The principle behind viability assays involves differentiating live cells from dead or dying cells based on specific biological markers. For example:
Membrane Integrity: Assays like the trypan blue exclusion test and ethidium homodimer staining rely on the intactness of the cell membrane to distinguish live cells (with intact membranes) from dead cells (with compromised membranes). Metabolic Activity: Assays like the MTT and ATP tests measure the metabolic processes occurring within live cells, providing an indirect measure of cell viability. Apoptosis Markers: The annexin V/PI assay identifies apoptotic cells by detecting externalized phosphatidylserine, a marker for early apoptosis, and PI uptake, indicating late apoptosis or necrosis.
