The central nervous system of ascidian larvae

Abstract

Ascidians are marine invertebrate chordates. Their tadpole larvae contain a dorsal tubular nervous system, resulting from the rolling up of a neural plate. Along the anterior-posterior (A-P) axis, the central nervous system (CNS) is organized into a sensory vesicle, neck, trunk ganglion, and tail nerve cord and consists of approximately only 330 cells, of which around 100 are thought to be neurons. The organization of distinct neuronal cell types and neurotransmitter gene expression within the CNS has been described. The unique developmental mode of ascidians, with a small number of cells and a fixed cell division pattern, allows individual cells to be traced throughout development. This feature has led to the complete documentation of the cell lineages of certain cell types in the CNS. Thus, a step-by-step understanding of nervous system development from the initial stages of neural induction to the neurogenesis of individual neurons is a feasible goal. The genetic control of neural fate induction and early neural plate patterning are now well understood. The molecular mechanisms specifying the cholinergic neurons of the trunk ganglion as well as the pigment cells of the sensory organs are also well elucidated. In addition, studies have begun on the morphogenetic processes of neurulation. Remaining challenges include building an embryonic atlas integrating gene expression patterns, cell lineage, and neuronal cell types as well as developing the gene regulatory networks of cell fate specification and integrating them with the genetic control of morphogenesis. WIREs Dev Biol 2016, 5:538-561. doi: 10.1002/wdev.239 For further resources related to this article, please visit the WIREs website.

The CNS connectome of a tadpole larva of Ciona intestinalis (L.) highlights sidedness in the brain of a chordate sibling

“…Many asymmetries have been uncovered by the developmental expression of Nodal and its signaling pathways (Hamada et al, 2002; Hudson, 2016). As in vertebrates, in ascidians, their sibling group (Satoh et al, 2014), Nodal expresses on the left hand side of the developing embryo (Boorman and Shimeld, 2002a, 2002b; Yoshida and Saiga, 2008).…”

Patterning of brain precursors in ascidian embryos

“…Morpholinos for Snail (MO1), Nodal and Msxb and Ciona Su(H) DBM are described previously (Hudson andYasuo, 2005, 2006;Hudson et al, 2015;Imai et al, 2006;Roure and Darras, 2016). SB431542 (Tocris) and DAPT (Calbiochem) treatments have been described previously (Hudson andYasuo, 2005, 2006).…”

“…SB431542 (Tocris) and DAPT (Calbiochem) treatments have been described previously (Hudson andYasuo, 2005, 2006). SB431542 was added to embryos at the 16-or 32-cell stage and DAPT at the 76-cell stage.…”

“…Ascidians belong to the urochordates, or tunicates, a phylum of invertebrate chordates closely related to vertebrates (Delsuc et al, 2006;Satoh et al, 2014). Ascidian embryos develop with very few numbers of cells and a fixed cell lineage, features enabling the stepby-step analysis of developmental cell fate choices with a single-cell level of precision (Hudson, 2016).…”

Parallel Visual Circuitry in a Basal Chordate

“…Ascidians, including those of the widely studied ascidian genus Ciona, are members of the chordate sub-phylum Tunicata, which comprise the closest extant relatives of the vertebrates [8]. While much smaller and containing many fewer cells, ascidian tadpole larvae resemble vertebrate larvae in having a prominent notochord running the length of a muscular tail and a dorsal CNS with regional anterior-to-posterior homology to vertebrate CNSs (see recent review, [9]). Despite these homologies with vertebrates the Ciona CNS contains only 177 neurons [1].…”