Somite

A somite is a division of the body of an animal. In vertebrates this is mainly discernible in the embryo stage, in athropods 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. Somites and Their Derivatives

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 o	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 o	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 o	Oscillating signal of Notch and Wnt pathways = clock 	Set before mesoderm becomes capable of forming somites o	Gradient of FGF that is rostral to caudal (nose to tail gradient) = wave o	Different timing for different organisms: 	In Chick: Somites are formed every 90 minutes 	In Mouse: Variable time frame o	Somites bud off rostrally at regular intervals 	Sometimes called somitomeres 	The outer cells become epithelium 	Inner cells remain as mesenchyme o	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 dependant and independent of embryo size (changed via surgery or genetic engineering) o	Chick: 50 o	Mice: 65 o	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) o	EphA4 is restricted to the boundaries of somites o	Ephrin-B2 also important for boundaries

Epithelialization of Somites •	Fibronectin and N-cadherin are key to epithelialization o	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 o	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 o	The internal cells of the somite are not predestined

Somites Make What? •	Rib and vertebrae cartilage •	Muscles: o	Rib cage o	Limbs o	Abdominal wall o	Back and tongue •	Dorsal skin dermis (back skin)

In crustaceans
In crustacean biology, a somite is a segment of the hypothetical primitive crustacean body plan. In current crustaceans, several of those somites may be fused.