The distribution of iron in the brain: a phylogenetic analysis using iron histochemistry

Histochemical procedures can be used to detect the cellular distribution of iron in the brain. The objective of the present study was to determine if the cellular distribution of iron enrichment is conserved between animals on different branches of the phylogenetic tree. This information can facilitate our understanding of the role of iron enrichment in cells of the brain. The animals studied were the mouse, rat, chicken, frog, fish and fly. In order to optimize the detection of iron, two histochemical staining methods and three fixatives per staining method were examined for each species. The results indicated that there was no single cell type that displayed iron enrichment in each of the species examined. In three out of five species in the phylum chordata, iron was enriched in oligodendrocytes; the exceptions to this were the fish and frog, which had iron enrichment in neurons but not oligodendrocytes. Iron was enriched in ependymal cells and endothelial cells in four out of the five species in the phylum chordata with the fish and the mouse being the respective exceptions. Myelin was stained in the mouse and rat, and microglia were occasionally observed in the rat and chicken. Astrocyte staining was not observed in any of the species examined. In the fly third instar larvae, iron enrichment was found in border glia and in neuropil. The absence of a conserved staining pattern between species suggests that iron enrichment probably does not play a role in the main functions that have been attributed to those cells that were stained. These findings, taken together with previously published data on the distribution of ferritin and transferrin, suggests that iron-enriched cells serve as stores of iron for the brain.