Genital embryology

Phenotypic sex

Phenotypic sex is ‘determined’ by whether the genital tubercle develops into a male or female pattern. In the male, urogenital swellings migrate ventrally and anteriorly to form the scrotum. The genital tubercle develops by elongation and cylindrical growth. At the same time, urethral folds close over the urethral groove, thereby establishing a urethra and a midline raphe. Mesenchymal tissue coalesces to surround the urethra and form the corpus spongiosum. This development is entirely under the influence (or absence) of testosterone, testosterone derivatives (i.e., dihydrotestosterone), and 5a-reductase and occurs between 6 and 13 weeks’ gestation (Fig. 13.1). The prepuce grows to cover the penile glans but is not influenced by dihydrotestosterone.

Fig. 13.1 The definitive phenotypic external genital growth that occurs in utero under the influence (or absence) of testosterone, dihydrotestosterone, and 5a-reductase. The influence of these virilizing hormones causes the genital tubercle to enlarge, the urethral folds to meet and close ventrally, and the scrotum to migrate medially and posteriorly. Any hormone deficit or receptor site inadequacy leads to an external female genitalia tendency (‘phenotype by default’).

In the female embryo, the lack of testosterone-influenced virilization holds the urogenital sinus and the genital tubercle in a fixed perineal position. The urethral groove remains unclosed (folds develop into labia minora), and the genital tubercle remains static in size but bends ventrally. The labia majora enlarge, migrate caudally, and fuse to form the posterior fourchette. It is this lack of closure of the ventral urethra (a ‘nonevent’) that causes the female perineum to be shorter and the introitus to be located in a more caudal position.

Genital anatomy

Male genital anatomy is unique in the human body and evolved phylogenetically as a means of protection from trauma and disease. This evolution has proved essential for the human race to continue procreation and elimination.

Genital fascia

Perhaps the most obvious protective mechanism is the testicular (and penile) withdrawal on exposure to physical stress (like hypothermia or blunt trauma), which causes the unique cremasteric muscles to contract, thereby withdrawing the testicles and shrinking the scrotum as close to the body as possible. Otherwise, the testicles hang free, ostensibly to provide the best milieu for sperm development. At the same time, the penile corporal bodies and the urethra also retract and shrink in size, although the penile skin does not have the same retraction properties of the scrotal skin. Both the penis and the scrotum have redundant skin coverage with their own separate blood supply and underlying supportive superficial fascial system.

The penis contains specially designed tunical tissues that surround the penile corporal bodies and have the ability to expand and hold the inflow of blood and to prevent, along with venous valve mechanisms, the egress of blood during erections. The tunica albuginea fascia envelops the corporal bodies tightly but is perforated by an intercavernosal membranous septum that allows blood flow between the corpora cavernosa. The tunical tissues are thick over the dorsal and lateral aspects of each corporal body but thin out in the ventral sulcus where the urethra and corpus spongiosum are located. The tunica also thins out beneath the glans penis cap where it has direct vascular contact with the glans.

Overlying the tunica is the deep penile fascia (Buck fascia), a strong laminar structure that tightly surrounds and binds the corpora cavernosa together and, in the case of the corpus spongiosum, envelops these tissues into a single-functioning entity. The urethra and its overlying corpus spongiosum are also protected proximally by surrounding muscles and by their location within the intercorporal groove distally. Buck fascia carries important neurovascular structures to the glans penis, including the deep dorsal vein and arteries, the deep dorsal nerves of the penis, the circumflex arteries and veins, and the penile lymphatics (Fig. 13.2).¹

Fig. 13.2 A cross-section of the penile shaft illustrates the superficial and deep fascial layers and their relationships to the corporal bodies and neurovascular structures.

(From: Quartey JK. Microcirculation of penile and scrotal skin. Atlas Urol Clin North Am. 1997;5:1–9.).

The penile glans itself is a vascular spongiosum containing unique sensory endings that are erogenous and tactile. The glans epithelium is a unique uroepithelium that contains sensory cells, particularly around the corona. The glans is naturally covered and protected by a prepuce that consists of inner and outer laminae. The inner lamina consists of uroepithelium that is similar to that of the glans and, in fact, developmentally separates from the glans in the last trimester and after birth. The outer lamina consists of epithelium that is consistent with the glabrous skin of the penile shaft. Superficial to Buck fascia but beneath the penile shaft skin and prepuce lays the superficial fascial system, defined by dartos fascia. This fascial layer is a continuation of the Scarpa fascia superiorly and the Colles fascia inferiorly and surrounds the penis from the penoscrotal and peno-pubic angles to the prepuce.

Dartos fascia contains its own vascular plexus that allows overlying skin islands to be elevated on its independent blood supply.

Colles fascia is a deep, tight, triangular fascial system that arises laterally from the inferior pelvic rami and posteriorly from the perineal membrane to protect the genitalia from toxins, trauma, and infections (and envelops both testicles circumferentially, as the tunica dartos). Colles fascia is analogous to the dartos fascia on the penis, and thus skin island flaps can be elevated on the vascular plexus carried on this fascia (see Fig. 9.11).²

Overlying both testicles, the epididymis, and the cord structures is a loose, well-vascularized superficial fascial layer. The tunica vaginalis invests the testicles, and the parietal tunica vaginalis acts as the ‘vaginal space,’ which can be likened to the peritoneal cavity. Although the testicles are anchored within the scrotum, they move separately and independently on their cremasteric systems. The neurovascular supply to the testicles is dedicated to the viability of the testicles, epididymis, and cord structures (vas deferens) as well as to continued sperm production.

Genital blood supply

The genitalia have two separate arterial sources. The first is the deep vascular system originating from the deep internal pudendal artery. The paired pudendal arteries originate from the internal iliac arteries, pass along the borders of the inferior pelvic rami, and then give off the perineal and scrotal branches before continuing as the common penile arteries. After exiting from the Alcock canal, a split in the obturator fascia that runs from the lesser sciatic foramen to the ischial tuberosity along the sidewall of the ischiorectal fossa, each common penile artery gives off three branches (bulbar, urethral, cavernosal), and terminates in the dorsal artery of the penis, which runs within Buck fascia distally to terminate in the balanitic arteries. Within Buck fascia, the dorsal penile arteries are coiled and tortuous compared with the deep dorsal vein, which is linear and straight. This anatomy may have something to do with erectile function (Fig. 13.3).³

Fig. 13.3 The deep arterial vascularization to the penis arises from branches of the common penile arteries.

(From: Quartey JK. Microcirculation of penile and scrotal skin. Atlas Urol Clin North Am. 1997;5:1–9.)

The perineal branch of the pudendal artery is just superficial to Colles fascia and has an unpredictable length, but its central location and strong collateral supply make it a mainstay of genitourinary flap reconstruction. The scrotal branch of the perineal artery passes along the fold between the lateral scrotum and medial thigh and arborizes within the tunica dartos (Fig. 13.4).

Fig. 13.4 The internal pudendal artery exits Alcock canal and then divides into the posterior scrotal and perineal arteries. The perineal artery closely follows the crural reflection of Colles fascia. This anatomy is favorable to dissection, elevation, and transposition of fasciocutaneous flaps that can be rotated into the perineum.

(From: Jordan GH, Stack RS. General concepts concerning the use of genital skin islands for anterior urethral reconstruction. Atlas Urol Clin North Am. 1997;5:23–44.)

The second blood supply of the penis is the superficial external pudendal system –branches of the femoral arteries. The femoral artery typically gives off a superficial external pudendal artery and a deep external pudendal artery. The superficial external pudendal artery supplies vascularity to the dartos fascia and genital skin. The deep external pudendal artery arises as a separate branch and passes into the genital skin as the lateral inferior pudendal artery, which then separates into a dorsolateral branch supplying the dorsal and lateral penile shaft skin and an inferior branch supplying the ventral penile skin and the anterior plane of the scrotum (the anterior scrotal artery) (Fig. 13.5). This arrangement allows surgeons to elevate long axial and transverse flaps with relative safety and still cover the shaft donor site with the adjacent skin.

Fig. 13.5 The deep external pudendal vascular system arises from the femoral artery and empties into the saphenous vein. The vessel passes onto the penile shaft and anterior scrotum to vascularize the skin and dartos fascia.

(From: Jordan GH, Stack RS. General concepts concerning the use of genital skin islands for anterior urethral reconstruction. Atlas Urol Clin North Am. 1997;5:23–44.)

The penile venous system also has an accompanying dual blood supply. The superficial system arises from the distal penile shaft and passes to the superficial dorsal vein within the dartos fascia to drain the penile shaft skin. In approximately 70% of anatomic studies, the superficial dorsal vein empties into the left saphenous vein. Other vascular patterns include connections into the right saphenous vein (10%), left femoral vein (7%), and inferior epigastric vein (3%); in 10%, the deep dorsal vein runs as a dual supply and empties into the saphenous veins bilaterally. These collateral veins are usually of different caliber and are asymmetric in their course (Fig. 13.6).

Fig. 13.6 The superficial dorsal vein of the penis usually empties into the left saphenous system – an anatomic fact that must be taken into account in planning fasciocutaneous flaps.

(From Jordan GH, Stack RS. General concepts concerning the use of genital skin islands for anterior urethral reconstruction. Atlas Urol Clin North Am. 1997;5:23–44.)

The vas deferens, epididymis, and testes are vascularized from the retroperitoneal blood supply, primarily the spermatic artery, which originates from the aorta, and the deferential artery, which supplies the vas deferens. In addition, collateral blood supply from the retroperitoneal cremasteric artery follows the vas to become the vasal artery. As the spermatic artery and its venae comitantes approach the testis, it divides into the internal testicular artery (which supplies the testis and the adjacent epididymal head and body) and the inferior testicular artery, which passes within the testis. The epididymal tail is supplied by branches of the epididymal, vasal, and testicular arteries.⁴

The veins form in the pampiniform plexus, which coalesces around the testis and epididymis to flow into the testicular veins. The testicular veins then pass in a retroperitoneal plane to empty into the inferior vena cava on the right side and the left renal vein on the left side.

Exstrophy and epispadias

Exstrophy of the bladder is an uncommon condition that occurs in approximately 1 of every 30 000 live births, of which boys predominate in a 3 : 1 ratio. The defining features of epispadias and exstrophy are an open and protruding bladder, an open urethra, and a foreshortened epispadiac penis. However, the associated spectrum of anomalies may extend to involve the musculoskeletal structures and the gastrointestinal tract. Classic exstrophy –defined by bladder exstrophy, epispadias, diastasis recti, absence of fusion of the pubic symphysis, and deformed pubic escutcheon – occurs in 60% of cases; epispadias alone occurs in 30% of cases, and 10% of cases are more extensive dysmorphias including cloacal exstrophy.

The etiology of exstrophy-epispadias is controversial, but it does not represent an arrest of a normal fetal developmental stage. It occurs in early gestation between the 3rd and 9th weeks. The anomaly is associated with the formation and normal retraction of the cloacal membrane. In the normal fetus, a mesodermal layer of tissue spreads medially to replace the thin cloacal membrane by the 9th week in utero. According to Muecke’s theory,⁵ the cloacal membrane persists and resists any medial migration of mesoderm. The membrane then ruptures, thereby producing a lack of mesodermal tissue to form the anterior abdominal wall and endodermal tissue to form the anterior wall of the bladder. This lack of mesodermal migration also has a profound effect on the musculoskeletal system. The pubic rami are widely separated, and the inferior pubic rami are consequently laterally rotated. This defect produces a widened and foreshortened urethra and bladder neck. It also produces an incompletely formed penis that remains rudimentary and, by definition, is a phallus. According to Mitchell and Bägli,⁶ the anomaly is that of a fetal abdominal wall hernia and can be recreated in the laboratory in chickens because they have a persistent cloaca by induction of a localized vascular accident (J. Sumfest, pers. comm. 2002).

The defining features of exstrophy-epispadias are: an open urinary tract with protruding bladder and foreshortened epispadiac penis (Fig. 13.7). The crural bodies are attached to the splayed pubic tubercles, producing a penis that is short, wide, and with dorsal chordee. Unlike in the normal anatomy, corporal bodies are independent of each other with no communication through the intercorporal septum. The neurovascular structures to the glans are laterally displaced but move medially at the distal end of the foreshortened penis; the glans is spade shaped and incompletely formed, and each side is totally dependent on the respective dorsal neurovascular supply for its viability. Little circulation passes through the corporal bodies into the glans, as opposed to a penis with normal development. The separated pelvic ring also produces a widened scrotum and lack of competent pelvic musculature. Therefore, the perineum is short and the anus can be patulous and anteriorly displaced. The rectus muscles are widely separated, and inguinal hernias are the rule.

Fig. 13.7 Exstrophy in a boy.

Although the initial postnatal diagnosis and treatment of bladder exstrophy and epispadias remain in the realm of the pediatric urologist and pediatric orthopedic surgeon, it is important that the plastic surgeon be prepared, if consulted, to help reconstruct such a child. The goals of initial closure are to reconstruct a functional genitourinary system, to reduce the risk of bladder squamous metaplasia, and to close the pelvic ring. This is carried out by direct closure of the bladder and reconstitution of the pelvic ring.

Different techniques have been described for penile reconstruction^6,7 and although the results of the urethral closure have drastically improved, the ideal surgical approach is still controversial: neonatal versus delayed closure and one stage versus multi-stage repair. Phallic length mainly depends on antenatal development and the majority of these patients end-up with small and undeveloped penises, despite the best efforts of their treating surgeons. As they pass through their post-adolescent period, many of these young men will benefit from further lengthening procedures or even complete penile reconstruction. In some patients; correction of unaesthetic scars and further release of insufficiently released corpora can help to gain length (Fig. 13.8).

Fig. 13.8 A Z-plasty on the dorsal aspect of the penis extending into the prepubic area can be useful to obtain maximal lengthening.

Exstrophy patients miss an umbilicus and often they are consulting for umbilical reconstruction. Different techniques have been described with good cosmetic outcomes. Neonatal preservation of the umbilicus and transposition to an abdominal position can overcome the loss of umbilicus.^8,9

Unfortunately, in some boys there just is not enough tissue due to underdevelopment or due to partial or complete loss of penile tissue after primary closure. These patients might be a good candidate to undergo a phallic construction or further penile reconstruction with the use of microsurgical tissue transplantation techniques (Fig. 13.9) or local pedicled perforator flaps (see below, Fig. 13.12A–C).

Fig. 13.9 (A) Bladder exstrophy patient with a severely underdeveloped penis who requested a complete penile reconstruction with a free radial forearm flap. (B,C) Postoperative result after radial forearm phalloplasty with the small glans incorporated at the base of the reconstructed penis.

Most of these patients have some form of different urinary diversion and therefore a nonfunctional urethra. Although the ejaculatory ducts are mostly intact, they are often abnormally positioned as a prepubic fistula. These anatomic facts create unpredictable sperm production, and most of these patients are unable to procreate naturally. For these reasons, the urethral reconstruction may be a moot point in this group of patients thus making a phalloplasty in these ‘boys without a penis,’ much more easy compared to a penile reconstruction in a patient who wants to void through his new penis. The different options and peculiarities for a phalloplasty in an exstrophy patient will be addressed further in this chapter.

After neonatal closure of the exstrophy, the pubic hairline can show an asymmetry. This can easily be reconstructed by creating and mobilizing skin flaps and correcting the pubic hairline.

Disorders of sex development (formerly “intersex”)

After standardization of the terminology, intersex conditions are nowadays defined as Disorders of Sexual Development (DSD).¹⁰

DSD is not within the scope of this text and therefore it is limited here to a short description of the conditions in which genital reconstruction is needed. Only conditions with genital ambiguity and those with the absence of Müllerian duct derived structures are mentioned.

46XX DSD: over-virilized females. The most prevalent condition here is congenital adrenal hyperplasia (CAH) where, due to a cortisol synthesis defect, androgens are overproduced in a female subject. This leads to virilization of the female genitals with urogenital sinus formation (confluence of urogenital tracts), labioscrotal fusion and clitoromegaly. Surgical correction is needed for separating the urogenital tracts bringing the urethra and the vagina separately to the perineum with a nerve sparing reduction of the clitoris size and a reconstruction of the labiae. Although the optimal timing of this surgery still is a point of controversy, most surgeons agree on early reconstruction.

46XY DSD: under-virilization in males. Testosterone synthesis defects and partial androgen receptor insensitivity are among the causes. Patients in this group present genital ambiguity with varying degrees of hypospadias, penoscrotal transposition and cryptorchidism. Surgical treatment early in life consists of hypospadias repair, correction of penoscrotal transposition and orchidopexy.

46XY/46XX DSD: consists of varying degrees of genital ambiguity and presence of both male and female or dysgenetic gonads. Reconstruction is done after gender assignment, which is not always obvious. Multidisciplinary teams with expertise in these pathologies are needed to guide the diagnosis and treatment in these children.

Genital conditions: this DSD classification includes conditions like penile agenesis, extreme penoscrotal transposition with rudimentary penis, Mayer-Rokitansky-Kuster-Hauser with absence of Müllerian duct derivates like vagina and uterus, cloacal exstrophy, micropenis and other genital malformations with normal chromosomes and gonads. All these conditions will eventually require genital reconstruction although due to their low prevalence these should be treated in centers of excellence with a lot of expertise in this field.

Buried penis

The buried penis deformity is present in both the pediatric and adult populations. A buried penis is defined as a penis that is of normal size for age but hidden within the peripenile fat and subcutaneous tissues (Fig. 13.10).

Fig. 13.10 Buried penis: retraction of the peripenile and pubic fat reveals a normal size penis.

In the pediatric population, the fat deposit is often part of the constellation of poor virilization. The abnormal mons fat pad (gynecoid mons pubis) may become associated with a generalized obesity in the adolescent patient, and the buried penis must be differentiated from a micropenis in this group. In adults, the problem is almost always associated with obesity and the development of pubic, scrotal, and peripubic ptosis, which must be addressed to correct the problem of the hidden penis. Liposuction and lipectomy are part of the treatment in adults however in children the fat resection is abandoned. With pubertal development the prepubic fat deposit often decreases in size. The focus is on the release of the penis from the fibrotic dartos tissue.^11,12 Many techniques are described but the most important steps include keeping all available skin from the start of the procedure, to resect all dartos tissue and to recover the released corpora with the skin (Fig. 13.11).

Fig. 13.11 (A) Typical buried penis in an infant. (B) Ventral incision of the skin with maximal preservation of skin at the start of the procedure. (C) After complete resection of the fibrotic dartos tissue the penis is released from its buried position and the skin is extendable. (D) Coverage of the released corpora with the extended skin creating a penis with normal length.

Reconstructive options for severe penile insufficiency

A clear definition of severe penile inadequacy has not yet been established but can be considered as an insufficient penile length and function to obtain successful sexual intercourse. This implies that puberty must be finished and that the patient must be sexually active. Conditions with penile insufficiency include: aphallia or penile agenesis, idiopathic micropenis (stretched penile length in a full-term newborn male <2.5 cm), 46XY DSD and bladder exstrophy. Reconstructive surgery in these mostly young patients is required because of the devastating effect on psychological and sexual function.

The development of perforator flaps has given rise to some new reconstructive options in patients with such severe penile insufficiency. These perforator flaps have the advantage of reducing the donor site morbidity, increasing the range of motion of the flap and combining different tissue flaps on one single pedicle. Although, overall, the free vascularized radial forearm flap (discussed further below) is still considered as the ‘standard technique’ in penile reconstruction, the pedicled anterolateral thigh (ALT) flap has been shown to provide a valuable phalloplasty alternative specifically in patients with congenital penile insufficiency. This flap is a skin flap based on a perforator from the descending branch of the lateral circumflex femoral artery, which is a branch from the femoral artery.

There are several reasons why in the ‘boys without a penis’ a pedicled ALT flap can be preferred above the standard radial forearm flap (Fig. 13.12):

Fig. 13.12 The reconstruction of a penis with a pedicled ALT flap used as a tube-within-a-tube technique (only possible in thin ‘boys without a penis’ and after defatting). No real urethra was reconstructed here since the patient had a urostomy. (A) Preoperative view. (B) After flap dissection. (C) The flap is tunneled underneath the rectus femoris muscle. (D) Suturing and nerve connection (ilioinguinal nerve to lateral femoral cutaneous nerve).

It is very important to preserve and incorporate any useful glandular, penile and cavernosal tissue at the basis of the newly reconstructed phallus in order to facilitate sexual stimulation and pleasure (Fig. 13.9C). If available, a dorsal penile nerve is identified and connected with a cutaneous nerve of the flap; if not available, the lateral femoral cutaneous nerve is connected to the ilioinguinal nerve.

Usually, it is recommended to perform a 3D angio CT-scan preoperatively to provide detailed information on the perforator vessel(s) and the subcutaneous tissue layer.

Unfortunately, similar as to the radial forearm flap, also the ALT flap has a rather high urological complication rate with frequent strictures and/or fistula formation. Secondary procedures might be needed to treat these complications and especially the treatment of urethral strictures is challenging and difficult.

There is still a lot of controversy whether or not to perform a phalloplasty in children. Penile construction in children is similar as in adults with one added requirement – growth through puberty to adulthood. Because the phallus is constructed of somatic tissues (showing linear growth) but replaces a penis that is formed by genital tissues (demonstrating a more exponential growth), the growth rates are temporally and quantitatively different during puberty.¹³ Care must be taken to accurately predict the anticipated growth rate and to design a phallic model that is larger and longer than normal genital size for that age group.¹⁴

Another issue that we are just now beginning to address is the ‘correct’ age at which to proceed with the insertion of penile prostheses. Once these boys have reached 18 years and the age for majority, they must be physically and psychologically prepared to manage a phallus that has previously been erectionless.

There is another nagging problem with operating on children – the lack of informed consent. Although the child’s best interest and surgery’s best intentions are usually served by early reconstruction, there are no long-term studies that have evaluated the results of this surgery over a lifetime or even a generation.

Finally, it comes as no surprise that the large majority of these genitally compromised boys require prolonged psychological therapy to deal with genital loss, surgical trauma, inadequacy, and scarring. These psychological issues are often closely commingled with the need for secondary surgery to complete reconstruction.

Nowhere in surgery is there a stronger need for parental and family support than with these late teenage boys who have essentially undergone years of ‘surgical abuse.’

General reconstructive options

Genital skin grafts

Genital skin loss occurs from burns, avulsion injuries, infections, and gangrene. As a rule, total excision of the necrotic genital tissues followed by early skin grafting produces the best results. When the wound is contaminated or infected, adequate debridement of necrotic tissues combined with wound bed preparation prior to skin grafting might be required and this can be performed with adequate (moist) wound dressings, by temporary coverage with allograft skin or with the use of topical negative pressure.

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