Cell death is a natural, tightly regulated process that removes damaged, infected, or unnecessary cells and helps maintain tissue homeostasis.

In our previous articles, we covered how cells grow and divide—starting with the cell cycle [article link] and then moving to mitosis and cytokinesis [article link], where cells separate their genetic material and complete division. When these steps fail, cells can undergo regulated cell death to prevent abnormal cells from persisting.

In this article, we provide a clear overview of major cell death pathways and show how live-cell imaging can track cytotoxicity in real time, capturing morphological changes as well as the timing and extent of cell death.

Table of Contents

1. Overview of Cell Death

2. Types of Regulated Cell Death

3. Methods for Measuring Cytotoxicity

4. Case study: Live-Cell Imaging of Nocodazole-Induced Cytotoxicity

1. Overview of Cell Death

Cell death can be broadly classified into two major forms: accidental cell death (ACD) and regulated cell death (RCD). These two forms differ in how they are triggered, how rapidly they progress, and how much cellular control is involved.

(1) Accidental Cell Death (ACD)

ACD is an uncontrolled process that occurs when cells are exposed to sudden and severe physical, chemical, or mechanical stress. Because the damage is sudden and overwhelming, ACD generally occurs rapidly and does not rely on regulated signaling pathways that control its timing or execution.

(2) Regulated Cell Death (RCD)

RCD is a controlled process in which cells are directed to die through specific molecular pathways. These pathways determine when and how cell death occurs and can be modulated by intracellular or extracellular signals. Unlike ACD, RCD enables damaged or unwanted cells to be eliminated in a more selective and controlled manner, and it can be influenced by genetic or pharmacological interventions.

2. Regulated Cell Death Types

Regulated cell death (RCD) comprises several distinct pathways, each defined by characteristic molecular and morphological features. Among these, five major types are commonly discussed in cell biology.

(1)  Apoptosis: A caspase-driven, programmed cell death marked by cell shrinkage, membrane blebbing, and apoptotic body formation. Common readouts include caspase-3/7 activation and PARP cleavage.

(2)  Necroptosis: A regulated, lytic cell death pathway often triggered when apoptosis is blocked, characterized by cell swelling and membrane rupture. A key marker is MLKL activation.

(3)  Pyroptosis: An inflammasome-related, inflammatory cell death caused by gasdermin pore formation, leading to rapid swelling and membrane rupture. Typical readouts include gasdermin cleavage and IL-1β release.

(4)  Ferroptosis: An iron-dependent cell death driven by lipid peroxidation and lipid ROS accumulation, sometimes accompanied by mitochondrial shrinkage. It is commonly monitored using lipid peroxidation/ROS assays and ferroptosis modulators.

(5)  Autophagy-dependent cell death: A regulated cell death that requires autophagy machinery and may show increased autophagic vacuoles. LC3-II and p62 changes can support interpretation, but should be considered alongside autophagic flux.

These regulated cell death pathways differ in their triggers, morphological changes, and typical markers, as summarized in the table below. 

While these pathways have distinct features, they can overlap in practice. Accurate assessment of cell death therefore requires appropriate measurement methods, as discussed in the following section.

3. Methods for Measuring Cell Death & Cytotoxicity

Accurately assessing cell death is essential in research areas such as drug testing, toxicity studies, and cell biology. Rather than relying on a single indicator, cytotoxicity is evaluated by monitoring cellular changes associated with loss of viability and cell death. Based on the type of change being measured, cytotoxicity assays can be broadly grouped into three main categories.

(1) Assessing membrane integrity
: One of the most direct ways to identify dead or dying cells is to evaluate plasma membrane integrity. In healthy cells, the plasma membrane remains intact and prevents membrane-impermeant dyes from entering the cell. When membrane integrity is compromised, fluorescent DNA-binding dyes enter the membrane and bind to DNA, producing a detectable fluorescent signal indicative of cell death. PI, EthD-1, and CellTox™ Green dye are commonly used to detect loss of membrane integrity. The representative image below shows HEK293 (GFP-tagged) cells stained with EthD-1.

▶ Watch the time-lapse video on YouTube

(2) Detecting enzymatic activity
: Another approach to evaluating cytotoxicity is to use enzyme activity–based readouts. Some dyes report cell viability by fluorescing only when intracellular enzymes remain active, with signals decreasing as cells lose viability. Other enzyme-based probes detect protease activity (e.g., caspases or granzymes) and become fluorescent when these pathways are activated, enabling early detection of cells undergoing cell death. Common dyes for enzymatic activity assays include Calcein-AM, CellTracker™ Green CMFDA, and CellEvent™ Caspase-3/7 detection dye. The example below shows HeLa cells stained with CellEvent™ Caspase-3/7 detection dye.

▶ Watch the time-lapse video on YouTube

(3) Measuring metabolic activity
: Cytotoxicity can also be assessed by measuring metabolic activity, which reflects overall cell function and viability. Metabolically active cells maintain ATP levels or convert substrates into detectable products, whereas decreased metabolic signals indicate reduced viability and declining cell health. Common dyes used to assess metabolic activity include JC-10 and BioTracker ATP-red live cell dye. The example below shows HeLa cells stained with JC-10 under control and CCCP-treated conditions.

▶ Read the application note on mitochondrial membrane potential analysis

Choosing the right assay is essential because each method captures a different aspect of cytotoxicity—such as cell death, cell viability, or overall cell health. Using an appropriate method ensures accurate interpretation of cellular responses. To illustrate how these approaches are applied in practice, the following section presents live-cell imaging–based examples of drug-induced cell death.

4. Observation Example
: Nocodazole-induced Cell Death

Live-cell imaging enables continuous monitoring of cellular responses to drug-induced cytotoxicity, capturing both morphological changes and cell death signals within the same experiment. This approach allows cytotoxic effects to be tracked over time rather than relying on a single endpoint measurement.

The table below summarizes the observation conditions used to monitor nocodazole-induced cytotoxicity using live-cell imaging.

Time-lapse imaging revealed clear, concentration- and time-dependent cytotoxic responses during nocodazole treatment. Brightfield images showed a progressive decline in cell confluency, while fluorescence imaging displayed increasing dead-cell signals, consistent with ongoing cell death.

To explore these changes in more detail, the corresponding Celloger® Mini Plus time-lapse video is available at the link below.
▶ Watch the time-lapse video on Youtube

Following these visual observations, the cytotoxic effects of nocodazole were further evaluated using quantitative analysis. Cell death was quantified over time using Celloger analysis software by calculating the ratio of fluorescent signal coverage (dead cells) to brightfield confluency (total cells).

The results show a clear concentration- and time-dependent increase in cytotoxicity. These quantitative findings align with time-lapse observations, showing reduced cell confluency and increased fluorescent dead cell signals over time.

By integrating real-time imaging with fluorescence-based quantification, this approach enables reliable assessment of the timing and progression of drug-induced cell death.

▶ Learn more about Celloger® Mini Plus:

In this article, we explored how cell death pathways can be triggered when normal cellular processes fail. We also showed how these dynamic changes can be observed over time using live-cell imaging. Rather than relying on static endpoint measurements, this approach enables continuous monitoring of cellular responses under experimental conditions. To learn more about live-cell imaging systems and their applications in cell research, visit the Curiosis website.