regeneration

Regeneration is the process of rebirth, reinstallation, and growth that makes genomes, cells, organisms, and ecosystems resilient to natural fluctuations or actions that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. Regeneration can either be complete where the new tissue is the same as the lost tissue, or incomplete where after the necrotic tissue comes fibrosis. At its most elementary level, regeneration is mediated by the molecular processes of DNA synthesis. Regeneration in biology, however, mainly refers to the morphogenic processes that describe the phenotypic plasticity of traits allowing multi-cellular organisms to repair and maintain the integrity of their physiological and morphological states. Above the genetic level, regeneration is fundamentally regulated by asexual cellular processes. Regeneration is different from reproduction. For example, hydra perform regeneration but reproduce by the method of budding.

The hydra and the planarian flatworm have long served as model organisms for their highly adaptive regenerative capabilities. Once wounded, their cells become activated and start to remodel tissues and organs back to the pre-existing state. The Caudata (urodeles) salamanders and newts), an order of tailed amphibians, is possibly the most adept vertebrate group at regeneration, given their capability of regenerating limbs, tails, jaws, eyes and a variety of internal structures. The regeneration of organs is a common and widespread adaptive capability among metazoan creatures. In a related context, some animals are able to reproduce asexually through fragmentation, budding, or fission. A planarian parent, for example, will constrict, split in the middle, and each half generates a new end to form two clones of the original. Echinoderms (such as the starfish), crayfish, many reptiles, and amphibians exhibit remarkable examples of tissue regeneration. The case of autotomy for example, serves as a defensive function as the animal detaches a limb or tail to avoid capture. After the limb or tail has been autotomized, cells move into action and the tissues will regenerate. Ecosystems are regenerative as well. Following a disturbance, such as a fire or pest outbreak in a forest, pioneering species will occupy, compete for space, and establish themselves in the newly opened habitat. The new growth of seedlings and community assembly process is known as regeneration in ecology.

Genetic Control of Regeneration

A number of genes have been found to be implicated in regeneration. One of the most potent of these is Wnt.

Injection of agents (e.g. antisense RNA molecules) that interfere with the Wnt/β-catenin pathway

blocks limb regeneration in salamanders.

promotes head formation in regenerating planarians, while

injection of agents that enhance the Wnt/β-catenin pathway

enable chicks (that, like mammals, are normally incapable of regenerating limbs) to regenerate a wing; as well as enabling a regenerating planarian to form a tail. Planaria exhibit an extraordinary ability to regenerate lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. In one experiment, T. H. Morgan found that a piece corresponding to 1⁄279th of a planarian could successfully regenerate into a new worm. This size (about 10,000 cells) is typically accepted as the smallest fragment that can regrow into a new planarian. Regeneration of planaria is epimorphic regeneration. After amputation, stump cells form blastema. lost or damaged body parts. Sponges can regenerate the entire organism from just a conglomeration of their cells.

This cnidarians can also regenerate its entire body from cells. The cells that do the job are totipotent stem cells residing in the animal's body. Blocking Wnt/β-catenin signaling by RNA causes a head to form where a tail should (producing a two-headed animal) while  blocking part of the β-catenin degradation complex (thus enhancing the pathway) causes a tail to develop where a head should (producing a two-tailed animal).

Thus it appears that the default pathway of neoblasts is to regenerate a head. In the amputated animal, a gradient of Wnt/β-catenin signaling extends from a high in the posterior leading to the formation of a tail and decreasing towards the anterior until the default pathway is no longer inhibited and a head can form.

Echinoderms can regenerate the entire organism from just one arm and the central disk. oyster fishermen used to dredge up sea stars from their oyster beds, chop them up in the hope of killing them, and then dump the parts back overboard. They soon discovered to their sorrow the remarkable powers of regeneration of these animals.