REORGANIZATION OF THE ENDOPLASMIC-RETICULUM DURING MEIOTIC MATURATIONOF THE MOUSE OOCYTE

The endoplasmic reticulum (ER) of live metaphase II mouse eggs and pro phase I-arrested oocytes was compared using the fluorescent, lipophili c dicarbocyanine dye, DiI. DiI, dissolved in soybean oil, was microinj ected into oocytes and eggs; the dye diffused throughout the cytoplasm to label the ER, which was imaged by confocal microscopy. The mature egg had a fine reticular network of ER throughout the cell and numerou s dense accumulations of membrane in the cortex. These ER accumulation s, 1-2 mu m in diameter, were generally absent deeper in the cytoplasm . A similar staining pattern was observed when the eggs were fixed wit hin 1 min of injection, providing evidence that the cortical accumulat ions of membrane are part of a continuous ER membrane system, since me mbrane trafficking could not occur in a fixed egg. Cortical ER accumul ations were localized to the same region of the egg as the cortical gr anules and were not observed in the cortical granule-free region adjac ent to the meiotic spindle. In contrast, ER accumulations were rarely found in the cortex of the immature, prophase I-arrested oocyte, but l arger and less well-defined membrane clusters were found throughout th e deeper cytoplasm of the oocyte. The appearance of ER clusters in the egg cortex following oocyte maturation correlates with an increased a bility of the mature egg to release calcium at fertilization. Since th e ER is a calcium store, structural reorganization of the ER may be ne cessary to permit the large release of calcium and resulting cortical granule exocytosis at fertilization. (C) 1995 Academic Press, Inc.