Histology, immunohistochemistry and ultrastructure of the bovine palatine tonsil with special emphasis on reticular epithelium.

The paired palatine tonsils, located at the junction of the nasopharynx and oropharynx, are ideally positioned to sample antigens entering through the nasal cavity or oral cavity. Entering antigens will first contact tonsilar epithelium. To better understand the cellular composition of this important epithelial layer, palatine tonsils were collected from six, 7-month-old calves and examined by light microscopy, immunohistochemistry and electron microscopy. Morphometric analysis showed that epithelium overlying lymphoid follicles (reticular epithelium) contained more B-cells, CD4+, CD8+, CD11c+, CD172a+ and gamma/delta TCR+ cells than non-reticular epithelium, with B-cells, CD4+ cells and CD11c+ cells being most numerous. Scanning and transmission electron microscopy of reticular epithelium identified an interrupted basement membrane and vascular elements within the epithelium, as well as cells with characteristics morphologically consistent with cells identified as M-cells in other species. Bovine palatine tonsilar reticular epithelium contains key immune cells, as well as potential M-cell-like cells; elements essential for antigen uptake, antigen processing and initiation of immune responses.