In Drosophila oogenesis, several morphogenetic determinants and other developmental factors synthesized in the nurse cells have been shown to accumulate in the oocyte during pre- to mid-vitellogenic stages. However, the mechanisms of the involved intercellular transport processes that seem to be rather selective have not been revealed so far. We have investigated in vitro, by means of video-enhanced contrast time-lapse microscopy, the transport of cytoplasmic particles from the nurse cells through ring canals into the oocyte during oogenesis stages 6–10A. At stage 7, we first observed single particles moving into the previtellogenic oocyte. The particle transfer was strictly unidirectional and seemed to be selective, since only some individual particles moved whereas other particles lying in the vicinity of the ring canals were not transported. The observed transport processes were inhibitable with 2,4-dinitrophenol, cytochalasin B or N-ethylmaleimide, but not with microtubule inhibitors. At the beginning of vitellogenesis (stage 8), the selective translocation of particles through the ring canals became faster (up to 130 nm/second) and more frequent (about 1 particle/minute), whereas during mid-vitellogenesis (stages 9–10A) the velocity and the frequency of particle transport decreased again. Following their more or less rectilinear passage through the ring canals, the particles joined a circular stream of cytoplasmic particles in the oocyte. This ooplasmic particle streaming started at stage 6/7 with velocities of about 80 nm/second and some reversals of direction at the beginning. The particle stream in the oocyte was sensitive to colchicine and vinblastine, but not to cytochalasin B, and we presume that it reflects the rearrangement of ooplasmic microtubules described recently by other authors. We propose that during stages 7–10A, a selective transport of particles into the oocyte occurs through the ring canal along a polarized scaffold of cytoskeletal elements in which microfilaments are involved. This transport might be driven by a myosin-like motor molecule. Either attached to, or organized into, such larger particles or organelles, specific mRNAs and proteins might become selectively transported into the oocyte.
Testing Strategies of the In Vitro Micronucleus Assay for the Genotoxicity Assessment of Nanomaterials in BEAS-2B Cells
