Cell viability is a key concept in biological research that assesses cell health and responsiveness. It assesses the proportion of living cells in a population, providing insights into cellular well-being. This measurement is important across various scientific disciplines, including drug discovery, toxicology, and regenerative medicine. It provides valuable information on how cells survive and thrive under different conditions. Measuring cell viability is a basic step in many experiments, ensuring reliable findings and guiding further investigation.
What is Cell Viability
Cell viability refers to the percentage of living cells within a given population. Viable cells are intact and retain their ability to perform normal functions. It is distinct from cell proliferation (cell multiplication) and cell death (irreversible loss of function and structural integrity).
It is important in drug discovery, assessing how potential therapeutic agents affect cells. In toxicology, it determines harmful effects of chemicals and contaminants. Researchers use viability assays to understand cellular responses to treatments or environmental factors, ensuring cell culture health. Knowing cell viability is often a preliminary step before complex cell-based assays. This ensures accurate and reliable experimental results, as dead cells can interfere with subsequent analyses.
Calculating Viability with Trypan Blue Exclusion
Trypan blue exclusion is a common method for assessing cell viability, relying on cell membrane integrity. Live cells with intact membranes exclude the dye, while damaged membranes allow it to enter, staining the cell blue. This differential staining distinguishes viable from non-viable cells.
To perform this assay, a cell suspension is mixed with trypan blue dye. After incubation, a sample is loaded onto a hemocytometer. This specialized counting chamber allows for microscopic visualization and counting of cells within defined squares. Researchers manually count unstained (viable) and blue-stained (non-viable) cells.
The percentage of viable cells is calculated using a formula: viable cells divided by total cells (viable + non-viable), multiplied by 100. For example, 80 unstained and 20 stained cells yield (80 / (80 + 20)) 100 = 80% viability. This method provides a direct and quick measure of cell membrane integrity, offering a snapshot of cell health.
Calculating Viability with Metabolic Assays
Metabolic assays assess cell viability by measuring the metabolic activity of living cells. Only metabolically active cells convert specific substrates into a detectable product. Common examples like MTT, XTT, and Resazurin assays rely on enzymes in viable cells, often mitochondrial, to transform compounds into colored or fluorescent products.
In an MTT assay, mitochondrial dehydrogenases in living cells reduce yellow MTT salt into purple formazan crystals. These crystals are dissolved, and the purple color intensity is measured spectrophotometrically. The amount of color produced is directly proportional to metabolically active cells; higher absorbance indicates more living cells.
Relative viability is calculated by comparing treated cell absorbance to untreated control cells. This determines the percentage of viability relative to a healthy, unmanipulated cell population. These assays are preferred for convenience and high-throughput screening, providing a quantitative measure of cell health based on metabolic function.
Interpreting Your Cell Viability Results
Interpreting cell viability results involves understanding what the calculated percentages signify. A high viability percentage (typically above 90%) indicates a healthy cell population suitable for further experimentation or application. A lower percentage suggests a significant proportion of non-viable cells, which could impact experimental outcomes. For instance, 50% viability after drug treatment implies half the cells were adversely affected.
Controls are important for accurate interpretation. A positive control (e.g., cells treated with a known cytotoxic agent) confirms assay function by demonstrating expected cell death. A negative control (untreated cells) establishes a 100% viability baseline. Comparing samples to these controls provides context for observed viability changes.
Several factors influence viability measurements, including cell type, experiment duration, and overall experimental conditions. For example, a cell line’s sensitivity to a treatment can lead to different viability outcomes. Cell viability is presented as a percentage, reflecting the proportion of living cells in a sample and providing information about cell health and response.