Somite 1036883 215394052 2008-05-28T00:14:10Z The Anome 76 /* In vertebrates */ fmt {{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]] [[de:Somit]] [[fr:Somite]] [[nl:Somiet]] [[sk:Somit]] [[fi:Somiitti]]