Editing Muscle cell (section)

== Evolution ==
{{Further|Evolution|Adaptation}}
The [[evolution]]ary origin of muscle cells in [[animals]] is highly debated: One view is that muscle cells evolved once, and thus all muscle cells have a single common ancestor. Another view is that muscles cells evolved more than once, and any [[Morphological convergence|morphological]] or structural similarities are due to [[convergent evolution]], and the development of shared genes that predate the evolution of muscle – even the [[mesoderm]] (the [[germ layer]]) that gives rise to muscle cells in vertebrates).

Schmid & Seipel (2005)<ref name="Seipel and Schmid"/> argue that the origin of muscle cells is a [[monophyletic]] trait that occurred concurrently with the development of the digestive and nervous systems of all animals, and that this origin can be traced to a single metazoan ancestor in which muscle cells are present. They argue that molecular and morphological similarities between the muscles cells in [[non-bilaterian]] [[Cnidaria]] and [[Ctenophora]], are similar enough to those of [[bilateria]]ns that there would be one ancestor in metazoans from which muscle cells derive. In this case, Schmid & Seipel argue that the last common ancestor of Bilateria, Ctenophora and Cnidaria, was a [[Triploblasty|triploblast]] (an organism having three germ layers), and that [[diploblasty]], meaning an organism with two germ layers, evolved secondarily, because of their observation of the lack of mesoderm or muscle found in most cnidarians and ctenophores. By comparing the morphology of cnidarians and ctenophores to bilaterians, Schmid & Seipel were able to conclude that there were [[myoblast]]-like structures in the tentacles and gut of some species of cnidarians and the tentacles of ctenophores. Since this is a structure unique to muscle cells, these scientists determined based on the data collected by their peers that this is a marker for [[Striated muscle tissue|striated muscles]] similar to that observed in bilaterians. The authors also remark that the muscle cells found in cnidarians and ctenophores are often contested due to the origin of these muscle cells being the [[ectoderm]] rather than the mesoderm or mesendoderm.

The origin of true muscle cells is argued by other authors to be the [[endoderm]] portion of the [[mesoderm]] and the endoderm. However, Schmid & Seipel (2005)<ref name="Seipel and Schmid"/> counter skepticism – about whether the muscle cells found in ctenophores and cnidarians are "true" muscle cells – by considering that cnidarians develop through a medusa stage and polyp stage. They note that in the hydrozoans' medusa stage, there is a layer of cells that separate from the distal side of the ectoderm, which forms the striated muscle cells in a way similar to that of the mesoderm; they call this third separated layer of cells the ''ectocodon''. Schmid & Seipel argue that, even in bilaterians, not all muscle cells are derived from the mesendoderm: Their key examples are that in both the eye muscles of vertebrates and the muscles of spiralians, these cells derive from the ectodermal mesoderm, rather than the endodermal mesoderm. Furthermore, they argue that since myogenesis does occur in cnidarians with the help of the same molecular regulatory elements found in the specification of muscle cells in bilaterians, that there is evidence for a single origin for striated muscle.<ref name="Seipel and Schmid">{{cite journal |first1 = Katja |last1 = Seipel |first2 = Volker |last2 = Schmid |date = 1 June 2005 |title = Evolution of striated muscle: Jellyfish and the origin of triploblasty |journal = Developmental Biology |volume = 282 |issue = 1 |pages = 14–26 |pmid=15936326 |doi = 10.1016/j.ydbio.2005.03.032 |doi-access = free}}</ref>

In contrast to this argument for a single origin of muscle cells, Steinmetz, Kraus, ''et al''. (2012)<ref name="independent evolution"/> argue that molecular markers such as the [[Myosin|myosin II]] protein used to determine this single origin of striated muscle predate the formation of muscle cells. They use an example of the contractile elements present in the Porifera, or sponges, that do truly lack this striated muscle containing this protein. Furthermore, Steinmetz, Kraus, ''et al''. present evidence for a [[Polyphyly|polyphyletic]] origin of striated muscle cell development through their analysis of morphological and molecular markers that are present in bilaterians and absent in cnidarians, ctenophores, and bilaterians. Steinmetz, Kraus, ''et al''. showed that the traditional morphological and regulatory markers such as [[actin]], the ability to couple myosin side chains phosphorylation to higher concentrations of the positive concentrations of calcium, and other [[Myosin|MyHC]] elements are present in all metazoans not just the organisms that have been shown to have muscle cells. Thus, the usage of any of these structural or regulatory elements in determining whether or not the muscle cells of the cnidarians and ctenophores are similar enough to the muscle cells of the bilaterians to confirm a single lineage is questionable according to Steinmetz, Kraus, ''et al''. Furthermore, they explain that the orthologues of the Myc genes that have been used to hypothesize the origin of striated muscle occurred through a gene duplication event that predates the first true muscle cells (meaning striated muscle), and they show that the Myc genes are present in the sponges that have contractile elements but no true muscle cells. Steinmetz, Kraus, ''et al''. also showed that the localization of this duplicated set of genes that serve both the function of facilitating the formation of striated muscle genes, and cell regulation and movement genes, were already separated into striated much and non-muscle MHC. This separation of the duplicated set of genes is shown through the localization of the striated much to the contractile vacuole in sponges, while the non-muscle much was more diffusely expressed during developmental cell shape and change. Steinmetz, Kraus, ''et al''. found a similar pattern of localization in cnidarians except with the cnidarian ''N.&nbsp;vectensis'' having this striated muscle marker present in the smooth muscle of the digestive tract. Thus, they argue that the pleisiomorphic trait of the separated orthologues of much cannot be used to determine the monophylogeny of muscle, and additionally argue that the presence of a striated muscle marker in the smooth muscle of this cnidarian shows a fundamental different mechanism of muscle cell development and structure in cnidarians.<ref name="independent evolution">{{cite journal |first1 = Patrick R.H. |last1 = Steinmetz |first2 = Johanna E.M. |last2 = Kraus |first3 = Claire |last3 = Larroux |first4 = Jörg U. |last4 = Hammel |first5 = Annette |last5 = Amon-Hassenzahl |first6 = Evelyn |last6 = Houliston |first7 = Gert |last7 = Wörheide |first8 = Michael |last8 = Nickel |first9 = Bernard M. |last9 = Degnan |display-authors = 6 |year = 2012 |title = Independent evolution of striated muscles in cnidarians and bilaterians |journal = Nature |volume = 487 |issue = 7406 |pages = 231–234 |pmc = 3398149 |pmid = 22763458 |doi = 10.1038/nature11180 |bibcode = 2012Natur.487..231S}}</ref>

Steinmetz, Kraus, ''et al''. (2012)<ref name="independent evolution"/> further argue for multiple origins of striated muscle in the metazoans by explaining that a key set of genes used to form the troponin complex for muscle regulation and formation in bilaterians is missing from the cnidarians and ctenophores, and 47&nbsp;structural and regulatory proteins observed, Steinmetz, Kraus, ''et al''. were not able to find even on unique striated muscle cell protein that was expressed in both cnidarians and bilaterians. Furthermore, the Z-disc seemed to have evolved differently even within bilaterians and there is a great deal of diversity of proteins developed even between this clade, showing a large degree of radiation for muscle cells. Through this divergence of the [[Sarcomere|Z-disc]], Steinmetz, Kraus, ''et al''. argue that there are only four common protein components that were present in all bilaterians muscle ancestors and that of these for necessary Z-disc components only an actin protein that they have already argued is an uninformative marker through its pleisiomorphic state is present in cnidarians. Through further molecular marker testing, Steinmetz et al. observe that non-bilaterians lack many regulatory and structural components necessary for bilaterians muscle formation and do not find any unique set of proteins to both bilaterians and cnidarians and ctenophores that are not present in earlier, more primitive animals such as the sponges and [[amoebozoa]]ns. Through this analysis, the authors conclude that due to the lack of elements that bilaterian muscles are dependent on for structure and usage, nonbilaterian muscles must be of a different origin with a different set of regulatory and structural proteins.<ref name="independent evolution"/>

In another take on the argument, Andrikou & Arnone (2015)<ref name=myogenesis/> use the newly available data on [[gene regulatory network]]s to look at how the hierarchy of genes and morphogens and another mechanism of tissue specification diverge and are similar among early deuterostomes and protostomes. By understanding not only what genes are present in all bilaterians but also the time and place of deployment of these genes, Andrikou & Arnone discuss a deeper understanding of the evolution of myogenesis.<ref name=myogenesis/>

In their paper, Andrikou & Arnone (2015)<ref name=myogenesis/> argue that to truly understand the evolution of muscle cells the function of transcriptional regulators must be understood in the context of other external and internal interactions. Through their analysis, Andrikou & Arnone found that there were conserved [[orthologues]] of the gene regulatory network in both invertebrate bilaterians and cnidarians. They argue that having this common, general regulatory circuit allowed for a high degree of divergence from a single well-functioning network. Andrikou & Arnone found that the orthologues of genes found in vertebrates had been changed through different types of structural mutations in the invertebrate deuterostomes and protostomes, and they argue that these structural changes in the genes allowed for a large divergence of muscle function and muscle formation in these species. Andrikou & Arnone were able to recognize not only any difference due to mutation in the genes found in vertebrates and invertebrates but also the integration of species-specific genes that could also cause divergence from the original gene regulatory network function. Thus, although a common muscle patterning system has been determined, they argue that this could be due to a more ancestral gene regulatory network being coopted several times across lineages with additional genes and mutations causing very divergent development of muscles. Thus it seems that the myogenic patterning framework may be an ancestral trait. However, Andrikou & Arnone explain that the basic muscle patterning structure must also be considered in combination with the [[Cis-Regulatory element|cis regulatory elements]] present at different times during development. In contrast with the high level of gene family apparatuses structure, Andrikou and Arnone found that the cis-regulatory elements were not well conserved both in time and place in the network which could show a large degree of divergence in the formation of muscle cells. Through this analysis, it seems that the myogenic GRN is an ancestral GRN with actual changes in myogenic function and structure possibly being linked to later coopts of genes at different times and places.<ref name=myogenesis>{{Cite journal |first1 = Carmen |last1 = Andrikou |first2 = Maria Ina |last2 = Arnone |title = Too many ways to make a muscle: Evolution of GRNs governing myogenesis |journal = Zoologischer Anzeiger |date = 1 May 2015 |volume = 256 |pages = 2–13 |series = Special Issue: Proceedings of the 3rd International Congress on Invertebrate Morphology |doi = 10.1016/j.jcz.2015.03.005}}</ref>

Evolutionarily, specialized forms of skeletal and [[cardiac muscle]]s predated the divergence of the [[vertebrate]] / [[arthropod]] evolutionary line.<ref name=evolution>{{cite journal |last1=OOta |first1=S. |last2=Saitou |first2=N. |year=1999 |title=Phylogenetic relationship of muscle tissues deduced from the superimposition of gene trees |journal=Molecular Biology and Evolution |volume=16 |issue=6 |pages=856–867 |issn=0737-4038 |doi=10.1093/oxfordjournals.molbev.a026170 |doi-access=free |pmid=10368962}}</ref> This indicates that these types of muscle developed in a common [[ancestor]] sometime before 700&nbsp;[[mya (unit)|million years ago (mya)]]. Vertebrate smooth muscle was found to have evolved independently from the skeletal and cardiac muscle types.

=== Invertebrate muscle cell types ===
The properties used for distinguishing fast, intermediate, and slow muscle fibers can be different for invertebrate flight and jump muscle.<ref name="Hoyle 1983">{{cite book | last=Hoyle | first=Graham | year=1983  | chapter=8. Muscle cell diversity | title=Muscles and Their Neural Control | publisher=John Wiley & Sons | location=New York, NY | pages=[https://archive.org/details/musclestheirneur0000hoyl/page/293 293–299] | isbn=9780471877097 | chapter-url=https://archive.org/details/musclestheirneur0000hoyl/page/293 }}</ref> To further complicate this classification scheme, the mitochondrial content, and other morphological properties within a muscle fiber, can change in a [[tsetse fly]] with exercise and age.<ref name="Anderson, M., and Finlayson, L. H. 1976">{{cite journal | last1 = Anderson | first1 = M. | last2 = Finlayson | first2 = L.H. | year = 1976 | title = The effect of exercise on the growth of mitochondria and myofibrils in the flight muscles of the Tsetse fly, Glossina morsitans | journal = J. Morphol. | volume = 150 | issue = 2 | pages = 321–326 | doi = 10.1002/jmor.1051500205 | s2cid = 85719905 }}</ref>