Fig. 1. Schematic representation of a cross-section of the human penis. (Adapted from ref. 282.)
Tunica Albugínea and Fibrous Skeleton ofthe Penis
The tunica albugínea is composed of elastic fibers forming a tough irregular lattice that is predominantly collagenous (both types I and III), although the detailed histological composition depends on its anatomical location and function (15). In the flaccid state, its average thickness is 2 to 3 mm. The tunica albuginea becomes much thicker ventrally, where it forms the groove to accommodate the corpus spongiosum. Surrounding the corpora cavernosa, the tunica is a bilayered structure. The inner layer is composed of circularly oriented bundles that support and contain the cavernous tissue. Intracavernosal pillars radiate from the inner layer into the corpora and act as struts to support the erectile tissue. The outer layer is oriented longitudinally, extending from the glans penis to the proximal crura, ultimately inserting on the inferior pubic ramus. As the crura diverge proximally, the circular fibers alone provide the support.
The tunica albuginea of the corpus spongiosum is much thinner than that of the corpora cavernosa and contains more elastic fibers. Cadaveric studies have demonstrated that the thinnest portion of the tunica is at the 6 o'clock position over the urethra, which explains the epidemiology of perforation during penile prosthesis implantation. Emissary veins run between the inner and outer bundles obliquely and, therefore, can be occluded easily by the shearing action of the tunical layers during erection. The outer layer appears to play an additional role in compression of the veins during erection. However, dorsal artery branches take a more direct perpendicular route and are protected by compression during erection by a peri-arterial fibrous sheath. The tunica albuginea provides tough uniform backing for engorged sinusoidal spaces.
Cavernosal design provides the penis with flexibility, rigidity, and strength. The caver-nosal tissue is spongelike and consists of a meshwork of interconnected cavernosal spaces that are lined by vascular endothelium and separated by trabeculae (16). These structures contain bundles of smooth muscle in a framework of mostly collagen (types I, IV, and, to a lesser extent, type III), elastin, and fibroblasts (17,18). Smooth muscle accounts for 45% of corpora cavernosal volume (19). Alterations in the relative quantities and structure of the cytoskeleton and contractile proteins may be responsible for changes in the active and passive characteristics of penile smooth muscle.
Ultrastructural examination of a smooth muscle cell reveals thin, thick, and intermediate filamentous structures. Thin filaments or light chains are composed primarily of actin (also called LC20 because its molecular weight is 20 kDa). Thick filaments are formed of myosin. Intermediate filaments contain either desmin or vimentin. Each type of filament has a specific function in corporal physiology.
The internal pudendal artery (the terminal branch of the hypogastric artery) is primarily responsible for the blood supply to the deep structures of the penis (20). The internal pudendal runs in a curve along the dorsolateral pelvic sidewall and enters the lesser pelvis through the lesser sciatic notch (see Fig. 3A). After entering the ischiorectal fossa, it courses along Alcock's canal along the inferior insertion of the obturator internus muscle. In men, the internal pudendal also gives rise to scrotal and bulbar branches (via the perineal branch) before continuing as the common penile artery. After piercing the urogenital diaphragm, the common penile artery continues along the medial margin of
the inferior ramus of the pubis. In the anterior perineum near the bulbospongiosus, the penile artery trifurcates; the branches include the bulbo-urethral artery, the cavernous artery (central or deep penile), and the dorsal artery of the penis (21). The glans penis can be separated completely from the corpora cavernosa without compromising blood supply (see Fig. 3B).
1. Bulbo-urethral artery. After giving off the bulbar artery, it continues as the urethral artery, which runs on the ventral surface of the corpus spongiosum beneath the tunica albuginea.
2. Dorsal artery. As the termination of the penile artery, it runs over each crus and continues along the dorsolateral surface of the penis distally toward the glans penis. It is situated as part of the neurovascular bundle, lateral to the dorsal vein and medial to the dorsal nerves. The course of the artery is somewhat tortuous; this course accommodates for elongation
Table 1 Penile Arterial System
Penile inflow (arterial)
Hypogastric (internal iliac) artery ♦ internal Pudenda artery ♦ common penile artery ♦ (1) bulbourethral (2) cavernous (3) dorsal artery Bulbourethral artery
• Bulbar: enters the bulb of the penis shortly after its origin. It supplies blood to bulbourethral (Cowper's) gland and the proximal urethral bulb.
• The urethral (spongiosal) artery runs longitudinally through the corpus spongiosum lateral to the urethra and also supplies glans.
Cavernous or deep penile artery: enters corpus cavernosum at base and runs eccentrically to the tip. Gives off two types of branches:
• Outer capillaries (nutritional) supply smooth muscle and nerve fibers.
• Inner helicine (supply) arteries are multiple muscular and helical shape arteries that open directly into the cavernous spaces without going into capillaries and act like resistance arteries (helical shape allows penis to elongate and dilate without decreasing flow).
Helicine arteries ♦ sinusoids ♦ post-cavernous venules ♦ sub-albugineal venous plexus ♦ emissary vein
Dorsal or superficial penile artery: runs deep to Buck's fascia, between the centrally located deep dorsal vein and the paired dorsal nerves.
during erection. This artery gives off circumflex branches to supply the spongiosum before its terminus at the glans. Distally, the dorsal artery runs ventrolaterally near the sulcus before entering the glans. The frenular artery branch of the dorsal artery curves around each side of the distal shaft to enter the frenulum and glans ventrally.
3. Cavernosal artery. The cavernosal artery is responsible for flow to the corporal bodies. It pierces the tunica, entering each cavernosum at the penile hilum, where the two corpora cavernosa come together. It gives off a short branch to the crus and then passes out the cavernous body almost to the tip, running medially toward the midseptum of the cavernous bodies until reaching the distal penis, when it is in the center of the corpus. Two types of branches arise from the cavernosal artery (22). Outer capillaries are responsible for penile nutrition during the flaccid state and supply the smooth muscle and nerve fibers. Inner helicine arteries open directly into cavernous spaces without entering capillaries, which are then emptied into the postcavernous venules. These inner arteries are shaped like corkscrews and allow the penis to elongate and dilate without compromising flow. Multiple layers of smooth muscle surround the helicine branches. This muscle is contracted while flaccid, allowing only small amounts of blood into the lacunar spaces. After the proper stimulus, muscle relaxation occurs and the arteries dilate and straighten, increasing blood flow and pressure to the lacunar spaces (23). Although the cavernosal arteries supply the bulk of the blood delivered to the corpora cavernosa, the dorsal artery contributes to a lesser degree via several circumflex branches to the middorsal corpora cavernosa. Therefore, a bypass into a proximally occluded dorsal penile artery can improve flow into the corpora cavernosa (see Table 1).
Anatomical variants are extremely common and include both cavernous arteries originating from the same side; hypoplasia, or absence of one dorsal penile artery and accessory penile arteries arising from obturator arteries external iliac; or others. Iatrogenic injury to these structures during deep pelvic surgery can lead to vasculogenic erectile dysfunction. Collateral vessels may open in the setting of obstruction, allowing communication between the cavernosal arteries or between cavernosal and dorsal vessels (24).
Penile skin is supplied by the external pudendal artery, a branch of the femoral artery. Each vessel divides to a dorsolateral and ventrolateral branch, which supply the skin of the shaft and prepuce.
Penile Venous System
There are three sets of veins that drain the penis: superficial, intermediate, and deep systems (25-28). The deep venous system drains both the corpora cavernosa and the cor-porus spongiosum. The postcavernous venules coalesce to form larger emissary veins that pierce the tunica albuginea. The emissary veins of the middle and distal penis join to form the circumflex veins, which empty into the deep dorsal vein. Both the emissary and circumflex veins have valves. The emissary veins of the proximal penis form the cavernous vein, which empties into the internal pudendal. The intermediate set of veins is deep to Buck's fascia. Veins from the glans penis form a retrocoronal plexus that drains into the deep dorsal vein. The deep dorsal vein courses proximally in the midline between the two corpora cavernosa and empties into the periprostatic plexus. The superficial dorsal vein drains the skin and the subcutaneous tissue superficial to Buck's fascia. This, in turn, drains into the superficial external pudendal vein. Emissary veins run between the inner and outer layers for a short distance, often piercing the outer bundles in an oblique manner; therefore, these emissary veins can be easily occluded by the shearing action of the tunical layers during erection (ref. 29; see Fig. 4; Table 2).
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