Architectural organization of filiform papillae in normal and black hairy tongue epithelium: dissection of differentiation pathways in a complex human epithelium according to their patterns of keratin expression.

Background An inadequate understanding of the complex morphologic characteristics of human filiform papillae has hampered the histopathological characterization of disorders affecting tongue keratinization. To better define the 3-dimensional cytoarchitecture of tongue epithelium, we performed detailed immunohistochemical analyses of normal and black hairy tongue tissues using a panel of antikeratin antibodies. Observations The dome-shaped base of the human filiform papilla (primary papilla) is surmounted by 3 to 8 elongated structures (secondary papillae). These secondary papillae are composed of a central column of epithelial cells expressing hair-type keratins and an outer rim of cells expressing skin-type keratins. The epithelium overlying the primary papillae and between the individual primary papillae express esophageal-type keratins. In black hairy tongue disease, there is a marked retention of secondary papillary cells expressing hair-type keratins. Conclusions Using a panel of antikeratin probes, we define the precise topographical localization of cell populations undergoing 3 distinct differentiation programs in dorsal tongue epithelium. Comparative analyses of black hairy tongue specimens indicate that defective desquamation of the cells in the central column of filiform papillae results in the formation of highly elongated, cornified spines or "hairs"—the hallmark of this disease.