Histology, Immunohistochemistry and Ultrastructure of the Tonsil of the Soft Palate of the Horse

The tonsil of the soft palate was an oval, flat structure located centro‐rostrally on the oral surface of the soft palate. Its stratified squamous non‐keratinized epithelium was perforated by holes or small crypts the deeper parts of which were loosely spongiform inter‐digitated with lymphoid tissue. These unusual features have not previously been reported in tonsils of any species. Crypts and reticulated epithelium as found in the lingual and palatine tonsils were not observed. Lectins showed varying affinities for specific layers of the epithelium. M cells were not observed. A few Langerhans cells were distributed among surface epithelial cells. Lymphoid tissue was arranged loosely and in isolated lymphoid follicles in the subepithelial lamina propria mucosae. Although IgA+ cells and macrophages were proportionately more numerous the amount of lymphoid tissue was much less than in the lingual and palatine tonsils. Most of the follicular germinal centres lacked a darkly stained corona. CD4 positive were more numerous than CD8+ lymphocytes and were distributed in the parafollicular and inter‐follicular areas. Large clusters of mucus acini positive for glycogen, acidic and neutral mucopolysaccharides separated lymphoid tissue from deeply placed striated muscle. Only a few high endothelial venules were observed in the parafollicular and inter‐follicular areas. These had relatively few vesiculo vacuolar or other organelles in their high endothelial cells and few lymphocytes attaching to their walls.