Nonidentical distribution of transferrin and ferric iron in human brain

Using the avidin-biotin immunoperoxidase technique and a diaminobenzidine intensification of the Prussian Blue method, we have compared the distribution of transferrin to that of ferric iron in five normal autopsy brains from adult human males. The observed distribution of transferrin was considerably more widespread than: (1) that of histochemically demonstrable ferric iron; (2) that reported for transferrin in the fetal and neonatal human brain; and (3) that reported for transferrin in other species. Transferrin immunoreactivity was present in neurons, oligodendrocytes, astrocytes, ependymal cells, and choroid plexus epithelial cells, although not in all cells of any type. Ferric iron, on the other hand, was demonstrable only in oligodendrocytes, in myelin sheaths, and possibly in axons. While staining for both transferrin and iron was relatively high in the basal ganglia and substantia nigra, the pattern of staining differed, with striatal efferent fibers staining more heavily than the neuropil for iron and less heavily than the neuropil for transferrin. The choroid plexus, which in the rat has been shown to synthesize transferrin, stained heavily for transferrin and not at all for iron. The findings of low iron and high transferrin in the choroid plexus suggest that the plexus may secrete transferrin into the cerebrospinal fluid, thereby facilitating the translocation of iron within the neuraxis. Furthermore, the nonidentical distribution of ferric iron and transferrin suggests that, in the human brain, transferrin may serve other functions besides the transport of iron from extracellular fluid to cytoplasm.