The pathology of adnexal tumors of the skin—including hair follicle and sebaceous, and sweat gland neoplasms—is a complex and voluminous subject. In keeping with the aims of Chap. 29 on sweat gland lesions, the following discussion of pilosebaceous tumors is meant to be practical and diagnostically oriented in its focus. Readers wishing to supplement the information presented here are referred to several comprehensive texts covering appendageal cutaneous neoplasms.1-3
For many years, the classification of adnexal tumors has been based on morphologic and histochemical findings. As a result, various authors have questioned the validity of historical classifications of adnexal neoplasms, particularly those specified as either apocrine or eccrine in light of a reexamination of criteria for classification.4 Particular aspects of the arguments against the traditional classification of these lesions include the developmental biology of adnexal neoplasms; neoplasms consistently showing 2 or more lines of differentiation; striking associations of particular entities; the anatomic distributions of various adnexal neoplasms; and finally the lack of specificity of many histologic, histochemical, immunohistopathologic, and ultrastructural findings.
It is currently accepted that the embryologic origin of the folliculo-sebaceous-apocrine units and eccrine units are different. Thus, whereas the sebaceous and apocrine ducts and glandular elements develop near the bulge of the hair follicular unit, the eccrine apparatus undergoes a separate embryologic development. These developmental differences provide the basis for the categorization of adnexal neoplasms into 2 general groups: (1) folliculo-sebaceous-apocrine and (2) eccrine. The occurrence of adnexal tumors with 2 or more lines of differentiation in the same tumor and the association of such tumors in a syndrome (Brooke-Spiegler syndrome) are additional factors suggesting a biologic relationship between hair follicular and apocrine-related neoplasms. The high frequency (or alternatively the absence) of particular adnexal neoplasms at the anatomic sites bearing a high concentration or exclusive concentration of particular adnexal structures provides clear circumstantial evidence favoring or arguing against particular lines of adnexal differentiation. For example, whereas sweat glandular neoplasms occurring in regions with numerous apocrine glands are most likely apocrine, those occurring on the palms and soles are most probably of eccrine origin.
Finally, one must consider to what extent one is helped by conventional microscopy, histochemistry, immunohistochemistry, and electron microscopy as directly applied to the classification and diagnosis of adnexal tumors. Hair follicular differentiation can be recognized by the presence of (1) basaloid epithelium closely resembling follicular germinative epithelium with closely associated dermal mesenchymal cells resembling hair papilla or (2) matrical corneocytes—that is, “shadow” or “ghost” cells. Sebaceous differentiation is typified by cells with coarsely vacuolated cytoplasm and a relatively central scalloped nucleus. On the other hand, histologic attributes clearly specific for either apocrine or eccrine differentiation may not exist. For example, decapitation secretion (“snouting”) has been considered by many to be specific for apocrine glandular differentiation. However, this finding may be observed in eccrine-derived neoplasms. In a similar fashion, an eosinophilic cuticle ...