Electron microscopic characterization of filiform papillae in the normal human tongue

The fine structure of filiform papillae on the normal human tongue was examined level by level, from the basal layer to the surface, in specimens taken from the dorsal side of the lingual body. Human lingual epithelia showed three distinct regions: epithelia on the anterior and on the posterior sides of filiform papillae and an interpapillary epithelium. While the basal and the squamous cell layers were similar throughout these three regions, differences were noted in the granular and the horny layers. The interpapillary epithelium actually lacked both the granular and the horny layers. The epithelium on the anterior side of filiform papillae was characterized by alternating layers of granular cells and of cornified cells. Granular cells possessed three types of keratohyaline-like granules within their cytoplasm: uniformly electron dense, relatively less electron dense, and a heterogeneous type. While the number of the keratohyaline-like granules was remarkably diminished in the epithelium on the posterior side of filiform papillae, a considerable amount of tonofibrils was present in the cytoplasm. In the uppermost portion of the anterior side of filiform papillae, coherence between adjacent epithelial cells depended mainly on the interlocking of cytoplasmic villi and poorly developed desmosomes on villi. On the other hand, epithelial cells on the posterior side of filiform papillae appeared to be more tightly adhesive compared with those on the anterior side. This was due to focal thickening of the plasma membrane and to desmosomes at the interface between the granular and cornified cells, and to the formation of a marginal band and increased intercellular cement presumably derived from lamellar bodies in the horny layer. These findings demonstrate distinct differences between the anterior and the posterior sides of filiform papillae in the human tongue with respect to keratinization patterns, structures associated with cell-to-cell adhesion and the strength of cellular cohesion in the uppermost portion, and the turnover of cornified cells. These differences may contribute to the formation of the unique external configurations of filiform papillae.