The centrosome cycle in syncytial Drosophila embryos analyzed by energy filtering transmission electron microscopy.

We have investigated the centrosome cycle in Drosophila syncytial embryos at the ultrastructural level by using a transmission electron microscope equipped with an electron energy filtering device (Omega filter). This new technique allows the study of uncontrasted thick sections with a high resolution. We have been able to characterize two classes of filamentous structures in the centrosomal apparatus that were not detectable on ultrathin sections. These new filamentous structures are: 1) a very orderly lattice that connects the two centrioles during mitosis; and 2) a fibrogranular connection between the centrosome and the nuclear envelope. The intercentriolar linkage could be involved in the precise timing of separation of the centrioles during late anaphase. The centrosome-nuclear envelope connection probably prevents the loss of centrosomes in this syncytial environment, and ensures the proper migration of the centrosomes along the surface of the nucleus. This connection may also couple the nuclei to the cytoskeleton, thus allowing their migration and their anchorage to the cortex at the blastoderm stage. This thick section analysis has also allowed us to precisely reconstitute the centrosome cycle. From their separation at telophase and throughout most of interphase, centrosomes are composed of a single centriole. We conclude that in the early Drosophila embryo there is an unusual delay between the separation of the parent centrioles and their duplication. This leaves a surprisingly short time to assemble a daughter centriole.