The Cortical Endoplasmic Reticulum (ER) of the Mouse Egg: Localization of ER Clusters in Relation to the Generation of Repetitive Calcium Waves

Abstract

The endoplasmic reticulum (ER) of the mature mouse egg consists of a fine tubular network and pronounced accumulations in the cortex. The ER was visualized both in intact eggs and with in vitro preparations of the cortex using the fluorescent lipophilic dye, DiI. Immunofluorescent labeling of the ER in isolated cortical preparations demonstrated that the ER clusters contain inositol 1,4,5-trisphosphate (IP3) receptors, indicating an important involvement in sperm-induced Ca2+ transients, which are triggered by IP3. We imaged the ER during fertilization and the subsequent Ca2+ transients and found that the clusters remained intact throughout this period. Recovery of fluorescence after photobleaching established that the ER clusters are continuous with the reticular ER network and that these structures remain stable and continuous throughout the time of fertilization-induced Ca2+ transients; continuity also remained during IP3 injection. These results indicate that, in contrast to echinoderm eggs, the ER of mouse eggs does not become disrupted when it releases Ca2+at fertilization. The localization and apparent stability of the cortical ER clusters may be important in generating Ca2+ oscillations, which are characteristic of fertilized mammalian eggs. Imaging of intracellular Ca2+ revealed that Ca2+ transients originate in the hemisphere of the egg that contains abundant ER clusters, thus the mouse contains a stable cortical pacemaker responsible for generating Ca2+ waves.