Somites are transient, segmentally organized structures. In the vertebrate embryo, the somites contribute to multiple tissues, including the axial skeleton, skeletal and smooth muscles, dorsal dermis, tendons, ligaments, cartilage and adipose tissue.
The somites also determine the migration paths of trunk neural crest cells and spinal nerve axons. As the primitive streak regresses and the neural folds begin to gather at the center of the embryo, the paraxial mesoderm separates into blocks of cells called somites. These structures are formed by budding off as epithelial spheres from the cranial end of the unsegmented paraxial mesoderm that lies on either side of the neural tube.
The total number of somites formed is species-specific in humans, 50 in chickens, 65 in mice and is used as an indicator of embryonic developmental stages. Once formed, the epithelial somite is patterned rapidly into distinct compartments that subsequently give rise to distinct cell lineages. In response to signals from surrounding tissues, the ventral portion of the epithelial somite de-epithelializes to form the mesenchymal sclerotome, whereas the dorsal portion, or dermomyotome, remains an epithelial sheet.
It forms a pair of cylinder-shaped epithelially-organized mesenchyma segments that are in the immediate vicinity of the neural tube and the notochord. After the beginning of the 3rd week, these cylinders become segmented from the cranial to the caudal end into so-called somitomeres process of metamerization.
Originally, each consists of a pseudostratified epithelium that is arranged around a central cavity, the somitocoel. The metameres are based on the division of the original embryo into several segments.
To be seen is the differentiation of the mesoblast into the paraxial, intermediate and lateral plate mesoderm. One can recognize the first space of the future intraembryonic coelom. For instance, in the chick embryo, a new somite is formed every 90 minutes, and the somites appear at exactly the same time on the two sides of the embryo.
Once the somites are formed, various regions within are specified to form only certain cell types and will eventually commit, creating a body plan for the organism. The commitment to specific tissue types occurs relatively late in the development of the somite. When the somite is still immature, it is multipotent, meaning each of its cells can become any of the somite-derived structures.
These structures include the cartilage of the vertebrae and ribs, the muscles of the rib cage, limbs, abdominal wall, back, and tongue, the tendons, the dermis of the dorsal skin, and vascular cells that contribute to the formation of the aorta and the intervertebral blood vessels. The somites eventually diverge into sclerotome cartilage , syndotome tendons , myotome skeletal muscle , dermatome dermis , and endothelial cells, each corresponding to different regions within the somite itself.
Each of these committed cell types is determined by induction and relative location within the organism. The processes that follow somitogenesis include myogenesis generation of muscle , osteogenesis generation of bone , tendon formation, and specification of the intermediate mesoderm i. Because the somites are an essential part of the developing body plan of vertebrates, any disruption in the cycle of formation or segmentation can result in anomalies such as congenital vertebral defects.
The myotome is that part of a somite that forms the muscles. 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.
The sclerotome forms the vertebrae and the rib cartilage and part of the occipital bone. It forms the musculature of the back, the ribs, and the limbs.
Learning Objectives Describe the functions of somites. Key Points The paraxial mesoderm is distinct from the mesoderm found more internally in the embryo. Alongside the neural tube, the mesoderm develops distinct paired structures called somites that develop into dermis, skeletal muscle, and vertebrae.
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