The classical example of morphallaxis is that of the Cnidarian hydra, where when the animal is severed in two (by actively cutting it with, for example, a surgical knife) the remaining severed sections form two fully functional and independent hydra. The notable feature of morphallaxis is that a large majority of regenerated tissue comes from already-present tissue in the organism. That is, the one severed section of the hydra forms into a smaller version of the original hydra, approximately the same size as the severed section. Hence, there is an "exchange" of tissue.
Researchers Wilson and Child showed circa 1930 that if the hydra was pulped and the disassociated cells passed through a sieve, those cells then put into an aqueous solution would shortly reform into the original organism with all differentiated tissue correctly arranged.
Morphallaxis is often contrasted with epimorphosis, which is characterized by a much greater relative degree of cellular proliferation. Although cellular differentiation is active in both processes, in morphallaxis the majority of the regeneration comes from reorganization or exchange, while in epimorphosis the majority of the regeneration comes from cellular differentiation. Thus, the two may be distinguished as a measure of degree. Epimorphosis is the regeneration of a part of an organism by proliferation at the cut surface. For example, in Planaria neoblasts help in regeneration.
The word comes from the Greek allazein, which means to exchange. The biological process was first discovered in hydra by Abraham Trembley, who was considered the father of environmental zoology. The process and mechanism of planarian regeneration was eventually renamed to 'Morphallaxis' by Thomas Hunt Morgan, the father of experimental genetics.
Regeneration in Cnidarian hydras
Hydras are a group of freshwater Cnidarians that are about 0.5 cm long. A hydra has a short, tubular shaped body. Hydras have a head that consists of a hypostome region and a foot that consists of a basal disc. The head portion of the hydra contains the mouth and tentacles which allows for the catching and eating of food. The foot portion of the hydra contains the basal disc which allows for the hydra to stick to rocks and other elements.
When a hydra gets cut in half, the head portion can regenerate and form a new foot with the basal disc, as well as, the foot portion can regenerate and form a new head with the hypostome region. If a hydra was severed into smaller pieces, the middle pieces would still form a head and foot at the appropriate regions of the hydra. This results in a smaller hydra that was regenerated by morphallaxis and occurs without cellular division.
The mechanism involved uses regenerative tissue remodeling. This allows the body's axes to repattern and form new tissue, as well as causes organs in the body to redevelop into different proportions.
Hydras contain a series of gradients that controls the formation of the correct head and foot regeneration. The head gradient permits the head to only form in one place and the foot gradient permits the basal disc to only form in another place. These gradients are driven by the polarity in the hydra. The hypostome in the head region inhibits the formation of another hypostome. This explains why two heads will not form on one hydra.
Types of Regeneration
Regeneration is divided into two different types: morphallaxis or epimorphosis. Epimorphosis is involved with newt limbs. In this process when a newt limb gets cut, a little portion of the limb remains attached which forms a mass of undifferentiated cells called a blastema. This blastema at the wound site then multiplies and is responsible for the regeneration of the tissues and organs that were lost. With epimorphosis, the undifferentiated cells aid in the regeneration of limbs. In contrast, there is morphallaxis that is best described in hydra regeneration. When a hydra gets cut, a blastema does not form. The remaining part of the hydra undergoes repatterning to regenerate all different portions of the body. As an example, remaining tail fragments of a hydra could form the pharynx in the central portion of the body.
- Dunbar HF (1946). Emotions and Bodily Changes. New York.
- Pellettieri J (March 2019). "Regenerative tissue remodeling in planarians - The mysteries of morphallaxis". Seminars in Cell & Developmental Biology. 87: 13–21. doi:10.1016/j.semcdb.2018.04.004. PMC 6195476. PMID 29631028.
- Gilbert SF (2000). "Regeneration". Developmental Biology (6th ed.).
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