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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
in biology, the renewal of lost or injured organs and tissues or of the entire organism from its parts. Regeneration occurs under natural conditions and may also be induced experimentally.
Regeneration in animals and man. In animals and man, regeneration is the formation of new structures to replace those removed or destroyed as a result of injury (reparative regeneration) or lost in the course of normal activity (physiological regeneration). It is a secondary development caused by the loss of an organ. A regenerated organ may have the same structure as the lost one, may differ from it, or may be completely unlike it (atypical regeneration).
The term “regeneration” was introduced in 1712 by the French scientist R. Réaumur, who studied the regeneration of legs in the crayfish. Regeneration of the entire organism from one of its pieces is possible in many invertebrates. This cannot occur in highly organized animals, in which only individual organs or parts of them regenerate. Regeneration may occur by means of tissue growth of a wound’s surface, by restructuring of the remaining part of an organ into a new organ, or by growth of the remainder of the organ without any change in its shape.
The belief that the capacity for regeneration diminishes as animals become more highly organized is erroneous, since the regenerative process is highly variable, depending both on the animal’s level of organization and on many other factors. It is also untrue that the capacity for regeneration decreases steadily with age. This capacity may even increase during ontogeny, although it often decreases in old age. During the last quarter century, Soviet and foreign scientists have demonstrated that although the external organs of mammals and man do not regenerate completely, their internal organs, muscles, skeleton, and skin are capable of regenerating. This phenomenon is under study at the organic, tissue, cellular, and subcellular levels. The development of techniques for intensifying or stimulating weak regeneration and for restoring a lost capacity for regeneration is causing regeneration to become more closely allied with medicine.
L. D. LIOZNER
Regeneration in medicine. Regeneration may be physiological, reparative, or pathological. In injuries and other pathological states accompanied by extensive cell death, tissue renewal is achieved by reparative regeneration. During reparative regeneration, if the lost part is replaced by equivalent, specialized tissue, there is complete regeneration. If nonspecialized connective tissue grows on the site of the defect, there is incomplete regeneration (substitution, or healing by cicatrization). In some cases of substitution, the organ’s function is restored by intensive neoformation of tissue, similar to the destroyed tissue, in the uninjured part. This neoformation occurs either by means of intensified cell reproduction or by intracellular regeneration—the renewal of subcellular structures with the number of cells remaining unchanged; examples are the cardiac muscle and nerve tissue.
The regenerative process may be weakened, intensified, or qualitatively altered by age, metabolic characteristics, the condition of the nervous and endocrine systems, diet, the rate of blood flow in injured tissue, and accompanying diseases. Such changes sometimes result in pathological regeneration, which is manifested by ulcers very slow to heal, failure of bone fractures to knit, excess tissue growth, or change of one type of tissue into another. The regenerative process is aided by stimulation of complete regeneration and prevention of pathological regeneration.
V. A. FROLOV
Regeneration in plants. In plants, regeneration may occur at the site of the lost part (restitution) or elsewhere (reproduction). The renewal of leaves in spring to replace those lost in autumn is an example of natural regeneration of the reproductive type. However, regeneration usually means only the renewal of parts torn away by force. In such regeneration, the plant uses first and foremost the basic means of normal development. Thus, regeneration of organs in plants occurs mainly by reproduction: the development of existing metameric vernations or of newly formed ones compensates for the lost organs. For example, when the tip of a shoot is cut off, the lateral shoots grow more rapidly. Plants or parts of them that do not develop metameri-cally regenerate more easily by restitution, as do portions of tissues. Thus, a wounded surface may become covered by periderm, while an injury on a trunk or branch may become scarred by callus. Plant propagation by means of cuttings is the simplest type of regeneration when the entire plant is regenerated from a small vegetative part.
Regeneration from pieces of root, rhizome, or thallus is also common. Plants may be grown from leaf cuttings, as in the case of begonias. In some plants, regeneration has been effected from isolated cells and even from individual isolated protoplasts. Some algae of the order Siphonales have been regenerated from small portions of their multinuclear protoplasm. Immaturity in plants usually promotes regeneration, but an organ may be incapable of regeneration in the earliest stages of ontogeny. Regeneration, a biological adaptation ensuring the healing of injuries, the restoration of accidentally lost organs, and often the process of vegetative propagation as well, is of major importance for plant and fruit growing, forestry, and landscape gardening. Information gained through the study of regeneration is also of help in the solution of theoretical problems, including the problem of the development of organisms. Growth regulators play a major role in regeneration.
N. P. KRENKE
REFERENCESVorontsova, M. A. Regeneratsiia organov u zhivotnykh. Moscow, 1949.
Studitskii, A. N. “Osnovy biologicheskoi teorii regeneratsii.” Izv. AN SSSR: Seriia biologicheskaia, 1952, no. 6.
Voprosy vosstanovleniia organov i tkanei pozvonochnykh zhivotnykh. (AN SSSR: Tr. ln-ta morfologii zhivotnykh, fasc. 11.) Moscow, 1954.
Vorontsova, M. A., and L. D. Liozner. Bespoloe razmnozhenie i regeneratsiia. Moscow, 1957.
Usloviia regeneratsii organov u mlekopitaiushchikh. Moscow, 1972.
Krenke, N. P. Regeneratsiia rastenii. Moscow-Leningrad, 1950.
Sinnott, E. Morfogenez rastenii. Moscow, 1963. (Translated from English.)
Hay, E. Regeneratsiia. Moscow, 1969. (Translated from English.)
Swingle, C. F. “Regeneration and Vegetative Propagation.” The Botanical Review, 1940, vol. 6, no. 7; 1952, vol. 18, no. 1.