majora
The outer surface of the labia majora and the perianal area are covered with hair‐bearing skin (Figure 2.16), but the inner aspects are hairless. The labia majora have a full complement of adnexal structures, but there is no hair in the pilosebaceous unit on the inner aspect. Small numbers of lymphocytes are often seen, and anogenital mammary‐like glands (AGMLG) are found most commonly in the interlabial sulci.
Figure 2.16 Histology of normal labium majus.
AGMLGs
It was originally thought that mammary tissue seen in the vulva was an embryological remnant of the mammary ridge. The AGMLGs were first described by van der Putte [31], and it is now clear that they are a distinct entity and are a normal component of anogenital skin [32]. They are found predominantly in the interlabial sulci of the vulva and have distinctive morphological, histological, and immunohistochemical features that are similar to the breast [33].
The morphology of the glands ranges from a simple wide duct with a few small diverticula to a complicated, convoluted tubular system that have features similar to those seen in the breast but without the lobules (Figure 2.17). Most glands, however, consist of a coiled duct with diverticular extensions at irregular intervals along their length. The glands all have a straight excretory duct that opens directly onto the skin surface. The anogenital glands extend to 3.9 mm into the dermis, which is deeper than eccrine or apocrine glands [34]. They are lined by simple columnar epithelium resting on a layer of myoepithelium. The columnar epithelium extends into the straight excretory duct until a short distance from the surface and then gives way to non‐keratinised, stratified squamous epithelium without a myoepithelial layer. The lining epithelium may show columnar cell change in these AGMLGs [34].
Figure 2.17 Lobulated anogenital mammary‐like gland with compact columnar epithelium
(courtesy of Dr James Scurry).
Labia minora
The inner and outer aspects of the labia minora are covered with non‐hair‐bearing skin, but sebaceous glands are still found, which open directly onto the skin. Eccrine glands are rarely seen. The dermis of the labia minora is composed mainly of elastic fibres and blood vessels with a rich innervation. The arrangement of blood vessels within the labia minora forms erectile tissue similar to that in the penile corpus spongiosus, their embryological counterpart in the male.
The epithelium on the inner aspects of the labia minora is still keratinised, but medially it merges with the vestibule, and this transition is seen clinically as Hart’s line.
Clitoris
The frenulum and prepuce of the clitoris are covered with keratinised stratified squamous epithelium which originates from the labia minora, and sebaceous and mucus‐secreting glands are present. The glans is covered by a thin keratinised epithelium but contains no adnexal or glandular structures.
Vestibule
The vulval vestibule is a mucosal surface and as such is covered by non‐keratinised stratified squamous epithelium rich in glycogen. It is devoid of pilosebaceous units and other adnexal structures.
Bartholin’s glands
Bartholin’s glands are lobulated and contain multiple acini grouped around the termination of each of the many branching ducts. The acini are lined with cuboidal epithelium and the ducts with stratified transitional epithelium. Argentaffin cells have been described in the epithelial lining of the main excretory duct of Bartholin’s gland and the paraurethral glands [35].
Minor vestibular glands
These are lined by a single layer of secretory cells.
Skene’s glands (paraurethral glands)
These glands have a mucinous columnar epithelium, and their ducts are lined by transitional‐type epithelium. Small cysts can occur in the duct.
Vestibular papillomatosis
They are composed of delicate fibrovascular cores lined by bland squamous epithelium that may have a thin layer of keratin. There is no koilocytosis.
Vagina
The vagina has an outer adventitial coat of fibroelastic tissue by which it is bound to the urethra and anchored to the pelvic walls by the pelvic ligaments. The intermediate coat of circular and longitudinal smooth muscle is intermingled with striated muscle from the pelvic floor. Between the muscular and inner epithelial layers is a layer of loose fibroelastic tissue in which there is an extensive network of venous channels. This venous network, with distension, changes the vaginal walls into erectile tissue and is the probable source of vaginal secretion during sexual intercourse. The inner aspect of the vagina is lined with non‐cornifying stratified squamous epithelium, the cells of which are heavily glycogenated. The vaginal surface of the cervix is covered with stratified squamous epithelium, while the cervical canal is lined by columnar epithelium in which there are numerous mucus‐secreting cells. The squamocolumnar junction may occur at the external os, but more often there is a transformation zone of variable extent situated around the external os.
Other epithelial cell types
In addition to keratinocytes there are three other cell types in the epidermis: melanocytes, Langerhans cells, and Merkel cells.
Melanocytes
Melanocytes are cells of neural crest origin that produce melanin. They appear as rounded cells with clear cytoplasm and are situated in the basal layer of the epidermis. They convert the amino acid tyrosine to melanin within membrane‐bound organelles called melanosomes. These are then transferred to keratinocytes through the melanocytes’ dendritic extensions. This transfer within the ‘epidermal melanin unit’ protects keratinocyte DNA from damage by ultraviolet radiation and certain toxins.
Melanocytes are also capable of other actions including secreting a number of signal molecules targeting keratinocytes, lymphocytes, fibroblasts, Langerhans cells, mast cells, and endothelial cells. Hormones profoundly influence human melanocyte activity, and there is a marked regional variation in the sensitivity of melanocytes to specific hormones. The vulva and perineum are sites that are very sensitive to hormonal factors.
Langerhans cells
Langerhans cells are found in the epidermis and are bone‐marrow‐derived dendritic cells, present in all layers of the epidermis. They lack pigment and represent 1–2% of the epidermal cell population and are located mainly in the suprabasal area. An analysis of the distribution of Langerhans cells in healthy tissue of the genital tract showed there