Comparative techniques for determining cellular iron distribution in brain tissues

Iron is essential for a number of normal brain functions and accumulates in high concentrations in specific regions of the brain. In pathological states, it may further accumulate in these and other areas that are typically low in iron content. The contribution of excess iron to potential central nervous system damage through its ability to donate an electron and to promote oxygen free radical formation has made the nature, location, extent, and process of iron deposition in the brain important areas of investigation. Nevertheless, there is relatively little information currently available on the cellular and subcellular distribution of iron in the central nervous system in either normal or diseased states. We describe and compare a number of the currently available techniques by which iron can be detected within the cellular components of the brain. Histochemical approaches, primarily in the form of the Perls' stain, yields information only on iron in its ferric state and is a relatively insensitive technique. Electron microscopy with x‐ray spectrometry can provide positive identification of iron but has limitations regarding morphological verification of the specific cells being probed and also has a relatively high lowest detection limit. Secondary ion mass spectrometry and proton‐induced x‐ray spectrometry are both expensive, highly complex techniques with greater detection sensitivity, but they have problems identifying the cellular components being analyzed. Finally, laser microprobe mass analysis combines histological localization and identification of probe sites in plastic‐embedded histological sections with detection limits in the single part per million range. It is likely that all these techniques will increasingly be employed in the future for investigation of brain iron in both normal and pathological specimens.