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		The Effects of Calcium-Free Seawater on the Development of Sea Urchin Embryos Amy Yanega '01 Objective The purpose of this experiment is to determine the effects of calcium-free seawater on the development of sea urchin embryos. Embryos were placed in calcium-free sea after fertilization and at the initiation of gastrulation Introduction Calcium is necessary in two processes shortly following fertilization. One process, the acrosomal reaction, is the fusion of the acrosomal vesicle and the sperm plasma membrane, which results in the extension of the acrosomal process. The acrosomal reaction is initiated by a fucose-containing polysaccharide within the egg jelly, which binds to the sperm and allows calcium to enter the sperm head. The second mechanism is the cortical granule reaction. Upon fertilization, the calcium ion concentration of the egg increases greatly, which causes the cortical granule membranes to fuse with the egg plasma membrane, releasing their contents. A wave of cortical, released from the endoplasmic reticulum, begins at the point of fertilization and travels completely around the egg. The calcium ions needed for these two mechanisms are not a result of an influx of calcium, but come from within the egg itself. Therefore withdrawing calcium during fertilization would not alter these processes. Later in development, calcium may play a role in cell adhesion. The major cell adhesion molecules are known as cadherins. Cadherins are calcium-dependent adhesion molecules. Cadherins are critical in establishing and maintaining intercellular connections, and appear to be crucial to the spatial segregation of cell types and to the organization of animal form. Calcium ions are needed for the adhesion of the same cadherin molecules between cells.