Cleavage in Xenopus Development

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0110985DanilchikOne of the most critical phases in the life of a new organism is cleavage, a series of rapid, synchronous cell divisions that populates the fertilized egg's volume with zygotic nuclei, clonally segregates its developmental potential, and reduces cell volume to that of somatic cells. Cleavage has long served as a favorite model for the study of animal-cell cytokinesis, since the egg's large size permits a variety of experimental manipulations and opportunities for observation unavailable in smaller cells. Nevertheless, cleavage is known to differ in several fundamental ways from somatic-cell mitosis, particularly by its synchrony, rapidity, lack of transcriptional activity, and reliance on maternally synthesized components. In amphibian embryos, cleavage is accompanied by the insertion of a new domain of plasma membrane between each new pair of cells. This membrane deposition has potential significance in events of early development, such as establishment of the body plan, since all receptor-mediated, cell-cell interactions depend on the composition of the membrane deposited between cells. Our discovery of a unique array of microtubules underlying the advancing cleavage furrow led to an investigation of the role of microtubules in the cleavage process. We now have evidence that the furrow microtubule array may be involved in delivery of vesicles to the site of new membrane expansion at the base of the cleavage furrow. Furthermore, we have detected novel membrane protrusive activity on either side of the expanding cleavage membrane domain that may be critical for pulling the new cells into close apposition as cleavage progresses. Our goal in this proposed project is to understand the function and integration of these processes in Xenopus cleavage. Specifically, our aims are to 1) learn the role of the furrow microtubule array in directing exocytotic vesicles to the growing cleavage plane, 2 ) analyze cytoskeletal assembly in furrows of living embryos, and 3) study the relationship between furrowing and membrane protrusive activity along the furrow margins.