The fine structure and localization of alkaline phosphatase activity of the small intestinal epithelium in the postnatal developing rat.

The morphological structure and the development of alkaline phosphatase activity of the small intestine of neo-natal rats were investigated by means of light and electron microscopy. Until around the 14th day of neo-natal life alkaline phosphatase activity of the duodenal and jejunal epithelial cells was observed on the microvilli, the apical vacuoles and tubules, and the tubular invagination. After the 15th day, the tubulo-vacuolar system below the microvilli disappears from the duodenal and jejunal cells. Also the enzyme activity becomes localized solely on the microvillous membrane. This cytochemical pattern in the jejunum shows no change with the advance of age. Around the 23rd day, alkaline phosphatase activities of the duodenal epithelium were observed in the same sites as those in the full grown rat, i.e. on the microvilli, the lateral cell membrane, and the Golgi apparatus. During the suckling period ultrastructural localization of alkaline phosphatase in the ileal cells can be seen in the microvillous membrane, the lateral cell membrane, the tubulo-vacuolar system, and the supranuclear vacuole. At the weaning period, however, the apical tubulo-vacuolar system and the supranuclear vacuole disappear, and all the absorptive cells in the ileal epithelium are free of reaction products as has been shown to be the case in the adult rat.