The sea urchin has been used extensively to study the events of fertilization and cleavage. The egg is big enough to be seen under the microscope, the gametes are produced in great abundance, and fertilization occurs naturally outside the body in the seawater. The cytoplasm is relatively clear, so fertilization is observed easily. .
On a brief history on fertilization, the sperm enters and binds to the egg surface. This induces a calcium wave that causes the cortical granules , to fuse with the plasma membrane, releasing their contents and causing the fertilization membrane to rise. The sperm is then pulled into the egg where it moves to the egg nucleus and fuses. DNA synthesis occurs along with duplication of the centrosome. Nuclear envelope breakdown and chromosome condensation leads to the metaphase plate. Anaphase leads to telophase and cell division. The process is then repeated.
There are a number of objectives of this lab, they include: investigation of the role of calcium ions in sperm and oocyte activation, to provide an opportunity to observe living gametes. Another is to have an opportunity to design experiments to test some of the ideas used to explain mechanisms of fertilization.
Calcium is an ion that is critically involved in many cellular processes, including many types of cell motility and exocytosis. In this experiment, we will test whether extracellular calcium is required for sea urchin fertilization. Ethylene Diamine Tetraacetic Acid (EDTA) is a potent chelator of Calcium, and can be used to eliminate calcium from the solution.
The slow block is found in many animals, including mammals. Unlike the transient fast block, changes in the egg that occur during the slow block to polyspermy are permanent. The slow block involves the cortical granule reaction. The slow block is triggered by release of Ca++ from internal stores upon sperm-egg contact.
In this experiment, there will be 3 different slides; a control experiment, a first and second experiment.