An Electron-histochemical Study on Protein Uptake and Distribution in the Colonic Mucosa of the Neonatal Rat

The uptake of exogenous protein by the colonic mucosa in neonatal rats was studied after enema infusions of horseradish peroxidase, of immunoglobulin conjugated with horseradish peroxidase, of unconjugated immunoglobulin, and of normal saline. Specimens of colonic mucosa were processed for the histochemical demonstration of peroxidase activity and then prepared for electron microscopy. Enzyme activity and, by inference, exogenous protein were located on the luminal plasma membrane of epithelial lining cells, in pinocytotic vesicles in their apical cytoplasm, in Golgi-associated sacs and in their secondary lysosomes. Protein was also located within lymphocytes which had invaded the epithelium. Transepithelial passage of protein was proven by its demonstration within the lysosomal systems of macrophages and nerves in the lamina propria. The demonstration of exogenous protein within the macrophage system of the colon is of interest in view of the putative allergic basis of certain bowel conditions.