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Morphallaxis is the regeneration of specific tissue in a variety of organisms due to loss or death of the existing tissue. The word comes from the Greek allazein, (αλλάζειν) which means to change.


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 food 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. [1]

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. [1] The biological process was first discovered in hydra by Abraham Trembley, who was considered the father of environmental zoology. Abraham Trembley was doing a research on a sample pond water and examined the lifestyle of hydra. He couldn’t decide if they belong to the animal or plant kingdom, so he cut them in half and planned to see whether they die, like animals would, or re-pattern, as plants. Even though the halves and smaller pieces gave rise to new individuals, he still believed that hydra is an animal, since all their features, like movements or feeding behavior matched with animals’. Trembley came to the conclusion, that some animals have the ability to regenerate. [2]

The process and mechanism of planarian regeneration was eventually renamed to 'Morphallaxis' by Thomas Hunt Morgan, the father of experimental genetics. [2]

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. [3]

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. [3]


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

There are three types of regenerations. One of them is Epimorphosis, which appears in Salamander limbs. This type of regeneration includes the formation of a new part, which is called the blastema. The amputation is sensed by a large number of somatic stem cells, that migrate to the wound they increase their division rate. At the wound blastema forms and the blastema cells proliferate to re-generate the lost tissues. There is no significant re-patterning of the remaining tissue. [4] [5]

Another type of regeneration is Morphallaxis, which is usually observed in Hydras. The main difference between the two types is that morphallatic regeneration does not include the formation of blastemal and there is no proliferation. Instead the existing tissue undergoes re-arrangement and it is transformed into the new organ. [4] [5]

The third type occurs for example in planarians. It was discovered once regeneration was observed in a cellular level. For very long biologists believed that planarians undergo epimorphosis, because regeneration on a trunk piece shows new tissue formation from a blastemal. However, on a tail piece next to the formation of a blastemal (which serves as a signaling center in this case), pharynx was re-arranged from a pre-existing tissue. The conclusion is that planarian regeneration cannot be listed neither to Epimorphosis or Morphallaxis. [5]

Related Research Articles

Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of stem cells in the adult organism.

<i>Hydra</i> (genus) Genus of cnidarians

Hydra is a genus of small, fresh-water organisms of the phylum Cnidaria and class Hydrozoa. They are native to the temperate and tropical regions. Biologists are especially interested in Hydra because of their regenerative ability – they do not appear to die of old age, or indeed to age at all.

Morphogenesis is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of developmental biology along with the control of tissue growth and patterning of cellular differentiation.

Cnidocyte Explosive cell containing one giant secretory organelle (cnida)

A cnidocyte is an explosive cell containing one giant secretory organelle called a cnidocyst that can deliver a sting to other organisms. The presence of this cell defines the phylum Cnidaria. Cnidae are used to capture prey and as a defense against predators. A cnidocyte fires a structure that contains a toxin within the cnidocyst; this is responsible for the stings delivered by a cnidarian.

Cellular differentiation

Cellular differentiation is the process in which a cell changes from one cell type to another. Usually, the cell changes to a more specialized type. Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types. Differentiation continues in adulthood as adult stem cells divide and create fully differentiated daughter cells during tissue repair and during normal cell turnover. Some differentiation occurs in response to antigen exposure. Differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals. These changes are largely due to highly controlled modifications in gene expression and are the study of epigenetics. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. Thus, different cells can have very different physical characteristics despite having the same genome.


Blastulation is the stage in early animal embryonic development that produces the blastula. The blastula (from Greek βλαστός is a hollow sphere of cells surrounding an inner fluid-filled cavity. Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.

Nerve net

A nerve net consists of interconnected neurons lacking a brain or any form of cephalization. While organisms with bilateral body symmetry are normally associated with a central nervous system, organisms with radial symmetry are associated with nerve nets. Nerve nets can be found in members of the Cnidaria, Ctenophora, and Echinodermata phyla, all of which are found in marine environments. Nerve nets can provide animals with the ability to sense objects through the use of the sensory neurons within the nerve net.

Planarian Flatworms of the Turbellaria class

A planarian is one of many flatworms of the traditional class Turbellaria. It usually describes free-living flatworms of the order Tricladida (triclads), although this common name is also used for a wide number of free-living platyhelminthes. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas.

Regeneration (biology) Biological process of renewal, restoration, and tissue growth

In biology, regeneration is the process of renewal, restoration, and tissue growth that makes genomes, cells, organisms, and ecosystems resilient to natural fluctuations or events 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.

Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation form the internal organs of the organism.

Blastema Mass of cells capable of enacting growth and regeneration

A blastema is a mass of cells capable of growth and regeneration into organs or body parts. Historically, blastemas were thought to be composed of undifferentiated pluripotent cells, but recent research indicates that in some organisms blastemas may retain memory of tissue origin. Blastemas are typically found in the early stages of an organism's development such as in embryos, and in the regeneration of tissues, organs and bone.

Within the field of developmental biology, one goal is to understand how a particular cell develops into a final cell type, known as fate determination. Within an embryo, several processes play out at the cellular and tissue level to create an organism. These processes include cell proliferation, differentiation, cellular movement and programmed cell death. Each cell in an embryo receives molecular signals from neighboring cells in the form of proteins, RNAs and even surface interactions. Almost all animals undergo a similar sequence of events during very early development, a conserved process known as embryogenesis. During embryogenesis, cells exist in three germ layers, and undergo gastrulation. While embryogenesis has been studied for more than a century, it was only recently that scientists discovered that a basic set of the same proteins and mRNAs are involved in embryogenesis. Evolutionary conservation is one of the reasons that model systems such as the fly, the mouse, and other organisms are used as models to study embryogenesis and developmental biology. Studying model organisms provides information relevant to other animals, including humans. New discoveries and investigations includen how RNAs and proteins are expressed differentially between cells types, temporally and spatially; and how they are responsible for cell fate determination contributing to the vast diversity of organisms.

Epimorphosis is defined as the regeneration of a specific part of an organism in a way that involves extensive cell proliferation of somatic stem cells, dedifferentiation, and reformation, as well as blastema formation. Epimorphosis can be considered a simple model for development, though it only occurs in tissues surrounding the site of injury rather than occurring system-wide. Epimorphosis restores the anatomy of the organism and the original polarity that existed before the destruction of the tissue and/or a structure of the organism. Epimorphosis regeneration can be observed in both vertebrates and invertebrates such as the common examples: salamanders, annelidas, and planarians.

Betty Hay Cell biologist and Developmental biologist

Elizabeth Dexter “Betty” Hay was an American cell and developmental biologist. She was best known for her research in limb regeneration, the role of the extracellular matrix (ECM) in cell differentiation, and epithelial-mesenchymal transitions (EMT). Hay led many research teams in discovering new findings in these related fields, which led her to obtain several high honors and awards for her work. Hay primarily worked with amphibians during her years of limb regeneration work and then moved onto avian epithelia for research on the ECM and EMT. Hay was thrilled by the introduction of transmission electron microscopy (TEM) during her lifetime, which aided her in many of her findings throughout her career. Moreover, Hay was a huge advocate of women in science during her lifetime.

Hox genes in amphibians and reptiles

Hox genes play a massive role in some amphibians and reptiles in their ability to regenerate lost limbs, especially HoxA and HoxD genes.

<i>Hydra viridissima</i>

Hydra viridissima is a species of cnidarian which are commonly found in freshwater, mostly ponds, rivers, and slow flowing parts of streams and rivers in the Northern temperate zone. They are also known as Hydra viridis and Chlorohydra viridissima. Green hydra are typically 10 mm long, and they have tentacles that are about half of their length. They feed on small crustaceans and are strictly carnivorous. Typically they feed on small crustaceans, insects and annelids. Hydra viridissima is often known as the green hydra, the species appears green because of the symbiotic relationship with Chlorella vulgaris, which is a green alga that lives within the organism. Hydra are normally sessile and live on aquatic vegetation. They secrete mucous to attach using their basal disc.

Non-neural cognition is the process by which cells other than neurons engage in information processing and rudimentary cognition. Non-neural cognition can manifest as cell memory, neurotransmitter signaling, or bioelectric communication and organization, and it is present in many organisms, from single-celled to humans.

Starfish regeneration Star-shaped organisms

Starfish, or sea stars, are radially symmetrical, star-shaped organisms of the phylum Echinodermata and the class Asteroidea. Aside from their distinguished shape, starfish are most recognized for their remarkable ability to regenerate, or regrow, arms and, in some cases, entire bodies. While most species require some part of the central body to be intact in order to regenerate arms, a few tropical species can grow an entirely new starfish from a portion of a severed limb. Starfish regeneration across species follows a common three-phase model and can take up to a year or longer to complete. Though regeneration is used to recover limbs eaten or removed by predators, starfish are also capable of autotomizing and regenerating limbs to evade predators and reproduce.

Dedifferentiation is a transient process by which cells become less specialized and return to an earlier cell state within the same lineage. This suggests an increase in a cell potency, meaning that after dedifferentiation, cells may possess an ability to redifferentiate into more cell types than it did before. This is in contrast to differentiation, where differences in gene expression, morphology, or physiology arise in a cell, making its function increasingly specialized.

Neoblast Planarian regeneration proliferative cells

Neoblasts (ˈniːəʊˌblæst) are non-differentiated cells found in planarians and responsible for regeneration. Neoblasts have little cytoplasm and a huge nucleus which is a characteristic of pluripotent cells. They are the only dividing growing cells in planaria. This mitotic characteristic is how they are detected by adding Bromodeoxyuridine (BrdU) and staining with anti-BrdU. They have a size between 5 µm to 8 µm in diameter. Neoblasts represent about 30 percent of all cells in planaria. They are not present in the anterior, posterior or pharynx.


  1. 1 2 Dunbar HF (1946). Emotions and Bodily Changes. New York.
  2. 1 2 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.
  3. 1 2 Gilbert SF (2000). "Regeneration". Developmental Biology (6th ed.). ISBN   978-0878932436.
  4. 1 2 Pellettieri, Jason (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. ISSN   1084-9521. PMC   6195476 . PMID   29631028.
  5. 1 2 3 Agata, Kiyokazu; Saito, Yumi; Nakajima, Elizabeth (2007-02-28). "Unifying principles of regeneration I: Epimorphosis versus morphallaxis". Development, Growth & Differentiation. 49 (2): 73–78. doi: 10.1111/j.1440-169x.2007.00919.x . ISSN   0012-1592. PMID   17335428.