Before the 7th week of development, the appearance of the external genital area is the same in males and females. Elongation of the genital tubercle into a phallus with a clearly defined terminal glans portion is noted in the 7th week, and gross inspection at this time may lead to faulty sexual identification. Ventrally and caudally, the urogenital membrane, made up of both endodermal and ectodermal cells, further differentiates into the genital folds laterally and the urogenital folds medially. The lateral genital folds develop into the labia majora, whereas the urogenital folds develop subsequently into the labia minora and prepuce of the clitoris.

The external genitalia of the fetus are readily distinguishable as female at about 12 weeks (Figure 3-1). In the male, the urethral ostium is located conspicuously on the elongated phallus by this time and is smaller, owing to urogenital fold fusion dorsally, which produces a prominent raphe from the anus to the urethral ostium. In the female, the hymen is usually perforated by the time delivery occurs.

FIGURE 3-1 Development of the external female genitalia. A: Indifferent stage (about 7 weeks). B: About 10 weeks. C: About 12 weeks.

The perineum represents the inferior boundary of the pelvis. It is bounded superiorly by the levator ani muscles and inferiorly by the skin between the thighs (Figure 3-2). Anteriorly, the perineum extends to the symphysis pubis and the inferior borders of the pubic bones. Posteriorly, it is limited by the ischial tuberosities, the sacrotuberous ligaments, and the coccyx. The superficial and deep transverse perineal muscles cross the pelvic outlet between the two ischial tuberosities and come together at the perineal body. They divide the space into the urogenital triangle anteriorly and the anal triangle posteriorly.

FIGURE 3-2 The perineum, showing superficial structures on the left and deeper structures on the right.

The urogenital diaphragm is a fibromuscular sheet that stretches across the pubic arch. It is pierced by the vagina, the urethra, the artery of the bulb, the internal pudendal vessels, and the dorsal nerve of the clitoris. Its inferior surface is covered by the crura of the clitoris, the vestibular bulbs, the greater vestibular (Bartholin’s) glands, and the superficial perineal muscles. Bartholin’s glands are situated just posterior to the vestibular bulbs, and their ducts empty into the introitus just below the labia minora. They are often the site of gonococcal infections and painful abscesses.

VULVA

The external genitalia are referred to collectively as the vulva. As shown in Figure 3-3, the vulva includes the mons veneris, labia majora, labia minora, clitoris, vulvovaginal (Bartholin’s) glands, fourchette, and perineum. The most prominent features of the vulva, the labia majora, are large, hair-covered folds of skin that contain sebaceous glands and subcutaneous fat and lie on either side of the introitus. The labia minora lie medially and contain no hair but have a rich supply of venous sinuses, sebaceous glands, and nerves. The labia minora may vary from scarcely noticeable structures to leaf-like flaps measuring up to 3 cm in length. Anteriorly, each splits into two folds. The posterior two folds attach to the inferior surface of the clitoris, at which point they unite to form the frenulum of the clitoris. The anterior folds are united in a hood-like configuration over the clitoris, forming the prepuce. Posteriorly, the labia minora may extend almost to the fourchette.

FIGURE 3-3 Female external genitalia.

The clitoris lies just in front of the urethra and consists of the glans, the body, and the crura. Only the glans clitoris is visible externally. The body, composed of a pair of corpora cavernosa, extends superiorly for a distance of several centimeters and divides into two crura, which are attached to the undersurface of either pubic ramus. Each crus is covered by the corresponding ischiocavernosus muscle. Each vestibular bulb (equivalent to the corpus spongiosum of the penis) extends posteriorly from the glans on either side of the lower vagina. Each bulb is attached to the inferior surface of the perineal membrane and covered by the bulbocavernosus muscle. These muscles aid in constricting the venous supply to the erectile vestibular bulbs and also act as the sphincter vaginae.

As the labia minora are spread, the vaginal introitus, guarded by the hymenal ring, is seen. Usually, the hymen is represented only by a circle of carunculae myrtiformes around the vaginal introitus. The hymen may take many forms, however, such as a cribriform plate with many small openings or a completely imperforate diaphragm.

The vestibule of the vagina is that portion of the introitus extending inferiorly from the hymenal ring between the labia minora. The fourchette represents the posterior portion of the vestibule just above the perineal body. Most of the vulva is innervated by the branches of the pudendal nerve. Anterior to the urethra, the vulva is innervated by the ilioinguinal and genitofemoral nerves. This area is not anesthetized adequately by a pudendal block, and repair of paraurethral tears should be supplemented by additional subcutaneous anesthesia.

The upper vagina, cervix, uterus, and fallopian tubes are formed from the paramesonephric (müllerian) ducts. Although human embryos, whether male or female, possess both paired paramesonephric and mesonephric (wolffian) ducts, the absence of Y chromosomal influence leads to the development of the paramesonephric system with virtual total regression of the mesonephric system. With a Y chromosome present, a testis is formed and müllerian-inhibiting substance is produced, creating the reverse situation.

Mesonephric duct development occurs in each urogenital ridge between weeks 2 and 4 and is thought to influence the growth and development of the paramesonephric ducts. The mesonephric ducts terminate caudally by opening into the urogenital sinus. First evidence of each paramesonephric duct is seen at 6 weeks’ gestation as a groove in the coelomic epithelium of the paired urogenital ridges, lateral to the cranial pole of the mesonephric duct. Each paramesonephric duct opens into the coelomic cavity cranially at a point destined to become a tubal ostium. Coursing caudally at first, parallel to the developing mesonephric duct, the blind distal end of each paramesonephric duct eventually crosses dorsal to the mesonephric duct, and the two ducts approximate in the midline. The two paramesonephric ducts fuse terminally at the urogenital septum, forming the uterovaginal primordium. The distal point of fusion is known as the müllerian tubercle (Müller’s tubercle) and can be seen protruding into the urogenital sinus dorsally in embryos at 9 to 10 weeks’ gestation (Figure 3-4). Later dissolution of the septum between the fused paramesonephric ducts leads to the development of a single uterine fundus, cervix, and, according to some investigators, the upper vagina.

FIGURE 3-4 Early embryologic development of the genital tract (A to C) and vaginal plate (D). MD, mesonephric duct; MT, müllerian tubercle; PD, paramesonephric duct; UVP, uterovaginal primordium; US, urogenital sinus; VP, vaginal plate.

(Redrawn from Didusch JF, Koff AK: Contrib Embryol Carnegie Inst 24:61, 1933.)

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