Skin and Receptor Apparatus
The skin of the organ is distinguished by minimal thickness and a complete absence of subcutaneous adipose tissue. Specialized sebaceous glands, also known as preputial glands, are localized in the glans region.
The organ is characterized by an extremely high density of sensory receptors at various levels:
- Epidermis: contains free nerve endings.
- Papillary dermis: rich in tactile Meissner corpuscles.
- Reticular dermis and tunica albuginea: include lamellar Pacinian corpuscles and Krause end bulbs.
Histology of Cavernous and Spongy Bodies
The internal structure of the organ is formed by two paired cavernous bodies (corpora cavernosa) and one unpaired spongy body (corpus spongiosum). Histologically, the terms "cavernous" and "spongy" are synonymous, describing the honeycomb architecture of the tissue. The corpus spongiosum encloses the urethra and features two expansions: the bulb at the base and the glans at the distal end.
Structure of the cavernous bodies (from outside to inside):
- Tunica albuginea. Composed of dense fibrous connective tissue interspersed with smooth myocytes and nerve endings. It acts as a mechanical limiter for organ volume during engorgement.
- Trabeculae. Connective tissue septa extending inward from the tunica albuginea.
- Lacunae (caverns). Spaces between the trabeculae, lined internally with endothelium. They function as blood-pooling reservoirs.
Large blood vessels run outside the lacunae and septa, whereas smaller vessels are located directly within the trabeculae.
Blood Supply and Mechanism of Erection
The angioarchitecture of the organ is strictly adapted for hemodynamic shifts. The deep artery, running centrally within the cavernous body, gives off two types of branches. Nutritive arteries transition into capillaries to 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 forming 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 lumens are closed. Blood is shunted into the venous system via arteriovenous anastomoses, bypassing the lacunae. Outflow through the deep veins (which possess three muscular layers) is completely unobstructed.
Hemodynamics during Erection:
- Filling phase. Parasympathetic impulses induce relaxation of the helicine artery sphincters. The vessels straighten and dilate, and blood rapidly fills the cavernous spaces.
- Tumescence/Maintenance phase. Outflow blockage occurs via two mechanisms: active (contraction of deep venous myocytes) and passive (compression of the subtunical venous plexus between the engorged erectile tissue and the tunica albuginea).
Blood engorgement of the corpus spongiosum is less pronounced during erection. This is physiologically necessary to prevent urethral compression and ensure the unobstructed passage of ejaculate. Following ejaculation, detumescence (return to the flaccid state) occurs: sympathetic tone is restored, the arteries spasm, and venous outflow opens.
Male Urethra and Associated Glands
The male urethra is divided into three segments:
- Prostatic urethra (~2.5 cm). Passes through the prostate gland. The seminal colliculus (colliculus seminalis) is located on its posterior wall, where the ejaculatory ducts open. During erection, the colliculus enlarges and occludes the urethra, preventing retrograde ejaculation of sperm into the urinary bladder.
- Membranous urethra (~1 cm). Traverses the urogenital diaphragm; it is the narrowest and least distensible segment.
- Spongy (penile) urethra (10–15 cm). Located within the corpus spongiosum, expanding distally into the navicular fossa.
The urethral wall 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) glands lie within the urogenital diaphragm. These are complex tubuloalveolar structures. Their ducts (3–4 cm) open into the proximal spongy urethra. During sexual arousal, prior to ejaculation, they secrete a viscous mucus rich in hexosaminoglycans, protecting the urethral mucosa from the irritating effects of urine and semen.