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
Development or developing may refer to:
Arts
*Development hell, when a project is stuck in development
*Filmmaking, development phase, including finance and budgeting
*Development (music), the process thematic material is reshaped
* Photograph ...
, 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, annelids, and planarians.
History
Thomas Hunt Morgan
Thomas Hunt Morgan (September 25, 1866 – December 4, 1945) was an American evolutionary biologist, geneticist, embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role that ...
, an evolutionary biologist who also worked with embryology, argued that limb and tissue reformation bore many similarities to embryonic development. Building off of the work of German embryologist
Wilhelm Roux
Wilhelm Roux (9 June 1850 – 15 September 1924) was a German zoologist and pioneer of experimental embryology.
Early life
Roux was born and educated in Jena, Germany where he attended university and studied under Ernst Haeckel. He also attended ...
, who suggested regeneration was two cooperative but distinct pathways instead of one, Morgan named the two parts of the regenerative process epimorphosis and
morphallaxis 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 morphall ...
. 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.
The key difference between the two forms of regeneration is that epimorphosis involves cellular proliferation and blastema formation, whereas morphallaxis does not.
In vertebrates
![Limb bud diagram](https://upload.wikimedia.org/wikipedia/commons/0/0e/Limb_bud_diagram.jpg)
In
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
s, epimorphosis relies on blastema formation to
proliferate cells into the new tissue. Through studies involving
zebrafish
The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family (Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ca ...
fins, the toetips of mice, and limb regeneration in
axolotl
The axolotl (; from nci, āxōlōtl ), ''Ambystoma mexicanum'', is a paedomorphic salamander closely related to the tiger salamander. Axolotls are unusual among amphibians in that they reach adulthood without undergoing metamorphosis. I ...
s, researchers at the
Polish Academy of Sciences
The Polish Academy of Sciences ( pl, Polska Akademia Nauk, PAN) is a Polish state-sponsored institution of higher learning. Headquartered in Warsaw, it is responsible for spearheading the development of science across the country by a society o ...
found evidence for epimorphosis occurring in a variety of vertebrates, including instances of mammal epimorphosis.
Limb regeneration
Limb regeneration
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 reg ...
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, the mesenchyme dedifferentiates into a blastema and a apical ectodermal cap forms, and the limb re-differentiates to form the full limb.
Processes in salamanders
Epidermal cells at the wound margins migrate to cover the wound and will become the wound epidermis.
No scar tissue forms, as it would in mammals. The mesenchymal tissues of the limb stump secrete matrix metalloproteinases (MMPs).
As the MMPs are secreted, the wound epithelium thickens
and eventually becomes an
apical ectodermal cap (AEC) that forms on the tip of the stump. This is similar to the embryonic
apical ectodermal ridge
The apical ectodermal ridge (AER) is a structure that forms from the ectodermal cells at the distal end of each limb bud and acts as a major signaling center to ensure proper development of a limb. After the limb bud induces AER formation, the AE ...
, which forms during normal
limb development
Limb development in vertebrates is an area of active research in both developmental and evolutionary biology, with much of the latter work focused on the transition from fin to limb.
Limb formation begins in the morphogenetic limb field, as mes ...
. Under the AEC, the nerves near the site of the limb destroyed are degraded. The AEC causes the
progress zone to re-establish; this means the cells under the AEC (including bone, cartilage, fibroblast cells, etc
)
dedifferentiate and become separated
mesenchymal
Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every ...
cells that form the blastema.
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.
The AEC also releases
fibroblast growth factors
Fibroblast growth factors (FGF) are a family of cell signalling proteins produced by macrophages; they are involved in a wide variety of processes, most notably as crucial elements for normal development in animal cells. Any irregularities in their ...
(FGFs) (including
FGF-4
Fibroblast growth factor 4 is a protein that in humans is encoded by the ''FGF4'' gene.
The protein encoded by this gene is a member of the fibroblast growth factor (FGF) family. FGF family members possess broad mitogenic and cell survival activi ...
and
-8) that drive the development of the new limb, essentially resetting the limb back to its embryonic development stage. However, even though some of the limb cells are able to dedifferentiate, they are not able to fully dedifferentiate to the level of
multipotent Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta.
According to Latin terms, Pluripotentia means the ability for many thin ...
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
A specification often refers to a set of documented requirements to be satisfied by a material, design, product, or service. A specification is often a type of technical standard.
There are different types of technical or engineering specificati ...
.
To begin the physical formation of a new limb, regeneration occurs in a distal to proximal sequence.
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.
In invertebrates
''Periplaneta americana''
The
American cockroach
The american cockroach (''Periplaneta americana'') is the largest species of common cockroach, and often considered a pest. In certain regions of the U.S. it is colloquially known as the waterbug, though it is not a true waterbug since it is not ...
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.
''Capitella teleta''
''C. teleta'' is a segmented worm found in North America that is capable of regenerating posterior segments after amputation. 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.
''Planaria vitta''
''P. vitta'' is a
flatworm of genus ''
Planaria
''Planaria'' is a genus of planarians in the family Planariidae. When an individual is cut into pieces, each piece has the ability to regenerate into a fully formed individual.
Description
Currently the genus ''Planaria'' is defined as fres ...
'' 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.
This mechanism activates the neoblasts which are
totipotent Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta.
According to Latin terms, Pluripotentia means the ability for many thin ...
stems cells 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
The dorsal and ventral epithelial cells then come to the site and become differentiated to begin regeneration.
The polarity of the planaria can be reestablished through an anterior-posterior gradient through Wnt/β-catenin signaling pathway.
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.
References
{{reflist
Developmental biology
Animal anatomy