The epithelium of canine palatine tonsils

The palatine tonsil is positioned to play a key role in protecting the body against ingested microorganisms. These microorganisms must traverse the tonsillar epithelium to initiate immune reactions, but no information is available from dogs on the structure of this epithelium. In this study, the morphology and ultrastructure of the epithelium of the palatine tonsil of dogs was examined using light and electron microscopy. The epithelium is of two types: reticular and non-reticular. Reticular epithelium, which is invaded by lymphoid cells, is located over the apices of nodules, separated indistinctly by islands of non-reticular squamous epithelium. The reticular epithelium contains M cells which are more abundant towards the periphery of lymphoid nodules. The apical membrane of the M cells forms folds from which microvilli extend into the lumen of the oropharynx. Abluminally, cytoplasmic processes enfold clusters of lymphocytes. Numerous desmosomes secure the marginal plasma membrane and lateral membranous interdigitations to adjacent epithelial cells, thereby maintaining the integrity of the tonsillar epithelium. Epithelial discontinuities, which are few, occupy similar positions to M cells and appear to contain migrating lymphocytes. A fenestrated basement membrane allows cell transport between the intraepithelial passageways of the reticular epithelium and the subepithelial lymphoid tissues.