VERGE 3 Lundeberg 1 Dependence of fertilization in sea urchins, Strongylocentrotus purpuratus, on microfilament formation and internal calcium concentration

Fertilization in sea urchins, Strongylocentrotus purpuratus, is the result of a series of interactions between the egg and sperm. Microfilament formation has been implicated in both the acrosomal process formation and the entry of the sperm into the egg. Changes in internal calcium concentrations have been linked to exocytosis reactions that occur in both the egg and the sperm. Therefore, the dependence of fertilization on microfilament generation and changes in internal calcium concentration were determined by examining the effects of cytochalasin and calcium ionophore A23187 on sea urchin fertilization. These experiments demonstrated the dependence of the fertilization process on microfilament generation and the changes in internal calcium concentration; the inhibition of fertilization by cytochalasin confirmed the anticipated dependence on microfilament generation, while the ability of calcium ionophore A23187 to induce fertilization membrane formation, by increased permeability to calcium, confirmed the importance of calcium as a signaling mechanism for exocytosis. INTRODUCTION Fertilization occurs as a result of a successful series of interactions between the egg and sperm. Initially the sperm is attracted to the egg by a chemotactic mechanism connected to the egg jelly layer. Contact with the jelly layer induces the acrosomal reaction, which results in the exocytosis of the acrosomal vesicle and the formation of the acrosomal process. The released enzymes from the acrosomal vesicle allows for the digestion through the jelly and vitelline layers, allowing the sperm to recognize and bind to the egg. Plasma membrane fusion and the entrance of the sperm into the egg complete the fertilization process (Gilbert 2003). Aside from sperm attraction, the egg must act to inhibit polyspermy following the success of a single sperm. The egg employs two mechanisms to achieve this inhibitory action: a large membrane