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The brain of the ascidian larva

We have worked on the central nervous system of the tadpole larva of the ascidian Ciona intestinalis. This tiny larva is built on a chordate body plan and has a brain that derives from a neural plate but comprises only about 330 cells. These characteristics and recent genomic studies as well as initial mutant screens, both of which are going on in a number of laboratories in the US, Europe and Japan, combine to promote the ascidian larva as a new model genetic organism (see ref. 86).


Reconstruction of the nuclei of cells in the central nervous system of the tadpole larva of a sea-squirt, Ciona intestinalis, made from serial semithin sections (by S. Stanley). Each cell type is recognised cytologically in the sections and colour-coded by type in this reconstruction. The same cells in the visceral ganglion (the main motor centre) of a different specimen have been reconstructed from serial EM to reveal their connections.


Reconstructions of the the larva at successive embryonic stages after neurulation, from confocal image stacks of preparations stained by nucleic acid markers, enables the arrangement and mitotic history of cells to be followed (by A. Cole).



We are also studying the cell types and synaptic circuits of the ascidian larval nervous system. A library of cell types has been reported from GFP labelling studies in which Ciona embryos were transfected with a plasmid incorporating a neuron-specific driver fused to GFP. The cell types are reported for the neurons of the central (see ref. 128) and peripheral (see ref. 129) nervous systems, from the work of Janice Imai.



Kerrianne Ryan is tracing the synaptic circuits formed by these neurons from serial-section EM, using a long series of sections to reconstruct these neurons from a single larva.