Somite
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The Anome
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{{Infobox Embryology |
Name = Somite |
Latin = |
GraySubject = 9 |
GrayPage = 52 |
Image = Image19.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: |accessdate=2007-10-13 |format= |work=}}</ref> |
Precursor = [[paraxial mesoderm]] |
GivesRiseTo = [[dermatome]], [[myotome]], [[sclerotome]] |
MeshName = Somites |
MeshNumber = A16.254.425.660.750 |
DorlandsPre = s_15 |
DorlandsSuf = 12743985 |
}}
A '''somite''' is a division of the body of an animal. In vertebrates this is mainly discernible in the embryo stage, in arthropods it is a characteristic of a hypothetical ancestor.
==In vertebrates==
In the developing [[vertebrate]] [[embryo]], somites (or '''primitive segments''' in older texts) are masses of [[mesoderm]] distributed along the two sides of the [[neural tube]] and that will eventually become dermis ([[dermatome]]), skeletal muscle ([[myotome]]), and vertebrae ([[sclerotome]]).
Because the sclerotome differentiates before the other two structures, the term "dermomyotome" is sometimes used to describe the combined dermatome and myotome.<ref>{{cite web |url=http://www.med.unc.edu/embryo_images/unit-mslimb/mslimb_htms/mslimb008.htm |title=unc |accessdate=2007-10-19 |format= |work=}}</ref>
==Somites and Their Derivatives==
{{wikify|date=December 2007}}
===Overview===
* [[Mesoderm]] forms at the same time as [[ectoderm]] and [[endoderm]]
* The mesoderm that is lateral (proximal) to the neural tube is called paraxial mesoderm (not the part underneath, that’s chordmesdorm that becomes the notochord)
* The paraxial mesoderm is initially called the “segmental plate” in chick or “unsegmented mesoderm” in other vertebrates
* As the primitive streak regresses (and neural folds gather, to eventually become the [[neural plate]]), the paraxial mesoderm separates into blocks
* Blocks are called somites
* Somites have three compartments:
* Sclerotome = [[vertebrae]] and rib cartilage
* Myotome = [[musculature]] of back, ribs and limbs
* Dermatome = [[skin]] on back
* Syndetome = [[tendons]] and some [[blood vessels]]
* Specify migration paths of [[neural crest]] cells and spinal nerve [[axons]]
* Specified by [[BMP]] gradient set up by [[Noggin]]
* Somites are transient but set up spinal axon paths and neural crest cell paths
===Formation of Somites===
* Clock and wave mechanism
* Oscillating signal of Notch and Wnt pathways = clock
* Set before mesoderm becomes capable of forming somites
* Gradient of [[FGF]] that is [[rostral]] to [[caudal]] (nose to tail gradient) = wave
* Different timing for different organisms:
** In Chick: Somites are formed every 90 minutes
** In Mouse: Variable time frame
* Somites bud off rostrally at regular intervals
**Sometimes called somitomeres
**The outer cells become [[epithelium]]
**Inner cells remain as [[mesenchyme]]
* Number of somites can be used to determine what stage of development the [[embryo]] is at (because rates of development can be affected by temperature or other factors, absolute age is not a good indicator of development)
* Somites appear on both sides of the [[neural tube]] simultaneously
* Flipping stuff around has no affect on which ends develop as rostral/caudal, even fully excising the tissue, it will still order itself properly and at the right times
* Somite formation can be induced by Noggin-secreting cells
* Number of somites is species dependent and independent of embryo size (changed via surgery or genetic engineering)
**Chick: 50
**Mice: 65
**Snake: 500
===Notch Signalling===
* Notch forms the boundaries of the somites
* Dll1 and Dll3 are Notch ligands, mutations of which cause various defects
* Notch regulates Hairy1 which sets up the caudal half of the somite
* Mesp2 induces EphA4 which causes repulsive interaction that separates somites (causes segmentation)
* EphA4 is restricted to the boundaries of somites
* Ephrin-B2 also important for boundaries
===Epithelialization of Somites===
* Fibronectin and N-cadherin are key to epithelialization
* Probably regulated by Paraxis and mesp2
* Mesp2 regulated by Notch signaling
* Paraxis regulated by processes involving the cytoskeleton
===Anterior-Posterior Axis Specification===
* Hox genes specify which somite becomes which feature
* Specification occurs very early
** After somites are made, they are set for what they will become, transplantation of somites results in the wrong type of vertebrae forming in the wrong place
** The internal cells of the somite are not predestined
Somites Make What?
* Rib and vertebrae cartilage
* Muscles:
**Rib cage
**Limbs
**Abdominal wall
**Back and tongue
*Dorsal skin dermis (back skin)
==In crustaceans==
In [[crustacea]]n biology, a somite is a segment of the hypothetical primitive [[crustacean body plan]]. In current crustaceans, several of those somites may be fused.
==References==
<references/>
==Additional images==
<gallery>
Image:Gray18.png|Chick embryo of thirty-three hours’ incubation, viewed from the dorsal aspect. X 30.
</gallery>
==External links==
* {{EmbryologyUNSW|Notes/week3_6}}
{{Embryology}}
[[Category:Embryology]]
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