Oral mucosa forms an important protective barrier between the external environment of the oral cavity and internal environments of the surrounding tissues. It is resistant to the pathogenic organisms that enter the oral cavity and to indigenous microorganisms residing there as microbial ﬂora. Epithelial cells, migratory neutrophils, and saliva all contribute to maintaining the health of the oral cavity and protecting the oral mucosa from bacterial, fungal, and viral infections. The protective mechanisms include several salivary antimicrobial peptides, the -defensins expressed in the epithelium, the -defensins expressed in neutrophils, and the secretory immunoglobin A (sIgA). However, in individuals with immunodeficiency or undergoing antibiotic therapy, in which the balance between microorganisms and protective mechanisms is disrupted, oral infections are rather common.
The tongue is a muscular organ projecting into the oral cavity from its inferior surface. Lingual muscles (i.e., muscles of the tongue) are both extrinsic (having one attachment outside of the tongue) and intrinsic (confined entirely to the tongue, without external attachment). The striated muscle of the tongue is arranged in bundles that generally run in three planes, with each arranged at right angles to the other two. This arrangement of muscle fibers allows enormous flexibility and precision in the movements of the tongue, which are essential to human speech as well as to its role in digestion and swallowing. This form of muscle organization is found only in the tongue, which allows easy identification of this tissue as lingual muscle. Variable amounts of adipose tissue are found among the muscle fiber groups.
Grossly, the dorsal surface of the tongue is divided into an anterior two thirds and a posterior one third by a V-shaped depression, the sulcus terminalis (Fig. 16.3). The apex of the V points posteriorly and is the location of the foramen cecum, the remnant of the site from which an evagination of the floor of the embryonic pharynx occurred to form the thyroid gland.
Papillae cover the dorsal surface of the tongue.
Numerous mucosal irregularities and elevations called lingual papillae cover the dorsal surface of the tongue anterior to the sulcus terminalis. The lingual papillae and their associated taste buds constitute the specialized mucosa of the oral cavity. Four types of papillae are described: filiform, fungiform, circumvallate, and foliate.
 Filiform papillae are the smallest and most numerous in humans. They are conical, elongated projections of connective tissue that are covered with highly keratinized stratified squamous epithelium (Fig. 16.4a and Plate 49, page 559). This epithelium does not contain taste buds. The papillae serve only a mechanical role. Filiform papillae are distributed over the entire anterior dorsal surface of the tongue, with their tips pointing backward. They appear to form rows that diverge to the left and right from the midline and that parallel the arms of the sulcus terminalis.
 Fungiform papillae, as the name implies, are mushroom-shaped projections located on the dorsal surface of the tongue (Fig. 16.4b). They project above the filiform papillae, among which they are scattered, and are just visible to the unaided eye as small spots (see Fig. 16.3 and Plate 50, page 561). They tend to be more numerous near the tip of the tongue. Taste buds are present in the stratified squamous epithelium on the dorsal surface of these papillae.