Anatomic Features
The conically shaped glans, covered by a pink smooth mucous membrane, is the most distal portion of the organ and shows in its central and ventral region, the meatus urethralis. The expanded anterior end of the corpus spongiosum, which has the shape of an obtuse cone similar to the cap of a mushroom, is the central and main tissue of the glans. It is molded over and attached to the blunt extremity of the corpora cavernosa and extends farther over their dorsal than their ventral surfaces (1). The base of this conus is an elevated rim or border, the corona, occupying 80% of the circumferential head of the glans; it is interrupted in the ventral portion of the glans by the mucosal fold of theFrenulum
The diameter of the corona is wider than the shaft and the remainder of the glans.Within the corpus spongiosum, there is the distal portion of the urethra that opens at the summit of the glans as a vertical, slitlike external orifice termed the urethral meatus. Normal anatomical structures frequently found in sexually active males and commonly located in the proximal glans are called pearly penile papules, hirsutoid papillomas, or papillomatosis corona penis or glandis. Grossly, they appear as 1- to 3-mm skin-colored, domed papules evenly distributed circumferentially around the corona and extending proximally on each side of the frenulum. They may be mistaken for warts and be the source of much anxiety for worried adolescents.Foreskin
Anatomic Levels of the Foreskin- Epidermis
- Dermis
- Dartos
- Lamina propria
- Epithelium
Proximal Penis (Or Shaft)
Anatomical Levels of the Penile Shaft- Epidermis
- Dermis
- Dartos
- Buck’s fascia
- Tunica Albuginea
- Corpora cavernosa
- Corpus spongiosum
The penile shaft, body, or corpus, of the penis is mainly composed of three cylindrical masses of erectile tissue, the two corpora cavernosa and a corpus spongiosum with central urethra. The posterior portion of the corpora cavernosa are two divergent and gradually tapering structures, called the crura, that insert in the ischiopubic bone from where they converge to fuse at the level of the inferior portion of the pubic symphysis. The distal three-fourths of the two corpora cavernosa are intimately bound together and make up the greater part of the shaft of the penis. They retain a uniform diameter in the shaft and terminate anteriorly in a bluntly rounded extremity, being embedded in a cap formed by the corpus spongiosum of the glans. The erectile tissue of the corpora cavernosa is a vast, spongelike system of irregular vascular spaces fed by the afferent arteries and drained by the efferent veins. In the flaccid condition of the organ, the cavernous spaces contain little blood and appear as collapsed irregular clefts. In erection they become large cavities engorged with blood under pressure.
The corpora cavernosa are surrounded by a firm, thick, fibrous envelope, the tunica albuginea. In the flaccid state the tunica albuginea measures 2 to 3 mm in thickness and becomes thinner (about 0.5 mm) during erection. On longitudinal sections of the organ, the albuginea covering the corpora cavernosa terminates in a “>”-shaped pattern variably ending beyond or at the level of coronal sulcus or, less frequently, behind it. The tunica albuginea enveloping the corpora cavernosa is thicker and less elastic than that surrounding the corpus spongiosum. The superficial longitudinal fibers of the tunica albuginea form a single tube that encloses both corpora cavernosa while the deep fibers are arranged circularly around each corpus, forming the septum of the penis by their junction in the median plane. The septum is thick and complete in the proximal shaft and discontinous distally. A shallow groove that marks their junction on the upper surfaces lodges the deep dorsal vein of the penis.
The corpus spongiosum and central urethra are located in the concave space on the undersurface of both corpora cavernosa. The middle portion of the corpus spongiosum located in the penile shaft is a uniform cylinder somewhat smaller than the corpus cavernosum. At its ends, it expands, the distal extremity forming the glans and the proximal forming the bulb. The urethra enters the corpus spongiosum 1 to 2 cm from the posterior extremity of the bulb by piercing the dorsal surface. The bulb is just superficial to the urogenital diaphragm, and its posterior portion projects backward towards the anus beyond the entrance of the urethra.
The three cylindrical structures forming the penile shaft are covered by a thin, delicate, and elastic skin. Beneath the dermis, there is a discontinuous smooth muscle layer called the dartos embedded in a thin layer of connective tissue corresponding to the superficial fascia of the classical descriptions. Between the dartos and the albuginea, there is a highly elastic yellowish tubelike sheath encasing all three corpora cavernosa and spongiosum; this is designated as Buck’s fascia (deep penile fascia of the classical descriptions). A septum of fascia extends inward between the corpora cavernosa and the corpus spongiosum, providing separate tubular investments for these columns of erectile tissue and dividing the penis into its dorsal (corpora cavernosa) and ventral (corpus spongiosum)
Skin
The skin covering the penile shaft is rugged and elastic. It shows a thin epidermis composed of a few cell layers and minimal keratinization. The epidermal papillae are thin and deep. The basal layer is hyperpigmented. Hair follicles are present in the dermis of the penile body and are more numerous in the proximal body. Hair follicles and other adnexa can extend out to the cutaneous foreskin in some individuals. They are scanty and contain no piloerector muscle. There are a few sebaceous glands not related to hair follicles. Occasionally there are also poorly developed sweat glands.Dartos
The penile dartos is composed of a discontinuous layer of smooth muscle fibers, variably arranged in transverse and longitudinal branches. Some bundles end at the balanopreputial sulcus while others run farther to become the preputial dartos. The dartos is embedded in a loose fibrovascular connective tissue with numerous nerve bundles that correspond to the superficial penile fascia of the classical anatomic description, and it is the skin equivalent of the penile hypodermis but without adipose tissue. The penile dartos, similar to scrotal smooth muscle fibers, produces a retraction of genital structures when the exterior temperature falls.Buck’s Fascia
The Buck’s fascia is a fibroelastic continuous membrane that encases the corpora cavernosa and the corpus spongiosum. It is composed of loose connective tissue with numerous blood vessels and peripheral nerve bundles running within and beneath it. Vater-Pacini corpuscles are often seen in the penile fascia. The yellow color is related to the numerous elastic fibers and adipose tissue present in the fascia. The skin and dartos slide over this fascia. The Buck’s fascia is a well-developed sheath of close up fibrovascular tissue that is very important from theTunica Albuginea
The tunica albuginea is a thick sheath of partially hyalinized collagen fibers covering both the corpora cavernosa and corpus spongiosum. It is a poorly vascular structure, with only a few branches of circumflex vessels traversing through it, as demonstrated by factor VIII and CD31 immunostains. It is mainly composed of collagen fibers arranged in an outer longitudinal and an inner circular layerThe outer layer, which appears to determine the variation in the thickness and strength of the tunica, is absent in the ventral portion of the corpus spongiosum, transforming this portion of tunica into a vulnerable area to perforation. This anatomic aspect probably explains why most prostheses tend to extrude in this area. The tunica albuginea forms an incomplete fibrous septum separating both corpora cavernosa. The collagen fibers are wavy in the flaccid state and become straight during erection. The fibers are arranged in such a way so as to permit some elasticity necessary for erection. Elastic fibers are rare in the tunica albuginea of the corpora cavernosa.
The tunica albuginea surrounding the corpus spongiosum is thinner and contains more elastic fibers than the one around the corpora cavernosa. In some unusual cases of Fournier’s gangrene, an infection of the lower urinary tract can spread to the corpus spongiosum. Eventually the tunica albuginea may be penetrated; and, with involvement of Buck’s fascia, the infection can rapidly spread to the dartos and directly extend to Colles’ scrotal fascia and Scarpa’s fascia of the anterior abdominal wall. The infection can spread to the buttocks, thigh, and ischiorectal space. The tunica albuginea is probably the real barrier to infiltration of squamous cell carcinoma, contrary to the old concept that the Buck’s fascia was the barrier to the spread of the cancer.
Corpora Cavernosa
The corpora cavernosa are the main anatomic structures used during erection. The substance of the corpora cavernosa consists of a three-dimensional network of trabeculae. These are composed of connective tissue and smooth muscle and are covered by endothelium, creating a network of interanastomosing vascular spaces between them. These spaces tend to be larger in the more central parts of each corpus cavernosum and smaller at their periphery. It seems that the smooth muscle bundles are the main component of the trabeculae in the corpora cavernosa. There is a highly structured criss-crossing of interconnected fibers and spaces that are tensed as the cylinder expands during erection. This creates an internal strength and rigidity that is far greater than that possible in a hollow tube filled to equivalent pressure. This specialized network appears to be necessary for erection. In the flaccid state, the vascular spaces are 1-mm slits that increase several times in diameter with erection. The interconnection between the venous sinuses is so wide that if a contrast is injected at one point, both corpora cavernosa can be immediately and completely visualized. The precise nature of vascular connections between the corpora cavernosa and corpus spongiosum remains controversial. Cavernospongious arterial anastomoses were described by different authors; however, their physiological role in erection remains unknown. These arterial anastomoses could explainCorpus Spongiosum
In the corpus spongiosum of the shaft, there are widely interconnected, branching vascular spaces separated by trabeculae. These vascular spaces of variable caliber are lined by endothelial cells and are surrounded by a thin layer of smooth muscle fibers. These fibers coalesce in various extraluminal parts of the vessels to form the subendothelial cushions or polsters. Toward the urethra, the lacunae become continuous with a mucosal plexus of veins; at the periphery, they communicate with the venous network of the albuginea. Compared to the corpus spongiosum of the shaft, the substance of the glans corpus spongiosum is made up of convolutions of large veins rather than spaces separated by trabeculae.The main differences between corpus spongiosum and corpora cavernosa from the penile shaft are that the blood spaces in the corpus spongiosum, unlike those of the corpora cavernosa, are the same size in peripheral and central areas and the trabeculae between them contain more elastic fibers, whereas smooth muscle bundles are relatively scarce when compared to the trabeculae of the corpora cavernosa. However, there is variability and sometimes it can be difficult to distinguish corpus cavernosum from spongiosum by histology alone.
Arteries
The arteries of the penis are branches of the internal pudenda, which is a branch of the iliac. There are two systems: the dorsal and the cavernous arteries. The dorsal arteries are located from the base of the penis near and on both sides of the dorsal profunda vein within Buck’s fascia and in the superior groove formed by the corpora cavernosa. Small-caliber branches, or circumflex arteries, irrigate the corpora cavernosa and the periurethral corpus spongiosum. They also perforate the albuginea to reach the corpora. The terminal branches irrigate the glans, and collateral branches provide the skin nutrients. Cavernous arteries penetrate the corpora cavernosa at the site where the corpora join, and they run longitudinally near the central septum, which divides the corpora. From the cavernous arteries originate the vasa vasorum, small arteries that irrigate the erectile tissues. The helicine branches also originate from the cavernous arteries, and they are responsible for filling the vascular spaces during the process of erection; their name derives from the fact that they are coiled and twisted along the trabeculae when the penis is flaccid. These arteries have thick muscular walls; and, in addition, many possess inner thickenings of longitudinal muscle fibers that bulge into their lumina. Many of the terminal branches of the helicine arteries open directly into the spaces of the erectile tissue.Veins
The superficial veins are irregularly distributed and easily noted under the skin. They end in the superficial dorsal vein; this vascular structure runs straight from the foreskin to the base of the penis. It drains the foreskin venous blood and the skin and is located in the space between the dermis and Buck’s fascia. The deeper venous system, for which the axis is the deep dorsal vein, runs along the superficial dorsal vein but in a plane separated by the Buck’s fascia. The circumflex veins originate in the periurethralcorpus spongiosum and terminate in the deep dorsal vein system. Similarly, there are veins originating in the corpora cavernosa that, after forming a small plexus at the base of the penis, terminate in the internal pudendal vein. The cavernous venous system, unlike the venous drainage from the glans penis, delays venous drainage and in doing so assists in maintaining erections (61).
Penile erection is a vascular phenomenon that results from trabecular smooth muscle relaxation, arterial dilation, and venous restriction. In further support of the concept of restriction of outflow is the observation that the walls of the circumflex veins are unusually muscular. In addition, these veins exhibit unique specializations of their lumina caller polsters. These are local accumulations of fibroblasts and smooth muscle cells beneath the endothelium that form conspicuous longitudinal thickenings or ridges that can be followed throughout hundreds of serial sections. These are believed to have a role in constricting the lumen and retarding venous outflow during erection. There are, however, controversies regarding the real presence and significance of the polsters in the penile veins and arteries, and some authors have proposed that they represent degenerative changes.
The deep dorsal vein of the penis has a connection with the vertebral veins, hence it is possible for metastases to make their way to the vertebrae or even to the skull and brain without going through the heart and lungs. Pyogenic organisms may be transported by the same route.
Nerves
The nerves originate in the sacral and lumbar plexuses. Peripheral nerves run along the arteries. Dorsal nerves are located external to the arteries, giving circumflex branches to the corpora cavernosa. The terminal branches end in the glans and foreskin. The dorsal nerve of the penis, the principal somatosensory nerve innervating the penis, consists of two populations of axons, one to innervate the penile shaft and urethra and the other to innervate the glans. Urethral innervation by the dorsal nerve of the penis supports the view that urethral afferent impulses are a component of reflex ejaculatory activity. The pattern of glanular innervation by the dorsal nerve of the penis identifies the glans as a sensory end organ for sexual reflexes. The undulating character of the dorsal nerve of the penis is a mechanism by which the nerve can accommodate to significant changes in penile length with erection. The dorsal nerve of the penis supplies the glans in most men, but branches of the perineal nerve can supply the ventral penis, frenulum, and periurethral area in some men.flylib.com/books/en/2.953.1.44/1/
The glans, however, has a higher concentration of nerve endings than the shaft of the penis and is thus particularly sensitive to physical stimulation.
Two other areas that are highly sensitive are the rim that separates the glans from the shaft of the penis, called the coronal ridge, and the small triangular region on the underside of the penis where a thin strip of skin called the frenulum attaches to the glans.
During sexual excitement, the blood vessels of the spongy tissue fill with blood and swell up. The rapid, forceful dilation of these blood vessels throughout the entire penis causes it to become firm and grow in size. This transition from a soft (flaccid) penis to a harder, stiffer penis is called getting an erection. The penis usually returns to a softer (non-flaccid) state shortly after ejaculation or after the penis is no longer being stimulated.
The erect penis is inserted into a woman’s vagina during the sexual act called intercourse or coitus.
When a man reaches the pinnacle of sexual excitement, his orgasm is often accompanied by spurts of semen from the opening at the tip of the glans. This is called ejaculation. The semen that is released from the penis during ejaculation contains sperm.
When one of the millions of sperm encounters an egg inside a woman’s body, their union can produce a pregnancy. The appearance of the penis varies considerably from one male to another. There can be differences in color, shape, size, and the status of the foreskin (circumcised or uncircumcised).
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