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		The Effects of Calcium-Free Seawater on the Development of Sea Urchin Embryos Amanda Weihbrecht & Lydia Hull, Millersville University Amy Yanega, Franklin & Marshall College Objective To test the effects of calcium-free seawater on gastrulation in sea urchin (Lytechinus variegatus) embryos during two stages of development. Background Sea urchins are a common indicator organism used in research, quickly showing signs of stress due to changes within their environments (“Sea Urchin Embryology”). We will use this property to assess the affect of removing calcium from their environment during two stages of development. Calcium plays a significant role in sea urchin development. It is necessary in two processes following fertilization. In the acrosomal reaction, it is involved in the fusion of the acrosomal vesicle and the sperm plasma membrane. This fusion results in the extension of the acrosomal process. The acrosomal reaction is initiated by a fructose-containing polysaccharide within the egg jelly, which binds to the sperm and allows calcium to enter the sperm head. The second mechanism involving calcium is the cortical granule reaction. This reaction involves an increase in the calcium ion concentration within the egg. This causes the cortical granule membranes to fuse with the egg plasma membrane, releasing their contents. Removing the calcium during fertilization would not alter these two processes, as the calcium for these early changes originate from within the egg (Gilbert, 2010). Later in development, the presence of calcium during embryogenesis allows cadherins to function properly, promoting cell-to-cell adhesion. Cadherins are calcium-dependent adhesion molecules. The embryo contains multiple cadherins, each having adhesive specificity toward its own kind. Cells will sort themselves out when mixed, based on cadherin type and ending in the most thermodynamically stable structure (Gilbert, 2010). In Lytechinus variegates, cadherins have a significant role in epithelial-mesenchymal transitions involving the delamination of epithelial cells. Changes in cadherin localization have been shown to accompany ingression of primary mesenchyme cells. Within the embryo the gut tissue must undergo convergence and extension to create the archenteron. The cells of the archenteron remain attached to each other by adherens junctions containing cadherin. This attachment, mediated by cadherins, may be necessary to maintain the archenteron through convergence and extension (Miller & McClay, 1997).