Homology of the Male and Female Genitalia
There are often many phenotypical differences between males and females of a particular species. These include differences in height, body mass, muscle tone, hair length, and in non-human species, skin or feather pigmentation. The most emphasized difference between the two members of the species is the capability (or lack thereof) to bear offspring. This difference would subsequently suggest that there would also be a difference between the structure of the male and female genitalia. While the differences between human male and female genitals will inadvertently be mentioned, the true aim of this article is to elucidate the similarities that exist between the two. As a result, embryological origins, as well as internal and external structures, along with some clinically relevant points, will be explored.
Although the genotypic sex of the individual is determined at the time of fertilization (46 XX or XY), phenotypic expression of sexual dimorphism in humans is subject to a myriad of genetic pathways. Furthermore, even though the end results appear structurally different, there are a lot of similarities between males and females embryologically. Both ovaries and testes begin as epithelial proliferation and mesenchymal condensation along the caudal segment of the future posterior abdominal wall (anteromedial to the mesonephros). This structure, the genital ridge, is infiltrated by primordial germ cells around the sixth gestational week. The arrival of these cells has been credited with the induction of gonadal differentiation. Additionally, as the primordial germ cells enter the gonadal ridge, the mesenchyme deep to the ridge is penetrated by the ridge’s proliferating epithelium. They continue to form primitive sex cords, making it impossible to distinguish between male gonads and female gonads at this developmental level. These primitive sex cords continue to form the rete testis in males and are replaced by vascular stroma of the ovarian medulla in females.
Both sexes are equipped with two paired tubular structures known as the paramesonephric (Müllerian) duct and the mesonephric (Wolffian) duct. The former arise anterolaterally, along the length of the urogenital ridge, while the latter is an intermediate mesodermal derivative between the levels of the upper thoracic and the upper lumbar region. The proximal end of the paramesonephric duct opens and communicates with the abdominal cavity while the distal end continues lateral to the mesonephric duct, before crossing it ventrally to course in the midline. In the midline, it approximates with the contralateral paramesonephric duct and subsequently fuses in females to later form the uterine canal (future fallopian tubes and uterus) and paramesonephric tubercle. In males, they degenerate to form the appendix testis.
Some aspects of the proximal part of the mesonephric ducts contribute to the formation of the renal system, while the majority degenerates during foetal life. A part of the distal mesonephric tube pierces the posterior part of the urogenital sinus and contribute to the formation of the seminal vesicle, ductus epididymis, ejaculatory ducts and ductus deferens. In females, these ducts degenerate, leaving cranial remnants in the mesovarium known as the epoöphoron and paroophoron, and caudal ones adjacent to the proximal vagina called Gartner’s cysts.
The external genitalia of both males and females also have similar origins. They arise from the genital tubercle that forms anterior to the cloacal folds (proliferating mesenchymal cells around the cloacal membrane). The caudal aspect of the cloacal folds further subdivides into the posterior anal folds and the anterior urethral folds. Bilateral to the urethral fold, genital swellings (tubercles) become prominent. These structures are the future scrotal swellings and labia majora in males and females, respectively. The genital tubercles of an eight week old embryo of either sex are identical. They both have a glans area, which will go on to form the glans clitoris (females) or glans penis (males), a urogenital fold and groove, and an anal tubercle. At around ten weeks, the external genitalia are still similar. At the base of the glans, there is a groove known as the coronal sulcus or corona glandis. It is the site of attachment of the future prepuce. Just anterior to the anal tubercle, the caudal end of the left and right urethral folds fuse to form the urethral raphe. The lateral part of the genital tubercle (called the lateral tubercle) grows longitudinally and is about the same length in either sex.
There are only a few similarities with respect to the internal reproductive organs of males and females. The gonads of both genders undergo decent from their initial position to different degrees. Testicular decent is far more dramatic and results in the male gonads being located outside the body cavity. On the other hand, the decent of the female gonads is less pronounced, and results in the ovaries being located in the true pelvis. A band of urogenital mesentery initially attaching the caudal end of the testis to the posterior abdominal wall, subsequently becomes ligamentous and is joined by a condensation of mesenchyme known as the gubernaculum. In males, the gubernaculum has both an intra-abdominal and an extra-abdominal component that promote testicular decent. The latter inserts into the scrotal swelling. It is noteworthy that a rudimentary gubernaculum also forms in females. Similarly, there are ligamentous attachments associated with the ovaries. Cranially, it is attached to the suspensory ligament of the ovary – a derivative of the cranial genital ligament. Caudally, it is attached to the ligament of the ovary proper and the round ligament of the uterus; both of which are products of the caudal genital ligament.
Finally, another similarity between males and females is the source of arterial blood for the gonads. In both sexes, the gonads receive arterial blood directly from the gonadal branch of the abdominal aorta. Also, their venous return is achieved by gonadal veins that drain directly to the inferior vena cava on the right and the left renal vein on the left.
In a fully developed male or female, the external genitalia may appear significantly different – which it is. However, here are some key homologies. The glans penis (colloquially referred to as the head of the penis) and glans clitoris are richly innervated portions of the genitalia that are primarily responsible for orgasms experienced during coitus. In males, the lateral tubercle grows rapidly during development to form the penile shaft. The rapid growth results in the approximation and subsequent fusion of the urethral folds, obliteration of the urogenital groove and the formation of the penile raphe on the ventral surface of the penis. In females, the growth of the lateral tubercle is less dramatic, resulting in the formation of a relatively smaller corpus clitoris (body of the clitoris). Furthermore, the urethral folds do not fuse in females, but grow to become the labia minora; while the urogenital sinus forms the vestibule of the vagina.
The internal makeup of male and female external genitalia has one major thing in common – cavernous tissue. The penis has two intercommunicating tubular structures called the corpora cavernosa that become engorged with blood during sexual arousal. The clitoris also has two cavernous cavities that allow it to become erect and pulsatile during sexual arousal.
At birth, both males and females have a skin covering the glans called the prepuce. For religious, hygienic or medical (e.g. phimosis – inability to fully retract the foreskin) reasons, the prepuce or foreskin in males is often removed surgically. The prepuce in females is also called the hood of the clitoris, and is formed dorsally from the cranial ends of the labia minora. In females, the labioscrotal swellings remain separated and progress to form the labia majora. In males, however, these swellings fuse and are separated internally by a septum forming isolated scrotal sacs – one for each testis.
Finally, the external urethral orifice in females is normally present within the vestibule of the vagina, superior to the vaginal orifice and inferior to the frenulum of the clitoris (on its ventral surface). However, in males, the external urethral opening typically appears at the apex of the glans penis. In some developmental anomalies, there is an ectopic urethral orifice either on the ventral surface of the penis (hypospadias) or on the dorsal surface of the penis (epispadias).