Ultrastructural observations on cortical endoplasmic reticulum and on residual cortical granules in the egg of Xenopus laevis

Abstract

Two kinds of cortical granules (CG) are present in the unfertilized egg of Xenopus laevis: CGa and CGv. The cisternae surrounding both CGs have been found to be interconnected through elements of ER. CG cisternae also anastomose with the subcortical ER. It is suggested that these connections establish a functional unity between the CGs and that the cortical network may be involved in the propagation of the activation stimulus leading to the gradual CG activation and rupture. About 2% of the CGs are left in the cortex following activation. They have never been found in the process of extrusion in eggs fixed later than 5 min after pricking. Notable ER modifications occur in the egg cortical cytoplasm within the first 30 min following activation. Residual CGs do not show preferential distribution with regard to their position in the cortical cytoplasm or with regard to their presence in the animal and vegetal hemispheres. Most CGv and a few CGa undergo changes, although the changes are different in the two classes of granules; both CG types lose most of their original contents. CGv eventually disaggregate, whereas CGa show a reduction of their contents and form one or more large invaginations. Autophagosomes are found in the animal hemisphere 20 to 30 min after activation. They are located in the cortical cytoplasm and have a granular content similar to modified CGs.