Trypan blue (TB) is used since 1937 as a "supravital" stain. It is called
"particulate" dye, because when in aqeous salt solutions it is in colloidal
form. Due to its "particulate" properties TB is excluded from live cells
that have plasma membrane integrity preserved. Loss of membrane integrity,
which characterizes necrotic cells, apoptotic cells at late phase ("necrotic
phase") of apoptosis, cells with mechanically damaged membranes and naked
nuclei, results that all these cells stain with TB. One has to be careful,
however, because live macrophages and some other other cells take up TB by
the process of phagocytosis. TB is then localized in granular structures
most likely primary and secondary lysosomes. In this capacity TB was used in
live animals to detect phagocytes in different organs. Interestingly, TB
fluoresces (red) when binds to proteins. Properties of TB were described in
old textbooks of histology (e.g. Bloom & Fawcett) histochemistry (Pearse)
and biological stains (Coons).
Zbigniew Darzynkiewicz
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