Experimental approaches to the study of the formation of axial structures during early development of <Emphasis Type="Italic">Xenopus laevis</Emphasis>

The effect of mechanical extension on the differentiation of axial mesoderm in double explants (sandwiches) of Xenopus laevis embryonic tissues isolated during the early gastrula–late neurula developmen-tal period is studied. In explants at the early gastrula stage, artificial extension orients and stimulates isolated differentiation of the notochord and somites as well as their joint formation. Moreover, extension facilitated the formation of the normal anatomical structure of the notochord and affected expression of Chordin gene. At the late gastrula stage, the effect of artificial extension on joint somite–notochord differentiation was weaker. At the stage of late neurula, somites were sometimes formed in explants lacking a notochord anlage. Thus, at earlier stages, the formation of somites was stimulated by contacts with the notochord and joint development of both structures was mechanical dependent, while at the later stages, somites developed inde-pendently of the notochord. Thus, the role of tissue extension is primarily the establishment of normal mor-phology and expression of Chordin was located in the direction of extension.