The Drosophila Egg Chamber: External Ionic Currents and the Hypothesis of Electrophoretic Transport

In insect egg chambers the nurse cells produce cytoplasm which is transported into the oocyte. It has been proposed that the transport is driven by an electrophoretic current resulting from a voltage gradient produced by the egg chamber. There are contradictory reports concerning the existence of significant voltage gradients. Any internal current must have a return pathway. An extracellular current surrounding the egg chamber that is in the opposite direction to that required for the return of an electrophoretic current has been reported, but other authors have not found such a consistent current. We used the vibrating probe to measure the extracellular currents surrounding 50 egg chambers during the stage of maximum transport. Our set of measurements was predominantly characterized by small and variable currents. Most of the measurements were on the order of 1 microampere/cm², and no particular pattern of current flow was consistently evident. Although it was possible to pick out some egg chambers that appeared to show patterns of current flow, approximately equal numbers of egg chambers showed patterns of opposite types, and most showed no pattern at all. Our data do not support the presence of a consistent pattern of ionic current.