In the developing vertebrateembryo, somites are masses of mesoderm distributed along the two sides of the neural tube and that will eventually become dermis (dermatome), skeletal muscle (myotome), and vertebrae (sclerotome). They originate from paraxial mesoderm which, towards the end of the third gestational week, becomes organized into loose masses of cells called somitomeres. Driven by changes in the expression of adhesion molecules, somitomeres compact and bud off to form the somites. Approximately 44 somites form and give rise to the bones of the face, vertebral column, associated muscles, and overlying dermis. Groups Conodonta Hyperoartia Petromyzontidae (lampreys) Pteraspidomorphi (early jawless fish) Thelodonti Anaspida Cephalaspidomorphi (early jawless fish) Galeaspida Pituriaspida Osteostraci Gnathostomata (jawed vertebrates) Placodermi Chondrichthyes (cartilaginous fish) Acanthodii Osteichthyes (bony fish) Actinopterygii (ray-finned fish) Sarcopterygii (lobe-finned fish) Actinistia (coelacanths) Dipnoi (lungfish) Tetrapoda Amphibia Amniota Sauropsida/(Reptiles) Aves (Birds) Synapsida Mammalia... Embryos (and one tadpole) of the wrinkled frog (Rana rugosa). ... The mesoderm is one of the three germ layers in the early developing embryo, the other two layers being the ectoderm and the endoderm. ... The neural tube is the embryonal structure that gives rise to the brain and spinal cord. ... Dermatome has three meanings: in surgery, a surgical instrument used for slicing skin in order to perform skin grafts. ... In vertebrate embryonic development, a group of tissues formed from somites that develop into the body wall muscle. ... In vertebrate embryonic development, a group of embryonic tissues formed from somites that develop into the vertebrae. ... A pregnant woman Pregnancy is the process by which a mammalian female carries a live offspring from conception until it develops to the point where the offspring is capable of living outside the womb. ... In the developing vertebrate embryo, the somitomeres are loose masses of paraxial mesoderm derived cells that form along each side of the neural tube towards the end of the third gestational week. ... The vertebral column seen from the side Different regions (curvatures) of the vertebral column The vertebral column (backbone or spine) is a column of vertebrae situated in the dorsal aspect of the abdomen. ...
Somites are the segmented precursors to the vertebral column and skeletal muscle within the trunk and tail of vertebrate embryos.
In the anterior of this field of somite precursor cells, the stripes of gene expression are stabilized and ultimately determine the position of the next segment border.
Different regions of the anterior/posterior axis require distinct genes to establish the somitic pattern: the Notch pathway mutations, in mouse, zebrafish and humans, mostly affect the posterior somites while the anterior somites are normal.
The sixth by the blood-vessels of the somite; the seventh by the bit of alimentary tract which traverses it; and the eighth by the neuromere (nerve ganglion pair, commissures, connectives and nerve branches).
The praegenital somite of the Arachnida is an example of a somite which is preserved in some members of the group and partially or entirely excalated in other cases, sometimes with fusion of its remnants to neighbouring somites.
Thus, a case of fusion of partially atrophied somites may simulate the appearance of incipient merogenesis or formation of new somites, and, vice versa, incipient merogenesis may be misinterpreted as a case of fusion of once separate and fully-formed somites.
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