A case of localized follicular hamartoma: an ultrastructural and immunohistochemical study

The study reported a case of a 22-year-old woman with a localized follicular hamartoma, presenting as a nevoid plaque on her face and scalp. The plaque, appearing at puberty, consisted of numerous skin-colored to light brown papules, sometimes with a single hair. Histological analysis revealed branched epithelial nests of squamoid and basaloid cells connected to the interfollicular epidermis and upper hair follicles, while the lower portions of the follicles appeared normal. Immunohistochemical findings showed keratin expression similar to infundibular epithelium, with S-100-positive cells in the nests and stroma, and numerous Factor XIIIa-positive dendritic cells near the nests. Ultrastructural examination identified immature melanocytes and Merkel cells in the nests, and stromal dendritic cells with cytoplasmic processes adhering to anchoring fibrils or basal lamina. The findings suggested the hamartoma was an abortive growth of secondary hair germs with limited differentiation to the upper follicular portion.