The modifications of cortical endoplasmic reticulum during in vitro maturation of Xenopus laevis oocytes and its involvement in cortical granule exocytosis.

In Xenopus laevis eggs, cisternae shells which surround cortical granules (CG) are part of a cortical endoplasmic reticulum (ER) network. In this paper the origin of such ER shells has been studied in full-grown, progesterone-exposed Xenopus oocytes. Furthermore, the possible role of the cortical ER in the activation process has been investigated by pricking maturing oocytes. It has been shown that in full-grown ovarian oocytes ER CG shells are absent and ER cisternae are extensively and randomly distributed throughout the peripheral cytoplasm, where they appear to be continuous with annulate lamellae (AL). Following hormone treatment, the AL completely disaggregate and the ER cisternae gradually migrate to the cortex where they surround the CG constituting the typical cortical network described in uterine eggs. Furthermore, it has been found that 8 h after progesterone treatment (before the first polar body extrusion) the response to pricking (CG exocytosis) occurs only at the animal half; there is no observable response in the vegetal half. At this time ER shells surround CG only in the animal hemisphere. A complete CG exocytosis occurs following the first polar body emission, when the cortical ER is well organized in the whole oocyte cortex. The correlation between the differentiation of the cortical ER and the arousal in the oocyte of the ability to respond to a pricking stimulus is discussed in the light of an involvement of the cortical ER in the propagation of CG exocytosis.