Calcein AM (Calcein acetoxymethyl ester) is a cell-permeable, non-fluorescent compound. Upon crossing the cell membrane, calcein-AM is rapidly hydrolyzed by cellular esterases inside live cells. The hydrolysis cleaves the AM group, converting the non-fluorescent calcein AM to a strongly green fluorescing calcein. Because the created calcein is more hydrophilic it is trapped inside the cells with intact membranes. Cells that do not possess active cytoplasmic esterases are unable to convert calcein AM to calcein, and therefore do not fluoresce green. This allows for a quick and easy detection of metabolically-active cells in a sample.
Since calcein-AM does not require DNA binding, it stains all metabolically-active cells and can be used to measure metabolic activity in non-nucleated cells, such as platelets. Because calcein AM is photostable, has low cytotoxicity and does not affect cellular functions, it is a popular stain for the examination of cell vitality and metabolic activity.