Aikaterini Deliveliotou and George Creatsas
Second Department of Obstetrics & Gynecology, University of Athens Medical School, Aretaieion Hospital, Athens, Greece
The vulva, or pudendum, is a collective term for the external female genital organs that are visible in the perineal area. Knowledge of the basic anatomy of the vulva is necessary in order to understand its physiology and appropriately recognize the wide spectrum of vulvar pathology. To achieve these goals the vulvar embryology is first presented before describing the anatomy of the vulva in women of reproductive age. Lifetime changes in the vulva from birth to adulthood are described in Chapter 3.
Early in the fifth week of embryonic life, the cloaca is divided by the urorectal septum, which gives rise to the perineum. Folds of tissue form on either side of the cloaca: the anterior folds are urogenital and the posterior folds are anal. The anterior folds meet at the midline to form the genital tubercle (1). The genital tubercle enlarges. In the male embryo, under the influence of androgens, the genital tubercle becomes the penis; in the female embryo, growth slows and it becomes the clitoris. On either side of the tubercle, the urogenital folds form the labia minora. In the indifferent stage, the labioscrotal swellings develop on either side of the urogenital folds. In the male embryo, under the influence of androgens, they differentiate into the scrotum; in the female, lacking androgenic stimulation, they remain largely unfused to become the labia majora. The definitive urogenital sinus gives rise to the vaginal vestibule, into which the urethra, vagina, and greater vestibular glands open.
The vulva consists of the mons pubis, the labia majora, the labia minora, hymen, the clitoris, the vestibule of the vagina, the urethral orifice, Skene's glands, Bartholin's glands, and the vestibular bulbs (Fig. 1).
Anular hymen Septate hymen Cribriform hymen
Figure 1 Anatomy of the adult vulva. Source: Courtesy of Elsevier. (See color insert p. 1.)
Anular hymen Septate hymen Cribriform hymen
Figure 1 Anatomy of the adult vulva. Source: Courtesy of Elsevier. (See color insert p. 1.)
The anterior and posterior boundaries of the vulva extend from the mons pubis to the anus, respectively; its lateral boundaries lie at the genitocrural folds. The vulvar epithelium exhibits regional differences in tissue structure based on embryonic derivation. The skin-bearing mons pubis, perineum, and labia are derived from the embryonic ectoderm. Vulvar skin, like skin at other sites, has a keratinized, stratified, squamous epithelial structure with hair follicles, sebaceous glands, and sweat glands. The thickness degree of keratinization of vulvar skin decreases progressively from the labia majora, over the clitoris, to the labia minora. The vulvar vestibule, derived from the embryonic endoderm, is nonkeratinized. Chapter 2 describes in detail the regional tissue structure of the vulva.
The mons pubis (mons Veneris) is the rounded eminence in front of the pubic symphysis, which is formed by a collection of adipose tissue beneath the integument. During puberty, it becomes covered with hair up to its junction with the abdominal wall. The hair pattern, or escutcheon, of most women is triangular. Genetic and racial differences produce a variety of normal hair patterns, with approximately one in four women having a modified escutcheon with a diamond pattern.
The labia majora are a pair of prominent longitudinal, cutaneous folds of fibro-adipose tissue that are homologous to the scrotum in the male. The structures bear epidermal tissue resembling the dartos tunic of the scrotum, as well as adipose tissue, areolar tissue, blood vessels, nerves, and glands. The labia majora also include the terminal extension of the round ligament and, occasionally, a peritoneal diverticulum, the canal of Nuck.
The size of the labia majora is related to fat content. Each is approximately 7 to 8 cm in length and 2 to 3 cm in width. The labia majora extend downward and backward from the mons pubis, thus forming the lateral boundaries of a fissure or cleft (the pudendal cleft or rima) into which the vagina and urethra open.
Each labium majus has two surfaces: the outer surface is pigmented, rugose, and bears pubic hair, sebaceous glands, apocrine glands, and eccrine glands. The inner surface is smooth; it bears sebaceous, apocrine, and eccrine glands but no hair follicles. Vulvar apocrine glands are similar to those of the breast and axillary areas.
The labia majora are thicker in front. Anterior to the clitoris, they join to form the anterior boundary of the pudendal cleft, known as anterior labial commissure. The labia majora do not surround the pudendal cleft fully; laterally, they remain approximately parallel to it and posteriorly, they gradually merge with the neighboring integument below the juncture of the labia minora (fourchette). The posterior ends of the labia majora and the connecting skin between them form the posterior boundary of the pudendum, known as posterior labial commissure. The interval between the posterior commissure and the anus is 2.5 to 3 cm in length and constitutes the perineum.
The labia minora (nymphae) are two small cutaneous folds that are situated between the labia majora and the vaginal orifice. The labia minora are homologous to the penile urethra and part of the skin of the penis in males. Laterally, they extend obliquely from the clitoris toward the rear for about 4 cm on either side of the vaginal orifice. They are shorter and thinner than the labia majora. At the clitoris, the anterior portion of each labium minus divides into two segments. Each upper segment passes anteriorly to the clitoris to meet its fellow of the opposite side, forming a fold, the preputium clitoridis, which overhangs the glans of the clitoris. Each lower segment passes beneath the clitoris, joining with its fellow to form the frenulum, which is attached to the inferior surface of the clitoris. The posterior portions of the labia minora surround the vestibule of the vagina. Their posterior juncture is the fourchette.
Histologically, the labia minora are composed of dense connective tissue, erectile tissue, and elastic fibers. Unlike the labia majora, they do not contain adipose tissue. The skin of the opposed surfaces of labia minora has numerous sebaceous glands but no hair follicles or sweat glands. Among women of reproductive age, there is significant variation in the size of the labia minora. They are relatively more prominent in children and postmenopausal women.
The clitoris is a short, cylindrical, erectile structure, 2 to 3 cm in length, at the superior portion of the vestibule. It is the female homologue of the penis. It is situated beneath the anterior labial commissure, partially hidden between the anterior segments of the labia minora. The clitoris consists of a base of two crura that attach to the periosteum of the symphysis pubis. Like the penis, the clitoris has a suspensory ligament and two small muscles, the ischiocavernosi, which are inserted into the crura of the clitoris. The body of the clitoris consists of two cylindrical corpora cavernosa composed of thin-walled, vascular channels that function as erectile tissue. The distal one-third of the clitoris is a small rounded tubercle (glans clitoridis) that consists of spongy erectile tissue with many nerve endings. Usually, only the glans is visible, with the body of the clitoris positioned beneath the skin surface. The normal glans clitoridis in adult women has a width less than 1 cm, with an average length of 1.5 to 2 cm. Age, weight, and oral contraceptive use do not change its anatomic dimensions. Childbearing may influence the size of the clitoris.
The hymen is a thin fold of mucous membrane situated at the entrance to the vagina. Between the hymen and the frenulum of the labia minora is a shallow depression, the navicular fossa. The inner edges of the hymen may be in contact with each other, such that the vaginal orifice appears as a cleft between them. The hymen is usually perforated, with many variations in its structure and shape. The most common forms are that of a ring, which is broadest posteriorly, or that of a semilunar fold, with a hollow margin turned toward the pubes. The hymen is rarely cribriform or has inner edges that form a membranous fringe. It can be completely absent or can appear as a complete septum across the lower end of the vagina, a condition known as an imperforate hymen. Small tags or nodules of firm fibrous material, termed carunculae myrtiformes, are the remnants of the hymen in sexually active women. However, the hymen can persist after the first sexual intercourse, so its presence cannot be considered a sign of virginity. Histologically, the hymen is covered by stratified squamous epithelium on both sides and consists of fibrous tissue with a few small blood vessels.
The vestibule is derived from the endoderm, the lowest portion of the embryonic urogenital sinus. It is the cleft posterior to the glans clitoridis and between the labia minora. It can be visualized by holding the labia minora apart. The vestibule extends from the clitoris to the posterior fourchette. Hart's line marks the juncture of the nonkeratinized epithelium of the vulvar vestibule and the keratinized epithelium of the inner surface of the labia minora. The urethral and vaginal orifices as well as the ducts of the greater vestibular glands open into the vestibule. The remnants of the hymen and numerous small mucinous glands are located within the area of the vestibule.
The female urethra, a membranous conduit for urine, runs from the urinary bladder to the vestibule and measures 3.5 to 5 cm in length. The mucosa of the distal one-third of the urethra is lined with stratified squamous epithelium, whereas the proximal two-thirds are lined with stratified transitional epithelium. The external urethral orifice is 4 to 6 mm in diameter and is immediately anterior to the vaginal orifice, approximately 2 to 3 cm beneath the glans clitoridis. Its mucosal edges grossly appear slightly everted, forming a short, sagittal cleft.
The vaginal orifice is a median slit below and posterior to the opening of the urethra; the hymen surrounds it, so that its size varies inversely with that of the hymen. It opens into the vagina, a neuromuscular vault connecting to the cervix of the uterus that unsheathes the penis during sexual intercourse and allows passage of the newborn infant during birth.
Skene's or paraurethral glands are homologous to the prostate in the male. They are branched, tubular glands, adjacent to the distal urethra. Usually, Skene's ducts run parallel to the long axis of the urethra for approximately 1 cm before opening into the distal urethra. Sometimes they open into the area just outside the urethral orifice. The duct of the Skene's gland presents an opening on its posterior surface. Skene's glands are the largest of the paraurethral glands; however, many smaller glands empty into the urethra.
The greater vestibular glands, or Bartholin's glands, are the homologues of the bulbourethral glands (Cowper's glands) in the male. They consist of two small, roundish, reddish-yellow bodies. Bartholin's glands are situated on the postero-lateral aspect of the vaginal orifice, in contact with the posterior end of each lateral mass of the bulb of the vestibule. Histologically, the gland is composed of cuboidal epithelium. The duct from each gland is approximately 2 cm in length and is lined by transitional epithelium. Bartholin's ducts open immediately lateral to the hymen into the groove between the hymen and the labia minora. Their mucus secretion helps maintain adequate lubrication. Infection of these glands can result in an abscess.
The vestibular bulbs are the homologues of the bulb and adjoining part of the corpus cavernosum urethrae of the male. They consist of two elongated masses of erectile tissue situated on either side of the vaginal orifice and are united to each other in front by a narrow median band termed the pars intermedia. Each lateral mass measures approximately 2.5 cm in length. The distal ends of the vestibular bulbs are adjacent to Bartholin's glands, whereas the proximal ends are tapered and joined to one another by the pars intermedia. Their deep surfaces are in contact with the inferior fascia of the urogenital diaphragm. Each bulb is immediately below the bulbocavernosus muscle.
Muscles of the Vulva
Three types of muscle exist in the vulva:
1. The ischiocavernosus muscle compresses the crura and lowers the clitoris. It originates from the ischial tuberosity and inserts at the ischio-pubic bone.
2. The bulbocavernosus muscle compresses the vestibular bulb and dorsal vein of the clitoris. It originates from the perineal body and inserts into the posterior aspect of the clitoris; some fibers pass above the dorsal vein of the clitoris in a sling-like fashion.
3. The superficial transverse perineal muscle holds the perineal body fixed. It originates from the ischial tuberosity and inserts at the central perineal tendon.
The vulva derives its blood supply from the femoral artery via the external and internal pudendal arteries. The venous drainage occurs via the internal pudendal veins.
Lymphatic Drainage of the Vulva
The vulva drains primarily to the superficial and deep inguinal nodes and along the dorsal vein of the clitoris, directly to the iliac nodes.
The innervation of the vulva derives from branches of several nerves, including the ilioinguinal nerve, the genital branch of genitofemoral nerve, the perineal branch of the lateral femoral cutaneous nerve of the thigh, and the perineal branch of the pudendal nerve.
This chapter provided a review of the embryology and anatomy of the vulva in women of reproductive age. This knowledge is necessary in order to understand the vulva's physiology and recognize the wide spectrum of vulvar pathology.
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