Penis

Skin and receptor apparatus

The skin of the organ is characterized by minimal thickness and the complete absence of subcutaneous adipose tissue. Specialized sebaceous glands, also known as preputial glands, are located in the region of the glans.

The organ is characterized by an extremely high density of sensory receptors at various levels:

• Epidermis: contains free nerve endings.
• Dermal papillae: rich in Meissner corpuscles.
• Reticular dermis and tunica albuginea: contain Vater-Pacini lamellar corpuscles and Krause end bulbs.

Histology of cavernous and spongy bodies

The internal structure of the organ is formed by two paired cavernous (erectile) bodies and one unpaired spongy (spongiosum) body. In a histological context, the terms "cavernous" and "spongy" are synonyms describing the honeycomb structure of the tissue. The spongy body contains the urethra inside and has two expansions: the bulb at the base and the glans at the distal end.

Structure of the cavernous bodies (from outside inwards):

1. Tunica albuginea (tunica albuginea). Composed of dense fibrous connective tissue with incorporated smooth myocytes and nerve endings. Acts as a mechanical limiter of organ volume during swelling.
2. Trabeculae. Connective tissue septa extending inwards from the tunica albuginea.
3. Lacunae (cavernae). Spaces between the trabeculae, lined internally with endothelium. Function as reservoirs for blood pooling.

Large blood vessels run outside the lacunae and septa, whereas small vessels are located directly within the trabeculae.

Blood supply and erection mechanism

The angioarchitecture of the organ is strictly adapted for hemodynamic changes. The deep artery, running in the center of the cavernous body, gives off two types of branches. Nutritive arteries transition into capillaries and supply the tissues with nutrients. Helicine arteries (aa. helicinae) open directly into the vascular lacunae. Their walls contain longitudinal and circular bundles of smooth myocytes that form a sphincter apparatus.

Hemodynamics at rest: The tone of the sympathetic nervous system predominates. Myocytes of the helicine arteries are contracted, the vessels are coiled, and their lumen is closed. Blood is shunted into the venous bed via arteriovenous anastomoses, bypassing the lacunae. Outflow through the deep veins (which have three muscular layers) is completely unobstructed.

Hemodynamics during erection:

• Filling phase. Parasympathetic impulses cause relaxation of the sphincters of the erectile helicine arteries. The vessels straighten and dilate, and blood rapidly fills the cavernous spaces.
• Maintenance phase. Outflow blockade occurs through two mechanisms: active (contraction of deep venous myocytes) and passive (compression of the cortical venous plexus between the engorged erectile tissue and the tunica albuginea).

Blood engorgement of the corpus spongiosum during erection is less pronounced. This is physiologically necessary to prevent urethral compression and ensure unobstructed passage of the ejaculate. Following ejaculation, detumescence (return to the resting state) occurs: sympathetic tone is restored, arteries undergo spasm, and venous outflow opens.

Male urethra and associated glands

The male urethra is divided into three segments:

1. Prostatic part (~2.5 cm). Passes through the prostate. The posterior wall features the seminal colliculus (colliculus seminalis), where the ejaculatory ducts open. During erection, the colliculus enlarges and occludes the urethra, protecting against retrograde flow of semen into the urinary bladder.
2. Membranous part (~1 cm). Lies within the urogenital diaphragm; it is the narrowest and least distensible segment.
3. Spongy part (10–15 cm). Located within the corpus spongiosum, expanding distally into the navicular fossa.

The wall of the urethra consists of mucosal, submucosal, and muscular layers. The connective tissue is rich in venous plexuses and elastic fibers. Small mucous Glands of Littré are present in the lamina propria.

Paired bulbourethral (Cowper's) glands are located within the thickness of the urogenital diaphragm. These are complex tubuloalveolar structures. Their ducts (3–4 cm) open into the proximal spongy urethra. During sexual arousal, even before ejaculation, they secrete a viscous mucus rich in hexosaminoglycans, protecting the mucosal lining of the canal from the irritating effect of semen.

Class and exam questions

What else to study on this topic

• Patterns of epithelial transition in different segments of the male urethra
• Histochemical composition of Cowper's gland secretion
• Structure of arteriovenous anastomoses in cavernous bodies
• Microscopic structure of intimal cushions in helicine arteries

Other topics in the "Male Reproductive System" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.