Intercellular and lymphatic pathways of the canine palatine tonsils

The palatine tonsils play a key role in initiating immune responses against antigenic material entering the mouth and their lymphatic pathways are important in disseminating immunological information to the lymph nodes and other mucosal surfaces. Scanning and transmission electron microscopy and Mercox casts were used to examine the intercellular and lymphatic pathways of the palatine tonsil in dogs. Intercellular fluid within the intraepithelial passageways of the reticular epithelium flows through pores in the basement membrane and intermingles with that of the subepithelial intercellular spaces. From there, tissue fluid enters initial lymphatics which form a plexus surrounding each follicle. No lymphatic vessels are seen entering or leaving the follicles but intercellular pathways of the follicles are continuous with those of the adjacent initial lymphatics. These pathways appear to provide the only route for lymphocytes leaving the follicles and directly entering the lymphatic pathway. Lymph then flows into sinuses adjacent to, and incompletely surrounding, the base of follicles; or it may enter a network of sinuses between and beneath the lymphoid follicles which convey lymph throughout the parafollicular tissue. Lymphoid cells enter the parafollicular sinuses which may be a major entry site of lymphocytes emigrating from the lymphoid parenchyma. Some lymph may bypass these sinuses by entering septal lymphatic vessels oriented perpendicular to the central connective tissue lamina of the palatine tonsil. All lymph is collected into basal lymphatic vessels where valves prevent retrograde flow. Basal vessels course within the central lamina and converge to form efferent lymphatic vessels which emerge at the caudal region of the palatine tonsil and convey lymph to the medial retropharyngeal lymph node.