The A to Z of Anatomical Histological and Medical terms
stain, such as Perl’s iron stain, or the Prussian blue reaction.
Carbon appears as anthracotic pigment in the lungs. It can be
distinguished from melanin by doing a Melanin bleach, which helps
to distinguish carbon from melanin. Poorly fixed tissues may contain
formalin-heme pigment, which is black and finely granular, but this is
widely scattered in the tissues without regard to cellular detail.
Formalin-heme pigment is also birefringent on polarization.
Silica is present in many minerals and building materials. Most
forms are very inert and cannot be stained in tissue but can be
demonstrated by white birefringence on polarization. It is most often
present in lung, but can make its way into lymph node.
Street drugs for injection often are diluted with compounds
containing minerals such as silica or talc. These crystals can be
found throughout the body, but especially in lymphoreticular tissues.
Tattoo pigment is usually black and is inert and non-polarizable.
Red tattoo pigment often contains cinnabar (which has mercury in
it). there are no specific stains for these materials, and in general,
minerals are best demonstrated by microincineration techniques or
by scanning electron microscopy with energy dispersive analysis
(SEM-EDA), which is also used in the analysis of gunshot residue,
because of its composition of antimony, barium, and lead.
Fat stains
The oil red O (ORO) stain can identify neutral lipids and fatty acids in
smears and tissues. Fresh smears or cryostat sections of tissue are
necessary because fixatives containing alcohols, or routine tissue
processing with clearing, will remove lipids. The ORO is a rapid and
simple stain. It can be useful in identifying fat emboli in lung tissue
or clot sections of peripheral blood.
Fungi stains
(Gomori methenamine silver stain = GMS)
Fungi and Pneumocystis carinii., have cell walls which stain black or
brown, outlining the organisms clearly. Disadvantages with this stain
is the large amount of background staining, so the morphology of the
organism needs to be known. Fungi also stains red with the PAS
method and blue with H&E.
Iron (haemosiderin) stains
Haemosiderin (storage iron granules) may be present in areas of old
haemorrhage or be deposited in tissues with iron overload
(haemosiderosis - physiological, haemochromatosis - pathological).
13 © A. L. Neill
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