The mammalian tongue filiform papillae: a theoretical model for primitive hairs

Abstract

The dorsal surface of the mammalian tongue is covered with numerous projections called filiform papillae. Immunohistochemical staining and immunoblotting data indicate that the interpapillary epithelium expresses esophageal-type keratins while the papillary epithelium expresses skin- and hair-type keratins. The hard keratins of mouse tongue are indistinguishable from the keratins of the pelage, while the hard keratin of human tongue corresponds to a minor hair-related keratin. Thus, the filiform papillae are constructed by combining two populations of keratinocytes that undergo skin and hair types of differentiation. They can therefore be interpreted as primary cutaneous appendages. Variations in the detailed lingual papillary structure among the different mammalian species, from a simple anterior/posterior type of compartmentalization (rodents) to a complicated concentric construction (primates) lead to propose a theoretical model to understand how mammalian hairs may have evolved from reptilian scales.