Morphological stages of pilomatricoma

In 1983, Ackerman proposed that pilomatricoma represents an infundibular-matrix cyst in its early stages. To study the evolution of this cystic neoplasm, we examined 118 lesions from 116 patients with pilomatricoma histopathologically and categorized the lesions into four distinct and chronological stages: early, fully developed, early regressive, and late regressive. Early lesions (eight cases) were small cystic structures lined by squamoid and basaloid epithelium containing keratin filaments and faulty hair matrix material composed of shadow cells. Fully developed lesions (27 cases) were large neoplasms lined by basaloid epithelium at their periphery, and within, composed of irregularly shaped, densely packed zones of cornified masses containing shadow cells. Early regressive lesions (37 cases) had no apparent epithelial lining but did have basaloid cell foci at the periphery; within, they were composed of pink hair matrix material with shadow cells surrounded by granulation tissue with inflammatory infiltrate and multinucleated histiocytic giant cells. Late regressive lesions (42 cases) had no epithelial component and were composed of irregularly shaped, partially confluent masses of faulty hair material, and calcified (and sometimes metaplastically ossified) shadow cells embedded in a desmoplastic stroma, with little or no inflammatory infiltrate. In four cases, there was a relatively large dermal nodule composed of several interconnected lobules that consisted largely of basaloid cells with only a few areas of shadow cells. We interpreted these lesions as proliferating pilomatricomas. Based upon our histopathologic findings, we propose that pilomatricomas may be categorized into four distinct morphological stages and that these stages reflect the "life" of a pilomatricoma. Thus, the lesion begins as an infundibular matrix cyst and ends up as a calcified and ossified nodule with no visible epithelial component.