Cellular basis of adult-type hypolactasia.

In the proximal jejunum and distal ileum of adult rabbits and rats, the lactase protein and lactase activity are present only in patches of enterocytes, located principally on the lower part of the villi, whereas in the mid-small intestine, lactase is present in all the villus enterocytes. In the proximal jejunum of adult rabbits, only a few vertical continuous sheets of lactase-positive enterocytes arise from the base of the villus, suggesting that the patchy expression of lactase may have a clonal origin. In the proximal jejunum of adult humans with persistent high lactase activity, all the villus enterocytes express lactase; on the contrary, a patchy expression of lactase protein and lactase activity is present in hypolactasic tissues. The patches of lactase-positive enterocytes are scattered randomly on the surface of the villus, suggesting that in hypolactasic humans the enterocyte heterogeneity occurs as a consequence of mechanisms that do not have a clonal origin.