Cell damage can cause death of individual cells, and ultimately to tissue or organ failure and even death of the organism. Harmful molecules are continually bombarding your body. The worst offenders are glucose (a type of sugar), and free radicals, by-products of energy production. Both can damage the proteins, fats and DNA that make up your cells. When you are young, your body is able to repair most of this damage. But as you get older, the repair process is less efficient. Some researchers think that boosting our natural defences against these chemical onslaughts may lessen the effects of ageing.
- Physical agents such as heat or radiation can damage a cell by literally cooking or coagulating their contents.
- Impaired nutrient supply, such as lack of oxygen or glucose, or the production of adenosine triphosphate (ATP) may deprive the cell of essential materials needed to survive.
- DNA damage: In human cells, both normal metabolic activities and environmental factors such as ultraviolet light and other radiations can cause DNA damage, resulting in as many as one million individual molecular lesions per cell per day.
- Membrane damage: damage to the cell membrane disturbs the state of cell electrolytes, e.g. calcium, which when constantly increased, induces apoptosis.
Types of damage 
Some cell damage can be reversed once the stress is removed or if compensatory cellular changes occur. Full function may return to cells but in some cases a degree of injury will remain.
Cellular swelling 
Cellular swelling may occur due to cellular hypoxia, which damages the sodium-potassium membrane pump; it is reversible when the cause is eliminated. Cellular swelling is the first manifestation of almost all forms of injury to cells. When it affects many cells in an organ, it causes some pallor, increased turgor, and increase in weight of the organ. On microscopic examination, small clear vacuoles may be seen within the cytoplasm; these represent distended and pinched-off segments of the endoplasmic reticulum. This pattern of nonlethal injury is sometimes called hydropic change or vacuolar degeneration. The ultrastructural changes of reversible cell injury include: • Blebbing • Blunting • distortion of microvilli • creation of myelin figures • loosening of intercellular attachments • mitochondrial changes • dilation of the endoplasmic reticulum
Fatty change 
Cell has been damaged and is unable to adequately metabolize fat. Small vacuoles of fat accumulate and become dispersed within cytoplasm.When mild, fatty change may have no effect on cell function but more severe fatty change may impair cellular function. In the liver, the enlargement of hepatocytes due to fatty change may compress adjacent bile canaliculi, leading to cholestasis. Depending on the cause and severity of the lipid accumulation, fatty change is generally reversible.
Progressive failure of essential metabolic and structural cell components usually in the cytoplasm. Necrosis generally involves a group of contiguous cells or occurs at the tissue level. Such progressive deterioration of the cell rapidly leads to cell death.
Process of self-destruction of the cell nucleus. It is not contiguous, but instead the dying cells are scattered throughout the tissue. In apoptosis the cells shrink from a decrease of cytosol and the nucleus, the organelles appear normal. The cell disintegrates into fragments referred to as apoptotic bodies. Apoptosis happens to everybody, in the average adult between 50 and 70 billion cells die each day due to apoptosis. Inhibition of apoptosis can result in a number of cancers, autoimmune diseases, inflammatory diseases, and viral infections. Hyperactive apoptosis can lead to neurodegenerative diseases, hematologic diseases, and tissue damage.
When a cell is damaged the body will try to repair or replace the cell to continue normal functions. If a cell dies the body will remove it and replace it with another functioning cell, or fill the gap with connective tissue to provide structural support for the remaining cells. The goal of the repair process is to fill the gap caused by the damaged cells to regain structural continuity. Normal cells try to regenerate the damaged cells but this cannot always happen.
Regeneration of parenchyma cells, or the functional cells, of an organism. The body can make more cells to replace the damaged cells keeping the organ or tissue intact and fully functional.
When a cell cannot be regenerated the body will replace it with stromal connective tissue to maintain tissue/organ function. Stromal cells are the cells that support the parenchymal cells in any organ. Fibroblasts, immune cells, pericytes, and inflammatory cells are the most common types of stromal cells.
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