Histological and ultrastructural examinations of porcine tonsils.

The histology and ultrastructure of porcine tonsils were studied. The porcine tonsils were lymphoepithelial organs situated at the opening of both the digestive and respiratory tracts. The tonsil of the soft palate in the oropharyngeal tract and the paraepiglottic tonsil in the laryngopharynx were mainly consisted of secondary lymphoid follicles encapsulated by connective tissue. The stratified squamous epithelia covering the tonsils and their crypts were frequently heavily infiltrated by lymphoid cells. The pharyngeal and tubal tonsils (TT) were situated in the nasopharyngeal tract. The cells of the pseudostratified columnar epithelia of the pharyngeal and TT were loosely connected, with large intercellular space. They consisted of scattered lymphoid follicles, aggregations of lymphoid cells and diffuse lymphoid tissues. Many high endothelial venules, specialized for the diapedesis of lymphoid cells into the tonsillar tissue, were detected in the four porcine tonsils. Therefore, the overall structures of the tonsils (the tonsil of the soft palate, the paraepiglottic tonsil, the pharyngeal and the TT) reflect their immune functionality in the oral and intranasal immunity.