Meiosis, Balbiani body and early asymmetry of Thermobia oocyte

The meiotic division guarantees maintenance of a genetic diversity; it consists of several stages, with prophase I being the longest and the most complex. We decided to follow the course of initial stages of meiotic division in ovaries of Thermobia domestica using modified techniques of squash preparations, semithin sections, and electron microscopy. We show that germaria contain numerous germline cells that can be classified into three categories: cystoblasts, meiotic oocytes, and growing previtellogenic oocytes. The cystoblasts are located most apically. The meiotic oocytes occupy the middle part of the germarium, and the previtellogenic oocytes can be found in the most basal part, near the vitellarium. Analyses of the semithin sections and squash preparations show that post leptotene meiotic chromosomes gather in one region of the nucleoplasm where they form the so-called bouquet. The telomeres of the bouquet chromosomes are attached to a relatively small area (segment) of the nuclear envelope. Next to this envelope segment, the nucleolar organizers are also located. We show that in concert to sequential changes inside the oocyte nuclei, rearrangement of organelles within the ooplasm (oocyte cytoplasm) takes place. This leads to the formation of the Balbiani body and consequent asymmetry of the ooplasm. These early nuclear and cytoplasmic asymmetries, however, are transient. During diplotene, the chromosome bouquet disappears, while the Balbiani body gradually disperses throughout the ooplasm. Finally, our observations indicate the presence of lampbrush chromosomes in the nuclei of previtellogenic oocytes. In the close vicinity to lampbrush chromosomes, characteristic spherical nuclear bodies are present.