CYTOPLASMIC TRANSPORT IN DROSOPHILA OVARIAN FOLLICLES - THE MIGRATIONOF MICROINJECTED FLUORESCENT-PROBES THROUGH INTERCELLULAR BRIDGES DEPENDS NEITHER ON ELECTRICAL CHARGE NOR ON EXTERNAL OSMOLARITY

Using video-intensified fluorescence microscopy and a pseudocolor disp lay of fluorescence intensity, we analyzed the distribution of microin jected 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 m ass of the microinjected fluorescent probe. As culture solutions, we u sed 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 wel l as of two cationic dyes (rhodamine 6G and tetramethylrhodamine dextr an-lysine) were iontophoretically microinjected either into a nurse ce ll or into the oocyte of stage-10 follicles. in the tissue-culture med ium, within a few seconds following microinjection, all tested dyes pa ssed through the intercellular bridges in both the anterior direction (to the nurse cells) and the posterior direction (to the oocyte), inde pendent of their electrical charge or molecular mass. In all simple sa lines, irrespective of their osmolarity, Lucifer Yellow CH was found t o preferentially migrate in the posterior direction and to accumulate in the oocyte due to progressive binding to yolk spheres. Thus, with t his sensitive method, no correlation was detectable between the extern al osmolarity, the electrical charge and the preferential direction of migration of a microinjected probe. Our results indicate that the ele ctrical gradient described by other authors does not exert significant influence on the migration of charged molecules through intercellular bridges in situ.