Regeneration is the process of renewal, restoration, and growth that makes genomes, cells, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans.Regeneration can either be complete where the new tissue is the same as the lost tissue, or incomplete where after the necrotic tissue comes fibrosis. At its most elementary level, regeneration is mediated by the molecular processes of DNA synthesis.Regeneration in biology, however, mainly refers to the morphogenic processes that characterize the phenotypic plasticity of traits allowing multi-cellular organisms to repair and maintain the integrity of their physiological and morphological states. Above the genetic level, regeneration is fundamentally regulated by asexual cellular processes.Regeneration is different from reproduction. For example, hydra perform regeneration but reproduce by the method of budding.
Pattern formation in the morphogenesis of an animal is regulated by genetic induction factors that put cells to work after damage has occurred. Neural cells, for example, express growth-associated proteins, such as GAP-43, tubulin, actin, an array of novel neuroptides, and cytokines that induce a cellular physiological response to regenerate from the damage.
Strategies include the rearrangement of pre-existing tissue, the use of adult somatic stem cells and the dedifferentiation and/or transdifferentiation of cells, and more than one mode can operate in different tissues of the same animal. All these strategies result in the re-establishment of appropriate tissue polarity, structure and form.
Simple animals like planarians have an enhanced capacity to regenerate because the adults retain clusters of stem cells within their bodies which migrate to the parts that need healing. Then, they divide and differentiate to grow the missing tissue and organs back. The process is more complex in vertebrates, but nevertheless,salamanders possess strong powers of regeneration, which begins immediately after amputation. Limb regeneration in the axolotl and newt has been extensively studied.
The mechanism for regeneration in Murphy Roths Large (MRL) mice has been found, and is related to the deactivation of the p21 gene.
At least two species of African Spiny Mice, Acomys kempi and Acomys percivali, are capable of completely regenerating the autotomically released or otherwise damaged tissue. These species can regrow hair follicles, skin, sweat glands, fur and cartilage.
In May 1932, L.H. McKim published a report in The Canadian Medical Association Journal, that described the regeneration of an adult digit-tip following amputation. A house surgeon in the Montreal General Hospital underwent amputation of the distal phalanx to stop the spread of an infection. In less than one month following surgery, x-ray analysis showed the regrowth of bone while macroscopic observation showed the regrowth of nail and skin. This is one of the earliest recorded examples of adult human digit-tip regeneration.
Toes damaged by gangrene and burns in older people can also regrow with the nail and toe print returning after medical treatment for gangrene
There have appeared claims that human ribs could regenerate if the periosteum, the membrane surrounding the rib, were left intact. In one study rib material was used for skull reconstruction and all 12 patients had complete regeneration of the resected rib.
The human liver is particularly known for its ability to regenerate.It is capable of doing so from only one quarter of its tissue,due chiefly to the unipotency ofhepatocytes.Resection of liver can induce the proliferation of the remaining hepatocytes until the lost mass is restored, where the intensity of the liver’s response is directly proportional to the mass resected.
Regenerative capacity of the kidney remains largely unexplored. The basic functional and structural unit of the kidney is nephron, which is mainly composed of four components: the glomerulus, tubules, the collecting duct and peritubular capillaries. The regenerative capacity of the mammalian kidney is limited compared to that of lower vertebrates.
Several animals can regenerate heart damage, but in mammals cardiomyocytes (heart muscle cells) cannot proliferate multiply and heart damage causes scarring and fibrosis. The long held view was that mammalian cardiomyocytes are terminally differentiated and cannot divide.