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The segmentation of the vertebrate body is established by somitogenesis, during which somite form sequentially in a rhythmic fashion from the paraxial presomitic mesoderm. The somites are transient embryonic segments giving rise to the vertebras, the skeletal muscles and dorsal epidermis. In vertebrates, all cells of the paraxial mesoderm go successively through the three phases of differentiation and matura- tion, which are tightly regulated at the spatio-temporal level. A growth phase during which progenitor cells are produced from mesodermal stem cells, which are localised in the primitive streak and latter in the tail bud, and become organised as two streaks of the mesenchymal cells bilaterally to the neural tube and notochord forming the presomitic mesoderm. A pattering phase occurring in the presomitic mesoderm during which the segmental pattern is established on the molecular level. A segmentation phase in which the somite boundaries are formed and epithelization of mesenchymal cells occurs. Recent data suggest that the molecular oscillatory [segmentation] clock and gradients of signalling molecules control the somitoge- nesis. The periodic expression of several genes, most of which are related to the Notch signalling pathway, is the base of the oscillatory clock. Dynamic gradient of retinoic acid and morphogens FGF6 and Wnt3a are coupled with molecular oscillatory clock. In this review the mechanism of oscillatory clock in zebra- fish, chick and mouse embryos is described.
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The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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