Aberrantly differentiated cells in benign pilomatrixoma reflect the normal hair follicle: immunohistochemical analysis of Ca-binding S100A2, S100A3 and S100A6 proteins.

BACKGROUND: Pilomatrixoma is a common benign cutaneous tumour containing differentiated hair matrix cells. This tumour is mainly composed of basophilic, transitional, shadow and squamoid cells. Although some S100 proteins are expressed in a tissue-specific manner in the hair follicle (e.g. S100A2 in the outer root sheath, S100A3 in the cortex and cuticle, and S100A6 in the inner root sheath), little information is available concerning their distribution in the aberrantly differentiated tissues of pilomatrixoma.

OBJECTIVES: To characterize the disordered epithelial elements of pilomatrixoma by localizing S100A2, S100A3 and S100A6 proteins.

METHODS: Immunohistochemistry and dual-immunofluorescence microscopy were performed on 22 pilomatrixoma specimens using antibodies specific to the three proteins.

RESULTS: Tissue-specific distribution of the S100 proteins investigated was preserved in the morphologically disordered tumour tissues. Anti-S100A2 antibody stained squamoid cells and putative outer root sheath cells; basophilic and potential hair matrix cells were occasionally stained. S100A3 staining was found in transitional cells and putative cortical cells, and was strong in both dispersed cells and hair-like structures surrounding cells which were presumably cuticular cells. Anti-S100A6 antibody labelled some S100A3-negative transitional cell strands, potentially inner root sheath cells.

CONCLUSIONS: The epithelial elements of pilomatrixoma can be characterized using S100 proteins as biochemical markers. Our results show that pilomatrixomas retain a certain degree of differentiation indicative of distinct hair-forming cells.