Background - Traditionally, the determination of cell growth is done by counting viable cells after staining with a vital
dye. Several approaches have been used in the past. Trypan blue staining is a simple way to evaluate cell membrane
integrity (and thus assume cell proliferation or death) but the method is not sensitive and cannot be adapted for highthroughput
screening. Measuring the uptake of radioactive substances, usually tritium-labeled thymidine, is accurate but
it is also time-consuming and involves handling of radioactive substances. Yellow MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-
diphenyltetrazolium bromide, a tetrazole) is reduced to purple formazan in the mitochondria of living cells. The
absorbance of this colored solution can be quantified by
measuring at a certain wavelength (usually between 500 and 600
nm) by a spectrophotometer. The absorption max is dependent
on the solvent employed. This reduction takes place only when
mitochondrial reductase enzymes are active, and therefore
conversion can be directly related to the number of viable
(living) cells. When the amount of purple formazan produced by
cells treated with an agent is compared with the amount of
formazan produced by untreated control cells, the effectiveness of the agent in causing death of cells can be deduced,
through the production of a dose-response curve. Solutions of MTT solubilized in tissue culture media or balanced salt
solutions, without phenol red, are yellowish in color. Mitochondrial dehydrogenases of viable cells cleave the tetrazolium
ring, yielding purple MTT formazan crystals which are insoluble in aqueous solutions. The crystals can be dissolved in
acidified isopropanol. The resulting purple solution is spectrophotometrically measured. An increase in cell number results
in an increase in the amount of MTT formazan formed and an increase in absorbance. The use of the MTT method does
have limitations influenced by: (1) the physiological state of cells and (2) variance in mitochondrial dehydrogenase activity
in different cell types. Nevertheless, the MTT method of cell determination is useful in the measurement of cell growth
in response to mitogens, antigenic stimuli, growth factors and other cell growth promoting reagents, cytotoxicity studies,
and in the derivation of cell growth curves.