In the developing

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 ...

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 ...

, the somitomeres (or somatomeres) are collections of cells that are derived from the loose masses 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 are found alongside the developing

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, ...

. In human embryogenesis they appear towards the end of the third

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregn ...

al week. The approximately 50 pairs of somitomeres in the human embryo, begin developing in the cranial (head) region, continuing in a caudal (tail) direction until the end of week four.

Development

The first seven somitomeres give rise to the striated muscles of the

face The face is the front of the head that features the eyes, nose and mouth, and through which animals express many of their emotions. The face is crucial for human identity, and damage such as scarring or developmental deformities may affect th ...

, jaws, and

throat In vertebrate anatomy, the throat is the front part of the neck, internally positioned in front of the vertebrae. It contains the Human pharynx, pharynx and larynx. An important section of it is the epiglottis, separating the esophagus from the t ...

.Larsen W.J. Human Embryology. Churchill Livingstone.Third edition 2001.Page 62. The remaining somitomeres, likely driven by periodic expression of the '' hairy'' gene, begin expressing adhesion proteins such as N-cadherin and

fibronectin Fibronectin is a high- molecular weight (~500-~600 kDa) glycoprotein of the extracellular matrix that binds to membrane-spanning receptor proteins called integrins. Fibronectin also binds to other extracellular matrix proteins such as col ...

, compact, and bud off forming

somite The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryogenesis, embryonic stage of somitogenesis, along the head-to-tail axis in segmentation (biology), segmented animals. ...

s. The somites give rise to 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 ...

(

sclerotome The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryogenesis, embryonic stage of somitogenesis, along the head-to-tail axis in segmentation (biology), segmented animals. ...

), associated muscles ( myotome), and overlying dermis (

dermatome Dermatome may refer to: * Dermatome (anatomy), an area of skin that is supplied by a single pair of dorsal roots * Dermatome (embryology), the portion of the embryonic paraxial mesoderm, the somite, which gives rise to dermis * Dermatome (instrument ...

). There are a total of 37 somite pairs at the end of the fifth week of development, after the first occipital somite and 5-7 coccygeal somites disappear from the original 42-44 somites.

References

Embryology {{developmental-biology-stub