Examination of the reticular epithelium of the bovine pharyngeal tonsil.

The pharyngeal tonsil (adenoid), located at the posterior of the nasopharynx is ideally positioned to sample antigens passing through the nasal cavity or oral cavity. Entering antigens will first contact tonsilar epithelium. To better understand the cellular organization of this important epithelial layer, pharyngeal tonsils were collected from six, 7-month-old calves and examined by light microscopy, immunohistochemistry, and electron microscopy. Morphometric analysis showed that the epithelium overlying lymphoid follicles (reticular epithelium) contained significantly more B-cells, CD4+, and CD11c+ cells than nonreticular epithelium. In contrast, nonreticular epithelium contained significantly more, γ/δ TCR+ cells than reticular epithelium. Scanning and transmission electron microscopy of reticular epithelium identified a heterogeneous population of epithelial cells, many of which displayed morphologic characteristics of M-cells. Moreover, putative M-cells were shown to possess the capacity for microparticle uptake. Bovine pharyngeal tonsilar reticular epithelium contains key immune cells, as well as M-cells; elements essential for antigen uptake, antigen processing, and initiation of immune responses. A better understanding of the morphology and function of tonsilar lymphoepithelium will strengthen our understanding of it's role in disease pathogenesis, and potential use as an induction site for mucosal immune responses to vaccination.