Genital Anatomy

Libby Edwards

A knowledge of anatomy and the various normal appearances of the genitalia helps with the recognition of both normal structures and pathologic findings.

Anogenital skin of both genders is composed of different structures and types of skin, each of which is predisposed to different diseases. An understanding of the location and appearance of different types of epithelium is useful in the diagnosis of genital skin diseases. Both male and female genitalia contain dry, keratinized skin with hair follicles (mons, scrotum, lateral labia majora) and modified mucous membranes that are partially keratinized with vestigial follicles and appendages (eg, labia minora, clitoral hood, glans penis), and the female vestibule is composed of a true mucous membrane, which is nonkeratinized and has no hair follicles or appendages.

A familiarity of the location of these different skin types is useful in the diagnosis of anogenital skin disease. Some dermatoses, such as psoriasis, prefer keratinized skin, some dermatoses, such as lichen planus, preferentially affect mucous membranes and modified mucous membranes while sparing keratinized skin, and some dermatoses, such as lichen sclerosus, most often affect modified mucous membranes and spare mucous membranes.

The normal appearance of anogenital skin varies as much from one individual to another as does the face. Most clinicians, despite substantial experience in the evaluation of patients, tend to not “see” an asymptomatic area. Puffiness, erythema, papillomatosis, and asymmetry generally are not noted if a patient has no complaints. However, if a patient reports pain or itching, these normal variants take on the stature of abnormalities. Similarly, patients do not notice these changes if they are comfortable, but they are certain that redness is new as soon as symptoms occur.

This is true of all body surfaces, but especially true of anogenital skin, which is often more difficult than the face or hands for the patient to see on a regular basis. Practitioners, as taught in training, are predisposed to listen to their patients, so that an individual who reports pain and redness is likely to exhibit an examination described by the provider as red. Therefore, it is important for an examiner to be familiar with the normal variation of the genitalia and to make judgments based on the objective findings without being unduly influenced by the patients’ perceptions.

A confident patient cannot be dissuaded from their descriptions of abnormalities, and reassurances are generally useless. Sometimes, interpretation of the abnormality can be useful, such as telling a patient that redness can occur with pain syndromes, such as vulvodynia or scrotodynia, but that the redness is not a sign of inflammation or infection in that case.

Female Genitalia

Vulva

The periphery of the vulva encompasses the mons anteriorly, the labia majora laterally, and the perineum posteriorly, extending centrally to the hymen, which marks the entrance to the vagina (Fig. 1-1). These structures compose the external genitalia.

The labia majora (singular = labium majus) are two fatty folds of skin that are derived from ectodermal tissues. The lateral aspect of the labia majora is covered with dry, keratinized, conspicuously hair-bearing skin. Each hair follicle is part of a pilosebaceous unit comprising
the follicle itself, its hair shaft, sebaceous gland, and the erector pili apparatus—a smooth muscle that contracts to form gooseflesh.

Fig. 1-1. Normal vulva.

The labia minora (singular = labium minus) are much thinner folds of connective tissue and squamous epithelium located medial to the labia majora. This area as well as the medial labia majora, the clitoral hood, and the clitoris are covered with the partially keratinized modified mucous membrane epithelium. The vestibule, or introitus, is mucous membrane that extends from the medial origin of the labia minora (Hart line) to the hymeneal ring. The mons and labia majora cover and protect more delicate structures such as the clitoris, the clitoral hood, the labia minora, and the vestibule.

Although the modified mucous membranes of the medial labia majora, the labia minora, and periclitoral skin are generally considered to exhibit hairless mucous membrane epithelium, these areas actually are covered with partially keratinized skin that contains several structures, including subtle hair follicles, apocrine sweat glands, and ectopic sebaceous glands (Fordyce spots). These Fordyce spots are often prominent, especially on the medial aspect of these skin folds, raising a concern for genital warts in some patients. These sebaceous glands appear as small, yellow to white, lobular papules (Fig. 1-2).

Hart line is a variably distinct line of demarcation at the base of the medial aspect of each labium minus, separating modified mucous membrane from the mucous membrane skin of the vestibule. Nonkeratinized, nonhair-bearing squamous mucosal epithelium with mucoussecreting glands extends from Hart line to and including the vagina and the external surface of the cervix. Mucosa is defined as a membrane that lines body structures that communicate to the air, and generally produce lubricating mucus, and is nonhair bearing. Both the vagina and the vestibule are covered with mucosae, whose surfaces are variably wet (depending on estrogen status) as a result of mucus produced by associated glands and the cervix.

Fig. 1-2. Fordyce spots here are cream-colored papules on the medial anterior labia majora.

Fig. 1-3. Pits that represent the ostia of vestibular glands occur regularly abutting the lateral aspect of the hymenal caruncles (arrow).

The vestibule, or introitus, extends from Hart line to the hymen. Variable numbers of mucous-secreting vestibular glands open onto the mucous membrane of this area. These glands are shallow pits lined with secretory cells; these primarily open circumferentially around the external aspect of the hymeneal ring and between the hymen and urethra (Fig. 1-3) but are occasionally visible on other areas of the vestibule (Fig. 1-4). They supplement lubrication in young, wellestrogenized women. Bartholin glands are paired glands that lie under the posterior portion of the vestibule, with duct openings just outside the hymen at the 5 and 7 o’clock positions. Skene glands exit into the distal urethra.

The well-estrogenized vagina is pink with prominent rugae, although this is variable. Vaginal secretions are normally present, and these are generally white, odorless, and fairly thin, with variable volume.

Fig. 1-4. Vestibular gland ostia also occur on other areas of the vestibule but are often very subtle. This vulva shows distinct ostia in the anterior vestibule.

The vulva undergoes marked change from birth until puberty (see Chapter 15). The skin of the mons and the lateral labia majora is characterized by fine vellus hair at birth, but with puberty, coarse terminal hair appears. In addition, the labia minora are almost absent until the onset of puberty, when estrogen appears and the labia minora elongate. The modified mucous membranes are smooth in prepubertal girls, before developing the normal sebaceous glands and redundant tissue of the estrogenized vulva. Apocrine glands become better developed with sexual maturity, and small, monomorphous papillae are common.

Likewise, after menopause and loss of estrogen, the labia minora shrink, hair diminishes, and the fat of the labia majora decreases (see Chapter 15). The normal pink color pales, sebaceous glands miniaturize, papillomatosis disappears, and the modified mucous membranes become smooth and flat. The vagina becomes pale and smooth, with minimal vaginal secretions.

Normal Variants

A normal vulva exhibits erythema of the modified mucous membranes and vestibule, as is true of the oral mucosa, lips, and conjunctivae. Many women also show erythema of the hair-bearing labia majora normally. The degree of erythema varies widely from patient to patient, so that its significance sometimes can be difficult to ascertain. Generally, very light complected individuals and red-headed women exhibit more marked erythema than darker individuals, and the vestibule of children is sometimes remarkable normally. In one reported series of premenopausal women, 43% showed erythema (Figs. 1-5 and 1-6).¹ This finding is often misinterpreted by patients and their physicians as indicative of inflammation. In addition, more than half of the women with vestibular erythema report no dyspareunia, but experience pain when the area is touched with a cotton-tipped applicator (the Q-tip test). This indicates that redness and a positive Q-tip test are normal findings and do not, alone and in the absence of real-life symptoms, constitute the vestibulodynia pattern of vulvodynia, formerly called the vulvar vestibulitis syndrome.

Fig. 1-5. The modified mucous membranes of the vulva normally exhibit mild erythema, as do mucous membranes of the mouth and lips. Some women, particularly those who are fair, display striking but still normal redness, so that true inflammation can be difficult to judge.

Fig. 1-6. Redness of the ostia of vestibular glands (arrow) is a common finding in vestibulodynia (formerly named vulvar vestibulitis). However, most often, this erythema is a normal finding, as is true in this asymptomatic woman.

Vulvar papillae (see Chapter 7) are also common normal variants, occurring in about one-third of premenopausal women (Figs. 1-7, 1-8, 1-9, 1-10, 1-11).¹^,² When these occur in the
vestibule, they are called vestibular papillae, but these also occur on the medial labia minora and occasionally even on the edges of the labia minora. These variants are sometimes mistaken for signs of disease, usually condylomata acuminata. Initial descriptions of vulvar papillomatosis reported biopsies consistent with human papillomavirus (HPV) infection as the cause. However, biopsies of even normal vulvar skin often show epithelial cells that contain perinuclear vacuoles that can mimic koilocytes of HPV infection. More recent studies have evaluated for the actual presence of the virus by polymerase chain reaction. The current consensus is that vestibular papillomatosis is a variant of normal, distinct from HPV infection.²^,³ And, normal vulvar papillomatosis can be distinguished without a biopsy by the differing morphology; these small, soft, monomorphous, tubular projections of vulvar papillomatosis are distributed in a symmetrical pattern, most often long the medial aspect of the vestibule. They are also common on the bilateral medial labia minora, where they are more likely to be dome shaped and less tubular. Vulvar papillae differ from condylomata acuminata by their rounded rather than acuminate tips and symmetric pattern. Vulvar papillae are discrete to the base, whereas genital warts generally are fused at the base to adjacent lesions. Moreover, genital warts are often keratinized and appear white in this moist area.

Fig. 1-7. Vulvar papillae are classically found in the vestibule and consist of symmetrical plaques of tubular papules with rounded tips, discrete to the base, unlike filiform papules of genital warts that are often fused at the base with pointed (“acuminate”) tips.

Fig. 1-8. Vulvar papillae can be short and closely set, giving a cob-blestoned appearance to the skin.

Fig. 1-9. Vulvar papillae sometimes form symmetrical lines, particularly at the edge of Hart line.

Fig. 1-10. Vulvar papillae can be more discrete and separated, and in some patients who are darkly complexioned, pigmented.

Occasionally, similar dome-shaped, smooth papules coalesce into a cobblestone texture on the inner labia minora.
Rarely, these lesions form papules on the edge of the labia minora. These changes are also often mistaken for warts. Although vulvar papillae were once believed to produce itching or pain, they are now known to be asymptomatic.

Fig. 1-11. Although vulvar papillae are found most often in the vestibule and medial labia minora, they can occur on edges of the labia minora, as seen here.

Fig. 1-12. The labia minora in this woman are very small and also display prominent Fordyce spots.

The whitening of vulvar skin following the application of 5% acetic acid has been believed to be pathognomonic of HPV infection and intraepithelial neoplasia. Although very sensitive, this is a nonspecific finding occurring with any condition that produces hyperkeratosis or thickening of the skin.⁴ Some investigators have found that 5% acetic acid predictably produces acetowhitening of all vulvar skin with prolonged contact.¹

Labia minora exhibit wide morphologic variability. These folds of skin can be large and pendulous, so small as to be nearly absent, or very asymmetric (Figs. 1-12, 1-13, 1-14). The anterior origin of the labia minora most often is the frenulum of the clitoris, but frequently, the anterior origin is from several structures, with a contribution from the skin lateral to the clitoral hood (Fig. 1-15) and the posterior aspect of the labia minora can be fused (Fig. 1-16).