During segmentation of the vertebrate hindbrain, a distinct population of boundary cells forms at the interface between each segment. Little is known regarding mechanisms that regulate the formation or functions of these cells. We have investigated a potential role of Notch signaling and find that in the zebrafish hindbrain, radical fringe is expressed in boundary cells and delta genes are expressed adjacent to boundaries, consistent with a sustained activation of Notch in boundary cells. Mosaic expression experiments reveal that activation of the Notch/Su(H) pathway regulates cell affinity properties that segregate cells to boundaries. In addition, Notch signaling correlates with a delayed neurogenesis at hindbrain boundaries and is required to inhibit premature neuronal differentiation of boundary cells. These findings reveal that Notch activation couples the regulation of location and differentiation in hindbrain boundary cells. Such coupling may be important for these cells to act as a stable signaling center.

No abstract

The formation of localised signalling centres is essential for patterning of a number of tissues during development. Previous work has revealed that a distinct population of boundary cells forms at the interface of segments in the vertebrate hindbrain, but the role of these cells is not known. We have investigated the function of the Wnt1 signalling molecule that is expressed by boundary and roof plate cells in the zebrafish hindbrain. Knockdown of wnt1 or of tcf3b, a mediator of Wnt signalling, leads to ectopic expression of boundary cell markers, rfng and foxb1.2, in non-boundary regions of the hindbrain. Ectopic boundary marker expression also occurs following knockdown of rfng, a modulator of Notch signalling required for wnt1 expression at hindbrain boundaries. We show that the boundary and roof plate expression of wnt1 each contribute to upregulation of proneural and deltagene expression and neurogenesis in non-boundary regions, which in turn blocks ectopic boundary marker expression. Boundary cells therefore play a key role in the regulation of cell differentiation in the zebrafish hindbrain. The network of genes underlying the regulation of neurogenesis and lateral inhibition of boundary cell formation by Wnt1 has a striking similarity to mechanisms at the dorsoventral boundary in the Drosophila wing imaginal disc.