Various microprobe techniques are used to quantify elemental concentra-
tions of metals at cellular and subcellular resolution [140]. Recent studies have
utilized frozen unfixed sections for microprob e studies [102], and the
environmental constraints on high resolution microscopy and spectro scopy
are improving, with the application of subcellular-resolution secondary ion
mass spectrometry (nanoSIMS) to biological samples [55], and the facility to
study fully hydrated samples with scanning electron microscopy [141]. Such
techniques require an understanding of the variable yields obtained from
inhomogeneous materials like human tissue if the spatial distribution of
specific elements is to be quantified. Another technique, Mo
¨
ssbauer spectro-
scopy, has been used to study both structural and magnetic aspects of
nanoscale iron compounds in tissue, including early structural studies of
hemosiderin [142], investigations of iron-rich regions to establish the
proportion of iron that was due to ferritin [56], and studies of the relative
magnetic properties of natural and synthetic ferritin cores [36, 143].
High energy X-ray absorption spectroscopy has been used to study aspects
of ferritin cores and their biomineralization [43, 107]. Aspects of this technique
are now be ing applied to tissue, where a highly sensitive (<ppm) method to
locate and characterize nanoscale iron compounds dispersed in autopsy tissue
has been developed using a combination of synchrotron X-ray fluorescence and
absorption spectroscopy [45, 58, 61]. This approach provides site-specific
information on the chemical and structural state of iron compounds at
subcellular resolution in the plane of measurement and can therefore identify
spatial distributions of iron biominerals including ferrihydrite, magnetite, and
wu
¨
stite-like structures [45].
18.5.3 Magnetic Characterization
The magnetic properties of iron compounds in the human brain are of interest
for several reasons. First, disrupt ion in iron metabolism appears to alter the
forms of iron compounds present [39, 40], and so understanding which
compounds occur is important for our understanding of, and ability to treat,
syndromes where disrupted iron metabolism is a feature. Applications include
the use of SQuID magnetometry in a preliminary study to demonstrate
differences in magnetite concentrations for AD and control cases [93], and,
incidentally, for the first demonstration of magnetite in the human brain [26].
Second, magnetic characterization techniques enable differences in iron
loading and the magnetic properties of bulk tissue samples and extracts to
be quantified in a nondestructive manner. For example, Dubiel et al. [56] used
SQuID magnetometry and transmission electron microscopy (TEM) to
demonstrate that the uniaxial magnetic anisotropy constant is an order of
magnitude higher in brain tissue from the globus pallidus than it is in liver.
Finally, the various magnetic properties of the nanoscale iron compounds
present give us a means to detect and distinguish between them; for examp le,
SQuID magnetometry can be used to quantify the various magnetic
NANOSCALE IRON COMPOUNDS RELATED TO NEURODEGENERATIVE DISORDERS 477