FLAGELLAR MOTILITY IS NOT INVOLVED IN THE INCORPORATION OF THE SPERM INTO THE EGG AT FERTILIZATION*

Cinemicrography of sea urchin fertilization reveals that the fertilizing sperm is one of the first sperm to attach to the egg. Just before the cortical reaction the fertilizing sperm ceases motility and then is incorporated into the egg without flagellar beating. The rate of incorporation is 5–11 μm/sec and is constant. Lytechinus pictus sperm rendered immotile by azide treatment can bind to and fertilize eggs but binding, and therefore fertilization, is blocked by azide treatment of Strongylocentrotus purpuratus gametes.     

Development in the Floating World: Defenses of Eggs and Embryos Against Damage from UV Radiation

Eggs and embryos of many aquatic organisms develop in the watercolumn and can experience ultraviolet radiation with potentiallydeleterious effects. This is especially vexing for floatingembryos that develop in the surface or neuston layer. Radiationdamage can be a particular problem for these embryos since thecell division cycle during the cleavage period is quite shortand often these cycles do not have mitotic checkpoints to insurefaithful transmission of DNA to the daughter cells. This couldresult in cell division with unrepaired DNA in the blastomeres,which could impact embryogenesis and the transmission of thegenome through the germ line. Described strategies to restrictradiation damage include mechanisms to limit oxidative damageand the use of sunscreens such as the mycosporines to curb radiationto sensitive targets. We describe a particularly ingenious useof sunscreens in the tunicate embryo, the use of extra-embryoniccells to shield the embryo from potentially harmful UV-A andUV-B radiation. We also raise questions regarding the natureof UV damage to embryos (is it DNA or also protein) and thecharacteristics of DNA repair in such embryos. It is likelythat unique mechanisms are present in floating embryos thatdevelop in this air-water interface to assure that cell andgenomic integrity are maintained in this challenging environment.     

The Program of and Mechanisms of Fertilization in the Echinoderm Egg

Current research on the mechanisms of sperm-egg fusion, theblock to polyspermy, and metabolic activation are described.A cinemicrographic analysis of fertilization reveals that fusionof sperm and egg occurs between non-motile gametes, indicatingthat the flagellar motion of sperm is not required. The blockto polyspermy is reviewed, emphasizing recent work on the roleof cortical granule protease in altering sperm receptors ofthe vitelline layer. Metabolic activation or derepression at fertilization is highlyregulated and occurs in a definite sequence. The primary eventappears to be release of intracellular Ca²⁺. The timing of metabolicderepression is different in starfish oocytes. Here, a partof the derepression occurs during maturation and another partat fertilization.     

Regulation of the pentose phosphate pathway at fertilization in sea urchin eggs

Summary

After fertilization of sea urchin eggs, there is a rapid increase in cellular levels of NADPH, a metabolite utilized in a variety of biosynthetic reactions during early development. Recent studies have shown that a dramatic increase in the activity of the pentose phosphate shunt occurs in vivo shortly after fertilization, consistent with the hypothesis mat this metabolic pathway is a major supplier of NADPH in sea urchin zygotes. One mechanism that may account, in part, for this increase in pentose shunt activity is the dissociation of glucose-6-phosphate dehydrogenase (G6PDH), the first enzyme of the shunt, from cell structural elements. In vitro, G6PDH is associated with the insoluble matrix obtained from homogenates of unfertilized eggs, and in this state, the enzyme is inhibited. Within minutes of fertilization, G6PDH is released as an active, soluble enzyme. A similar solubilization and activation of G6PDH occurs after fertilization of eggs of other marine invertebrates and in mammalian cells in culture stimulated by growth factors. The occurrence of this phenomenon in such diverse cell types, in response to different stimuli, suggests that the redistribution of G6PDH between insoluble and soluble locations may be involved in the regulation of the pentose phosphate shunt during cell activation in general.     

Cortical Granules of Sea Urchin Eggs do not Undergo Exocytosis at the Site of Sperm-egg Fusion*

Microscopic observations of sea urchin egg fertilization (phase contrast, Nomarski and transmission electron microscope) reveal that the cortical granules in the area of sperm egg-fusion do not undergo exocytosis. These intact granules remain associated with the sperm, moving into the egg cytoplasm with the entering sperm. This sperm-cortical granule association occurs before the sperm centriole affects microtubule organization and the sperm-cortical granule association is not affected by cytochalasin D or griseofulvin. We discuss the possibility that a reorganization of the egg cytoplasm ensues from the sperm-egg interaction at the site of sperm-egg fusion. Other possibilities are that the retention of cortical granules is not related to egg reorganization, but is necessary for successful sperm incorporation or reflects an unrelated component of the activation process.