The morphologic features of trichofolliculoma are variable, reminiscent of the anagen, catagen, and telogen phases of a normal hair follicle in its cycle. We recently described an early, fully developed stage and late stages of trichofolliculoma. Using immunohistochemical examination, we sought to demonstrate hyperplasia of Merkel cells in all three stages of trichofolliculoma. We found this to be the most striking in small lesions of the late stage. The distribution of the Merkel cells in several stages of trichofolliculoma coincided with the known arrangement of normal follicular Merkel cells during the follicular cycle. However, antibodies against neurofilaments failed to detect innervated Merkel cells, in contrast to normal follicular Merkel cells. Antibodies against Ki67 did not reveal proliferative Merkel cells in any of the trichofolliculomas, but for unknown reasons, a distinct cytoplasmic staining of Merkel cell processes sometimes occurred. Nuclear Ki67 was strongly expressed in the nuclei of follicular keratinocytes of the fully developed trichofollicullomas, whereas those at a late stage showed a markedly decreased staining pattern. Our finding of Merkel cells in all trichofolliculomas underlines their classification as hamartomas with follicular differentiation. Hyperplasia of Merkel cells, even in trichofolliculomas at a late stage, as regressing lesions might implicate hitherto unknown regulatory functions of this neuroendocrine cell.
Adult Child, Preschool