, an organism
is said to regenerate
a lost or damaged part if the part regrows so that the original function is restored.
Regenerative capacity is related to complexity: in general, the more complex an animal is the less regeneration it is capable of. Whereas newts, for example, can regenerate severed limbs mammals cannot. Limb regeneration in newts 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. Simpler animals like planaria have an enhanced capacity to regenerate because the adults retain clusters of stem cells within their bodies which migrate to the parts of the body that need healing then divide and differentiate to provide the required missing tissue.
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 . 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 . 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.
In August 2005, Lee Spievack, then in his early sixties, accidentally sliced off the tip of his right middle finger just above the first phalange. His brother, Alan, was researching regeneration and provided him with powdered extracellular matrix
. Mr. Spievack covered the wound with the powder, and the tip of his finger re-grew in four weeks. The news was released in 2007. Lee Spievack is the first documented case of an adult human regenerating fingertips; however, Ben Goldacre
has described this as "the missing finger that never was", claiming that fingertips regrow and quoted Simon Kay, professor of hand surgery at the University of Leeds, who from the picture provided by Goldacre described the case as seemingly "an ordinary fingertip injury with quite unremarkable healing" and as "junk science".
Purported regeneration of human ribs
There have appeared some claims that human ribs could regenerate if the periosteum, the membrane surrounding the rib, were left intact. However, the given source (J Am Podiatr Med Assoc 96(6): 508–512, 2006: ) just states that a piece of the human rib, transplanted to the foot, could survive in the new place, if the periosteum was transplanted with it. There have also been attempts to sustain the claim by noting that some back muscles may be transplanted (destroying its old functionality, but hopefully restoring other muscle functionality to the patient: ).
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. Resection of liver can induce the proliferation of the remained hepatocytes until restore the loosed mass, where the intensity of the liver’s response is directly proportional to the mass resected. For almost 80 years surgical resection of the liver in rodents has been a very useful model to the study of cell proliferation
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.
Regeneration in the mammalian kidney
In the mammalian kidney, the regeneration of the tubular component following an acute injury is well known. Recently regeneration of the glomerulus has also been documented. Following an acute injury, the proximal tubule is damaged more, and the injured epithelial cells slough off the basement membrane of the nephron. The surviving epithelial cells, however, undergo migration, dedifferentiation, proliferation, and redifferentiation to replenish the epithelial lining of the proximal tubule after injury. Recently, the presence and participation of kidney stem cells in the tubular regeneration has been shown. However, the concept of kidney stem cells is currently emerging. In addition to the surviving tubular epithelial cells and kidney stem cells, the bone marrow stem cells have also been shown to participate in regeneration of the proximal tubule, however, the mechanisms remain highly controversial.
Regeneration in the lower vertebrate kidney
Like other organs, the kidney is also known to regenerate completely in lower vertebrates such as fish. Some of the known fish that show remarkable capacity of kidney regeneration are goldfish, skates, rays, and sharks. In these fish, the entire nephron regenerates following injury or partial removal of the kidney.
Regeneration in MRL mice
Adult mammals have limited regenerative capacity compared to most vertebrate embryos/larvae, adult salamanders and fish. The MRL mouse is a strain of mouse that exhibits remarkable regenerative abilities for a mammal. Study of the regenerative process in these animals is aimed at discovering how to duplicate them in 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 not, however, protect them 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.
- In John Milton's, "Paradise Lost" (Book VI) the character Satan, possesses the power of accelerated healing.
- In the television series Doctor Who, Time Lords can 'regenerate' up to 12 times. They become totally new people, by replacing every cell in their body, but retaining the brain. Due to the 13-lives limitation (12 regenerations + original body), they are not immortal, but can survive 12 otherwise-fatal incidents like a mythical "cat with nine lives".
- In the book A Planet Called Treason by Orson Scott Card, the main character is descended from a race of people whose most famous trait is complete regeneration, the only way to kill them being beheading.
- In the comics and films of X-Men, Wolverine has the ability of accelerated healing. This ability is shared by a number of comic book heroes including Deadpool, The Hulk, Lady Deathstrike, and Sabertooth, to name a few.
- In the television series Heroes, the characters Claire Bennet (the cheerleader) and Adam Monroe (an antagonist in the show's second season) both can "regenerate". The ability has enabled Monroe to live for about 400 years without apparent aging, because his cells continually regenerate. It is not yet clear whether this applies to Claire as well, as she is still young.
- In the anime series DragonBall Z, the character Piccolo can regenerate any part of his body immediately.
- In the anime series Naruto, the character Tsunade can regenerate any part of her body. However, the use of this ability reduces the life span of the character.
- In the anime series Hellsing, practically all supernatural characters can regenerate any part of their body.
- Various species in the Star Wars universe, including Trandoshans, have the ability to regenerate limbs.
- In the Patternist series of books by Octavia Butler, the character Anyanwu is able to change the shape of her own body and as a result consciously induce rapid healing in herself. Her descendants, whom the later books in the series deal with, are eventually able to use this ability on others as well as themselves.
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- Medicine's Cutting Edge: Re-Growing Organs