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Somites (red) in a human embryo (back view). Labeling in Dutch

A follow (from Latin . Somitus ) is the Ur segment ( "somites"), which temporarily in the embryonic development of vertebrates occurs and from the somitomere is formed.

The somites are tied off in the head-to-tail direction ( craniocaudal ) from the mesoderm to the side of the midline ( paraxial ). They are therefore located in two strands to the right and left of the axial structures of the notochord and neural tube .

The Thus consists initially of epithelium with a mesenchymal cavity, the Henceocoel . Later the ventromedial (towards the middle of the abdomen) part is called mesenchymal and called the sclerotome . The dorsolateral part (facing the lateral back), remaining epithelial, is called the dermatomyotome .

Derivatives of Somites

The dermatomyotomes are differentiated into a dermatome , from which the dermis (dermis) and subcutis (subcutaneous tissue ) arise, and into a myotome (from the Greek mys “muscle”), which is the starting point for the trunk and limb muscles.

From the sclerotomes go including the vortex produced. Resegmentation takes place in that the rear part of one sclerotome forms a vertebra with the front part of the next. The vertebrae are thus shifted by half a segment compared to the somites ("original vortices"). The segmental back muscles, which arise from the dermomyotome and are not resegmented, attach to neighboring vertebrae.

Articulated animals

Especially in the arthropods , “somites” are primarily understood to be uniform body segments that are normally sclerotized ( impregnated with calcium carbonate (lime)), with sclerites (cover plates made of chitin ).

Individual evidence

  1. ^ Ulrich Lehmann: Paleontological Dictionary . 4th edition. Ferdinand Enke Verlag, Stuttgart 1996, p. 222 .