Asymmetric distribution of mitochondria in the vegetal marginal zone of the ascidian embryo

다국어 초록 (Multilingual Abstract)

The mechanisms by which embryo exactly distributes mitochondria into the blastomeres during embryogenesis are one of the important issues in developmental biology. Although the mechanisms has been thought to be important for the proper embryonic development, our understanding has remained limited. In the present study, the distribution of mitochondria was examined in embryos of the ascidian, Halocynthia roretzi, by immunohistochemical staining with three-types of the mitochondria-specific antibodies and vital staining of mitochondria with a fluorescent probe, DiOC2(3). Results of the immunohistochemical staining coincided with that of vital staining, which is able to detect the distribution of mitochondria in cytoplasm of the embryo. Mitochondria was mainly segregated into the B4.1 posterior-vegetal blastomeres at the 8-cell stage. During the next stages, mitochondria was preferentially partitioned into cells of the B-line muscle and the A-line nerve cord precursor compared with each sister cell, endoderm in the 5th cleavage stage, and mesenchyme and notochord in the 6th cleavage stage. However, the mitochondria-rich cytoplasm is divided equally among the blastomeres of the animal hemisphere between the 8-cell and the 64-cell stages. When B6.2 blastomeres were isolated at the early 32-cell stage embryo and cultured in seawater, until control embryos reached the 64-cell stage, pattern of mitochondria distribution was similar to results of the coisolated B7.3 and B7.4 blastomeres from the 64-cell stage embryos. Therefore, it is likely that mitochondria are asymmetrically segregated into the marginal cells in the vegetal hemisphere of the ascidian embryo without cell-cell interaction.