Ultrastructural cytochemical identification of the siderophilic enterocyte.

Specific iron-binding sites in the gut were visualized by the sequential incubation of glutaraldehyde-fixed specimens in iron nitrilotriacetate (FeNTA) and acid ferro-cyanide (AF). In rat duodenum, jejunum, and ileum, approximately half of the enterocytes from the crypt to the distal villus contained FeNTA-AF-reactive material. The staining intensity of individual enterocytes appeared to decrease progressively in more distal locations in the gut. Maximal FeNTA-AF staining was observed in cells in the upper half of the villus, and was localized primarily in microvilli, apical cytoplasm, and lateral membranes. In duodenal crypt cells, stain deposits were present primarily in the microvilli. FeNTA-AF stained sites in the cytoplasmic and microvillus matrix were approximately 100-fold greater in number than were sites of intrinsic iron stained with AF alone. FeNTA-AF and AF staining in human duodenal enterocytes was similar to that observed in rat duodena, demonstrating the applicability of this methodology to human samples. In rat duodena, the distribution of AF staining alone in specimens taken 10 min after the in vivo intraduodenal administration of FeCl2 was similar to that of FeNTA-AF staining in tissue from fasted animals not given iron. The distribution and frequency of iron-binding sites stainable with the FeNTA-AF method which occur in a subpopulation of enterocytes can be correlated closely with physiologic, histologic, and ultrastructural parameters of inorganic iron absorption previously reported.