Potassium-dependent p-nitrophenyl phosphatase and carbonic anhydrase reactivities suggest that lymphoid follicles in the large intestine of lambs are lined with a uniform type of epithelial cell distinct from the absorptive epithelium.

The epithelium covering the large intestinal lymphoid follicles in fetal and postnatal lambs was examined for potassium-dependent p-nitrophenyl-phosphatase (K(+)-NPPase), carbonic anhydrase, magnesium-dependent adenosine triphosphatase (Mg(2+)-ATPase) and acid phosphatase. Reactivities for these enzymes indicated a homogenous population of cells in the follicle-associated epithelium (FAE), distinct from the absorptive epithelium. There were essentially no differences in the enzyme reactivities of the large intestinal FAE between fetuses in late gestation and postnatal lambs. The FAE showed a weak reaction for K(+)-NPPase and a variable staining for Mg(2+)-ATPase and acid phosphatase. In contrast, the adjacent absorptive epithelium demonstrated strong reactions for these enzymes. Carbonic anhydrase gave a strong reaction at the luminal and apparent basolateral cell borders of the large intestinal FAE. This distribution of reactivity for carbonic anhydrase resembled that found in the ileal FAE. In absorptive epithelial cells, only the luminal cell border reacted strongly for carbonic anhydrase. Serial sections of large intestinal tissue showed a variation in the basolateral staining of FAE from one section to the next, a finding which suggested that the reaction may be associated with transcytosis. The lymphoid follicles and domes of the large intestine showed a variable granular pattern of carbonic anhydrase staining, which also suggested a dependence on epithelial transcytosis.