Cell viability is a fundamental measurement in biology and medicine, defined as the percentage of live, functional cells within a total cell population. This metric provides a snapshot of the overall health and responsiveness of cells in a given environment. Scientists use this quantification to determine a cell culture’s ability to survive and carry out its intended biological functions. The proportion of cells that are alive is typically expressed as a percentage.
Defining Cellular Health
A cell’s health is determined by several measurable physiological properties that indicate its ability to sustain life and respond to its environment. One basic indicator is the integrity of the cell membrane, which acts as a selective barrier controlling what enters and leaves the cell. A healthy cell actively maintains this barrier, while a compromised or dying cell loses its ability to regulate molecular traffic.
Cellular health also depends heavily on metabolic activity, the complex network of chemical reactions that produce energy. Specifically, the activity of enzymes within the mitochondria, the cell’s powerhouses, serves as a reliable marker of viability. Cells that are metabolically active are producing the adenosine triphosphate (ATP) required for cellular processes, including growth and maintenance. The ability to replicate, or proliferate, is another sign of a robust cell population.
When cells are damaged or stressed, they generally follow one of two distinct pathways toward death. The first is necrosis, an uncontrolled and accidental form of cell death occurring due to severe injury, such as physical trauma or toxic exposure. Necrotic cells rapidly swell and rupture, releasing their internal contents and triggering an inflammatory response. This process is characterized by an early loss of plasma membrane integrity.
The second pathway is apoptosis, a highly regulated, genetically programmed process often described as cellular suicide. Apoptosis involves the orderly dismantling of the cell, where the cell shrinks and its internal components are packaged into small membrane-bound vesicles called apoptotic bodies. Because the plasma membrane remains intact throughout the initial stages, this type of cell death does not cause inflammation.
Common Measurement Techniques
The most common methods for quantifying cell viability rely on assessing the physiological properties that distinguish live cells from dead or dying cells. One widely used approach targets the integrity of the cell membrane through dye exclusion assays. The principle behind these assays, such as the Trypan Blue exclusion method, is that the dye cannot pass through the intact membrane of a live cell.
In a sample, viable cells actively exclude the Trypan Blue dye and remain clear. Cells with compromised membranes, characteristic of necrosis, absorb the dye and appear blue when viewed under a microscope. This method provides a direct count of cells that have lost their membrane barrier. However, because cells undergoing early-stage apoptosis still have an intact membrane, they are often incorrectly counted as viable.
Other assays focus on metabolic activity, providing a more functional measure of cellular health. Tetrazolium reduction assays, including the popular MTT and XTT methods, are based on the cell’s ability to reduce a chemical compound. These assays use a yellow tetrazolium salt taken up by the cells.
Viable cells contain active mitochondrial enzymes, specifically dehydrogenases, which convert the yellow tetrazolium compound into a purple, insoluble formazan product. The accumulation of this colored product is directly proportional to the number of metabolically active cells. By measuring the intensity of the purple color using a plate reader, researchers quantify the metabolic capacity of the population.
Why Cell Viability Matters
Measuring cell viability is a necessity that underpins numerous applications across biological research and industry. In drug discovery and development, viability assays are routinely used to test the safety and efficacy of new therapeutic compounds. Researchers treat cell lines with varying concentrations of a potential drug to determine the dose that kills target cells without causing unacceptable toxicity to healthy cells.
Cell viability is also used in toxicology screening to assess the potential hazards of environmental chemicals, pollutants, or cosmetics. By exposing human or animal cells to these substances, scientists establish safe exposure levels and understand the mechanisms by which a substance causes harm. This data is used by regulatory bodies to protect public health and inform safety guidelines.
In biological manufacturing, particularly for cell-based therapies and regenerative medicine, viability measurements are a fundamental quality control step. When manufacturing therapeutic cells for transplantation, it is necessary to confirm that the final product contains a high percentage of viable, functional cells before administration. Ensuring the health of the cell population at every stage, from initial culture to final cryopreservation, is important for product potency and patient safety.