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Myogenic cells are potential muscle cells which divide mitotically. Following their exit from the cell cycle at phase G1/G0 they enter the stage of post-mitotic myoblasts. Expression of regulatory proteins of MRF family (MyoD, Myf5, myogenin and MRF-4), which form heterodimers with E family proteins E (E12 and E47) aided by MEF family proteins, starts in their nuclei. The system activates muscle-specific genes. Before the fusion the myoblasts elongate, become linearly arranged, adhere to each other and fuse into an elongated myotube. Transmembrane proteins, e.g. cadherins and integrins, partici- pate in adhesion and fusion. Myogenic cells of non-segmented mesoderm and of newly formed somites are affected by signal proteins, Shh and Wnt, emitted from the neighbouring tissues. These proteins trigger the activity of genes of MRF family, Myf5 and MyoD. Besides, myogenic cells display a great morphogenetic mobility. In Amniota they translocate under the dermatomyotome. Pioneer myoblasts of the myotome grow and elongate. In some amphibian species during somitogenesis myogenic cells rotate from the perpendicular position to the position parallel to the body axis. In other amphibians they elongate along the body axis. As a result of rotation and elongation they occupy the whole length of the myotome and then differentiate into mononucleate myotubes. In fishes myogenic cells of non-segmented mesoderm migrate from paraxial position onto the myotome surface, while „external cells” migrate into the myotome. In amphibians and fishes mesenchymal cells migrate into the myotomes via miosepts. Myoblasts of mesenchymal origin fuse with the myotube. Moreover, myogenic cells are capable of migration for rather long distances. In Amniota cells of the ventral lip of dermatomyotome, and in lower chordates cells of the ventral part of the somite migrate e.g. to primordial limbs, fins and ventral muscles. The migrating cells show an expression of regulatory proteins Pax3 and Lbx1.
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Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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