Cytoplasmic transport in Drosophila ovarian follicles: the migration of microinjected fluorescent probes through intercellular bridges depends neither on electrical charge nor on external osmolarity.

Using video-intensified fluorescence microscopy and a pseudocolor display of fluorescence intensity, we analyzed the distribution of microinjected molecules within the nurse-cell/oocyte syncytium of Drosophila ovarian follicles. We varied the composition and the osmolarity of the culture solution as well as the electrical charge and the molecular mass of the microinjected fluorescent probe. As culture solutions, we used four simple salines (IMADS) and a complex tissue-culture medium (R-14) that matched the osmolarity of adult hemolymph. Small amounts of two anionic dyes (Lucifer Yellow CH and Lucifer Yellow dextran) as well as of two cationic dyes (rhodamine 6G and tetramethylrhodamine dextran-lysine) were iontophoretically microinjected either into a nurse cell or into the oocyte of stage-10 follicles. In the tissue-culture medium, within a few seconds following microinjection, all tested dyes passed through the intercellular bridges in both the anterior direction (to the nurse cells) and the posterior direction (to the oocyte), independent of their electrical charge or molecular mass. In all simple salines, irrespective of their osmolarity, Lucifer Yellow CH was found to preferentially migrate in the posterior direction and to accumulate in the oocyte due to progressive binding to yolk spheres. Thus, with this sensitive method, no correlation was detectable between the external osmolarity, the electrical charge and the preferential direction of migration of a microinjected probe. Our results indicate that the electrical gradient described by other authors does not exert significant influence on the migration of charged molecules through intercellular bridges in situ.