligozoospermia. Potential causes of this condition are congenital cysts such as paramesonephric (Mullerian) cysts. ^[4]

Seminal vesicles

The seminal vesicles may be absent or atrophic. These conditions are often associated with malformations such as bladder exstrophy, cloacal abnormalities, or ambiguous genitalia.

Prostate

The prostate may show variation in form that is usually determined by rectal examination. The posterior surface of the prostate has a midline sulcus that may be wider, narrower, deeper or shallower than normal, or be absent altogether. ^[11] Serous malformations of the prostate are often associated with epispadias, hypospadias, or bladder exstrophy. Enlargement of the prostate with age is common and not necessarily of any concern if the patient has no urinary retention and is asymptomatic; prostate cancer risk is not correlated to gland size.

Urethra

Absence of the urethra may be a true absence or may be associated with the absence of the penis. Urethra atresia and duplicity may occur. Accessory urethral canals occur when an accessory channel lies dorsal or ventral to the true urethra. A bifid urethra forms when accessory channels originate from the true urethra and form an additional meatus at the glans penis. Paraurethral ducts are blind pits that open on the glans penis and are usually a few millimeters in depth but may connect back to the true urethra.

Posterior urethral valves are one of the most serious anomalies associated with the neonatal period, often leading to decreased renal function and incontinence and occurring in 1 in 8000-25,000 live births. ^[32] Young's classification Type 1 makes up 95% of posterior urethral obstruction, where there is a ridge on the floor of the urethra, continuous with the seminal colliculus. ^[32]  Anterior urethral valves are rarer than posterior urethral valves and often occur in the form of a diverticulum of the urethra. These valves form when there is a defect in the corpus spongiosum, leading to a thin-walled urethra that balloons during urination and subsequent obstruction. ^[32]

Penis

See the list below:

• Microphallus
• Congenital absence or duplication
• Hypospadias is relatively common and occurs in 1 in 250 male newborns ^[32] and is due to an abnormal ventral opening of the urethral meatus anywhere from the glans penis to the perineum.
• Epispadias is less common and occurs in 1 in 117,000 of male newborns ^[32] and is due to an abnormal dorsal opening of the urethral meatus and may occur on the glans, penile shaft, or the penopubic region. Patients with epispadias are incontinent and have complete epispadias in 70% of the cases. ^[33]
• Phimosis occurs when the opening of the prepuce is too small for it to be retracted back over the glans penis. A dramatic variation of phimosis is a buried penis in which the entire penis is concealed beneath a phimotic foreskin and a suprapubic fat pad, most commonly seen in overweight patients.
• Paraphimosis occurs when the prepuce of an uncircumcised male is trapped behind the glans, and cannot be returned to its usual configuration.

Scrotum

See the list below:

• A communicating hydrocele is a collection of peritoneal fluid that accumulates between the layers of the tunica vaginalis when the processus vaginalis fails to obliterate between the deep inguinal ring and the superior border of the scrotum. Most communicating hydroceles are smaller in the morning and increase in size throughout the day when the individual is in the upright position. Diagnosis is based on physical examination and transillumination of the hydrocele sac.
• A non-communicating hydrocele is fluid that accumulates between the layers of the tunica vaginalis, but without a patent connection to the abdominal cavity. These hydroceles stay at a constant size throughout the day, and tend to gradually and painlessly enlarge.
• A spermatocele is a large cyst or collection of cysts that arise from the epididymis. They are often clinically indistinguishable from non-communicating hydroceles.
• Fournier's gangrene is a form of rapidly advancing necrotizing fasciitis affecting the scrotum and often extending to the perineum, penis, and abdominal wall. Patients with diabetes mellitus, liver disease, local trauma, periurethral extravasation of urine, and perianal infection are predisposed to Fournier's gangrene. The initial diagnosis can be made based on a swollen, erythematous, and tender infection of the scrotum and the most common pathogens are facultative organisms, anaerobes, and group A streptococcus.

Testis

See the list below:

• Testicular carcinoma
• Testicular trauma
• Testicular torsion may occur if the testis twists on the suspending spermatic cord. This is a surgical emergency as the blood supply needs to be restored to the testis within 6 hours of symptom onset to decrease the risk for testicular loss. The testis should subsequently be sutured to the scrotal wall (orchiopexy) to prevent recurrence, in addition to orchiopexy of the contralateral testis.
• Most acute presentations of scrotal pain and swelling can be attributed to epididymitis, testicular torsion, or torsion of a testicular appendage. In many cases, torsion of a testicular appendage, although a benign condition, may present identically to testicular torsion, a true urologic emergency. Ultrasound may be used to aid in diagnosis; however, a normal clinical exam of a nontender testis in the presence of a paratesticular nodule at the superior pole may be more diagnostic for appendiceal torsion. Classically, a blue dot may be seen through the skin in the area of pain (blue dot sign); however, this is only present in 20% of cases.
• Varicocele is a common condition characterized by dilation of the pampiniform plexus in the spermatic cord. Varicocele is most commonly caused by defective venous valves and is quite common, almost always on the left side due to the differences in testicular venous drainage on that side. Very rarely, it may be due more serious causes such as testicular vein compression by an abdominal tumor; for that reason, an isolated right-sided varicocele is an indication for abdominal imaging. Symptoms of varicocele include a dull, aching pain in the scrotum, testicular heaviness, testicular atrophy, and visible and palpable veins in the scrotum commonly referred to as feeling like "a bag of worms." However, most cases are asymptomatic and require no treatment. Varicoceles are sometimes implicated in male factor infertility by decreasing semen quality parameters, and surgical correction can be pursued for that reason.

Epididymis

Epididymitis is inflammation of the epididymis. It may be due to an infectious process, commonly in males aged 19-35 years. In this age group, antibiotic treatment should be initiated for Neisseria gonorrhea and Chlamydia trachomatis. In young children and males over 65 years of age, Escherichia coli is the most common pathogen. Furthermore, other pathogens include Ureaplasma, Mycobacterium tuberculosis, and the drug amiodarone, commonly used for cardiac rate control. However, many cases are viral or idiopathic and tend to be self-limiting in that case.

Prostate

See the list below:

• Prostate carcinoma
• BPH
• Prostatitis

Urethra

Urethral stricture disease refers to anterior urethral disease leading to scarring of the penile urethra and secondary reduction of the urethral lumen. Any process that leads to injury of the urethral epithelium can be an etiological factor for urethral stricture. The most common cause is urethral trauma, usually saddle trauma; however, iatrogenic trauma does occur during the course of urehtral catheterization or endoscopy.

Inflammatory strictures secondary to gonorrhea were historically the most common cause of urethral stricture; however, they still do take place. Other infectious causes of urethral stricture include chlamydia and Ureaplasma urealyticum. Treatment of urethral stricture disease is arduous as stricture formation is common. The most common treatment modalities include internal urethrotomy (transurethral incision of the stricture), laser ablation, and open stricture excision and urethral anastomosis. Grafts obtained from oral (buccal) mucosa are used to great effect to treat severe strictures.

Injury to the anterior urethra (bulbous urethra, penile urethra, fossa navicularis) is most commonly due to blunt trauma, penetrating injuries, and instrumentation, and injury to the posterior urethra (prostatic and membranous urethra) is most commonly associated with pelvic fractures. ^[32] Urinary extravasation may result from urinary trauma. If the deep fascia (Buck's fascia) of the penis is not ruptured, urine extravasation is usually limited to the penis; however, if the fascial plane is breached, urine may extravasate to the penis, scrotum, perineum, and abdominal wall.

Penis

See the list below:

• Penile carcinoma
• Penile fracture
• Bicycle seat neuropathy, where the pudendal nerve is compressed for a prolonged period, leading to erectile and sensory dysfunction.