MECHANISMS LEADING TO CORTICAL REACTION IN THE MAMMALIAN EGG

Activation of the mammalian egg results in cortical reaction (CR), whi ch is correlated with an increase in intracellular Ca2+ concentration and PKC activation. The CR is a gradual rather then an ''all or none'' response, and can be regulated by different concentrations of parthen ogenetic activators. To evaluate the biological significance of parthe nogenetic induced CR, rat eggs were fertilized or activated by differe nt concentrations of ionomycin and TPA. Cortical granules (CG) were mo nitored by electron microscopy, while the CG exudate was visualized by Lens culinaris lectin and Texas Red, using light and confocal microsc opy. The ability of the CR to trigger a full block to polyspermy was e xamined in an IVF system. Our study demonstrates the existence of ligh t and dark CG, which differ by number, distribution in the egg cortex, and sensitivity to parthenogenetic activators. Sperm penetration or h igh concentration of activators, trigger depletion of both light and d ark CG, leading to a full CR. Low concentration of activators altered the CG density, the ratio of dark/light CG, and induced partial CR tha t was sufficient to cause a block to polyspermy. The results imply tha t Ca2+ rise or PKC activation have different effects on light and dark CG. In recently fertilized or parthenogenetically activated eggs, CG exudate appeared as evenly distributed spots, whereas in more advanced stages of fertilization the exudate was scattered as patchy aggregate s. This observation suggests a difference in the dispersion of CG exud ate after fertilization as compared to parthenogenetic activation. (C) 1998 Wiley-Liss, Inc.