The potential role of the ovine pharyngeal tonsil in respiratory tract immunity: a scanning and transmission electron microscopy study of its epithelium.

The mucosal epithelium of the pharyngeal tonsil was studied in 10 conventionally raised sheep by scanning and transmission electron microscopy. The surface of the tonsil was covered by two types of topographically and ultrastructurally distinct epithelium. The epithelium overlying the lymphoid follicle region (follicle-associated epithelium) was compressed by adjacent ciliated epithelium and consisted of predominantly non-ciliated cells of varying height. The majority of these cells possessed microvilli or microfolds of varying number, height and density on their surface. Small numbers of squamous cells and, occasionally, intermediate cells were also present in these areas. Ultrastructurally, the microvillous cells showed features similar to M cells which occur in other mucosal sites. They contained cytoplasmic vesicles and vacuoles and formed intercellular digital junctions. Between these cells was a heavy infiltration of lymphoid cells. Focal disintegration of the mucosal epithelium was occasionally seen. The epithelium covering non-follicle areas consisted of mainly ciliated cells interspersed with some goblet cells and squamous cells, morphologically resembling those elsewhere in the respiratory tract. These findings have provided evidence which confirms that the ovine pharyngeal tonsil is part of the respiratory tract-associated lymphoid tissue.