Embryologic, Histologic, and Anatomic Aspects
Apocrine units in humans are found in the axillae, areolae, periumbilical region, perineal and circumanal areas, prepuce, scrotum, mons pubis, labia minora, and external auditory canals (ceruminous glands), and on the eyelids (Moll’s glands). All of them are true apocrine glands; none is “modified,” as so often is asserted. Nor is the breast a “modified apocrine gland” or a “modified sweat gland.” The breast simply is a distinctive region of the skin and subcutaneous tissue, the latter housing a specific type of apocrine gland, i.e., a mammary gland specialized for manufacture of colostrum at the time of parturition. A mammary gland can be identified as an apocrine gland because it exhibits striking “decapitation” secretion. The duct of a mammary gland, named the lactiferous duct, looks just like any other apocrine duct. Uncommonly, a few apocrine units may be found ectopically on the face and scalp. Apocrine glands are small and nonfunctional until puberty, at which time they enlarge and begin to secrete their product.
Histologically, an apocrine unit consists of (1) a coiled secretory portion, i.e., the gland itself, situated in the lower part of the reticular dermis or in the subcutaneous fat, (2) a straight duct that empties into an infundibulum at a level above the entrance of a sebaceous duct, and (3) a duct that spirals through infundibular epidermis, i.e., the apocrine acrosyringium (Fig. 1.49). In cross section, the diameter of an apocrine gland is about 10 times greater than that of an eccrine gland (Fig. 1.50).
An apocrine gland consists of two parts: (1) a coiled secretory structure situated in the lower part of the dermis or in the subcutaneous fat and (2) a straight duct that enters an infundibulum above the site at which a sebaceous duct enters the base of it.
Cross section of an apocrine gland reveals its diameter to be several times greater than that of an eccrine gland. (x176)
The lumen of an apocrine gland is lined by a single row of columnar cells with abundant eosinophilic cytoplasm and a round nucleus situated near the base (Fig. 1.51). The convex apical border of the secretory cells projects to variable extent into the lumen, depending on the particular stage in the secretory cycle. The apical portion of glandular cells shows changes specific for apocrine secretion, namely, the appearance of being decapitated or pinched off (Fig. 1.52). “Decapitation secretion,” “pinching-off secretion,” “snouts,” and “apocrine secretion” are synonyms. Surrounding the secretory cells are (1) a layer of contractile myoepithelial cells (Fig. 1.53), (2) a basement membrane, and (3) collagen bundles and elastic fibers of the periadnexal dermis.
“Decapitation secretion” is characteristic of a cell of an apocrine gland.
Lumen of an apocrine gland is lined by a layer of cuboidal or columnar cells that possess a round nucleus situated near the base of abundant, pale, eosinophilic cytoplasm. A convex apical border of glandular cells projects into the lumen. The manner of secretion of apocrine glands is “pinching off” or “decapitation” of apical cytoplasm. (x763)
Secretory cells of an apocrine gland are surrounded by a single layer of myoepithelial cells.
The apocrine duct, like the eccrine duct, is composed of two layers of cuboidal cells and an inner periluminal cuticle, but is devoid of myoepithelial cells. Although apocrine glands are distinguished easily from eccrine glands histologically, apocrine and eccrine ducts are indistinguishable from one another when viewed by conventional microscopy. Distally, the epithelium of an apocrine duct merges with infundibular epidermis and cornifies independent of it. The intrainfundibular portion of an apocrine duct spirals in a manner similar to that of the eccrine duct through surface epidermis (Fig. 1.54).
An apocrine duct enters the infundibulum of an epidermis above the entry of a sebaceous duct, which is at the junction of infundibulum and isthmus. The duct then spirals through infundibular epidermis in a fashion similar to that of the eccrine duct through surface epidermis.
Apocrine glandular cells have attributes ultrastructurally that are typical of secretory epithelia, namely, prominent rough endoplasmic reticulum and Golgi apparatus, numerous ribosomes, mitochondria, and lysosomes (Fig. 1.55). Many secretory granules are situated near the luminal border of those cells. The luminal portion of apocrine glandular cells, together with their secretory granules, appears by electron microscopy to be “pinched-off” or “decapitated,” with the cleaved terminal portion seeming to lie free within the lumen. The exact function of apocrine secretion is not known. The secretory granules of apocrine glandular epithelium resemble zymogen ones and are reactive histochemically for iron, lipofuscin, and neutral mucopolysaccharides. Apocrine secretion and eccrine sweat are unrelated wholly to one another, as are the glands that make them. The apocrine gland in humans is not a sweat gland, although it is in horses.
An active columnar cell of an apocrine gland displays dense secretory granules and villi at the free border of the lumen. (x7500) (Courtesy of Ken Hashimoto, M.D.)
Apocrine units do not play an important role in inflammatory diseases of the skin. Even Fox-Fordyce disease, referred to often as “apocrine miliaria,” is not analogous to true eccrine miliaria (rubra); it seems to result from a plug of corneocytes in contiguous infundibula that reside in “apocrine regions,” i.e., axillae and pubis, and, therefore, really is an abnormality of epidermis rather than a malady of apocrine units per se. Hidradenitis suppurativa also is a misnomer; it is a suppurative infundibulitis that affects apocrine units only secondarily.