Radioautographic Observations on Iron Absorption by the Duodenum of Mice With Iron Overload, Iron Deficiency, and X-linked Anemia

The uptake of iron by the absorptive cells of the duodenum, and its subsequent transfer to the lamina propria, has been studied using ⁵⁵Fe and high-resolution radioautography in mice rendered iron deficient by diet, in mice with dietary iron overload, and in mice with hereditary malabsorption of iron (s/a). In all, as in normally iron replete mice, two phases of iron absorption can be distinguished. There is an early active phase of uptake and transfer, followed by a second "storage" phase. The pathway of iron absorption is qualitatively similar to the normal with rapid uptake by the brush border, passage through the terminal web to the rough endoplasmic reticulum, and areas rich in free ribosomes. Iron passes through the lateral cell membrane, intercellular spaces and epithelial basement membrane to the vessels of the lamina propria. The rough endoplasmic reiculum and areas rich in free ribosomes form the major localization of iron at all stages of absorption. Only a relatively small amount of radioactive iron is found over ferritin in iron-loaded mice and mice with s/a; morphologically recognizable ferritin was not observed at any stage in iron-deficient animals. Significant numbers of grains have not been seen over mitochondria in any group of animals studied. In iron-deficient mice, the rate of uptake and of transfer of iron is increased. With the 10 µg dose of iron used, all iron taken up by the absorptive cell is transferred to the lamina propria within 3 hr, with none remaining in the cell during the second, storage phase. In mice with iron overload, transfer of iron is decreased, resulting in considerable storage of iron in the cell. In s/a mice, uptake does not appear to be unduly reduced, but transfer is reduced, thereby resulting in increased storage of iron in the absorptive cell. The rough endoplasmic reticulum and areas rich in free ribosomes appear to play an important role in the uptake, transfer, and storage of iron by the absorptive cell regardless of the state of the body iron stores. Ferritin would seem to have a less important part in iron absorption, possibly acting as a storage or detoxifying mechanism.