regeneration

Our Living Language  : Regeneration of parts or, in some cases, nearly the entire body of an organism from a part, is more common than one might think. Many protists like the amoeba that have been cut in half can grow back into a complete organism so long as enough of the nuclear material is undamaged. Severed cell parts, such as flagella, can also be regrown in protists. New plants can be grown from cuttings, and plants can often be regenerated from a mass of fully differentiated cells (such as a section of a carrot root), which, if isolated in a suitable environment, turn into a mass of undifferentiated cells that develop into a fully differentiated organism. The capacity for regeneration varies widely in animals, with some able to regenerate whole limbs and others not, but the capacity is reduced significantly in more complex animals. Certain simple invertebrates like the hydra are always regenerating themselves. If cut into tiny pieces that are then mixed up, the pieces can reorganize themselves and grow back into a complete organism. Flatworms have the capacity to regenerate themselves from only a small mass of cells. If they are chopped up into fine pieces, each piece has the capacity to develop into an entire organism. Starfish, which are echinoderms, can regenerate their entire body from their central section and a single arm. Newts and salamanders can regenerate lost legs and parts of eyes, but many other amphibians such as frogs and toads cannot. Certain lizards can regenerate their tails. In many animals, these regenerated body parts are not as large as the originals but are usually sufficient to be functional. Many higher animals such as mammals regularly regenerate certain tissues such as hair and skin and portions of others such as bone, but most tissues cannot be regenerated. About 75 percent of the human liver can be removed, and it will regenerate into a functional organ. The physiological reasons for this are still not understood. Regeneration in this case takes the form of the enlargement of the remaining structures rather than the re-creation of the lost ones. Thus, there are four mechanisms for tissue regeneration in animals: the reorganization of existing cells (as in the hydra), the differentiation of stored stem cells into the specific tissues needed (as in the salamander), the dedifferentiation of neighboring tissue cells and their subsequent regrowth as cells of the needed type (as in plants as well as certain animals like the salamander), and the compensatory growth of the surviving cells of the specific tissue (as in the human liver). There is a great interest in stem cells because of their potential use in regenerating body tissues, such as nerve cells and heart muscle. The biochemical mechanisms for dedifferentiation are also the subject of intense study.