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Cell regeneration is a biological feature of all living organisms from bacteria to plants and amphibians to mammals. It is the act of renewal, growth, or restoration of cells involved in maturation, wound healing, tissue repair, and similar biological functions. Cellular regeneration in its most extreme form is what allows starfish, flatworms, and lizards to regrow broken limbs, tails, or in the case of flatworms, clone entire body structures for the purpose of reproduction. Humans have certain limited cellular regeneration abilities that allow for the replacement of worn or damaged tissues.
While all organisms, including bacteria, fungi, and yeast, have the biological ability to regenerate cells, the process presents differently in each organism. Maintaining an organism's biological integrity is the primary purpose of cellular regeneration, although some organisms also use cell regeneration as a form of asexual reproduction. For example, yeast propagates and repairs itself through an asexual cell regeneration process known as cell budding. A new cell grows as a nub attached to an old cell, gathering DNA information to reproduce an exact duplicate cell. Upon maturity, the new cell breaks off and becomes independent of its host cell, thus allowing yeast and similar fungi to reproduce, grow, or repair damage.
Certain reptiles and amphibians have the capability for complex cellular regeneration, allowing entire tissue structures to regrow after damage through a process known as autotomy. When an injury occurs or such creatures are in danger from predators, adult cells within tails, fins, and other appendages can separate from the main body, leaving the appendage behind. As part of the creature's natural biochemical process, cells at the edges of such injuries morph back into stem cells, allowing for a cell regeneration process identical to the initial growth and development of the lost appendage.
In humans, cell regeneration presents a slightly different process. Stem cells, the generic cellular building blocks that allow an embryo to eventually form specific organs, tissues, and appendages, are present only in vitro. Once cells develop into mature cells, they cannot revert again to stem cells, as seen in certain reptiles and amphibians. Rather, mature brain cells, skin cells, nerve cells, and other cellular classifications can only split and reproduce like cells, thus limiting cell regeneration in humans.
While limited, cell regeneration in humans plays an important role in development, healing, and tissue repair. Cells in humans naturally die at a rate of billions per day due to either necrosis, the death of cells due to damage or injury, or through apoptosis. Apoptosis is a form of programmed cellular death that allows cells to fragment or otherwise die as part of the normal biochemical process involved in development, growth, and aging. Without some form of cell regeneration, necrosis and apoptosis would eventually result in the destruction of entire organs and tissue regions. Instead, cell regeneration allows the body to grow new cells to replace dead, dying, or otherwise damaged cells by splitting a single healthy cell into two separate, cells.
Although humans retain the ability to regenerate cells based on certain conditions, the ability to completely regenerate entire structures is limited to certain tissues and organs such as the liver and skin. Brain cells, for example, slowly regenerate over time, but a human could not grow a new brain through cell regeneration. Alternatively, the human body can regenerate the liver, provided at least one quarter of the organ remains intact. Likewise, skin can regrow to cover large areas of damage, provided there is a sufficient percentage of skin left from which to replicate new cells.