When added to cells, Fura-2AM crosses cell membranes and once inside the cell, the acetoxymethyl groups are removed by cellular esterases. Removal of the acetoxymethyl esters regenerates "Fura-2", the pentacarboxylate calcium indicator.
Adding Fura-2AM to cells allows it to cross cell membranes. Once inside, the acetoxymethyl groups are broken down by cellular enzymes, restoring Fura-2, a calcium indicator.