STRUCTURAL-CHANGES IN THE ENDOPLASMIC-RETICULUM OF STARFISH OOCYTES DURING MEIOTIC MATURATION AND FERTILIZATION

The endoplasmic reticulum (ER) of live starfish oocytes was observed d uring meiotic maturation and fertilization, The ER was visualized by i njection into the cytoplasm of an oil drop saturated with the fluoresc ent lipophilic dye DiI; DiI spread throughout the oocyte endoplasmic r eticulum and the pattern was imaged by confocal microscopy. The ER in the immature (germinal vesicle stage) oocyte was composed of interconn ected membrane sheets. In response to 1-methyladenine, the sheets of E R appeared to become associated with the yolk platelets, forming spher ical shells. A few of these spherical shells could sometimes be seen i n immature oocytes, but their number was much greater in the egg at th e first meiotic spindle stage. At about the time that the first polar body formed, the spherical shells disappeared, and the ER returned to a form like that of the immature oocyte. The spherical shells did not reappear during the second meiotic cycle. During maturation, the ER al so began to move; the movement was apparent by the time of germinal Ve sicle breakdown and continued throughout both meiotic cycles and in eg gs with second polar bodies. When eggs at the first meiotic spindle st age were fertilized, the form of the ER changed. Within I min after sp erm addition to the observation chamber, the circular cross sections o f the spherical shells of the unfertilized egg ER were no longer disti nct. At this point, the form of the ER could not be discerned with the resolution of the light microscope; however, the rate of spreading of DiI from an injected oil drop decreased, providing strong evidence th at the ER had become fragmented. The ER remained in this form for seve ral minutes and then gradually, the appearance of the ER and the rate of DiI spreading returned to be like those of the unfertilized egg, In jection of inositol trisphosphate caused a similar change in the ER st ructure. These results indicate that the ER is a dynamic structure, th e form of which changes during oocyte maturation and fertilization. (C ) 1994 Academic Press, Inc.