Enamel
proteins can be useful markers for assessment of the functional differentiation of neoplastic epithelium and the nature of extracellular matrices in
odontogenic tumors. In the present study, we examined immunohistochemical localization of sheathlin, a recently cloned enamel sheath
protein, in various
odontogenic tumors to evaluate functional differentiation of
tumor cells and the nature of hyalinous or calcified matrices in odontogenic
neoplasms. Distinct immunolocalization of sheathlin was observed in the immature enamel of the tooth germ at the late bell stage. Secretory ameloblasts facing the enamel matrix also showed positive staining in their cytoplasm. Definite localization of sheathlin was demonstrated in the enamel matrix in
odontogenic tumors with inductive dental hard tissue formation such as ameloblastic
fibroodontomas and
odontomas. Immunoexpression of sheathlin was, furthermore, demonstrated in eosinophilic droplets in solid nests of
adenomatoid odontogenic tumor (AOT) and ghost cells in the epithelial lining of
calcifying odontogenic cyst (COC). In AOT, cells facing the eosinophilic droplets also expressed the
protein in their cytoplasm. There was neither intracellular staining for sheathlin in the
tumor cells nor extracellular staining in the matrix of
ameloblastomas and calcifying epithelial
odontogenic tumors. Dentin, dysplastic dentin-like hyaline material and cementum in the
tumors examined were negative for sheathlin. These results show that immunodetection of sheathlin is a useful marker for functional differentiation of secretory ameloblasts and enamel matrix, which is often hard to differentiate from other hard tissues in
odontogenic tumors. Our findings from the view point of sheathlin expression support that the
tumor cells of
ameloblastomas do not attain full differentiation into functional ameloblasts. It is very interesting that epithelial cells in
odontogenic tumors can differentiate into functional ameloblasts without induction by odontogenic mesenchyme, as shown by immunoexpression of sheathlin in eosinophilic droplets within solid epithelial sheets in AOT and ghost cells in the epithelial lining of COC where inductive participation of mesenchymal cells was most unlikely.