Structural And Functional Modifications of the Nucleus During Oogenesis in the Mosquito Aedes Aegypti

Abstract

The observations are based on reconstructions from electron micrographs of complete serial sections of some 20 oocytes at different stages of development. At eclosion the Aedes aegypti egg chambers each contain 8 interconnected cells, one oocyte and 7 nurse cells. During the next few days the egg chambers develop up to a previtellogenic resting stage. The most advanced oocytes are now in the diplotene stage of meiosis. Following the first blood meal vitellogenesis commences and lasts for 3-4 days. During this time the oocyte increases from a sphere with a diameter of 30 µm to an elongate spheroid of the dimensions 190 x 600 µm. The nucleus expands with the oocyte, first branching then forming a cup-shaped structure, and finally elongating to a large sheet-like, cylindrical form. The chromosomes, condensed into a karyosphere, remain at the anterior pole close to the nurse cells. Towards the end of vitellogenesis meiosis is resumed, and shortly thereafter the envelope of the large nucleus disintegrates. For a short period the chromosomes, which are in first meiotic metaphase, are without an enveloping membrane, then several layers of vesiculate membrane reassemble around them, trapping some of the cytoplasm in the process. At this stage the oocyte may be ovulated or it may rest unchanged for some time. Thirty minutes after oviposition meiosis is complete, and the masses of membranes assemble in a few regions which later degenerate with the polar bodies.