[The roles of two kinds of membrane vesicles in the formation of annulate lamellae and nuclear envelopes in a cell-free system from Xenopus egg extracts].

The eggs of Xenopus laevis were activated to complete their second meiosis and enter interphase by calcium ionophore A 23187. Then they were centrifuged at 10,000 g to get activated Xenopus egg extracts. Chromatin structure could form in such an extracts after exogenous Lambda DNA was added, and nuclear envelopes (NEs) could assemble around the chromatin. Furthermore, annulate lamellae (AL), a NE-like membrane structure, could form in chromatin-free regions simultaneously. After large amounts of observation and analysis at ultrastructural level, we propose that both AL and NEs assemble in the extracts by the fusion of membrane vesicles. There were two kinds of membrane vesicles in Xenopus egg extracts, small rough vesicles with diameters of 200 nm and large smooth vesicles. They had many differences in morphology, size and membrane structure. However, they both participate in the formation of AL and NEs, 200 nm membrane vesicles first fused each other to become parallel double membrane cisternae, nuclear pore complexes assembled in the double membranes in the mean time. The fusion of large smooth vesicles with the double membrane cirternae led to the growth of AL and the maturity of NEs. During the formation of NEs, large smooth vesicles were easily observed to link two neighboring double membrane fragments by fusing with them, so that integrated nuclear envelope could form. 200 nm small rough vesicles could attach to the surface of chromatin to start the assembly of nuclear envelopes, while others could fuse each other at chromatin-free regions and these fusion resulted in the formation of annulate lamellae.