Changes in organization of the endoplasmic reticulum during Xenopus oocytematuration and activation

The organization of the endoplasmic reticulum (ER) in the cortex of Xenopus oocytes was investigated during maturation and activation using a green fl uorescent protein chimera, immunofluorescence, and electron microscopy. Den se clusters of ER developed on the vegetal side (the side opposite the meio tic spindle) during maturation. Small clusters appeared transiently at the time of nuclear envelope breakdown, disappeared at the time of first polar body formation, and then reappeared as larger clusters in mature eggs. The appearance of the large ER clusters was correlated with an increase in rele aseability of Ca2+ by IP3. The clusters dispersed during the Ca2+ wave at a ctivation. Possible relationships of ER structure and Ca2+ regulation are d iscussed.