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 trave rse the tonsillar epithelium to initiate immune reactions, but no info rmation is available from dogs on the structure of this epithelium. In this study, the morphology and ultrastructure of the epithelium of th e palatine tonsil of dogs was examined using light and electron micros copy. The epithelium is of two types: reticular and non-reticular. Ret icular epithelium, which is invaded by lymphoid cells, is located over the apices of nodules, separated indistinctly by islands of non-retic ular squamous epithelium. The reticular epithelium contains M cells wh ich are more abundant towards the periphery of lymphoid nodules. The a pical 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 margina l plasma membrane and lateral membranous interdigitations to adjacent epithelial cells, thereby maintaining the integrity of the tonsillar e pithelium. Epithelial discontinuities, which are few, occupy similar p ositions to M cells and appear to contain migrating lymphocytes. A fen estrated basement membrane allows cell transport between the intraepit helial passageways of the reticular epithelium and the subepithelial l ymphoid tissues.