| Notochord and floor plate, and in anamniote embryos hypochord, are midline cell types that arise from the dorsal organizer during gastrulation. Secreted molecules from the notochord and floor plate specify cell types of the neural tube and somites, whereas hypochord cells secrete molecules that are important for aorta development. Previous work suggested that the Delta ligand-Notch receptor signaling pathway promotes the development of an appropriate number of midline cells, and we hypothesized that it does so by regulating midline cell fate decisions. We tested this hypothesis using a combination of fate mapping, gene expression analysis, and loss- and gain-of-function tests. We show here that notochord precursors occupy a large area at the dorsal blastoderm margin, and these are closely associated with floor plate precursors in a domain a few cell diameters above the dorsal margin. Hypochord precursor cells express a downstream target of Notch signaling, her4, and arise from the lateral edge of the organizer and are also closely associated with notochord precursors. delta genes expressed in paraxial mesoderm, and notch1a and the Notch modulator lunatic fringe, both expressed in axial mesoderm, are required for hypochord specification. Conditional blockage of Notch signaling during gastrulation caused a significant reduction of floor plate and hypochord, whereas blocking Notch signaling after gastrulation had a lesser effect on midline development. Notch inhibition caused floor plate cells to proliferate less, suggesting that Notch is required for floor plate proliferation rather than specification. In contrast, constitutive activation of Notch at the beginning of gastrulation caused expansion of hypochord at the expense of notochord, but floor plate was not expanded. These studies support the idea that Delta signals from paraxial mesoderm specify hypochord at the lateral edge of notch and lunatic fringe expressing axial mesoderm. We also found that her9, which is regulated Nodal signaling, is expressed during early gastrulation in floor plate precursor cells and promotes floor plate specification and inhibits notochord. Together, these results indicate that distinct signaling events promote floor plate and hypochord specification by inhibiting notochord. |
