Intercellular and lymphatic pathways associated with tonsils of the soft palate in young pigs

Tonsils of the soft palate of pigs are the main oropharyngeal lymphoid tissues that protect the body against antigens entering through the mouth. The aim of this work was to elucidate the intercellular and lymphatic pathways by which lymph and cells are transported through these tonsils. Tonsillar tissue from freshly-killed pigs was examined using light microscopy and electron microscopy, or was injected with Mercox for scanning electron microscopy of corrosion casts. Intercellular fluid passes between epithelial cells and is continuous with that of the subepithelium. Fluid from the subepithelium flows into sinuses that form a network around the apex of follicles. These sinuses are continuous with parafollicular sinuses that penetrate the parafollicular tissue between the follicles. Some parafollicular sinuses are traversed by a complex network of cell processes, whereas others appear to lack such processes. Some parafollicular sinuses are closely located (10 microm) to venules; others lie adjacent to the follicle capsule. No lymphatics enter or leave the follicles. All lymph from the tonsils must traverse parafollicular sinuses before entering septal vessels, and these are continuous with basal vessels. Basal vessels coalesce to form efferent vessels that transport lymph from the tonsil to the primary lymph nodes. Septal, basal and efferent lymphatic vessels contain prominent valves and many lymphocytes. Lymphatic sinuses appear to be a significant pathway for lymphocytes migrating from the tonsillar lymphoid tissue.