| Abstract | Cells are the basic units of life, and one of their most awe-inspiring roles is to generate the signals and forces that build an embryo from an unstructured ball of cells--a process called morphogenesis. We use the ascidian, a basal chordate, as a simple model of morphogensis. Ascidians undergo the same gastrulation and neurulation movements as other chordates, but with tens of cells instead of thousands. This means that individual cells are large in proportion to embryo size and their contributions to global deformations during morphogenesis can be observed at high resolution. We take a multi-faceted approach to explore the mechanisms by which cells change shape, rearrange, and cleave to drive morphogenetic movements, using confocal microscopy, micromanipulation, laser ablation, pharmacological inhibition, and computer simulations of localized cortical contractility and adhesion. We found that endoderm invagination occurs by a mechanism of "collared rounding" by which apically narrow and columnar cells rapidly round to cause the invagination. Further, searches of tension parameter space in a two-dimensional computer simulation indicate that this is the only mechanism by which localized cortical contractilty can drive endoderm invagination on the ascidian geometry. Although these results contradict the textbook view that apical constriction drives invagination, the similarity of ascidian endoderm invagination to the well-studied ventral furrow and posterior midgut invaginations in Drosophila suggests it may be a general mechanism. We have recently begun to investigate neural plate invagination and closure in ascidians. Our preliminary results suggest that neural plate invagination requires basal crawling and that closure requires a coordination of localized cortical contractility, cell crawling, and formation of new adhesive contacts. The ascidian embryo, long seen as a model system for cell fate determination, is revealing itself to be an equally excellent system in which to investigate the cellular basis for morphogenesis. |
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