Epimorphosis

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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, [1] dedifferentiation, and reformation, [2] as well as blastema formation. [3] 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. [4] 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. [4] Epimorphosis regeneration can be observed in both vertebrates and invertebrates such as the common examples: salamanders, annelidas, and planarians. [5]

Contents

History

Thomas Hunt Morgan, an evolutionary biologist who also worked with embryology, argued that limb and tissue reformation bore many similarities to embryonic development. [6] Building off of the work of German embryologist Wilhelm Roux, who suggested regeneration was two cooperative but distinct pathways instead of one, Morgan named the two parts of the regenerative process epimorphosis and morphallaxis. Specifically, Morgan wanted epimorphosis to specify the process of entirely new tissues being regrown from an amputation or similar injury, with morphallaxis being coined to describe regeneration that did not use cell proliferation, such as in hydra. [7] The key difference between the two forms of regeneration is that epimorphosis involves cellular proliferation and blastema formation, whereas morphallaxis does not. [7]

In vertebrates

The apical ectodermal ridge in embryonic development is very similar to the apical ectodermal cap in limb regeneration. The progress zone can be seen near to the zone of polarizing activity, which instructs cells on how to orient the limb. Limb bud diagram.jpg
The apical ectodermal ridge in embryonic development is very similar to the apical ectodermal cap in limb regeneration. The progress zone can be seen near to the zone of polarizing activity, which instructs cells on how to orient the limb.
Dorsal and ventral views of a newt that has had a limb amputated and regrown, from "The elements of experimental embryology" by Julian Huxley and Gavin de Beer. The elements of experimental embryology (1963) (21260217591).jpg
Dorsal and ventral views of a newt that has had a limb amputated and regrown, from "The elements of experimental embryology" by Julian Huxley and Gavin de Beer.

In vertebrates, epimorphosis relies on blastema formation to proliferate cells into the new tissue. Through studies involving zebrafish fins, the toetips of mice, and limb regeneration in axolotls, researchers at the Polish Academy of Sciences found evidence for epimorphosis occurring in a variety of vertebrates, including instances of mammal epimorphosis. [10]

Limb regeneration

Limb regeneration occurs when a part of an organism is destroyed, and the organism must reform that structure. The general steps for limb regeneration are as follows: epidermis covers the wound which is called the wound healing process, [11] the mesenchyme dedifferentiates into a blastema and a apical ectodermal cap forms, and the limb re-differentiates to form the full limb. [12]

Processes in salamanders

Epidermal cells at the wound margins migrate to cover the wound and will become the wound epidermis. [13] No scar tissue forms, as it would in mammals. The mesenchymal tissues of the limb stump secrete matrix metalloproteinases (MMPs). [14] As the MMPs are secreted, the wound epithelium thickens [14] and eventually becomes an apical ectodermal cap (AEC) that forms on the tip of the stump. [15] This is similar to the embryonic apical ectodermal ridge, which forms during normal limb development. Under the AEC, the nerves near the site of the limb destroyed are degraded. [16] The AEC causes the progress zone to re-establish; this means the cells under the AEC (including bone, cartilage, fibroblast cells, etc [13] ) dedifferentiate and become separated mesenchymal cells that form the blastema. [13] [14] Some tissues express specialized genes (like muscle cells) and so if there is damage to these tissues, the genes become downregulated and the proliferation genes are unregulated. [13] The AEC also releases fibroblast growth factors (FGFs) (including FGF-4 and -8) that drive the development of the new limb, essentially resetting the limb back to its embryonic development stage. [17] However, even though some of the limb cells are able to dedifferentiate, they are not able to fully dedifferentiate to the level of multipotent progenitor cells. During regeneration, only cartilage cells can form new cartilage tissue, only muscle cells can form new muscle tissue, and so on. The dedifferentiated cells still retain their original specification. [13] To begin the physical formation of a new limb, regeneration occurs in a distal to proximal sequence. [18] The distal part of the limb is established first, and then the distal part of the limb interacts with the original proximal part of the limb to form the intermediate portion of the limb known as intercalation. [18]

In invertebrates

Periplaneta americana

The American cockroach is capable of regenerating limbs that have been damaged or destroyed, such as legs and antennae, as well parts of its compound eye. It does this with lectin—a protein made for binding proteins—named regenectin, which shares a family with other lipopolysaccharide (LPS) binding proteins. Regenectin carries both a regenerative and a system defense function, and it is produced by the cockroach's paracrine system to work with muscle reformation. [19]

Capitella teleta

C. teleta is a segmented worm found in North America that is capable of regenerating posterior segments after amputation. [20] This regeneration uses the interaction of several sets of Hox genes, as well as blastema formation. All of the Hox genes concerned in epimorphosis are present in the abdominal area of the worm, but not in the anterior portion. However, the genes do not, themselves, direct the anterior-posterior patterning of the worm's thorax. [21]

Planaria vitta

P. vitta is a flatworm of genus Planaria that, when needed, can draw upon both morphallaxis and epimorphosis to regrow itself; in P. vitta, epimorphosis precedes morphallaxis and lasts about ten days. Planaria begin epimorphosis by the epidermis contracting immediately after the worm is cut at the head as a predator reactionary mechanism in order to decrease the surface area at the site of the cut. [22] [23] This mechanism activates the neoblasts which are totipotent stems cells [24] which allows rhabdites to secrete materials to make a protective mucosal covering and epithelium to gather at the site through spreading of the cells rather than proliferation that occurs in vertebrates [23] The dorsal and ventral epithelial cells then come to the site and become differentiated to begin regeneration. [25] The polarity of the planaria can be reestablished through an anterior-posterior gradient through Wnt/β-catenin signaling pathway. [26] Polarity can be described in planarians that the anterior part of the wound site will create a head of a planaria, and the posterior side will create the tail. [26]

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.

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.

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.

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.

Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the correct structures form in the correct places of the body. For example, Hox genes in insects specify which appendages form on a segment, and Hox genes in vertebrates specify the types and shape of vertebrae that will form. In segmented animals, Hox proteins thus confer segmental or positional identity, but do not form the actual segments themselves.

Retinoic acid A metabolite of vitamin A

Retinoic acid (used simplified here for all-trans-retinoic acid) is a metabolite of vitamin A1 (all-trans-retinol) that mediates the functions of vitamin A1 required for growth and development. All-trans-retinoic acid is required in chordate animals, which includes all higher animals from fish to humans. During early embryonic development, all-trans-retinoic acid generated in a specific region of the embryo helps determine position along the embryonic anterior/posterior axis by serving as an intercellular signaling molecule that guides development of the posterior portion of the embryo. It acts through Hox genes, which ultimately control anterior/posterior patterning in early developmental stages.

The limb bud is a structure formed early in vertebrate limb development. As a result of interactions between the ectoderm and underlying mesoderm, formation occurs roughly around the fourth week of development. In the development of the human embryo the upper limb bud appears in the third week and the lower limb bud appears four days later.

GDF11

Growth differentiation factor 11 (GDF11) also known as bone morphogenetic protein 11 (BMP-11) is a protein that in humans is encoded by the growth differentiation factor 11 gene. GDF11 is a member of the Transforming growth factor beta family.

HOXB4

Homeobox protein Hox-B4 is a protein that in humans is encoded by the HOXB4 gene.

HOXD13

Homeobox protein Hox-D13 is a protein that in humans is encoded by the HOXD13 gene. This gene belongs to the homeobox family of genes. The homeobox genes encode a highly conserved family of transcription factors that play an important role in morphogenesis in all multicellular organisms.

HOXD1

Homeobox protein Hox-D1 is a protein that in humans is encoded by the HOXD1 gene.

Zone of polarizing activity

The zone of polarizing activity (ZPA) is an area of mesenchyme that contains signals which instruct the developing limb bud to form along the anterior/posterior axis. Limb bud is undifferentiated mesenchyme enclosed by an ectoderm covering. Eventually, the limb bud develops into bones, tendons, muscles and joints. Limb bud development relies not only on the ZPA, but also many different genes, signals, and a unique region of ectoderm called the apical ectodermal ridge (AER). Research by Saunders and Gasseling in 1948 identified the AER and its subsequent involvement in proximal distal outgrowth. Twenty years later, the same group did transplantation studies in chick limb bud and identified the ZPA. It wasn't until 1993 that Todt and Fallon showed that the AER and ZPA are dependent on each other.

Bat wing development

The order Chiroptera, comprising all bats, has evolved the unique mammalian adaptation of flight. Bat wings are modified tetrapod forelimbs. Because bats are mammals, the skeletal structures in their wings are morphologically homologous to the skeletal components found in other tetrapod forelimbs. Through adaptive evolution these structures in bats have undergone many morphological changes, such as webbed digits, elongation of the forelimb, and reduction in bone thickness. Recently, there have been comparative studies of mouse and bat forelimb development to understand the genetic basis of morphological evolution. Consequently, the bat wing is a valuable evo-devo model for studying the evolution of vertebrate limb diversity.

<i>Capitella teleta</i> Species of annelid

Capitella teleta is a small, cosmopolitan, segmented annelid worm. It is a well-studied invertebrate, which has been cultured for use in laboratories for over 30 years. C. teleta is the first marine polychaete to have its genome sequenced.

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.

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.

References

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