Development of the central nervous system of the larva of the ascidian, Ciona intestinalis L. II. Neural plate morphogenesis and cell lineages during neurulation.

We describe the lineage and morphogenesis of neural plate cells in the ascidian, Ciona intestinalis, from reconstructed cell maps of embryos at 12-min intervals during and after neurulation, between 31 and 61% of embryonic development. Neurulation commences in a posterior to anterior wave following in the wake of the ninth cleavage, when all cells, except possibly four, are in their 10th generation. The neural plate then comprises 76 cells, in up to four posterior rows each of eight vegetal-hemisphere cells, and eight anterior rows each of six animal-hemisphere cells. Two cells are lost from the neural plate to the muscle cell line during neurulation and four cells are gained from ectoderm outside the plate. All cells become wedge-shaped. Simple, stereotyped positional changes transform cells from lateral locations in the plate to posterior locations in the tube; bilateral partners shear their midline positions to form the keel, and ectodermal cells zipper up dorsally to form the capstone, of a tube which is four cells in cross section posteriorly, but more complex anteriorly. Neither cell death nor migration occur during neurulation. Divisions become asynchronous and the cell-cycle extends; 170 10th- to 12th-generation cells exist by the time the neural tube becomes completely internalized. Generally, only one further division is required to complete the lineage analysis, two at the most. Neural plate cell divisions were invariant using our observational methods, and their lineage is compared with that from recent studies of H. Nishida (1987, Dev. Biol. 121, 526-541).