Distribution of F-actin during cleavage of the Drosophila syncytial blastoderm.

Abstract

The process of cleavage during the syncytial blastoderm stage of the Drosophila embryo was studied in fixed whole-mounts using a triple-staining technique. Plasmalemma was stained with Concanavalin A conjuated to tetramethylrhodamine isothiocyanate, the underlying cortical F-actin with a fluorescein derivative of phalloidin and nuclei with 4,6-diamidine-2-phenylindole dihydrochloride. The surface caps, which overlie the superficial nuclei at this stage, were rich in F-actin as compared with the rest of the cortex. After the caps formed, they extended over the surface and flattened. While this was occurring the F-actin network within the caps became nore diffuse. By the end of the expansion process F-actin had become concentrated at both poles of the caps. The caps then split in 2. The cleavage was not accompanied by the formation of any apparent contractile ring of microfilaments across the cap, rather the break region was depleted in F-actin. The cortical actin associated with each half of the old cap then became reogranized around a nucleus to form a new daughter cap, and the cycle began again.