ABSTRACT
BACKGROUND: Pilomatricoma (PM) is benign follicular tumor composed of the basophilic cells, transitional cells, shadow cells, squamoid cells and keratin filaments/amorphous debris. At present, PM is assumed to differentiate toward hair-forming cells of hair follicles but definite direction is not clear. OBJECTIVES: This study was made in order to investigate the pathways of cell differentiation associated with sudden keratinization in PM. METHODS: In the present study, 19 cases of human PM was histopathologically examined and classified into 4 groups according to the chronological stages. RESULTS: In the chronological stages according to Kaddu's classification, there were 2 cases of early lesion, 6 cases of fully developed lesion, 7 cases of early regressive lesion and 4 cases of late regressive lesion. The basophilic cells changed into the shadow cells or amorphous debris through the transitional cells moving toward the exterior of the PM, as well as toward the interior. As keratinization occurs, some inner basophilic cells which had been located in marginal areas of keratinization lost their tight cell-cell bonding. These cells showed edematous/vesicular and squamoid changes. High molecular weight cytokeratin was expressed in a linear pattern in some early and fully developed lesions. There were fewer layers of basophilic cells between the stroma and squamoid cells/amorphous debris than between the stroma and shadow cells. Ki-67 was expressed strongly both basal and overlying basophilic cells. Apoptotic bodies were detected in most transitional cell layers and some amorphous debris zones. CONCLUSIONS: The present study suggests dual pathways of cell differentiation in PMs. In the sudden keratinization pathway, the basophilic cells, transtional cells, shadow cells, and squamoid cells are suddenly keratinized, and the basophilic cells become early the transitional cells or squamoid cells. Cytoplasmic expressions of Ki-67 and cytokeratin in the basophilic cells show that the basophilic cells differentiate toward the innermost layer of the outer root sheath cells.
