Which assay is best to detect apoptotic cells?
Viability assays measure different aspects of cell health. MTT measures metabolic activity of mitochondria, trypan blue assesses membrane permeability, chromium release measures cytotoxic lysis. None specifically identify programmed cell death early stage. Annexin V FITC flow cytometry detects externalized phosphatidylserine, earliest membrane event in apoptosis while membrane remains impermeable to PI, allowing distinction of early and late apoptosis versus necrosis. Quantitative fluorescence per cell and population percentages make it sensitive, specific and standard for apoptosis kinetics in drug screening and immunology research laboratories.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.