A Cell Kinetic and Cytological Study on the Asymmetric Cell Division of Thymic Lymphoblasts of the Embryonic Rat

Cell division of thymus lymphoid cells from embrynonic and young rats was investigated cytologically on cell smears, focusing attention on asymmetric cell division. Some of thymic lymphoblasts displayed features implicating asymmetric cell division. At the telophase of such cells, two immature daughter cells looked dissimilar: one of them was smaller in size and possessed a more condensed nucleus, compared with the counterpart cell. Furthrmore, in most cases the cytoplasm of the smaller daughter cell was stained with Giemsa more deeply. It was suggested that the asymmetry of the nucleus emerges at anaphase and telophase probably due to some polarized situation of the cytoplasm. Asymmetrically-dividing cells were relatively frequently observed during the developmental period when large lymphoblasts actively transform into smaller lymphocytes :16% to 17% of whole dividing cells were under asymmetric cell division on days 16 and 17 of gestation, while less than 5% on day 19 or thereafter. In correlation with this observation, asymmetrically-dividing cells were more frequently observed among large lymphoblasts than among other smaller cell fractions. These results support the view that the asymmetric cell division may play some essential role in the transformation of large lymphoblasts into smaller lymphocytes.