An electron microscopic study of heme uptake by rat duodenum.

The uptake of heme by mucosal absorptive cells of rat duodenum was studied by electron microscopy; normal and iron-deficient animals were used. Heme was administered as hemoglobin or hemin chloride by means of an intragastric tube and also by injection into a closed duodenal loop. The peroxidatic activity of heme was utilized to locate it on the cell surface and within the cell; aldehyde-fixed mucosal tissue was incubated in a solution of 3,3'-diaminobenzidine. Heme was observed first on the surfaces of the microvilli of the absorptive cell and in apical pits between individual microvilli. Shortly thereafter, heme appeared within membrane-bound tubules which apparently communicated with the apical pits. At later intervals, secondary lysosomes, located in apical cytoplasm and in the supranuclear region of the cell, contained heme; the manner of heme transfer from the apical pit-tubular system to the lysosomal compartment was not defined. Gradual loss, with time, of 3,3'-diaminobenzidine reaction product from secondary lysosomes indicated cleavage of heme therein. Normal and iron-deficient rats handled heme in a similar manner.