Advances in understanding of mammalian penile evolution, human penile anatomy and human erection physiology: clinical implications for physicians and surgeons

Abstract

Recent studies substantiate a model of the tunica albuginea of the corpora cavernosa as a bi-layered structure with a 360° complete inner circular layer and a 300° incomplete outer longitudinal coat spanning from the bulbospongiosus and ischiocavernosus proximally and extending continuously into the distal ligament within the glans penis. The anatomical location and histology of the distal ligament invites convincing parallels with the quadrupedal os penis and therefore constitutes potential evidence of the evolutionary process. In the corpora cavernosa, a chamber design is responsible for facilitating rigid erections. For investigating its venous factors exclusively, hemodynamic studies have been performed on both fresh and defrosted human male cadavers. In each case, a rigid erection was unequivocally attainable following venous removal. This clearly has significant ramifications in relation to penile venous surgery and its role in treating impotent patients. One deep dorsal vein, 2 cavernosal veins and 2 pairs of para-arterial veins (as opposed to 1 single vein) are situated between Buck's fascia and the tunica albuginea. These newfound insights into penile tunical, venous anatomy and erection physiology were inspired by and, in turn, enhance clinical applications routinely encountered by physicians and surgeons, such as penile morphological reconstruction, penile implantation and penile venous surgery.

Figure 1

Schematic illustration of the traditional penile anatomy. (A) Lateral view. The glans penis is exclusively composed of uniform sinusoids only? The deep dorsal vein (DDV) is sandwiched in by a pair of dorsal arteries (DA)? The 2: 1 ratio of arteries to veins is the same as in the umbilicus vessel. (B) Cross-section of a pendulous portion in the human penis. The tunica albuginea of the corpora cavernosa is consistently described as a one-layered coat with uniform thickness. The median septum is complete. There is one single DDV and two DAs between the tunica albuginea and Buck’s fascia. Thus the penile vascular system still complies with the general anatomical rule that veins number more than arteries do.

Figure 2

Schematic illustration of the fibroskeleton in the human penis. The tunica albuginea of the corpora cavernosa is a bilayered structure. The inner circular layer completely contains the sinusoids and, together with the intracavernosal pillars, supports them. There is a paucity of outer layer bundles at the region between the 5 and 7 o’clock positions where there is close contact with the corpus spongiosum. Distally, they are grouped into the glans penis forming the distal ligament, located at the 12 o’clock position of the distal urethra. The median septum is incomplete with dorsal fenestration at the pendulous portion of the penis and is merely complete where the penile crura are nearly formed.

Figure 3

Schematic illustration of the three-dimensional human penis. (A) Lateral view. The penis leans upon the suspensory ligament which is an extension of the linea alba. Proximally, it is capped by the glans penis, and the corpus spongiosum is held by the bulbospongiosus muscle in which the fibers are mostly transverse. The corpora cavernosa are surrounded by a bilayered tunica albuginea which is composed of inner circular and outer longitudinal collagen bundles. The intra-cavernosal pillars, which are considerably more numerous distally, are a continuation of the inner circular layer. The corpus cavernosum is entrapped in the ischiocavernosus muscle with the muscle fibers aligned in longitudinal arrangements. (B) Medial view. The distal ligament is segregated from the collagen bundles of the outer longitudinal layer of the tunica albuginea. It is an inelastic fibrous structure which forms the trunk of the glans penis. The incomplete septum is dorsally fenestrated. The corpus spongiosum contains the urethra. (C) Ventral aspect. The three-dimensional structure of the human penis is evident. The ischiocavernosus muscle is paired and situated at the lateral boundary of the perineum. Each segment covers its ipsilateral penile crus. Meanwhile, anterior fibers of the bulbospongiosus muscle partially spread out to encircle the corpus cavernosum (not shown here) and mostly insert into the ventral thickening of the tunica albuginea.

Figure 4

Schematic illustration showing advanced anatomy of the erection-related veins in the human penis. (A) Lateral view: The deep dorsal vein is consistently located in the median position and receives blood of the emissary veins from the corpora cavernosa and of the circumflex vein from the corpus spongiosum. It is sandwiched between the cavernosal veins, although these lie at a deeper position. Bilaterally, each dorsal artery is respectively sandwiched by its corresponding medial and lateral para-arterial veins. Note that the lateral para-arterial vein merges with the medial one proximally. The deeper color of the veins indicates the deepest part of the vasculature. (B) Cross section of the mid-penis. Note that the number of veins is seven, not one as was traditionally believed. (Although the number becomes four at the level of the penile hilum because each pair of the nomenclature veins merges.) Erection-related veins are arrayed in an imaginary arc on the dorsal aspect of the tunica albuginea.