What Happens When Cells and Tissues Are Damaged?
Cells and tissues are damaged all the time, by things as ordinary as a cut, a burn or a short break in blood supply. Mild damage can often be undone by the cell itself, but severe damage kills cells. The body then senses the harm, clears away the dead cells and rebuilds the tissue, either with new cells or with a scar.
By Chris Morais, MSc, MPhil, PhD
Published · Updated
When a cell is injured, it often runs low on energy, takes in water and swells. If the harm is mild and short, the cell can recover. If it is severe or lasts too long, the cell dies, often by bursting. Dead cells release warning signals that start inflammation, and the tissue is then rebuilt with new cells or scar tissue.
- What Is Cell and Tissue Damage?
- What Causes Cells and Tissues to Become Damaged?
- What Happens Inside a Cell When It Is Injured?
- Can a Damaged Cell Recover?
- How Do Badly Damaged Cells Die?
- How Does the Body Know Tissue Is Damaged?
- How Does the Body Repair Damaged Tissue?
- What Happens When Tissue Damage Lasts a Long Time?
- Frequently Asked Questions
What Is Cell and Tissue Damage?
Cell and tissue damage is harm to cells, or to the material around them, that stops them from working normally. Doctors and scientists often call it cell injury. In this article, "damage" and "injury" mean the same thing.
A cell is the smallest living unit of the body. Each one has an outer membrane, a nucleus that holds its DNA, and small working parts that make energy and build proteins. You can read more in What Is a Cell and How Does It Work?
A tissue is a group of similar cells that work together, such as muscle or the lining of the gut. A tissue also includes the support material between the cells and the blood vessels that feed them. Tissue damage happens when many cells are harmed at once, or when the support material or blood supply is harmed.
Damage is not all or nothing. It ranges from mild changes that a cell can fix, to severe harm that kills the cell. What happens next depends on three things: how strong the harm is, how long it lasts and what kind of cell is hit.
What Causes Cells and Tissues to Become Damaged?
Cells and tissues are damaged mainly by a lack of oxygen, physical injury, harmful chemicals, infections and attacks by the body's own immune system[1]. The main causes are:
- Lack of oxygen. This is one of the most common causes. It happens when blood flow to a tissue is blocked, as in a heart attack or a stroke, or when the blood carries too little oxygen. Without oxygen, cells cannot make enough energy to keep working.
- Physical injury. Cuts, crushing, burns and freezing all damage cells directly[1].
- Harmful chemicals. High doses of poisons and other toxins can damage cells[1]. Alcohol and some medicines can also harm cells, mainly when the amount or the length of use is too high. Doctors choose medicine doses so that the benefit is much greater than this risk.
- Infections. Some viruses burst the cells they infect[1]. Many bacteria release toxins that harm nearby cells. You can read more in How Do Bacteria Cause Harm in the Human Body?
- Immune attack. Parts of the immune system can punch holes in cells and kill them[1]. This normally helps fight germs, but it can also harm the body's own cells, as happens in autoimmune diseases.
- Oxidative stress. Cells make small amounts of reactive, unstable forms of oxygen, often called free radicals, as they use oxygen. When too many build up, they damage fats, proteins and DNA inside the cell.
- Other causes. A shortage of key nutrients, and faulty genes that make faulty proteins, can also leave cells damaged or weak.
What Happens Inside a Cell When It Is Injured?
Inside an injured cell, energy supplies often fall first, and the cell then takes in extra water and swells[1]. This swelling is one of the earliest signs of damage.
Cells run on a fuel molecule called ATP. Most ATP is made by the mitochondria, the cell's energy-making parts, and they need oxygen to do it. Pumps in the cell's outer membrane use ATP to keep the right balance of salts inside and outside the cell. When ATP falls too low, these pumps stop working properly[1]. Salt then builds up inside the cell, water follows, and the cell and its parts swell[1].
Other early changes can follow. Some cells, such as liver cells, collect small droplets of fat when they are under stress[1].
If the harm goes on, the damage spreads. The mitochondria swell and stop working, so even less energy is made[1]. Calcium, which is normally kept at very low levels inside the cell, starts to build up[1]. High calcium switches on enzymes that break down the cell's own membranes, proteins and DNA. Harmful free radicals can add to this damage. In the end, the outer membrane breaks[1].
Low oxygen is a common way this chain of events begins, and it will be explained in more detail in a separate article on hypoxia.
Can a Damaged Cell Recover?
Yes. A damaged cell can often recover if the harm is mild and the cause is removed soon enough[1]. Early swelling and fat build-up are examples of damage that can be undone[1].
When the cause goes away, the cell can return to normal. For example, if blood flow comes back, the mitochondria make ATP again. The membrane pumps restart, extra salt and water are pushed out, and the swelling goes down. The cell can also break down and replace parts that were harmed. In this way, it gets back to the steady inner balance that cells need to work, which is part of homeostasis.
Cells can also change to cope with stress that goes on for a long time. For example, a cell may grow smaller and use less energy when it receives less food or blood. This kind of change can protect the cell, as long as the stress is not too strong.
When Does Cell Damage Become Permanent?
Cell damage becomes permanent when the cell passes a "point of no return," after which it will die even if the cause is removed[1]. Damage that keeps going, such as a long shortage of oxygen, pushes a cell past this point[1].
There is no single sign that marks the exact moment. But cells that have passed it usually show severe damage to their mitochondria, a build-up of calcium and a broken outer membrane[1]. Once the membrane is broken, the cell can no longer hold its contents in.
How long it takes to reach this point depends on the type of cell. Brain cells, which use a lot of energy, can be harmed within minutes of losing oxygen. Other cells, such as those in skin and connective tissue, can survive much longer.
How Do Badly Damaged Cells Die?
Badly damaged cells die in two main ways: by necrosis, in which the cell swells and bursts, or by apoptosis, in which the cell takes itself apart in a tidy, controlled way[2].
In apoptosis, the cell shrinks and breaks into small, sealed packages. Nearby immune cells eat these packages, so the cell's contents do not leak out. Because of this, apoptosis does not set off inflammation[2]. Cells use apoptosis to remove themselves when their damage is too great to fix, and the body also uses it every day to get rid of old or unneeded cells. Apoptosis will be covered in full in a separate article.
Scientists have also found other forms of controlled cell death. Some of them look like necrosis, because the cell bursts, even though the cell itself starts the process.
What Is Necrosis?
Necrosis is the death of cells caused by injury, in which the cells swell, burst and spill their contents into the tissue around them[1],[2]. It is the main way cells die after severe harm such as a lack of oxygen, extreme heat or cold, high doses of toxins, physical injury or some virus infections[1].
Necrosis usually affects a group of cells at once, not just one cell. As each cell swells and breaks open, its working parts spill out[1]. The nucleus slowly fades away as its DNA breaks down[1]. The spilled contents start inflammation, and white blood cells called neutrophils are drawn to the area[1]. This inflammation helps clear the dead tissue, but it can also harm nearby healthy cells.
Dead tissue does not always look the same. The table below shows the main patterns of necrosis that doctors see[1].
| Type of necrosis | What the dead tissue is like | Where it is often seen |
|---|---|---|
| Coagulative | Firm; the outlines of dead cells stay for a while | Heart muscle after a heart attack |
| Liquefactive | Turns soft and liquid as white blood cells digest it | The brain; infected dead tissue |
| Caseous | Soft, crumbly and cheese-like | Tuberculosis and similar infections |
| Fat | Fatty tissue broken down by fat-digesting enzymes | An inflamed pancreas; injured fatty tissue |
| Gangrene | A large area of dead tissue, either dry or wet | Toes, feet or limbs with poor blood supply |
Over time, immune cells called macrophages move in and clear away the dead tissue[1]. The area is then rebuilt, as explained below.
How Does the Body Know Tissue Is Damaged?
The body knows tissue is damaged because injured and dying cells release warning signals that immune cells can detect[3]. These signals are often called danger signals. Their scientific name is damage-associated molecular patterns, or DAMPs.
Danger signals are usually normal molecules that stay hidden inside healthy cells. Examples include ATP, DNA, a protein called HMGB1 that normally sits in the nucleus, uric acid and heat shock proteins[3]. When a cell bursts, these molecules escape into the space around it. Immune cells such as macrophages carry special sensors, called receptors, that recognize these molecules[3]. Once the sensors are switched on, the immune cells release chemical messengers that start inflammation[3]. You can read more about these immune cells in How Does the Immune System Work?
Damaged cells and mast cells, a type of immune cell found in tissues, also release chemicals such as histamine. These chemicals widen nearby blood vessels and make them leaky[2]. Fluid and white blood cells then move into the damaged area[2]. This is why damaged tissue often becomes red, warm, swollen and painful[2]. These signs of inflammation will be explained in full in a separate article.
How Does the Body Repair Damaged Tissue?
The body repairs damaged tissue in one of two ways: by regeneration, which replaces lost cells with new cells of the same type, or by forming a scar[2],[4].
Repair follows a general order. First, bleeding is stopped by a clot[4]. Next, white blood cells clear away dead cells, debris and any germs[2]. New small blood vessels then grow into the area, and cells called fibroblasts move in[2],[4]. Together, they form a soft, healing tissue called granulation tissue[2],[4]. The fibroblasts lay down collagen and other support material, which slowly replaces the granulation tissue[4]. Over the following weeks and months, this new tissue matures and gets stronger. Wound healing will be explained step by step in a separate article.
Whether a tissue regrows or scars depends mostly on whether its cells can divide[4]:
- Cells that divide all the time. Skin cells and the cells lining the gut are always being replaced, so these tissues regrow well[4].
- Cells that divide when needed. Liver cells and some kidney cells do not usually divide, but they can start dividing after injury[4]. The liver is very good at regrowing lost tissue[4].
- Cells that cannot divide. Some cells cannot divide, even after injury[4]. When they are lost, the gap is filled with scar tissue, and the tissue may not work as well as before[4].
Regeneration also works best when the injury is small and the tissue's support framework is still in place. Large or deep injuries are more likely to heal with a scar. Why some tissues regrow while others scar will be covered in a separate article.
What Happens When Tissue Damage Lasts a Long Time?
When tissue damage lasts a long time or keeps coming back, inflammation does not settle, and the body keeps laying down scar-like material in a process called fibrosis[2],[5].
Short-lived inflammation usually settles once the tissue has healed[2]. When the harm goes on, inflammation becomes long-lasting and can lead to disease[2]. Common examples of long-lasting harm include heavy alcohol use over many years, which can damage the liver, and long-term high blood sugar, which can damage the kidneys.
In fibrosis, too much support material, mainly collagen, builds up in the tissue[5]. It is made mostly by cells called myofibroblasts, which are fibroblasts that have taken on some features of muscle cells[5]. Over time, this material replaces working cells and makes the tissue stiff[5]. Fibrosis can happen in almost any solid organ[5]. When it is severe, it can lead to failure of the liver, lungs, kidneys or heart[5]. Severe fibrosis has been estimated to account for up to 45% of deaths in developed countries[5].
Scars that form after injury can be remodeled, and some fibrosis can lessen if the cause is removed early[4]. But when damage keeps going for years, the scarring can become permanent[4]. Fibrosis will be covered in full in a separate article.
Frequently Asked Questions
What is gangrene? Gangrene is the death of a large area of tissue, often in a toe, foot or limb. Dry gangrene happens when the blood supply is slowly blocked, as can happen with blood vessel disease linked to diabetes[1]. The tissue becomes dry, shrunken and dark. Wet gangrene happens when bacteria infect the dead tissue, which becomes soft and liquid[1]. Wet gangrene can spread quickly and needs urgent medical care.
Can dead tissue come back to life? No. Once cells have died, they cannot be brought back to life. Immune cells clear away the dead tissue, and it is then replaced by new cells or by scar tissue[1],[4]. Doctors sometimes remove dead tissue during treatment, because it can slow healing and allow infection.
Can heart muscle and brain cells repair themselves? Only to a very small extent. Heart muscle cells and nerve cells in the brain cannot divide to replace themselves, even after injury[4]. When many of these cells die, for example after a heart attack or a stroke, the lost cells are replaced by scar tissue, and some function may be lost[4]. This is why fast treatment to restore blood flow is so important.
Does inflammation always mean there is an infection? No. Danger signals from damaged cells can start inflammation even when no germs are present. This is called sterile inflammation[3]. It happens after injuries such as bruises and burns. When sterile inflammation lasts a long time, it is linked to diseases such as rheumatoid arthritis, atherosclerosis and some brain diseases[3].
Is DNA damage the same as cell damage? DNA damage is one kind of cell damage. Cells check their DNA all the time and repair most of the damage they find. If the damage is too great, the cell may destroy itself by apoptosis. If damaged DNA is not repaired and the cell keeps dividing, the changes can be passed on to new cells and may help lead to cancer. You can read more in How Do Genetic Changes Turn a Normal Cell into Cancer?
Morais C. What Happens When Cells and Tissues Are Damaged? [Internet]. BiologyWithin.com; 2026 Jul 30 [updated 2026 Oct 18]. Available from: https://www.biologywithin.com/2026/07/what-happens-when-cells-and-tissues-are-damaged.html
- Kendall T. Necrosis. Pathologia. The University of Edinburgh.
- Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, Korol O, Johnson JE, Womble M, DeSaix P. 4.6 Tissue Injury and Aging. Anatomy and Physiology 2e. OpenStax. 2022.
- Roh JS, Sohn DH. Damage-Associated Molecular Patterns in Inflammatory Diseases. Immune Netw. 2018;18(4):e27.
- Kendall T. Wound healing – regeneration and scarring. Pathologia. The University of Edinburgh.
- Jun JI, Lau LF. Resolution of Organ Fibrosis. J Clin Invest. 2018;128(1):97-107.
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