再生

更仔细的观察

再生的部分,或者在某些情况下,几乎整个有机体的身体部分,远比人们想象的更为常见。许多原生生物像变形虫减半能长回一个完整的有机体,只要足够的核材料的损伤。切断了细胞部分,如鞭毛,还可以再生的原生生物。从岩屑新植物生长,植物通常可以再生从大量完全分化的细胞(如胡萝卜根的部分),而如果孤立在合适的环境中,变成一个大规模的未分化细胞发展成一个完全分化的器官。在动物再生的能力差别很大,一些能够重新生成整个肢体,有些没有,但能力显著降低在更复杂的动物。某些简单的无脊椎动物,就像九头蛇总是再生。如果切成小块,然后混合起来,碎片整理自己,可以重新生长出来成一个完整的有机体。扁虫有能力自己只有一小团细胞再生。如果他们被切成细块,每一块有能力发展成一个完整的器官。海星是棘皮动物,可以再生他们的整个身体从中央部分和一个胳膊。 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.