SHERRARD, KM*; MUNRO, EM; BECKHAM, Y; ODELL, GM; Univ. of Washington, Seattle; Univ. of Washington, Seattle; Univ. of Washington, Seattle; Univ. of Washington, Seattle: New wrinkles: Cytoskeletal mechanics of endoderm invagination

Invagination, the folding inwards of a formerly flat epithelium, is a common and fundamental mode of morphogenesis. Invagination operates during gastrulation, neurulation, eye formation, and many other contexts in animal development; yet its mechanical basis remains largely unexplained. New tools, including advances in confocal microscopy, the ability to introduce fluorescently-labeled gene constructs into developing embryos, and inexpensive, powerful computers, make it possible to approach this problem anew. We know that cytoskeletal and adhesive proteins drive cell shape changes and crawling, but explaining how these myriad local forces work across many cells to generate the large-scale deformations of invagination requires both detailed observations and an equally detailed model. Using gastrulating ascidian embryos, we have begun imaging acto-mysosin dynamics in living and fixed embryos during invagination, and started modifying an existing model of cellular dynamics to incorporate the behavior of the cytoskeleton in the invaginating embryos. During ascidian gastrulation, the ten invaginating cells first bulge upwards, and their nuclei move apically. Subsequently their apices shrink, their bases widen, and their nuclei descend. Contrary to the oft-cited apical constriction hypothesis , we have found no evidence of an apical concentration of actin in the cell cortex. However, clouds of actin surrounding the nuclei could potentially drive apical constriction. We explore this and other possible cytoskeletal/adhesive mechanisms of invagination using the computational model.