Cell membrane formation during the cellularization of the syncytial blastoderm of Drosophila.

The early blastoderm of Drosophila is a syncytium in which about 6000 nuclei become localized in the peripheral cytoplasm. During cycle 14 interphase, a wave of membrane formation encircles each nucleus inside its own plasma membrane, thereby generating an intact epithelial layer. The details of this process of cellularization have been unclear. Using an improved method of fixation of the embryos for electron microscopy, we show by morphological observations that a large number of membrane-bounded, electron-transparent vesicles, of diameters ranging from 0.05 micron to 0.5 micron, are present in the periplasm and become redistributed during cellularization so as to provide the membrane mass required at each phase of the process. We recognize three phases. In the first two phases, the vesicles that were present in the apical periplasmic space at earlier stages become concentrated and aligned between the nuclei. The vesicles then undergo concerted but not precisely synchronous fusion to form double membranes, starting at furrows in the plasma membrane of the embryo and extending about 7 microns into the periplasmic space. Subsequently, in the third phase vesicles are recruited to the basal periplasmic space but do not become aligned between the nuclei as in the first phase. We presume that these vesicles fuse individually with the growing ends of the double membranes until encirclement of each nucleus is complete. We speculate that these vesicles are all derived from the Golgi apparatus and are moved about in the blastoderm by interactions with components of the cytoskeleton.