Abstract
We provide the first analysis of how a segmentally reiterated pattern of neurons is specified along the anteroposterior axis of the vertebrate spinal cord by investigating how zebrafish primary motoneurons are patterned. Two identified primary motoneuron subtypes, MiP and CaP, occupy distinct locations within the ventral neural tube relative to overlying somites, express different genes and innervate different muscle territories. In all vertebrates examined so far, paraxial mesoderm-derived signals specify distinct motoneuron subpopulations in specific anteroposterior regions of the spinal cord. We show that signals from paraxial mesoderm also control the much finer-grained segmental patterning of zebrafish primary motoneurons. We examined primary motoneuron specification in several zebrafish mutants that have distinct effects on paraxial mesoderm development. Our findings suggest that in the absence of signals from paraxial mesoderm, primary motoneurons have a hybrid identity with respect to gene expression, and that under these conditions the CaP axon trajectory may be dominant.
Highlights
The vertebrate nervous system consists of many specialized cell types that form at distinct characteristic positions
Our findings demonstrate that signals from paraxial mesoderm are required to specify middle primary (MiP) and caudal primary (CaP), and they suggest that additional signals from the somites are required to fine-tune or maintain correct spatial organization of primary motoneurons (PMNs) subtypes
Because CaPs normally have turned off islet1 expression by the time they extend axons, our results suggest that in tri;kny mutants most PMNs have a hybrid identity with respect to gene expression but a CaP-like identity based on axon trajectory
Summary
The vertebrate nervous system consists of many specialized cell types that form at distinct characteristic positions. In all vertebrates examined so far, additional signals from paraxial mesoderm specify distinct subpopulations of MNs that occupy specific motor columns at particular anteroposterior (AP) axial levels (Eisen, 1999; Ensini et al, 1998; Liu et al, 2001). In addition to distinct motor columns at particular AP axial levels (Eisen, 1994), zebrafish have a more fine-grained, segmentally reiterated pattern of different PMN subtypes along the spinal cord AP axis. Such a finegrained, reiterated pattern of distinct MN subtypes has not yet been described in other vertebrates, probably because there are many more MNs, and individual cells cannot be recognized. Many aspects of PMN development have been characterized (Lewis and Eisen, 2003), it is still unclear how these different subtypes are specified
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