TUNEL staining detects DNA strand breaks by using terminal deoxynucleotidyl transferase to add labeled nucleotides to exposed 3′-OH DNA termini, generating either microscopic staining in fixed cells or tissue sections, or fluorescence/cytometric signal in cell suspensions. TUNEL positivity reflects DNA fragmentation but should not be interpreted alone as definitive apoptosis, because TUNEL can also label necrotic, autolytic, mechanically damaged, or DNA-repair-associated DNA breaks.
Organelle Staining
Organelle staining techniques aim to precisely localize and dynamically assess the morphology and functional status of subcellular structures. These include fundamental nuclear DNA counterstaining and nuclear morphology staining, as well as methods for analyzing DNA content and cell cycle phases via flow cytometry. Among DNA staining techniques, the TUNEL assay is widely used; it reflects the cell death status by specifically detecting DNA fragmentation associated with apoptosis. Regarding energy metabolism and substance degradation, key methods include staining for mitochondrial membrane potential and mitochondrial mass, as well as live-cell staining techniques targeting lysosomes and acidic vesicles. These diverse staining approaches provide powerful visualization tools for investigating the dynamic changes of organelles during physiological and pathological processes.
Related Experimental Schemes
Flow cytometric DNA-content cell-cycle staining measures the fluorescence intensity of DNA-bound fluorochromes in single cells or nuclei to estimate DNA content distributions, allowing assignment of populations to G0/G1, S, and G2/M phases by DNA histogram deconvolution. Propidium iodide (PI) intercalates into DNA, and PI fluorescence is proportional to cellular DNA content when staining is performed under conditions that make DNA accessible and minimize non-DNA signal. Cells with G2/M DNA content are expected to show approximately twice the fluorescence intensity of G0/G1 cells, while S-phase cells occupy intermediate fluorescence values. PI-based DNA-content analysis can also detect cells with fractional DNA content, often reported as sub-G1, when DNA fragmentation and extraction during staining reduce retained DNA signal in apoptotic cells. DAPI is an alternative DNA fluorochrome for univariate DNA-content analysis, while bivariate approaches combining DNA content with proliferation
Nuclear DNA counterstaining uses DNA-binding fluorescent dyes to visualize nuclei and chromatin so that nuclei can be located, counted, segmented, and evaluated for morphology; Hoechst 33342, DAPI, propidium iodide, and DRAQ5 are commonly reported nuclear stains, while live-cell DNA labeling is better supported for Hoechst dyes and DRAQ5 than for propidium iodide in intact viable cells. Nuclear morphology staining can detect apoptosis-associated nuclear changes, including chromatin condensation, nuclear shrinkage, nuclear fragmentation, reduced nuclear area/perimeter/axis length, and increased nuclear fluorescence intensity; these morphology readouts have been compared with apoptosis markers such as TUNEL and caspase-3 immunofluorescence.
Mitochondrial membrane potential staining measures the electrochemical polarization across the mitochondrial inner membrane in live cells using lipophilic cationic fluorescent probes; early rhodamine-based work showed that selective mitochondrial dye accumulation is lost when the mitochondrial transmembrane potential is dissipated. JC-1 reports mitochondrial polarization by shifting from green monomer fluorescence to red J-aggregate fluorescence as dye concentration increases within energized mitochondria; therefore, the red/green fluorescence ratio is used as a relative readout of mitochondrial membrane potential. TMRE or TMRM staining provides a single-channel relative readout because these cationic rhodamine esters accumulate in polarized mitochondria, and lower fluorescence indicates reduced mitochondrial polarization when acquisition and dye-loading conditions are controlled. Mitochondrial mass staining is commonly performed with MitoTracker Green FM or related MitoTracker dyes as
Lysosome and acidic-vesicle live-cell staining detects acidic intracellular compartments by using membrane-permeant acidotropic probes that accumulate in low-pH vesicles, including lysosomes, late endosomes, autolysosomes, and acidic phagosomes. LysoTracker staining is commonly used as an intensity-based readout of acidic lysosomal compartment abundance or enlargement, while acridine orange produces green fluorescence in less concentrated compartments and red fluorescence after concentration-dependent accumulation in acidic vesicular organelles. Loss or reduction of acridine-orange red signal can be used as a readout of lysosomal membrane permeabilization or reduced acidic-vesicle integrity. This protocol is designed for live cultured cells and can be adapted for fluorescence microscopy, high-content imaging, plate-reader readout, or flow cytometry when the selected literature supports the readout. Because these dyes report acidotropic accumulation rather than lysosome identity alone,
