Cleavage in Xenopus Development

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Danilchik 96-31765 Fertilized eggs become multicellular through a series of rapid, synchronous cell divisions, or cleavages, that serve to populate the egg's volume with zygotic nuclei, clonally segregate the egg's developmental potential, and reduce daughter cells' volume to that of somatic cells. In animal cells, cleavage furrowing is generally recognized to be a microfilament-dependent process. Recently, however, Dr. Danilchik discovered a novel microtubule-containing array in the early cleavage furrows in eggs of the frog Xenopus laevis. This structure consists of numerous thick, short bundles of radially arranged, acetylated microtubules that emanate from the vicinity of the contractile ring of all cleavage furrows until the midblastula stage. Dr. Danilchik hypothesizes that the microtubule array functions to recruit membrane vesicles to the site of new membrane formation at the furrow's leading edge, and perhaps also to draw a thick layer of peripheral cytoplasm inward along each cleavage furrow, a process known as ingresson. Since all receptormediated, cell-cell interactions depend on the composition of the membranes deposited between cells, it is clear that the directed membrane deposition and ingression have potentially significant roles in events of early development, such as specification of the primary tissue layers. The specific aims of this proposal are to carry out: 1) a structural analysis, to define further the cytoskeletal organization of the furrow, 2) functional studies, to understand how membrane deposition and ingression are accomplished, and 3) developmental gene-expression studies, in which vesicle delivery or ingression will be locally inhibited to learn the developmental significance of cleavage. Results from this project will contribute to our understanding of how animal cells divide, one of the most fundamental problems in Cellular and Developmental Biology.