ИСТИНА

The notochord is required for floor plate induction and establishment of the dorso-ventral patterning of the neural tube and is believed to have organizer properties. The inductive properties are related to the function of sonic hedgehog (shh) which diffuse from the notochord, forms a gradient and autoinduces shh expression in the floor plate. However reported data is inconsistent and detailed spatio-temporal pattern of shh expression has not been studied across species. Therefore we studied dynamics of shh expression in Xenopus leavis, chicken the rabbit embryos using in situ hybridization. Technovit sections reveal divergent pattern: in Xenopus shh is expressed at the beginning of gastrulation in distinct area above the dorsal blastopore lip adjacent to the prospective neuroectoderm whereas the floor plate is positive at the end of gastrulation, in the chick embryo shh is expressed in the prospective neuroectoderm prior to the notochord formation and even prior to mesoderm migration, whereas in the rabbit the classical pattern was found in that shh is first expressed in the notochord and the floor plate domain is then induced during somitogenesis stages. These findings are discussed within the framework of loss of function experiments and known mutants and morphants with disturbed notochord development and shh expression. We propose that the mode of floor plate induction adapts to differences in topography of embryonic tissues during gastrulation and neural induction.