Calcein AM is converted into green fluorescence via intracellular esterases. If some cell compartments do not fluoresce this either means the compartment does not have intracellular esterases or that the compartment is not permeable to Calcein AM. Does anyone have a thought on which one of these explanations is most appropriate and on what else it might means for the properties of an intracellular compartment if Calcein AM signal is not visible? Thanks!