Title: Redefining the dorsal hindbrain based on genetic lineage
Description: Development of the vertebrate central nervous system depends on the generation of specific neural cells in appropriate numbers at defined times. Towards understanding such developmental events, it is essential to link progenitor cell coordinate position and genetic profile in the embryo to a final fate in the adult. We develop and apply genetic fate mapping methodologies to examine progenitor-progeny cell relationships for the rhombic lip (RL)---a hindbrain germinal zone productive of essential neural cells in the brainstem, for which experimental study has been challenging given its deployment of progeny cells across complex, long-distances. We determine that the lower RL (LRL) is subdivided along its dorsoventral axis into molecularly-defined territories, each corresponding to a particular fate: Lmx1a/Gdf7 expression define the territory which produces the hindbrain roof plate epithelium (hRPe) and hindbrain choroid plexus epithelium (hCPe); Math1 defines the territory which produces the mossy fiber afferent system; and Ngn1 likely defines the primordium for a subset of climbing fiber precerebellar afferents. These findings, taken together with loss-of-function studies, support the model that specification events are enacted within the LRL. Cell types emerge from the LRL at distinct intervals of time; temporal specificity of gene expression represents a separate axis for fate regulation. To address how progeny cell types deploy from the RL over time, we develop and apply an inducible genetic fate mapping approach. Having identified that the Gdf7 +/ Lmx1a + subdomain within the RL harbors progenitors for both hRPe and hCPe, we study further the development of these organizing centers important for dorsal hindbrain patterning. It is unclear how they are related with respect to lineage and gene expression. We address how cells in the hRPe are organized and whether they contribute to the hCPe.

We find that the hRPe is comprised of three distinguishable fields, each differing in tissue organization, proliferative state, order of emergence from the RL, and molecular profile---only two fields contribute to the hCPe. We determine that the RL produces hCPe cells directly until late in embryogenesis. We further determine that hindbrain cells in the Gdf7 , but not Math1 lineage hyperproliferate in response to constitutively active Notch1.