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In biology, regeneration is an organism's ability to replace body parts. Aside from being used to generally describe any number of specific healing processes, regeneration also is a specific method of healing that is noted for its ability to regrow lost limbs, severed nerve connections, and other wounds. It can be seen in the organisms of planaria and starfish.
Regeneration occurs in many, if not all vertebrate types , and is present in some adult animals such as salamanders ( e.g. ). Mammals exhibit limited regenerative abilities, although not as impressive as salamanders. Examples of mammalian regeneration include antlers, finger tips and holes in ears. Finger tip regeneration has been well characterized, and these studies have resulted in the first demonstration of a genetic pathway controlling regeneration in a mammal. Several species of mammals can regenerate ear holes; a phenomenon that has been most studied in the MRL mouse. If the processes behind regeneration are fully understood, it is believed this would lead to better treatment for individuals with nerve injuries (such as those resulting from a broken back or a polio infection), missing limbs, and/or damaged or destroyed organs.
Perhaps the most impressive example of regenerative power is the starfish. Starfish can regenerate one or more arms that are cut off. Even more impressive is the fact that, in certain starfish, an amputated arm can regenerate an entire new starfish. A starfish arm can only regenerate into a whole new organism if some of the central ring of the starfish is part of the chopped off arm. Each arm contains enough organs and biological structure to sustain itself. For example, each arm has gonads and one eye.
Regeneration of a lost limb occurs in two major steps, first de-differentiation of adult cells into a stem cell state similar to embryonic cells and second, development of these cells into new tissue more or less the same way it developed the first time . Some animals like planarians instead keep clusters of non-differentiated cells within their bodies, which migrate to the parts of the body that need healing.
Regeneration in salamanders
In urodele amphibians (salamanders), the regeneration process begins immediately after amputation. Limb regeneration in the axolotl and newt have been extensively studied. After amputation, the epidermis migrates to cover the stump in less than 12 hours, forming a structure called the apical epidermal cap (AEC). Over the next several days there are changes in the underlying stump tissues that result in the formation of a blastema (a mass of dedifferentiated proliferating cells). As the blastema forms, pattern formation genes – such as HoxA and HoxD – are activated as they were when the limb was formed in the embryo [8,10]. The Distal tip of the limb (the autopod, which is the hand or foot) is formed first in the blastema. The intermediate portions of the pattern are filled in during growth of the blastema by the process of intercalation [7,8]. Motor neurons, muscle, and blood vessels grow with the regenerated limb, and reestablish the connections that were present prior to amputation. The time that this entire process takes varies according to the age of the animal, ranging from about a month to around three months in the adult and then the limb becomes fully functional.
In spite of the historically small size of the number of researchers studying limb regeneration, remarkable progress has been made recently in establishing Ambystoma (the axolotl) as a model genetic organism. This progress has been facilitated by advances in genomics, bioinformatics, and somatic cell transgenesis in other fields, that have created the opportunity to investigate the mechanisms of important biological properties, such as limb regeneration, in the axolotl . The Ambystoma Genetic Stock Center (AGSC) is a self-sustaining, breeding colony of the Mexican axolotl (Ambystoma mexicanum) supported by the National Science Foundation as a Living Stock Collection. Located at the University of Kentucky, the AGSC is dedicated to supplying genetically well-characterized axolotl embryos, larvae, and adults to laboratories throughout the United States and abroad. An NIH-funded NCRR grant has led to the establishment of the Ambystoma EST database, the Salamander Genome Project (SGP) that has led to the creation of the first amphibian gene map and several annotated molecular data bases, and the creation of the research community web portal (www.ambystoma.org).
Regeneration of human fingers
Studies in the 1970s showed that children up to the age of 10 or so who lose fingertips in accidents can regrow the tip of the digit within a month provided their wounds are not sealed up with flaps of skin -- the de facto treatment in such emergencies.
Regeneration of human ribs
Human ribs can regenerate if the periosteum, the membrane surrounding the rib, is left intact (Nadia Rosenthal in Howard Hughes Medical Institute "The 2006 Holiday Lectures on Science"). For this reason, ribs are used as a source of bone in reconstructive surgery. 
Regeneration of human liver
The human liver is one of the few glands in the body that has the ability to regenerate from as little as 25% of its tissue. This is largely due to the unipotency of hepatocytes.
Regeneration in MRL mice
Adult mammals have a limited regenerative response as compared to most vertebrate embryos/larvae and adult salamanders and fish. Among adult mammals, the MRL mouse is a strain of mice that exhibits enhanced regenerative abilities, and for this reason it has been a well studied model system for mammalian regeneration. Since adult salamanders exhibit such a remarkable regenerative ability, and species of mammals, such as the MRL mouse, also have regenerative abilities, it is thought that it should be possible to enhance the innate regenerative ability of humans.
By comparing the differential gene expression of scarless healing MRL mice and poor healing C57BL/6 mice strain, 36 genes have been identified that are good candidates for studying how the healing process differs in MRL mice and other mice.
The regenerative abilities of MRL mice does however not protect against myocardial infarction. MRL mice show the same amount of cardiac injury and scar formation as normal mice after a heart attack.
In some fictional stories, the possibility for enhanced human regeneration is explored. Comic books, especially, have featured characters with such abilities. In these stories, human healing from injury is treated as a superpower. Usually, this "healing factor", as it is called, allows for rapid regeneration from injury in a very short period of time; usually a few seconds. Even normally fatal injuries are often overcome with relative ease. While the specifics sometimes differ, the factors are often presented as an inherent ability gained through human mutation/evolution, deliberate engineering or magic.
- Spallanzani's mouse: a model of restoration and regeneration
- Mice that regrow hearts in the news
- DARPA Grant Supports Research Toward Realizing Tissue Regeneration
- The Geniuses Of Regeneration in BusinessWeek, May 24, 2004
- UCI Limb Regeneration Lab
- Biochem Biophys Res Commun. 2005 Apr 29;330(1):117-22. PMID 15781240
- Wound Repair Regen. 2005 Mar-Apr;13(2):205-8. PMID 15828946
- Tanaka EM. Cell differentiation and cell fate during urodele tail and limb regeneration. Curr Opin Genet Dev. 2003 Oct;13(5):497-501. PMID 14550415
- Nye HL, Cameron JA, Chernoff EA, Stocum DL. Regeneration of the urodele limb: a review. Dev Dyn. 2003 Feb;226(2):280-94. PMID 12557206
- Yu H, Mohan S, Masinde GL, Baylink DJ. Mapping the dominant wound healing and soft tissue regeneration QTL in MRL x CAST. Mamm Genome. 2005 Dec;16(12):918-24. PMID 16341671
- Odelberg SJ.Unraveling the molecular basis for regenerative cellular plasticity.PLoS Biol. 2004 Aug;2(8):E232. PMID 15314652
- Bryant, S.V., Endo, T. and Gardiner, D.M. Vertebrate limb regeneration and the origin of limb stem cells. Int. J. Dev. Biol. 2002 46:887-896. PMID 12455626
- Gardiner DM, Blumberg B, Komine Y, Bryant SV. Regulation of HoxA expression in developing and regenerating axolotl limbs. Development. 1995 Jun;121(6):1731-41. PMID 7600989
- Mullen LM, Bryant SV, Torok MA, Blumberg B, Gardiner DM. Nerve dependency of regeneration: the role of Distal-less and FGF signaling in amphibian limb regeneration. Development. 1996 Nov;122(11):3487-97. PMID 8951064
- Torok MA, Gardiner DM, Shubin NH, Bryant SV. Expression of HoxD genes in developing and regenerating axolotl limbs. Dev Biol. 1998 Aug 15;200(2):225-33. PMID 9705229
- Endo T, Bryant SV, Gardiner DM. A stepwise model system for limb regeneration. Dev Biol. 2004 Jun 1;270(1):135-45. PMID 15136146
- Putta S, Smith JJ, Walker JA, Rondet M, Weisrock DW, Monaghan J, Samuels AK, Kump K, King DC, Maness NJ, Habermann B, Tanaka E, Bryant SV, Gardiner DM, Parichy DM, Voss SR, From biomedicine to natural history research: EST resources for ambystomatid salamanders. BMC Genomics. 2004 Aug 13;5(1):54. PMID 15310388
- Wieland, C, Regenerating ribs. Creation. 1989 September;21(4):46-47. 
- Weintraub, Arlene , The Geniuses Of Regeneration. 2004 MAY 
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