Abstract:The present study was carried out to determine the time-dependent changes in the cortical granule (CG) distribution and the glycoprotein compositions of zona pellucida (ZP) after egg activation by electrostimulation in porcine oocytes matured in vitro. CG exocytosis was observed by staining with fluorescein isothiocyanate-labeled peanut agglutinin (FITC-PNA) and laser confocal microscopy, and ZP modification was analyzed by using enhanced chemiluminescent (ECL) detection of the biotinylated ZP derived from a single oocyte. In the oocytes matured in vitro, CGs staining with FITC-PNA had formed a continuous monolayer underlying the oolemma, and three major bands (ZP1, ZP2 and ZP3) were observed in a biotinylated ZP subjected to SDS-PAGE followed by ECL detection. Electrostimulation to induce artificial activation caused a decline in the fluorescent intensity of the CGs with a concomitant decrease in the amounts of ZP1 and ZP2 bands, but CG exocytosis according to oocyte activation occurred slowly, and the incidence of oocytes with complete CG exocytosis was first observed 1.0 h after electrostimulation (9%). Similarly, the complete ZP modification, i.e. a maximum decrease in amounts of ZP1 and ZP2 bands, required more than 3.5 h after artificial activation, and the ZP dissolution time caused by 0.1% protease action increased apparently as incubation time was prolonged after electrostimulation. Results of these experiments suggest that the slower ZP modification due to the delayed CG exocytosis in porcine oocytes may reflect the remarkable increase in polyspermy frequency in in-vitro fertilization. Key words: Cortical granule exocytosis, Zona reaction, Zona hardening, ECL detection, Porcine oocyte.All mammalian eggs are surrounded by a relatively thick, insoluble extracellular coat called the zona pellucida (ZP) [1]. After fertilization, the ZP plays an important role in the subsequent activation events [2]. The porcine ZP is composed of three glycoprotein families, ZP1 (92 kDa), ZP3α (55 kDa), and ZP3β (55 kDa), which were named with a nomenclature based on apparent molecular weight [3,4], and ZP1 is split into two smaller molecules, ZP2 (69 kDa) and ZP4 (23 kDa) under reducing conditions [5]. The porcine ZP3 families have been shown to comprise approximately 60% of the total glycoprotein content [4,6]. In particular, ZP3α seems to be responsible for the binding of boar spermatozoa to oocytes, since preincubation of boar sperm with solubilized and purified ZP3α inhibits subsequent sperm attachment to porcine ZP [7].In mammalian species, sperm activates oocytes by triggering calcium oscillations that persist for more than an hour after sperm penetration [8][9][10], and artificially increasing intracellular Ca 2+ by application of electrostimulation in the presence of Ca 2+ [11], or injection of CaCl 2 [12] is capable of inducing parthenogenetic activation in porcine oocytes. The increase in cytoplasmic free-Ca 2+ is recognized as the primary intracellular signal responsible for initiating the activation pr...