Gross Anatomy
The penile shaft is composed of three erectile columns, the two corpora cavernosa and the corpus spongiosum, as well as the columns' enveloping fascial layers, nerves, lymphatics, and blood vessels, all covered by skin (see the following images). The two suspensory ligaments, fundiform ligament and suspensory ligament proper, [1] which are composed of primarily elastic fibers support the penis at its base. [2]
The paired corpora cavernosa contain erectile tissue and are each surrounded by the tunica albuginea, a dense fibrous sheath of connective tissue with relatively few elastic fibers. The corpora cavernosa communicate freely through an incomplete midline septum. Proximally, at the base of the penis, the septum is more complete; ultimately, the corpora diverge, forming the crura, which attach to the ischiopubic rami. The ischiocavernosus muscles play an active role in erection by compressing the crura, thereby propelling blood distally into the corpora cavernosa. [3]
The tunica albuginea consists of two layers, the outer longitudinal and the inner circular (see the image below). Both layers contain different types of collagen bundles. [4]
The outer longitudinal fibers extend from the glans penis to the proximal crura, where they anchor into the pubic ramus. Notably, there is a paucity of outer longitudinal fibers between the 5 and 7 o'clock positions, corresponding to the ventral groove that houses the corpus spongiosum. [4] This anatomical feature renders this area more vulnerable to perforation or prosthesis extrusion in surgical contexts. [5]
The inner circular layer surrounds the endothelial-lined sinusoids (cavernous spaces) and is reinforced by intracavernous pillars that radiate from this layer. These pillars act as struts, providing support to the erectile tissue and augmenting the septum's structural integrity. This design facilitates the hydraulic function of the corpora cavernosa during erection. [5]
The tunica albuginea becomes thicker ventrally where it forms a groove to accommodate the corpus spongiosum. The tunica albuginea of the corpus spongiosum is considerably thinner (< 0.5 mm) than that of the corpora cavernosa (approximately 2 mm). Along the inner aspect of the tunica albuginea, flattened columns or sinusoidal trabeculae composed of fibrous tissue and smooth muscle surround the endothelial-lined sinusoids (cavernous spaces). In addition, a row of structural trabeculae arises near the junction of the three corporal bodies and inserts in the walls of the corpora about the midplane of the circumference. [6]
Histologically, elastic fibers within the tunica albuginea form an irregular lattice that supports collagen fibers, contributing to its tensile strength and flexibility. [4]
The erectile tissue within the corpora contains arteries, nerves, muscle fibers, and venous sinuses lined with flat endothelial cells, and it fills the space of the corpora cavernosa. The cut surface of the corpora cavernosa looks like a sponge. There is a thin layer of areolar tissue that separates this tissue from the tunica albuginea.
Blood flow to the corpora cavernosa is via the paired deep arteries of the penis (cavernosal arteries), which run near the center of each corpora cavernosa (see the following image).
The single corpus spongiosum lies in the ventral groove between the two corpora cavernosa. The urethra passes through the corpus spongiosum. The corpus spongiosum possesses a much thinner and more elastic tunica albuginea to allow for distention of the corpus spongiosum for passage of the ejaculate through the urethra. The thinner tunica albuginea of the corpus spongiosum also allows the corpus to become less rigid during erection. Hence, the distal extension of the spongiosum, the glans penis, covers the tips of the corpora cavernosa to provide a cushioning effect. The urethral meatus is positioned just slightly on the ventral surface of the glans and is slitlike. The edge of the glans overhangs the shaft of the penis, forming a rim called the corona.
The three erectile bodies are surrounded by deep penile (Buck) fascia, the dartos fascia, and the penile skin. The deep penile (Buck) fascia is a strong, deep, fascial layer that is immediately superficial to the tunica albuginea. It is continuous with the deep fascia of the muscles covering the crura and bulb of the penis, the ischiocavernosus and bulbospongiosus.
On the dorsal aspect of the corpora cavernosa, the deep dorsal vein and paired dorsal arteries and branches of the dorsal nerves are contained within the deep penile (Buck) fascia. This fascia splits to surround the corpus spongiosum, and it extends into the perineum as the deep fascia of the ischiocavernosus and bulbospongiosus muscles. The deep penile (Buck) fascia encloses these muscles and each crus of the corpora cavernosa and the bulb of the corpus spongiosum, adhering these structures to the pubis, ischium, and the urogenital diaphragm.
Penile skin
The penile skin is continuous with that of the lower abdominal wall. Distally, the penile skin is confluent with the smooth, hairless skin covering the glans. At the corona, it is folded on itself to form the prepuce (foreskin), which overlies the glans. The outer surface of the prepuce is covered by keratinized squamous epithelium, while its inner mucosal layer is variably keratinized depending on fractional exposure. The prepuce is attached to the glans via the frenulum on its ventral side. [7]
The subcutaneous connective tissue of the penis and scrotum has abundant smooth muscle and is called the dartos fascia, which continues into the perineum and fuses with the superficial perineal (Colle) fascia. In the penis, the dartos fascia is loosely attached to the skin and deep penile (Buck) fascia and contains superficial arteries, veins, and nerves of the penis.
Dartos fascia plays a critical role in penile mobility and thermoregulation. Its loose attachment allows for significant mobility of penile skin over underlying tissues, facilitating functions such as erection and maintaining hygiene. Additionally, its smooth muscle fibers contribute to scrotal temperature regulation by contracting or relaxing in response to environmental changes, thereby optimizing conditions for spermatogenesis. [8]
Vasculature
Arterial supply
Blood supply to the skin of the penis is from the left and right superficial external pudendal arteries, which arise from the femoral artery (see the image below). The superficial external pudendal arteries branch into dorsolateral and ventrolateral branches, which collateralize across the midline. In addition, branches in the skin form an extensive subdermal vascular plexus. Blood supply to the ventral penile skin is based on the posterior scrotal artery, a superficial branch of the deep internal pudendal artery.
Blood supply to deep structures of the penis is derived from a continuation of the internal pudendal artery after it gives off the perineal branch. Three branches of the internal pudendal artery flow to the penis as follows:
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The artery of the bulb (bulbourethral artery) passes through the deep penile (Buck) fascia to enter and supply the bulb of the penis and penile (spongy) urethra.
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The dorsal artery travels along the dorsum of the penis between the dorsal nerve and deep dorsal vein and gives off circumflex branches that accompany the circumflex veins; the terminal branches are in the glans penis.
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The deep penile (cavernosal) artery is usually a single artery that arises on each side and enters the corpus cavernosum at the crus and runs the length of the penile shaft, giving off the helicine arteries, which are an integral component of the erectile process.
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In approximately 28-35% of individuals, an accessory pudendal artery contributes to penile blood supply. It most commonly originates from either the obturator or inferior vesical arteries and may function as a crucial source of supplementary blood flow to deep penile structures. Preservation of this vessel during procedures such as radical prostatectomy is crucial for maintaining erectile function. [9]
Venous drainage
The penis is drained by three venous systems, the superficial, intermediate, and deep (see the image below).
Superficial veins are contained in the dartos fascia on the dorsolateral surface of the penis and coalesce at the base to form a single superficial dorsal vein, which usually drains into the great saphenous veins via the superficial external pudendal veins.
The intermediate system contains the deep dorsal and circumflex veins, lying within and beneath the deep penile (Buck) fascia. Emissary veins begin within the erectile tissue of the corpora cavernosa and course through the tunica albuginea and drain into the circumflex or deep dorsal veins. The circumflex veins arise from the spongiosum, ventrum of the penis, and the emissary veins often drain into them.
The circumflex veins course laterally around the cavernosa, passing beneath the dorsal arteries and nerves and draining into the deep dorsal vein. The deep dorsal vein lies in the midline groove between the two corpora cavernosa and is formed from 5-8 veins emerging from the glans penis, forming the retrocoronal plexus. It receives blood from the emissary and circumflex veins and passes underneath the symphysis pubis at the level of the suspensory ligament, leaving the shaft of the penis at the crus and draining into the prostatic plexus.
Deep venous drainage is via the crural and cavernosal veins. The crural veins arise in the midline, in the space between the crura. The cavernosal veins are consolidations of the emissary veins, which join to form a large venous channel that drains into the internal pudendal vein. Three or four small cavernosal veins course laterally between the corpus spongiosum and the crus of the penis for 2-3 cm before draining into the internal pudendal veins.
Lymphatics and Nerve Supply
Lymphatic drainage from the glans penis drains into large trunks in the area of the frenulum. These lymphatic vessels then circle to the dorsum of the corona and unite, coursing proximally beneath the deep penile (Buck) fascia, terminating mostly in the deep inguinal nodes of the femoral triangle. Some lymphatic drainage is to the presymphyseal lymph nodes and the lateral lymph nodes of the external iliac lymphatics.
Nerves to the penis are derived from the pudendal and cavernous nerves. The pudendal nerves supply somatic motor and sensory innervation to the penis. The cavernous nerves are a combination of parasympathetic and visceral afferent fibers and provide nerve supply to the erectile tissue. The cavernous nerves run in the crus and corpora of the penis, primarily dorsomedial to the deep penile arteries. The dorsal nerve of the penis, a branch of the pudendal nerve, provides rich sensory innervation to the glans penis. [1]
Microscopic Anatomy
Tunica albuginea
The tunica is composed of elastic fibers that form an irregular, latticed network on which the collagen fibers rest. The tunica albuginea is composed of an inner circular layer and an outer longitudinal layer.
The inner circular layer consists primarily of circularly arranged collagen fibers. These fibers provide structural integrity and support to the corpora cavernosa, helping maintain their shape during erection. [5]
The outer longitudinal layer is composed of longitudinally oriented collagen bundles. This layer contributes to the tensile strength and elasticity of the tunica, allowing it to accommodate changes in size and shape during erection. [5]
Emissary veins travel between the inner and outer layers of the tunica and often exit the outer layer in an oblique manner. The outer layer of the tunica compresses the emissary veins when the penis becomes engorged with blood.
Corpora cavernosa
The corpora cavernosa are two spongy cylinders. Within the tunica albuginea are the interconnected sinusoids separated by smooth muscle trabeculae and surrounded by elastic fibers, collagen, and loose areolar tissue. The terminal cavernous nerves and helicine arteries are intimately associated with smooth muscle. The sinusoids are larger in the center and smaller in the periphery.
Histologically, fibroblasts constitute a significant portion of the cavernosal cellular population. These fibroblasts regulate microenvironment homeostasis through paracrine signaling networks. [10]
Corpus spongiosum
The structure of the corpus spongiosum is similar to that of the corpora cavernosa, except that the sinusoids are larger and a much thinner outer layer of the tunica albuginea is present. The glans has no tunical covering.
Microscopically, the corpus spongiosum consists of four distinct layers: [11]
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Transition zone - A collagen-rich layer adjacent to the urethral epithelium, highly vascularized
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Elastin-rich layer - Provides elasticity to accommodate changes in blood flow
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Vascular layer - Contains veins, arteries, and vascular spaces crucial for engorgement during erection
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Outer layer - A collagen-rich tunica albuginea interwoven with short elastic fibers, offering structural support
The vascular lumen occupies a significant portion of the corpus spongiosum, with its area increasing during erection. Studies have shown a transverse area increase of 129% and a longitudinal area increase of 140% during erection, highlighting its capacity for distension. The vascular architecture includes incomplete septa, particularly in frontal sections, which allow for efficient blood flow and expansion. [12]
Erectile tissue vessels
The helicine arteries, branches of the deep penile artery, supply the trabecular tissue and sinusoids. They are contracted and tortuous in the flaccid state and dilated and straight in the erect state. Venous drainage from the erectile tissue originates in the venules starting at the peripheral sinusoids beneath the tunica albuginea. They travel in the trabeculae between the tunica and the peripheral sinusoids, forming the subtunical venular plexus before exiting as the emissary veins.
Neurotransmitters and Receptors
Adrenergic nerve fibers and receptors are present in the cavernous trabeculae, and they surround the deep penile arteries. Noradrenaline is the major neurotransmitter that controls penile flaccidity and tumescence. [13, 14] Sympathetic contraction is thought to be mediated by activation of postsynaptic alpha(1)-adrenergic receptors that increase intracellular calcium levels and enhance the contractile apparatus' sensitivity to calcium. This process is modulated by presynaptic alpha(2)-adrenergic receptors that inhibit the release of vasodilatory neurotransmitters, contributing to the maintenance of flaccidity. [15]
Acetylcholine plays a crucial role in vascular smooth muscle relaxation. It facilitates the release of nitric oxide (NO) from nonadrenergic, noncholinergic (NANC) neurons and endothelial cells, while also inhibiting noradrenaline release from sympathetic fibers. Acetylcholine can directly contract penile smooth muscle in vitro but primarily acts as a modulator for NO release. [16]
NO appears to be the principal neurotransmitter that causes penile erection. Nonadrenergic, noncholinergic (NANC) neurons release NO. The release of NO increases the production of cyclic guanosine monophosphate (cGMP), which relaxes cavernosal smooth muscle. [17, 18, 19]
Other neurotransmitters include: [20]
Vasoactive intestinal peptide (VIP): Released by NANC fibers, VIP exerts a relaxant effect on cavernosal smooth muscle, enhancing penile vasodilation. [21]
Calcitonin gene-related peptide (CGRP): Found in nerve fibers within the cavernous bodies, CGRP promotes arterial inflow, smooth muscle relaxation, and cavernous outflow occlusion, playing a supportive role in erection. [22]
Prostaglandins: Locally synthesized in penile tissues, prostaglandins such as prostaglandin E1 contribute to smooth muscle relaxation and are clinically used in erectile dysfunction therapy. [23]
Other peptides: Other peptides such as substance P may also have vasorelaxant effects. [24]
With relaxation of the smooth muscles in the trabeculae and the arterial wall, the following events occur, which lead to an erection:
- Arterial inflow increases as a result of dilatation of the arterioles and arteries
- The sinusoids within the corpora cavernosa distend with blood
- Subtunical venular plexuses are compressed between the tunica albuginea and the distended sinusoids, leading to decreased venous outflow
- The tunica albuginea is stretched to its capacity, compressing emissary veins, and thus further decreasing venous outflow; as a result, intracavernous pressure increases and is further increased by contraction of the ischiocavernous and bulbospongiosus muscles, resulting in full rigidity. [25]
Pathophysiologic Variants
Penile agenesis
Congenital absence of the penis, or aphallia, is a rare anomaly caused by developmental failure of the genital tubercle. The approximate incidence of this condition is 1 case per 30 million population. The phallus is completely absent, including the corpora cavernosa and corpus spongiosum; however, some children have been reported to have small portions of corpora cavernosa. The urethra opens at any point of the perineal midline from over the pubis to, most frequently, the anus or anterior wall of the rectum.
See also Genital Anomalies.
Penile duplication
Duplication of the penis, or diphallia, is another rare anomaly resulting from incomplete fusion of the genital tubercle. A new classification system proposes four forms of penile duplication. The most common form, hemiphallus, is associated with bladder-exstrophy complex. The patient exhibits a bifid penis, which consists of two separated corpora cavernosa that are associated with two separate hemiglans.
True diphallia, is an extremely rare congenital condition, comprising complete penile duplication. Other forms include partial duplication anomalies and pseudodiphallus.
Microphallus
The term microphallus, or micropenis, is applicable only to a normally formed yet abnormally short penis. Specifically, the term applies to a penis with a stretched length of more than 2.5 standard deviations less than the mean for age.
Penile torsion
Penile torsion is a rotational abnormality of the penis. The embryologic abnormality is often an isolated skin and dartos, but it may also be related to abnormalities in the orientation of the cavernosal bodies.
Webbed penis
Penoscrotal webbing is a condition in which the scrotal skin extends onto the ventral penile shaft and buries the penile shaft in the scrotum.
Buried penis
In hidden (buried, concealed) penis, the penile shaft is buried below the surface of the prepubic skin. This happens in children and adults with obesity because the prepubic fat is very abundant and hides the penis. The condition may also derive from poor anchorage of penile skin to the deep fascia or be acquired when the shaft of the penis is entrapped in scarred prepubic skin following extreme circumcision or other trauma.
Buried penis is a common condition in which the penis is partially or completely concealed and may be related to a prominent suprapubic fat pad or due to laxity of underlying penile tissue, the dartos.
Absence of the corpora cavernosa and corpora cavernosa plus corpus spongiosum
Congenital absence of the corpora cavernosa and all three corporal bodies result in dilatation of the posterior urethra, megalourethra. Scaphoid megalourethra is related to absence of the corpus spongiosum and is more common than fusiform megalourethra, which is the result of absence of all three corporal bodies.
Curvature of the penis
Curvature of the penis may be congenital or acquired. Congenital curvature may be classified as chordee without hypospadias or true congenital curvature of the penis. Chordee without hypospadias is a term implying that although the meatal location is normal, curvature is present due to inappropriate fetal development of the ventral penile structures. With congenital curvature of the penis, although the urethra, corpus spongiosum, and fascial layers are normally developed, one aspect of the tunica albuginea of the corpora cavernosa has relative shortness or inelasticity.
Typically, ventral curvature is present throughout life. Abnormalities of the ventral penile skin may also exist. In most of these patients, the penis is curved because of inelasticity of the ventral aspect of the corpora cavernosa. In some patients, the corpus spongiosum may become atretic distal on the shaft, with no coverage around the distal urethra.
Individuals with congenital curvature of the penis can have ventral, lateral, or less commonly, dorsal curvature. The curvature tends to involve the entire pendulous portion of the penile shaft.
Peyronie Disease
Peyronie disease is an inflammatory condition that is characterized by the formation of fibrous, noncompliant nodules within the tunica albuginea. [26, 27, 28, 29, 30, 31, 32] One of the most likely causes of Peyronie disease may be repeated tunical mechanical stress and microvascular trauma as well as abnormal wound healing. [27] The tunica albuginea is a multilayered structure consisting of inner circular and outer longitudinal layers of connective tissue encompassing the corpora cavernosa (see the following image). [33, 34]
The tunica albuginea is composed of fibrillar (mainly type I but also types III and V) collagen in organized arrays interlaced with elastic fibers. [34, 35] Although collagen has great tensile strength, it is unyielding. Indeed, it is the elastin content that provides the compliance of the tunica albuginea. The fibrotic plaques (composed of collagen but not elastin) seen in patients with Peyronie disease are produced most likely by tunical fibroblasts in response to cytokine stimulation. [27]
Erectile Dysfunction
Erectile dysfunction may be from a psychogenic or organic component. The erectile process is a neurovascular event, requiring functioning cavernous nerves, arteries, and veins. Injury to the cavernous nerves — such as that occurring during radical prostatectomy and certain colorectal surgeries (abdominal perineal resection [APR] and low anterior resection) — may result in erectile dysfunction. Cardiovascular disease may contribute to arterial insufficiency. Lastly, diseases such as Peyronie disease, which affect the tunica albuginea, may lead to inadequate compression of the emissary veins and a resultant venous leak. The penile curvature associated with Peyronie disease may also make sexual penetration difficult.
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Male reproductive organs, sagittal section.
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Male reproductive organs, cross-section.
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Structure of the tunica albuginea.
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Arterial supply of the penis.
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Venous drainage of the penis.
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Buried penis is a common condition in which the penis is partially or completely concealed and may be related to a prominent suprapubic fat pad or due to laxity of underlying penile tissue, the dartos.
