Institut für Genetik, Universität Mainz, Saarstraße 21, D-55122 Mainz,
In addition to the activation of genes that define the boundaries of the mesodermal primordium, information is required for the specification of the various mesodermal cell fates. Recent data suggest that the positions occupied by the mesodermal cells following invagination and subsequent dorsal migration over the inner surface of the ectoderm is critical for their determination and that ectodermal signals control various aspects of mesodermal patterning.
Mesodermal progenitors are in direct contact with the mesectodermal midline cells which form a single row on either side between the anlagen of the mesoderm and the ectoderm and which give rise to a characteristic set of lineages in the midline region of the CNS. We provide experimental evidence that the CNS midline progenitors induce the formation of identified mesodermal cells (dorsal median cells, DM cells) and are required for proper patterning of ventral somatic muscles.
In particular we demonstrate that 1. the mesodermal precursors of the DM cells are direct neighbours of the CNS midline precursors, 2. in single minded (sim) mutants and midline deficient regions of the wildtype the DM cells are missing and the pattern of ventral somatic muscles is disturbed, 3. upon implantation of wildtype midline cells into the midline region of sim mutants this phenotype is rescued, 4. implantation of additional midline cells into the wildtype midline leads to the induction of supernumerary DM cells as well segment boudary muscles, and 5. the DER signalling pathway is involved in this inductive process.
Supported by the DFG (Te 130/7-1)