The cortical reaction in the egg of Discoglossus pictus: A study of the changes in the endoplasmic reticulum at activation

Abstract

In Discoglossus pictus previous ultrastructural observations have shown that at the animal dimple, where sperm fuse with the egg, cortical granules (CG), vacuoles, and tightly packed clusters of small cisternae are present. At fertilization the clusters open (i.e., become loose) and give rise to longer cisternae arranged in whorls and chains which migrate toward the plasma membrane. The vacuoles fuse to form cisternae and exocytose along with the CG. In the rest of the egg periphery, while exocytosis occurs, the clusters do not open as a result of activation ( C. Campanella, R. Talevi, U. Atripaldi, and L. Quaglia (1986). In “Molecular and Cellular Biology of Fertilization” (J. L. Hedrick, Ed.). Plenum, New York ). We have recently conducted electrophysiological studies which have detected inward currents at the dimple center, outward current at the rest of the egg surface, and an eightfold increase in [Ca 2+] i which propagates from the site of activation throughout the egg ( R. Nuccitelli, D. Kline, W. Busa, R. Talevi, and C. Campanella (1988). Dev. Biol. 130, 120–132 ). In this paper we have asked whether the anionic current and the Ca 2+ increase could be causally related to the changes of the smooth endoplasmic reticulum (SER) at activation. The results obtained by activating the eggs in ion-substituted Ringers indicate that (1) the migration of cisternae is not dependent on the polarity of the activation current crossing the dimple, but is strongly impaired, together with CG exocytosis, by 5× Cl − Ringer; (2) TMB-8, a drug which partially blocks calcium release ( C. Y. Choiu and M. J. Malagodi (1975). Brit. J. Pharmacol. 53, 279–288 ), partially inhibits opening of cisternae clusters and the formation of an SER network in the dimple. This suggests a causal relationship between the Ca 2+ rise and the cluster transformation at activation.