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Cysts are one of the most common specimens received by pathologists. By definition, a cyst is any cavity that is lined by an epithelium. Cysts may be found in virtually any organ in the body and are often lined by epithelium that is native to that particular organ. Although cysts may be classified on the basis of several different criteria, the diagnosis of cutaneous cysts is predominantly based on the nature of the epithelial lining. Recognition of the nature of the epithelial cyst wall lining is also important for understanding the histopathogenesis of these lesions, which usually arise from normal counterparts in the skin, predominantly adnexal structures.

Classification of cutaneous cysts may be approached in a variety of ways. One may classify them based on their presumed appendage of origin—whether hair, eccrine gland, apocrine gland, or salivary gland—or on whether they are developmentally or nondevelopmentally derived. There is considerable overlap between what one might consider a cyst and what one might consider an appendageal tumor. For example, several authors classify apocrine and eccrine hidrocystomas as appendageal tumors1 because they arise from apocrine and eccrine glands, respectively. Some authors consider endometriosis and endosalpingiosis to be cutaneous cysts even though they represent heterotopic rests of normal tissue that have implanted at distant sites. Because of the extensive overlap between classification schemes of cutaneous cysts, a simple algorithm for the diagnosis of cutaneous cysts based on the nature of their epithelial lining is presented (Fig. 25-1). Tables 25-1, 25-2, 25-3, 25-4, 25-5 summarize the clinical and histologic features of cutaneous cysts.

TABLE 25-1Classification of Cutaneous Cysts
TABLE 25-2Clinical and Histologic Features of Cysts Lined by Stratified Squamous Epithelium

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