regeneration

regeneration [rē‚jen·ə′rā·shən]

(biology)

The replacement by an organism of tissues or organs which have been lost or severely injured.

(chemistry)

Restoration of the activity of a deactivated catalyst.

(control systems)

positive feedback

(electronics)

Replacement or restoration of charges in a charge storage tube to overcome decay effects, including loss of charge by reading.

(nucleonics)

Restoration of contaminated nuclear fuel to a usable condition.

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Regeneration (biology)

The process by which an animal restores a lost part of its body. Broadly defined, the term can include wound healing, tissue repair, and many kinds of restorative activities. Within the field of developmental biology, however, most research in regeneration involves systems in which removing a complex structure or major part of an organism initiates a chain of events that produces a structure that duplicates the missing part both functionally and anatomically.

The best-known and most widely studied examples of regeneration are those involving epimorphosis, in which the lost structure is reproduced directly by a combination of cell proliferation and redifferentiation of new tissue. Examples can be found throughout the animal kingdom. Research on regenerating systems yields information regarding basic mechanisms of animal development. Noteworthy has been the progress in understanding the factors that control pattern formation in the development of complex structures, such as vertebrate limbs.

Mammals, birds, and reptiles have a much more poorly developed ability than amphibians and fish to regenerate complete organs, but nevertheless can reform missing tissue and restore function after partial removal of certain organs. For example, if part of the liver is cut away, the remaining portions increase in size to compensate for the missing tissue and to restore the normal functional capacity of the organ. The process of liver regeneration involves the triggering of active growth in the remaining liver cells, in cells of bile ductules, and in unspecialized cells called stem cells, all of which are usually quiescent in the normal liver. Proliferation of these cells and their subsequent differentiation are key events in the process by which the missing liver mass is replaced and adequate hepatic function restored. In the musculoskeletal system, different populations of quiescent stem cells allow efficient replacement of damaged or partially removed bones and muscles. See Bone, Liver, Muscle

Of all vertebrates, amphibians have the most highly developed capacity for regeneration. Certain species have the ability to regenerate not only limbs and tails but also parts of the eye, lower jaw, intestine, and heart. Complete regeneration of amputated limbs can occur throughout the lifetime of most salamanders and newts. In frogs and toads, the ability to regenerate limbs is lost during metamorphosis to the adult form.

Protozoa and simple multicellular animals, including sponges, coelenterates, and flatworms, display remarkable capacities for regeneration following various experimental manipulations. Regenerative ability in such organisms correlates closely with their capacity to reproduce asexually, most commonly by fission or by budding, and the mechanisms of growth involved in regeneration are often very similar to those of asexual propagation. For example, just as complete ciliated protozoa will develop after fission, which divides the nucleus and organelles between daughter cells, intact individuals will also be reconstituted from fragments of a single organism if the fragment contains a complete set of the genetic material and a portion of the original cell's cortical cytoplasm. Similar rules regarding the importance of the nucleus apply to regenerative processes in all protozoa.

Most annelids, such as the earthworm, can readily regenerate segments after their removal: some species can regenerate whole organisms from any fragment. Like more primitive invertebrates, certain annelids can reproduce asexually by transverse fission. The capacities for fission and for reconstitution from fragments in annelids are remarkable, considering the anatomical complexity of animals in this phylum. When an earthworm is cut transversely into two parts, the anterior part can regenerate several posterior segments. The ability of the posterior half to regenerate anterior segments is, however, more limited and is absent altogether in some species. Experiments in which components of the nervous system are removed surgically have revealed the importance of a neural influence for segment regeneration, presumably mediated by a growth-stimulating hormone secreted from neural cells. See Annelida

The ability of certain echinoderms, such as starfish, to regenerate missing arms is well known. Cutting such an animal into several pieces results in each piece forming a new organism, a phenomenon that usually requires the presence of at least some of the central portion of the body. Equally remarkable is a regenerative response shown by another echinoderm, the sea cucumber. When this animal is strongly irritated, it eviscerates itself through its anus or through a rupture of its body wall. This phenomenon produces a nearly empty sack of skin and muscle, which then proceeds to regenerate all the internal organs, beginning with the digestive tract. See Echinodermata

The capacity for appendage regeneration is widespread among the many diverse members of the phylum Arthropoda. In these complex animals with well-developed exoskeletons and no asexual mode of reproduction, regeneration shows a close correlation with the molting process. See Arthropoda, Developmental biology