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

We report the case of a 22-year-old woman with a nevoid plaque that we term ed localized follicular hamartoma. The plaque was noticed at puberty on a u nilateral site of the face and scalp. Clinically, it revealed numerous, ski n-colored to light brown papules alone and in groups, occasionally bearing a single hair. Histologically branched epithelial nests of squamoid and/or basaloid cells were revealed in connection with the interfollicular epiderm is and the upper portions of hair follicles, of which the lower portions sh owed normal structures. Immunohistochemically, the epithelial nests showed the keratin expression consisted with that of the infundibular epithelium. S-100-positive cells ere found in the epithelial nests and the stroma. Fact or XIIIa-positive dendritic cells were numerous in adjacent to the epitheli al nests. Ultrastructurally, immature melanocytes with a small number of pr emelanosomes and Merkel cells were found in the nests. Stromal dendritic ce lls showed the adherent features of the cytoplasmic processes to anchoring fibrils or basal lamina of the epithelial nests. From these findings, our c ase is a hamartoma, which seems to be an abortive growth of secondary hair germs with a limited differentiation to the upper follicular portion.