The Epithelium Covering Peyer’s Patches in Young Milk-Fed Calves: An Ultrastructural and Enzyme Histochemical Investigation

Between the ordinary villi over Peyer’s patches there are small domes or “pseudovilli” caused by bulges in the lymphoid tissue. These “pseudovilli” were studied in 5 healthy milk-fed, about 3-week-old, pre-ruminant calves. Scanning electron microscopy revealed that the “pseudovilli” were covered by a specialized follicle associated epithelium (FAE). The FAE had poorly developed microvilli and often extensive folding of the cell surface close to the cell borders. By transmission electron microscopy the tips of the marginal folds of the FAE seemed to fuse, probably in the process of enfolding bulk material from the lumen. The FAE apical cytoplasm contained numerous thick-walled and bristle-coated invaginations, tubules and vesicles indicative of micropinocytosis. Multivesicular bodies and large vacuoles were frequent. Indications of extracellular unloading of residual bodies were found. Intraepithelial lymphocytes tended to group together, and some were rich in rough endoplasmic reticulum. Enzyme histochemistry showed weak reactions of adenosine triphosphate splitting enzyme and aminopeptidase in the FAE luminal cell border. Cytoplasmic acid phosrphatase showed a marked basal-apical decrease along the “pseudovillus” probably caused by the onset of endocytosis. The results of this study appear compatible with the concept that the FAE takes up macromolecules from the lumen by pinocytosis and sensitizes lymphocytes.