Abstract
Ascidians are invertebrate chordates and the closest living relative to vertebrates. In ascidian embryos a large part of the central nervous system arises from cells associated with mesoderm rather than ectoderm lineages. This seems at odds with the traditional view of vertebrate nervous system development which was thought to be induced from ectoderm cells, initially with anterior character and later transformed by posteriorizing signals, to generate the entire anterior-posterior axis of the central nervous system. Recent advances in vertebrate developmental biology, however, show that much of the posterior central nervous system, or spinal cord, in fact arises from cells that share a common origin with mesoderm. This indicates a conserved role for bi-potential neuromesoderm precursors in chordate CNS formation. However, the boundary between neural tissue arising from these distinct neural lineages does not appear to be fixed, which leads to the notion that anterior-posterior patterning and neural fate formation can evolve independently.
Highlights
Along-held traditional view of vertebrate CNS formation is that neural tissue is induced from the ectoderm germ layer in a process called “neural induction”, cells choosing between neural and epidermis
Much of the vertebrate spinal cord is believed to arise from bi-potential precursors, termed neural-mesoderm precursors (NMps), which choose between spinal cord and paraxial mesoderm fates [15,17,18,98,100,101]
Wnt, FGF- and RAsignals are involved in patterning the epidermis and associated peripheral nervous system of the ascidian larvae and FGF8/17/18 and Wnt-5 ligands are required for correct tail tip cell morphology and tail elongation [172,175,176]
Summary
Along-held traditional view of vertebrate CNS formation is that neural tissue is induced from the ectoderm germ layer in a process called “neural induction”, cells choosing between neural and epidermis This neural induction step induces neural fate with anterior character, part of which is “transformed” into posterior fates by posteriorizing signals (reviewed in [15,16]). In vertebrates, while some posterior neural tissue does form via a process known as “posteriorization”, it has become clear that the posteriormost CNS originates from a distinct ontology to the anterior CNS and is associated with mesoderm fates [15,17,18,19] We provide a detailed description of the embryonic origin of the ascidian larval neuromesoderm-derived CNS and discuss to what extent this is similar to the situation in vertebrates, with particular focus on mice, chick and zebrafish as vertebrate examples
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