Molecularly and temporally separable lineages form the hindbrain roof plate and contribute differentially to the choroid plexus

Both hindbrain roof plate epithelium ( hRPe) and hindbrain choroid plexus epithelium ( hCPe) produce morphogens and growth factors essential for proper hindbrain development. Despite their importance, little is known about how these essential structures develop. Recent genetic fate maps indicate that hRPe and hCPe descend from the same pool of dorsal neuroectodermal progenitor cells of the rhombic lip. A linear developmental progression has been assumed, with the rhombic lip producing non- mitotic hRPe, and seemingly uniform hRPe transforming into hCPe. Here, we show that hRPe is not uniform but rather comprises three spatiotemporal fields, which differ in organization, proliferative state, order of emergence from the rhombic lip, and molecular profile of either the constituent hRPe cells themselves and/ or their parental progenitors. Only two fields contribute to hCPe. We also present evidence for an hCPe contribution directly by the rhombic lip at late embryonic stages when hRPe is no longer present; indeed, the production interval for hCPe by the rhombic lip is surprisingly extensive. Further, we show that the hCPe lineage appears to be unique among the varied rhombic lip- derived lineages in its proliferative response to constitutively active Notch1 signaling. Collectively, these findings provide a new platform for investigating hRPe and hCPe as neural organizing centers and provide support for the model that they are themselves patterned structures that might be capable of influencing neural development along multiple spatial and temporal axes.