Development and immunophenotyping of the pharyngeal tonsil (adenoid) in cattle

Immunoperoxidase staining and electron and light microscopy were used to characterize the development of the pharyngeal tonsil in 98 cattle aged between 30 days of gestation and 12 years. The rugae of the pharyngeal tonsil were poorly formed before 95 days of gestation. Microvillous (M) cells associated with intra-epithelial leucocytes (lympho-epithelium) were scattered among ciliated and goblet cells covering most of the surface in post-natal animals. Intra-epithelial leucocytes were rare in fetuses, but ciliated and M cells could be distinguished. Leucocytes of the lamina propria started to accumulate at approximately 120 days of gestation. A loose accumulation of mononuclear cells progressed into a B-cell rich upper and T-cell rich lower layer, with typical lymphoid tissue organization in post-natal animals and lymphoid involution in aged cattle. Primary lymphoid follicles formed at 5 months of gestation, but germinal centres did not form until 2 to 4 weeks after birth. Except for null cells, the relative number of cells staining for each leucocyte phenotype or MHC class II antigen increased with age, especially during the neonatal period. The early development, strategic location and specialized structure of the pharyngeal tonsil suggest an important role in modulating inhaled antigens in cattle. Fetal and neonatal calves had minimal lymphoid tissue priming, as indicated by lack of secondary follicles, low MHC class II expression and few intra-epithelial leucocytes. The phenotypic differences may be relevant to the increased susceptibility of calves to infectious diseases shortly after birth.