View source for Somite

{{short description|Each of several blocks of mesoderm that flank the neural tube on either side in embryogenesis}}
{{Infobox embryology
| Name          = Somite
| Latin         = somitus
| Image         = Gray19 with color.png
| Caption       = Transverse section of half of a chick embryo of forty-five hours' incubation. The dorsal (back) surface of the embryo is towards the top of this page, while the ventral (front) surface is towards the bottom.
| Image2        = Gray20.png
| Caption2      = Dorsum of human embryo, 2.11 mm in length. (The older term ''primitive segments'' is used to identify the somites.)
| System        =
| CarnegieStage = 9
| Days          = 20<ref name="umedumt">{{cite web |url=http://staff.um.edu.mt/acus1/Gasrtrulation.htm |title=The Third Week Of Life |access-date=2007-10-13 |last=Cuschieri|first=Alfred|website=[[University of Malta]]}}</ref>
| Precursor     = [[Paraxial mesoderm]]
| GivesRiseTo   = [[#Dermatome|Dermatome]], [[#Myotome|myotome]],
[[#Sclerotome|sclerotome]], [[#Syndetome|syndetome]]
}}
The '''somites''' (outdated term: '''primitive segments''') are a set of bilaterally paired blocks of [[paraxial mesoderm]] that form in the [[embryogenesis|embryonic stage]] of [[somitogenesis]], along the head-to-tail axis in [[segmentation (biology)|segmented]] animals. In [[vertebrate]]s, somites subdivide into the
[[#Dermatome|dermatomes]], [[#Myotome|myotomes]],
[[#Sclerotome|sclerotomes]] and [[#Syndetome|syndetomes]] that give rise to the [[vertebrae]] of the [[vertebral column]], [[rib cage]], part of the [[occipital bone]], [[skeletal muscle]], [[cartilage]], [[tendon]]s, and [[dermis|skin]] (of the back).<ref name="Larsen">{{cite book |last1=Larsen |first1=William J. |url=https://archive.org/details/humanembryology0003lars |title=Human embryology |date=2001 |publisher=Churchill Livingstone |isbn=978-0-443-06583-5 |edition=3. |location=Philadelphia, Pa. |pages=53–86 |url-access=registration}}</ref>
The word ''somite'' is sometimes also used in place of the word ''[[Metamerism (biology)|metamere]]''. In this definition, the somite is a [[Homology (biology)|homologously]]-paired structure in an animal [[body plan]], such as is visible in [[annelids]] and [[arthropods]].<ref>{{cite web |url=http://www.merriam-webster.com/dictionary/metamere|title=Metamere|author=<!--Staff writer(s); no by-line.--> |year=2012|work=Dictionary and Thesaurus-Merriam-Webster Online|publisher=Merriam-Webster|access-date=11 December 2012}}</ref>
==Development==
[[File:Gray18.png|thumb|Chick embryo of thirty-three hours' incubation, viewed from the dorsal aspect. X 30]]
The [[mesoderm]] forms at the same time as the other two [[germ layer]]s, the [[ectoderm]] and [[endoderm]]. The mesoderm at either side of the neural tube is called [[paraxial mesoderm]]. It is distinct from the mesoderm underneath the neural tube, which is called the [[chordamesoderm]] that becomes the notochord. The paraxial mesoderm is initially called the "segmental plate" in the chick embryo or the "unsegmented mesoderm" in other vertebrates. As the [[primitive streak]] regresses and neural folds gather (to eventually become the [[neural tube]]), the paraxial mesoderm separates into blocks called somites.<ref name="Gilbert, S.F. 2010 413–415">{{cite book|author=Gilbert, S.F.|title=Developmental Biology|url=https://archive.org/details/developmentalbio00gilb_892|url-access=limited|year=2010|pages=[https://archive.org/details/developmentalbio00gilb_892/page/n437 413]–415|publisher=Sinauer Associates, Inc. | edition=9th|isbn=978-0-87893-384-6}}</ref>
===Formation===
[[File:Gray64.png|thumb|left|Transverse section of a human embryo of the third week to show the differentiation of the primitive segment. ao. Aorta. m.p. Muscle-plate. n.c. Neural canal. sc. Sclerotome. s.p. Dermatome]]
The pre-somitic mesoderm commits to the somitic fate before mesoderm becomes capable of forming somites. The cells within each somite are specified based on their location within the somite. Additionally, they retain the ability to become any kind of somite-derived structure until relatively late in the process of [[somitogenesis]].<ref name="Gilbert, S.F. 2010 413–415"/>
The development of the somites depends on a clock mechanism as described by the [[clock and wavefront model]]. In one description of the model, oscillating [[Notch signaling pathway|Notch]] and [[Wnt signaling pathway|Wnt]] signals provide the clock. The wave is a gradient of the [[fibroblast growth factor]] protein that is [[Anatomical terms of location#Cranial and caudal|rostral to caudal]] (nose to tail gradient). Somites form one after the other down the length of the embryo from the head to the tail, with each new somite forming on the caudal (tail) side of the previous one.<ref name="pmid16546158">{{Cite journal
| last1 = Baker | first1 = R. E. | author1-link = Ruth Baker
| last2 = Schnell | first2 = S.
| last3 = Maini | first3 = P. K. | author3-link = Philip Maini
| doi = 10.1016/j.ydbio.2006.01.018
| title = A clock and wavefront mechanism for somite formation
| journal = Developmental Biology
| volume = 293
| issue = 1
| pages = 116–126
| year = 2006
| pmid = 16546158
| url = https://ora.ox.ac.uk/objects/uuid:d854bc87-2cc3-4582-95a5-77acb06f4638 | doi-access = free
}}</ref><ref>{{Cite journal |last1=Goldbeter |first1=A. |last2=Pourquié |first2=O. |year=2008 |title=Modeling the segmentation clock as a network of coupled oscillations in the Notch, Wnt and FGF signaling pathways |url=http://www.ulb.ac.be/sciences/utc/ARTICLES/2008_Goldbeter_J_Theor_Biol.pdf |journal=Journal of Theoretical Biology |volume=252 |issue=3 |pages=574–585 |bibcode=2008JThBi.252..574G |doi=10.1016/j.jtbi.2008.01.006 |pmid=18308339 |via={{Lang|fr|[[Université libre de Bruxelles]]|italic=no}}}}</ref>
The timing of the interval is not universal. Different species have different interval timing. In the [[chicken|chick]] embryo, somites are formed every 90 minutes. In the [[mouse]] the interval is 2 hours.<ref>{{cite journal |last1=Wahi |first1=Kanu |title=The many roles of Notch signaling during vertebrate somitogenesis |journal=Seminars in Cell and Developmental Biology |date=2016 |volume=49 |pages=68–75|doi=10.1016/j.semcdb.2014.11.010 |pmid=25483003 |s2cid=10822545 }}</ref>
For some species, the number of somites may be used to determine the stage of embryonic development more reliably than the number of hours post-fertilization because rate of development can be affected by temperature or other environmental factors. The somites appear on both sides of the neural tube simultaneously. Experimental manipulation of the developing somites will not alter the rostral/caudal orientation of the somites, as the cell fates have been determined prior to somitogenesis. Somite formation can be induced by [[Noggin (protein)|Noggin]]-secreting cells. The number of somites is species dependent and independent of embryo size (for example, if modified via surgery or genetic engineering). Chicken embryos have 50 somites; mice have 65, while snakes have 500.<ref name="Gilbert, S.F. 2010 413–415"/><ref>{{cite journal|author=Gomez, C|year=2008|journal=Nature|volume=454|pages=335–339|pmid=18563087|doi=10.1038/nature07020|title=Control of segment number in vertebrate embryos|issue=7202|bibcode=2008Natur.454..335G|s2cid=4373389|display-authors=etal}}</ref>
As cells within the paraxial mesoderm begin to come together, they are termed [[somitomeres]], indicating a lack of complete separation between segments. The outer cells undergo a [[mesenchymal–epithelial transition]] to form an [[epithelium]] around each somite. The inner cells remain as [[mesenchyme]].
{{Clear}}
====Notch signalling ====
{{Main|Notch signaling pathway}}
The Notch system, as part of the clock and wavefront model, forms the boundaries of the somites. [[DLL1]] and [[DLL3]] are Notch [[ligand]]s, mutations of which cause various defects. Notch regulates [[HES1]], which sets up the caudal half of the somite. Notch activation turns on [[LFNG]] which in turn inhibits the Notch receptor. Notch activation also turns on the HES1 gene which inactivates LFNG, re-enabling the Notch receptor, and thus accounting for the oscillating clock model. [[MESP2]] induces the [[EPHA4]] gene, which causes repulsive interaction that separates somites by causing segmentation. EPHA4 is restricted to the boundaries of somites. [[EPHB2]] is also important for boundaries.
====Mesenchymal-epithelial transition====
[[Fibronectin]] and [[N-cadherin]] are key to the mesenchymal–epithelial transition process in the developing embryo. The process is probably regulated by paraxis and MESP2. In turn, MESP2 is regulated by Notch signaling. Paraxis is regulated by processes involving the [[cytoskeleton]].
====Specification====
[[File:Gray65.png |thumb|right|Scheme showing how each vertebral centrum is developed from portions of two adjacent segments. Myotome labelled in upper left.]]
The [[Hox gene]]s specify somites as a whole based on their position along the anterior-posterior axis through specifying the pre-somitic mesoderm before somitogenesis occurs. After somites are made, their identity as a whole has already been determined, as is shown by the fact that transplantation of somites from one region to a completely different region results in the formation of structures usually observed in the original region. In contrast, the cells within each somite retain plasticity (the ability to form any kind of structure) until relatively late in somitic development.<ref name="Gilbert, S.F. 2010 413–415"/>
=== Derivatives ===
[[File:End_of_week_4_Embryo_with_somites.jpg|thumb|Human embryo at the end of week 4 with somite development.]]
{{Anchor|Syndetome}}
In the developing vertebrate [[embryo]], somites split to form dermatomes, skeletal muscle (myotomes), [[tendon]]s and cartilage (syndetomes)<ref name="pmid12705871">{{cite journal |vauthors=Brent AE, Schweitzer R, Tabin CJ |title=A somitic compartment of tendon progenitors |journal=[[Cell (journal)|Cell]] |volume=113 |issue=2 |pages=235–48 |date=April 2003  |pmid=12705871 |doi= 10.1016/S0092-8674(03)00268-X|s2cid=16291509 |doi-access=free }}</ref> and [[bone]] (sclerotomes).<!-- According to the linked source for Syndetomes, they are the tendon progenitors (and also derive from sclerotome). Cartilage should be part of sclerotome, just like bone -->
{{Anchor|Dermomyotome}}
Because the sclerotome differentiates before the dermatome and the myotome, the term '''dermomyotome''' refers to the combined dermatome and myotome before they separate out.<ref>{{cite web |title=Embryo Images |url=http://www.med.unc.edu/embryo_images/unit-mslimb/mslimb_htms/mslimb008.htm |access-date=2007-10-19 |website=[[University of North Carolina School of Medicine]]}}</ref>
====Dermatome====
The '''dermatome''' is the dorsal portion of the paraxial mesoderm somite which gives rise to the skin ([[dermis]]). In the human embryo, it arises in the third week of [[Human embryogenesis|embryogenesis]].<ref name="Larsen" /> It is formed when a dermomyotome (the remaining part of the somite left when the sclerotome migrates), splits to form the dermatome and the myotome.<ref name=Larsen /> The dermatomes contribute to the skin, [[fat]] and [[connective tissue]] of the [[neck]] and of the trunk, though most of the skin is derived from [[lateral plate mesoderm]].<ref name=Larsen />
====Myotome====
The '''myotome''' is that part of a somite that forms the muscles of the animal.<ref name=Larsen /> Each myotome divides into an [[epaxial]] part (''epimere''), at the back, and a hypaxial part (''hypomere'') at the front.<ref name=Larsen /> The [[myoblast]]s from the hypaxial division form the muscles of the thoracic and anterior abdominal walls. The epaxial muscle mass loses its segmental character to form the [[extensor muscles]] of the neck and trunk of mammals.
In fishes, salamanders, caecilians, and reptiles, the body musculature remains segmented as in the embryo, though it often becomes folded and overlapping, with epaxial and hypaxial masses divided into several distinct muscle groups.{{citation needed|date=November 2014}}
====Sclerotome====
{{Anchor|Cutis plate}}
The '''sclerotome''' (or '''cutis plate''') forms the [[vertebra]]e and the rib cartilage and part of the occipital bone; the myotome forms the [[musculature]] of the back, the ribs and the limbs; the syndetome forms the tendons and the dermatome forms the [[skin]] on the back. In addition, the somites specify the migration paths of [[neural crest]] cells and the [[axon]]s of [[spinal nerve]]s. From their initial location within the somite, the sclerotome cells migrate medially towards the [[notochord]]. These cells meet the sclerotome cells from the other side to form the vertebral body. The lower half of one sclerotome fuses with the upper half of the adjacent one to form each vertebral body.<ref name="Walker">
Walker, Warren F., Jr. (1987) ''Functional Anatomy of the Vertebrate''  San Francisco: Saunders College Publishing.</ref> From this vertebral body, sclerotome cells move dorsally and surround the developing [[spinal cord]], forming the vertebral arch. Other cells move distally to the costal processes of [[thoracic vertebrae]] to form the ribs.<ref name="Walker"/>
== In arthropods==
In [[crustacean]] development, a somite is a segment of the hypothetical primitive crustacean body plan. In current crustaceans, several of those somites may be fused.<ref>{{Cite journal |last=Ferrari |first=Frank D. |last2=Fornshell |first2=John |last3=Vagelli |first3=Alejandro A. |last4=Ivanenko |first4=V. N. |last5=Dahms |first5=Hans-Uwe |date=2011 |title=Early Post-Embryonic Development of Marine Chelicerates and Crustaceans with a Nauplius |url=https://repository.si.edu/bitstream/handle/10088/17108/iz_2011FerrariEtAl_Crustaceans_Chelicerates.pdf |journal=Crustaceana |volume=84 |issue=7 |pages=869–893 |issn=0011-216X}}</ref><ref>{{Cite journal |last=Manning |first=Raymond |date=1998 |title=A new genus and species of pinnotherid crab (Crustacea, Decapoda, Brachyura) from Indonesia |url=https://sciencepress.mnhn.fr/en/periodiques/zoosystema/20/2/un-nouveau-genre-et-une-nouvelle-espece-de-crabe-pinnotheridae-crustacea-decapoda-brachyura-d-indonesie |journal=Zoosystema |language=en |volume=20 |issue=2 |pages=357–362}}</ref>
==See also==
*[[Developmental biology]]
== References ==
{{Reflist}}
== External links ==
* {{EmbryologyUNSW|Notes/week3_6}}
{{Embryology}}
{{Authority control}}
[[Category:Embryology]]