A Revaluation of Folliculosebaceous Cystic Hamartoma: The Histopathological and Immunohistochemical Features

The histogenesis of folliculosebaceous cystic hamartoma (FSCH), including the origin of the frequently associated adipocytes and other mesenchymal components, remains unclear. There are controversial problems regarding FSCH, such as the relationship between FSCH and trichofolliculoma (TF), and the exact concept of sebaceous TF. Fourteen FSCHs and 1 sebaceous TF were revaluated for the histopathology and studied for immunohistochemical profile of various cytokeratins, hair follicular stem cell markers, and others. The nestin expression was partly upregulated in the sebaceous duct structures and the proliferating spindle cells in the surrounding connective tissue in some lesions. S-100 protein staining clarified the presence of lipogenesis in these nestin-expressed lesions, and the nestin-positive spindle cells were also seen around the immature adipocytes. Some FSCHs showed the focal follicular differentiation, where no signs of catagen or telogen stage were seen. One peculiar case showed both FSCH and TF features equally. The possibility that the adipocytes and other mesenchymal components in FSCHs originated from the nestin-positive multipotent stem cells was suggested. It is not convincing that FSCH and TF represent a chronological change in the spectrum of the same condition. Each FSCH and TF is therefore considered to be a distinctive entity. They may develop under a similar pathogenesis differing from each other in the direction of fundamental differentiation. In contrast to the TF lesions, the unusual upregulation of nestin expression is occasionally seen in FSCH lesions, including their stromas, which may thus result in the production of various kinds of mesenchymal components in FSCHs.