Somite
Encyclopedia
A somite is a division of the body of an animal. In vertebrate
s this is mainly discernible in the embryo stage; in arthropod
s it is a characteristic of a hypothetical ancestor.
embryo
, somites 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.
forms at the same time as ectoderm
and endoderm
. The mesoderm that is lateral to (at the side of) the neural tube is called paraxial mesoderm. It is distinct from the mesoderm underneath the neural tube which is called the chordamesoderm
and 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. These somites have four compartments: the sclerotome forms the vertebrae and the rib cartilage; the myotome forms the musculature
of the back, the ribs and the limbs; the dermatome
forms the skin
on the back; and the syndetome forms the tendon
s and some blood vessel
s. In addition the somites specify the migration paths of neural crest
cells and spinal nerve axon
s.
". Somitogenesis depends on a clock and wave mechanism. Ocsilating Notch and Wnt signaling pathway
s provide the clock. The wave is a gradient of the FGF
protein that is 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 those already in existing somites.
The timing of the interval is not universal. Different species have different interval timing. In the chick
embryo somites are formed every 90 minutes. In the mouse
the interval is variable.
n biology, a somite is a segment of the hypothetical primitive crustacean body plan. In current crustaceans, several of those somites may be fused.
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s this is mainly discernible in the embryo stage; in arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s it is a characteristic of a hypothetical ancestor.
In vertebrates
In the developing vertebrateVertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
, somites are masses of mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
distributed along the two sides of the neural tube
Neural tube
In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
and that will eventually become dermis (dermatome
Dermatome
Dermatome may refer to:*Dermatome , a surgical instrument used to produce thin slices of skin*Dermatome , an area of skin that is supplied by a single pair of dorsal roots*Dermatome...
), skeletal muscle (myotome
Myotome
In vertebrate embryonic development, a myotome is a group of tissues formed from somites that develop into the body wall muscle.Each myotome divides into a dorsal epaxial part and a ventral hypaxial part....
), and vertebrae (sclerotome
Sclerotome
A sclerotome is part of a somite, a structure in vertebrate embryonic development. Sclerotomes eventually differentiate into the vertebrae and most of the skull...
).
Because the sclerotome differentiates before the other two structures, the term "dermomyotome" is sometimes used to describe the combined dermatome and myotome.
Overview
The mesodermMesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
forms at the same time as ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
and endoderm
Endoderm
Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...
. The mesoderm that is lateral to (at the side of) the neural tube is called paraxial mesoderm. It is distinct from the mesoderm underneath the neural tube which is called the chordamesoderm
Chordamesoderm
Chordamesoderm, also known as axial mesoderm, is a type of mesoderm that lies along the central axis, under the neural tube.* will give rise to notochord* starts as the notochordal process, whose formation finishes at day 20....
and 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
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
regresses and neural folds gather (to eventually become the neural tube
Neural tube
In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
), the paraxial mesoderm separates into blocks called somites. These somites have four compartments: the sclerotome forms the vertebrae and the rib cartilage; the myotome forms the musculature
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
of the back, the ribs and the limbs; the dermatome
Dermatome
Dermatome may refer to:*Dermatome , a surgical instrument used to produce thin slices of skin*Dermatome , an area of skin that is supplied by a single pair of dorsal roots*Dermatome...
forms the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
on the back; and the syndetome forms the tendon
Tendon
A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...
s and some blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s. In addition the somites specify the migration paths of neural crest
Neural crest
Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia....
cells and spinal nerve axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
s.
Formation of somites
The cells which will become somites are committed before mesoderm becomes capable of forming somites. The formation of somites is called "somitogenesisSomitogenesis
Somitogenesis is the process by which somites are produced. Somites are bilaterally paired blocks of mesoderm that form along the anterior-posterior axis of the developing embryos of segmented animals, often originating in an anterior to posterior direction. In vertebrates, somites give rise to...
". Somitogenesis depends on a clock and wave mechanism. Ocsilating Notch and Wnt signaling pathway
Wnt signaling pathway
The Wnt signaling pathway is a network of proteins best known for their roles in embryogenesis and cancer, but also involved in normal physiological processes in adult animals.-Discovery:...
s provide the clock. The wave is a gradient of the FGF
FGF
FGF may refer to:* Federación Gallega de Fútbol, the governing body of the sport of football in Galicia, Spain* Fédération Guinéenne de Football, the governing body of football in Guinea...
protein that is rostral to caudal
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...
(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 those already in existing somites.
The timing of the interval is not universal. Different species have different interval timing. In the chick
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...
embryo somites are formed every 90 minutes. In the mouse
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
the interval is variable.
-
- Sometimes called somitomeres, indicating a lack of complete separation between segments
- The outer cells become epitheliumEpitheliumEpithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
- Inner cells remain as mesenchymeMesenchymeMesenchyme, or mesenchymal connective tissue, is a type of undifferentiated loose connective tissue that is derived mostly from mesoderm, although some are derived from other germ layers; e.g. some mesenchyme is derived from neural crest cells and thus originates from the ectoderm...
- Number of somites can be used to determine what stage of development the embryoEmbryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
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 tubeNeural tubeIn the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
simultaneously - Flipping stuff around has no effect on which ends develop as rostral or caudal, even fully excising the tissue has no effect, it will still order itself properly and at the right times
- Somite formation can be induced by NogginNoggin (protein)Noggin, also known as NOG, is a protein which in humans is encoded by the NOG gene.Noggin inhibits TGF-β signal transduction by binding to TGF-β family ligands and preventing them from binding to their corresponding receptors. Noggin plays a key role in neural induction by inhibiting BMP4, along...
-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 DLL3DLL3Delta-like 3 , also known as DLL3, is a protein which in humans is encoded by the DLL3 gene. Two transcript variants encoding distinct isoforms have been identified for this gene.- Function :...
are Notch ligands, mutations of which cause various defects - Notch regulates Hairy1, which sets up the caudal half of the somite
- Notch activation turns on Lunatic FringeLFNGLFNG O-fucosylpeptide 3-beta-N-acetylglucosaminyltransferase, also known as LFNG and Lunatic Fringe, is a human gene.-External links:* -Further reading:...
which inhibits the Notch receptor - Notch activation also turns on HESHESHES may refer to* human embyonic stem cells* Environment, Health and Safety * Harvard Extension School* Health Survey For England, an annual statistical survey in England* Hypertext Editing System...
which inactivates Lunatic Fringe, re-enabling the notch receptor, accounting for the oscillating clock model - MESP2MESP2Mesoderm posterior protein 2 , also known as class C basic helix-loop-helix protein 6 , is a protein that in humans is encoded by the MESP2 gene.- Function :...
induces EPHA4 which causes repulsive interaction that separates somites (causes segmentation) - EPHA4 is restricted to the boundaries of somites
- EPHB2 also important for boundaries
Epithelialization of somites
- FibronectinFibronectinFibronectin is a high-molecular weight glycoprotein of the extracellular matrix that binds to membrane-spanning receptor proteins called integrins. In addition to integrins, fibronectin also binds extracellular matrix components such as collagen, fibrin and heparan sulfate proteoglycans...
and N-cadherin are key to epithelialization - Probably regulated by paraxis and MESP2
- MESP2 is regulated by Notch signaling
- Paraxis is regulated by processes involving the cytoskeletonCytoskeletonThe cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic 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 produce
- Rib and vertebrae cartilage
- Muscles (through MyoDMyoDMyoD is a protein with a key role in regulating muscle differentiation. MyoD belongs to a family of proteins known as myogenic regulatory factors . These bHLH transcription factors act sequentially in myogenic differentiation. MRF family members include MyoD, Myf5, myogenin, and MRF4 .MyoD is one...
protein formation):- Rib cage
- Limbs
- Abdominal wall
- Back and tongue
- Dorsal skin dermis (back skin)
In crustaceans
In crustaceaCrustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
n biology, a somite is a segment of the hypothetical primitive crustacean body plan. In current crustaceans, several of those somites may be fused.