The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of
paraxial mesoderm
Paraxial mesoderm, also known as presomitic or somitic mesoderm, is the area of mesoderm in the neurulating embryo that flanks and forms simultaneously with the neural tube. The cells of this region give rise to somites, blocks of tissue running ...
that form in the
embryonic stage of
somitogenesis
Somitogenesis is the process by which somites form. Somites are bilaterally paired blocks of paraxial mesoderm that form along the anterior-posterior axis of the developing embryo in vertebrates. The somites give rise to skeletal muscle, cartila ...
, along the head-to-tail axis in
segmented animals. In
vertebrate
Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebra ...
s, somites subdivide into the
dermatomes,
myotomes,
sclerotomes and
syndetomes that give rise to the
vertebrae
Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...
of the
vertebral column
The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...
,
rib cage
The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great ve ...
, part of the
occipital bone
The occipital bone () is a neurocranium, cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lob ...
,
skeletal muscle
Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...
,
cartilage
Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...
,
tendon
A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...
s, and
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different ...
(of the back).
The word ''somite'' is sometimes also used in place of the word ''
metamere''. In this definition, the somite is a
homologously-paired structure in an animal
body plan
A body plan, (), or ground plan is a set of morphology (biology), morphological phenotypic trait, features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many.
This term, usually app ...
, such as is visible in
annelids
The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...
and
arthropods
Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
.
Development

The
mesoderm
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...
forms at the same time as the other two
germ layer
A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce tw ...
s, the
ectoderm
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...
and
endoderm
Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gastr ...
. The mesoderm at either side of the neural tube is called
paraxial mesoderm
Paraxial mesoderm, also known as presomitic or somitic mesoderm, is the area of mesoderm in the neurulating embryo that flanks and forms simultaneously with the neural tube. The cells of this region give rise to somites, blocks of tissue running ...
. 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
In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural folds become elevated, ...
), the paraxial mesoderm separates into blocks called somites.
Formation

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
Somitogenesis is the process by which somites form. Somites are bilaterally paired blocks of paraxial mesoderm that form along the anterior-posterior axis of the developing embryo in vertebrates. The somites give rise to skeletal muscle, cartila ...
.
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 and
Wnt signals provide the clock. The wave is a gradient of the
fibroblast growth factor 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 the previous one.
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
A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus'' ...
the interval is 2 hours.
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-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.
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
Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
around each somite. The inner cells remain as
mesenchyme
Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood, or bone. The interactions between mesenchyme and epithelium help to form nearly ever ...
.
Notch signalling
The Notch system, as part of the clock and wavefront model, forms the boundaries of the somites.
DLL1 and
DLL3 are Notch
ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
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
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...
.
Specification
The
Hox gene
Hox genes, a subset of homeobox, homeobox genes, are a gene cluster, group of related genes that Evolutionary developmental biology, specify regions of the body plan of an embryo along the craniocaudal axis, head-tail axis of animals. Hox protein ...
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.
Derivatives

In the developing vertebrate
embryo
An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...
, somites split to form dermatomes, skeletal muscle (myotomes),
tendon
A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...
s and cartilage (syndetomes)
and
bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
(sclerotomes).
Because the sclerotome differentiates before the dermatome and the myotome, the term dermomyotome refers to the combined dermatome and myotome before they separate out.
Dermatome
The dermatome is the dorsal portion of the paraxial mesoderm somite which gives rise to the skin (
dermis
The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
). In the human embryo, it arises in the third week of
embryogenesis
An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...
.
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.
The dermatomes contribute to the skin,
fat
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers specif ...
and
connective tissue
Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...
of the
neck
The neck is the part of the body in many vertebrates that connects the head to the torso. It supports the weight of the head and protects the nerves that transmit sensory and motor information between the brain and the rest of the body. Addition ...
and of the trunk, though most of the skin is derived from
lateral plate mesoderm
The lateral plate mesoderm is the mesoderm that is found at the periphery of the embryo. It is to the side of the paraxial mesoderm, and further to the axial mesoderm. The lateral plate mesoderm is separated from the paraxial mesoderm by a narrow r ...
.
Myotome
The myotome is that part of a somite that forms the muscles of the animal.
Each myotome divides into an
epaxial part (''epimere''), at the back, and a hypaxial part (''hypomere'') at the front.
The
myoblasts 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.
Sclerotome
The sclerotome (or cutis plate) forms the
vertebra
Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...
e and the rib cartilage and part of the occipital bone; the myotome forms the
musculature
Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...
of the back, the ribs and the limbs; the syndetome forms the tendons and the dermatome forms the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different ...
on the back. In addition, the somites specify the migration paths of
neural crest
The neural crest is a ridge-like structure that is formed transiently between the epidermal ectoderm and neural plate during vertebrate development. Neural crest cells originate from this structure through the epithelial-mesenchymal transition, ...
cells and the
axon
An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...
s of
spinal nerve
A spinal nerve is a mixed nerve, which carries Motor neuron, motor, Sensory neuron, sensory, and Autonomic nervous system, autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each s ...
s. From their initial location within the somite, the sclerotome cells migrate medially towards the
notochord
The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
. 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.
[
Walker, Warren F., Jr. (1987) ''Functional Anatomy of the Vertebrate'' San Francisco: Saunders College Publishing.] From this vertebral body, sclerotome cells move dorsally and surround the developing
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...
, forming the vertebral arch. Other cells move distally to the costal processes of
thoracic vertebrae
In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae of intermediate size between the ce ...
to form the ribs.
In arthropods
In
crustacean
Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...
development, a somite is a segment of the hypothetical primitive crustacean body plan. In current crustaceans, several of those somites may be fused.
See also
*
Developmental biology
Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of Regeneration (biology), regeneration, asexual reproduction, metamorphosis, and the growth and di ...
References
External links
*
{{Authority control
Embryology