Formation of actin filament bundles in the ring canals of developing Drosophila follicles

Growing the intracellular bridges that connect nurse cells with each other and to the developing oocyte is vital for egg development. These ring canals increase from 0.5 mu m in diameter at stage 2 to 10 mu m in diameter at stage 11. Thin sections cut horizontally as you would cut a bagel, show that there is a layer of circumferentially oriented actin filaments attached to the plasma membrane at the periphery of each canal. By decoration with subfragment 1 of myosin we find actin filaments of mixed polarities in the ring such as found in the ''contractile ring'' formed during cytokinesis. In vertical sections through the canal the actin filaments appear as dense dots. At stage 2 there are 82 actin filaments in the ring, by stage 6 there are 717 and by stage 10 there are 726. Taking into account the diameter, this indicates that there is 170 mu m of actin filaments/canal at stage 2 (pi x 0.5 mu m x 82), 14,000 mu m at stage 9 and similar to 23,000 mu m at stage 11 or one inch of actin filament! The density of actin filaments remains unchanged throughout development. What is particularly striking is that by stages 4-5, the ring of actin filaments has achieved its maximum thickness, even though the diameter has not yet increased significantly. Thereafter, the diameter increases. Throughout development, stages 2-11, the canal length also increases. Although the density (number of actin filaments/mu m(2)) through a canal remains constant from stage 5 on, the actin filaments appear as a net of interconnected bundles. Further information on this net of bundles comes from studying mutant animals that lack kelch, a protein located in the ring canal that has homology to the actin binding protein, scruin. In this mutant, the actin filaments form normally but individual bundles that comprise the fibers of the net are not bound tightly together. Some bundles enter into the ring canal lumen but do not completely occlude the lumen. All these observations lay the groundwork for our understanding of how a noncontractile ring increases in thickness, diameter, and length during development.