Cells undergo mitosis for a handful of interconnected reasons, all of which boil down to the body’s need to make more of itself: growing larger during development, replacing cells that wear out or get damaged, and mounting immune defenses against infection. These are not separate programs running in isolation. The same molecular machinery drives division in a healing skin wound, a growing embryo, and a gut lining that replaces itself every few days, though the signals that flip the switch differ in each case. What makes mitosis interesting is not just why it happens, but how the body decides when, where, and how fast it should happen, and what goes wrong when those decisions break down.
Building a Body From a Single Cell
The most obvious reason for mitosis is growth. A fertilized egg is one cell. An adult human body contains roughly 37 trillion. Every one of those cells traces back to successive rounds of mitotic division, each producing two daughter cells with the same genetic information as the parent. During embryonic and fetal development, mitosis runs at an extraordinary pace, generating the sheer cell numbers needed to form tissues and organs. That pace slows after birth in most tissues, but it never truly stops: children and adolescents continue dividing cells to lengthen bones, expand organs, and add muscle mass until growth plates close and adult size is reached.
Growth-related mitosis is not just about getting bigger. It also depends on cells dividing in the right orientation. Research on wound repair and tissue formation has shown that the direction in which a cell divides matters for how tissues take shape, and that this orientation is guided by a mix of internal and external cues including cell geometry and polarity. When the orientation of division goes awry, it can lead to abnormal tissue development.
1Europe PMC. Oriented cell division: new roles in guiding skin wound repair and regenerationReplacing Cells That Wear Out
Your body is constantly losing cells. Skin cells slough off. The lining of your gut faces a harsh chemical environment and turns over rapidly. Red blood cells circulate for about 120 days before being broken down and replaced. Mitosis is the process that keeps these populations topped up. Without it, tissues that experience heavy wear would simply erode away.
The replacement rate varies enormously by tissue. Cells lining the small intestine are replaced roughly every three to five days. Skin cells cycle over a few weeks. Liver cells, by contrast, are remarkably long-lived and typically sit quietly in a non-dividing state. But the liver has a trick up its sleeve: when it loses functional mass due to injury, hepatocytes that are normally quiescent re-enter the cell cycle and proliferate until the organ is restored.
2PubMed Central. Regulation of hepatocyte identity and quiescenceThis liver regeneration is unusual in another way. Most organs that regenerate rely on stem cells, undifferentiated reserves that can divide and produce specialized daughter cells. The liver largely skips that step. Its already-differentiated hepatocytes do the dividing themselves, and many of them are polyploid, meaning they carry extra copies of the genome, yet still manage to execute proper cell division.
3PubMed Central. Breaking the rules of cell biology: Lessons from the liver’s exceptional regenerative capacityWound Healing and Tissue Repair
When you cut your finger or break a bone, mitosis ramps up locally to close the gap. A wound triggers a cascade of chemical signals, including growth factors released by platelets and immune cells, that tell nearby cells to start dividing. This is not just a matter of filling space with more cells. The divisions have to be coordinated in timing and orientation to rebuild structured tissue rather than producing a disorganized mass. The direction of cell division during wound repair is regulated by a combination of intrinsic cellular cues and signals from the surrounding environment, and errors in this coordination can impair healing or produce abnormal scar tissue.
1Europe PMC. Oriented cell division: new roles in guiding skin wound repair and regenerationRepair-driven mitosis is typically self-limiting. Once the wound closes and cell density returns to normal, division slows and stops. Part of this control comes from contact inhibition: when cells crowd together at sufficient density, physical contact between neighbors sends signals that suppress further proliferation.
4PubMed. Density-dependent regulation of cell growth: an example of a cell-cell recognition phenomenonFighting Infection
Your immune system uses mitosis as a weapon. When a virus or bacterium invades, a small number of immune cells that recognize the pathogen need to multiply rapidly to mount an effective defense. This is called clonal expansion, and it represents one of the most dramatic bursts of mitosis in the body. During a viral infection, the specific immune cells that recognize the virus can expand massively in number. Studies of lymphocytic choriomeningitis virus infection in mice showed that CD8+ T cells, a type of immune cell that kills infected cells, expanded greatly, and most of that expansion was specific to the virus itself.
5PubMed Central. Massive expansion of antigen-specific CD8+ T cells during an acute virus infectionAfter the infection clears, most of those newly minted immune cells die off through programmed cell death, leaving behind a smaller population of memory cells ready to expand again if the same pathogen returns. This cycle of rapid division followed by culling is one of the body’s most tightly controlled uses of mitosis, and it happens on a timeline of days rather than the weeks or months involved in tissue turnover.
How Stem Cells Use Mitosis Differently
Not every mitotic division produces two identical daughter cells. Stem cells often divide asymmetrically, generating one daughter that remains a stem cell and one that is committed to becoming a specialized cell type. This balance between self-renewal and differentiation is essential for maintaining tissues over a lifetime.
6PubMed Central. Cellular and molecular mechanisms of asymmetric stem cell division in tissue homeostasisStem cells achieve this asymmetry in two main ways. They can distribute cell-fate determinants, proteins that push a cell toward specialization, unevenly so that only one daughter inherits them. Or they can orient the plane of division so that one daughter stays in contact with the stem cell niche, the local environment that maintains stem cell identity, while the other daughter is pushed away and begins to differentiate.
7Cell. Mechanisms of Asymmetric Cell DivisionThis means mitosis in stem cells is not simply about making more cells. It is about making the right kinds of cells in the right proportions, a balancing act that, when disrupted, can lead to tissue degeneration on one hand or uncontrolled growth on the other.
What Tells a Cell to Divide
Cells do not divide spontaneously. They require permission, delivered in the form of chemical signals called mitogens, growth factors that bind to receptors on the cell surface and trigger internal signaling cascades. Research over several decades has shown that the strength and duration of these signals matter enormously: short, strong bursts of mitogenic signaling tend to push cells toward proliferation, while weak, sustained signaling is more often associated with differentiation, the process of becoming a specialized cell type. If signals are too strong and last too long, cells can enter a state of permanent arrest called senescence rather than continuing to divide.
8Science Advances. Mitogen signaling strength and duration can control cell cycle decisionsGrowth factor signals are not the whole picture. A cell also checks its internal nutrient and energy status before committing to division. Producing a complete copy of the genome and all the structural components for a new cell is metabolically expensive. The nutrient-sensing pathway centered on a protein complex called mTOR integrates information about energy availability, amino acid levels, and growth factor stimulation to coordinate whether a cell grows and divides or stays put.
9PubMed Central. Cell cycle regulation by the nutrient-sensing mammalian target of rapamycin (mTOR) pathwayEven after a cell commits to dividing, it passes through a series of internal checkpoints that verify conditions are safe to proceed. These checkpoints are driven by enzymes called cyclin-dependent kinases, which phosphorylate key proteins to push the cycle forward.
10PubMed Central. Cell Cycle Regulation by CheckpointsOne particularly important checkpoint occurs during mitosis itself: the spindle assembly checkpoint, which prevents the cell from splitting until every chromosome is properly attached to the structure that will pull the chromosomes apart. Defects in this checkpoint lead to uneven distribution of chromosomes between daughter cells, a hallmark of cancer cells.
11PubMed Central. Monitoring the fidelity of mitotic chromosome segregation by the spindle assembly checkpointPhysical Constraints on Cell Size
There is a less obvious reason cells divide that has nothing to do with external signals: geometry. All cells face a fundamental physical constraint related to their surface area relative to their volume. As a cell grows larger, its volume increases faster than its surface area, which would eventually limit its ability to take in nutrients and export waste through its membrane. One way cells solve this problem is by dividing before they get too large.
Interestingly, growing mammalian cells seem to have evolved a workaround for this constraint during the growth phase between divisions. Research has found that proliferating cells maintain a nearly constant ratio of surface area to volume as they grow, enabled by increased folding of the plasma membrane in larger cells. Electron microscopy confirmed that bigger cells have more extensively folded membranes, keeping effective surface area high enough to support functions like nutrient uptake, signaling, and eventually division itself.
12Europe PMC. Plasma membrane folding enables constant surface area-to-volume ratio in growing mammalian cellsCells That Refuse to Divide
Understanding why cells undergo mitosis also requires understanding when they do not. Some of the body’s most important cells exit the cell cycle permanently and never divide again under normal circumstances. The clearest example is the heart. Cardiac muscle cells proliferate rapidly during fetal development but exit the cell cycle shortly after birth in mammals, entering a state of permanent arrest called terminal differentiation.
13PubMed Central. Cardiac myocyte cell cycle control in development, disease, and regenerationThis is why heart attacks are so damaging: the dead muscle cells are replaced by scar tissue rather than by new heart cells. Neurons in most of the adult brain similarly do not divide, though limited neurogenesis occurs in a few specific regions. The existence of non-dividing cells highlights that mitosis is not a universal default state. It is an active decision, and some cells are locked out of it by design, presumably because the risks of dividing, such as introducing errors during DNA replication, outweigh the benefits for tissues whose function depends on stability and long-lived connections.
Some tissues take a middle path: instead of dividing, cells replicate their DNA without actually splitting in two, a process called endoreplication. The result is polyploid cells, larger cells with multiple copies of the genome. This is a normal part of development in tissues like the placenta and occurs in parts of the liver and other organs. Endoreplication can achieve some of the goals of mitosis, such as increasing a cell’s capacity to produce proteins, without the structural disruption of actually dividing.
14PubMed Central. Endoreplication: The Good, the Bad, and the UglyHow Cells Remember Their Identity Through Division
When a cell divides, it faces a problem: during mitosis, the chromosomes condense tightly and most gene activity shuts down. How does a skin cell’s daughter “know” it should behave like a skin cell, and not a liver cell, when the transcription machinery is largely offline? The answer involves a phenomenon called mitotic bookmarking. Certain transcription factors and chemical marks on chromosomes are retained even during the condensed mitotic state, effectively tagging genes that need to be switched back on as soon as division is complete.
Research has identified three types of molecular bookmarks that work together to preserve cell identity through division: transcription factors that maintain lineage commitment stay attached to mitotic chromosomes, specific chemical modifications to histone proteins keep the DNA accessible for reactivation, and components of the gene-reading machinery remain positioned at genes that need to turn on quickly after division.
15PubMed Central. Mitotic Gene Bookmarking: An Epigenetic Program to Maintain Normal and Cancer PhenotypesThis bookmarking is particularly critical in stem cells, which cycle rapidly and constantly balance self-renewal with differentiation. Work on pluripotent stem cells has shown that master regulators of “stemness,” along with specific histone marks, remain associated with the enhancers of stem cell genes during mitosis, promoting faithful transmission of stem cell identity to daughter cells.
16Cell Reports. Widespread Mitotic Bookmarking by Histone Marks and Transcription Factors in Pluripotent Stem CellsOther transcription factors involved in specialized functions, such as the vitamin D receptor, have also been confirmed as mitotic bookmarking factors, suggesting that this mechanism operates broadly across cell types to ensure daughters inherit the correct functional program.
17PubMed. Mitotic genome bookmarking by nuclear receptor VDR advocates transmission of cellular transcriptional memory to progeny cellsWhen the Body’s Clock Decides It Is Time to Divide
Mitosis does not happen randomly throughout the day. In many tissues, cell division follows a circadian rhythm, peaking at certain times and dropping at others. The body’s internal clock, which governs roughly 24-hour cycles in hormone release, metabolism, and sleep, also directly regulates when cells enter mitosis. Disruption of this circadian clock machinery has been linked to cancer, suggesting that clock-controlled timing of cell division is not just a curiosity but an essential layer of regulation.
18PubMed Central. The circadian clock and cell cycle: interconnected biological circuitsThe mechanism has been studied in detail in the regenerating mouse liver, where the circadian clock controls expression of genes that regulate the activity of a key enzyme needed for entering mitosis. One of these genes, wee1, was found to be directly regulated by molecular components of the circadian clockwork, creating a one-way link from the biological clock to the cell division cycle.
19PubMed. Control mechanism of the circadian clock for timing of cell division in vivoCircadian gating of mitosis has also been found in the adult brain. In the hippocampus, a brain region involved in memory, quiescent neural precursor cells proliferate in a rhythmic fashion. The clock protein PERIOD2 acts as a gate on cell-cycle entry: when it is absent, precursor cells enter the cycle without normal restraint. Loss of another clock protein, BMAL1, results in abnormally high proliferation and delayed exit from the cell cycle.
20PubMed. The circadian molecular clock regulates adult hippocampal neurogenesis by controlling the timing of cell-cycle entry and exitThe circadian regulation of mitosis has practical implications that researchers are still working out. If cell division in tumors also follows circadian patterns, then the timing of chemotherapy, many forms of which target dividing cells, could matter for how effective treatment is. This idea, called chronotherapy, remains an active area of investigation, though its translation into standard clinical practice has been slow. For healthy tissues, the clock’s influence on mitosis is one more reminder that cell division is not just a response to damage or growth signals but a process embedded in the body’s broader rhythmic biology.