In the developing

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

embryo An embryo is an initial stage of development of 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 spe ...

, the somitomeres (or somatomeres) are collections of

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

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 fold become elevated, ...

. In

human embryogenesis Human embryonic development, or human embryogenesis, is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of de ...

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 pr ...

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 an animal's 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 aff ...

, 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 pharynx and larynx. An important section of it is the epiglottis, separating the esophagus from the trachea (windpip ...

.Larsen W.J. Human Embryology. Churchill Livingstone.Third edition 2001.Page 62. The remaining somitomeres, likely driven by periodic expression of the ''

hairy Hairy may refer to: * people or animals covered in hairs or fur * plants covered in trichomes * insects covered in setae * people nicknamed "the Hairy" * Hairy (gene) The hairy localisation element (HLE) is an RNA element found in the 3' UTR of ...

'' gene, begin expressing adhesion proteins such as

N-cadherin Cadherin-2 also known as Neural cadherin (N-cadherin), is a protein that in humans is encoded by the ''CDH2'' gene. CDH2 has also been designated as CD325 (cluster of differentiation 325). Cadherin-2 is a transmembrane protein expressed in multip ...

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 coll ...

, 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 embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide ...

s. The somites give rise to the

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

(

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

), associated muscles (

myotome A myotome is the group of muscles that a single spinal nerve innervates. Similarly a dermatome is an area of skin that a single nerve innervates with sensory fibers. Myotomes are separated by myosepta (singular: myoseptum). In vertebrate embryon ...

), and overlying dermis ( dermatome). 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 The coccyx ( : coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the vertebral column in all apes, and analogous structures in certain other mammals such as horses. In tailless primates (e.g. humans and other ...

somites disappear from the original 42-44 somites.

References

External links

* - look for Fig. 16 Embryology {{developmental-biology-stub