Asymmetric division of immature lymphoid cells of the embryonic mouse thymus

Cell division of thymus lymphoid cells from 15- to 19-day-old embryonic mice, as well as from 2- and 7-day-old mice was investigated on cell smears stained with Giemsa or with the Feulgen reaction followed with malachite green. Asymmetrically dividing cells were observed during the development. At the anaphase and telophase of such cells, the nucleus of one of two immature daughter cells was apparently smaller and more condensed than that of the counterpart cell. The asymmetry was already apparent at the level of sister chromosomes at anaphase. Sometimes a difference also existed in the cytoplasm: the cytoplasm of the smaller immature daughter cell with condensed nucleus was stained with Giemsa more deeply than that of the counterpart. The frequency of cells displaying apparent asymmetric division among total dividing cells at the telophase was around 10% between 15 and 19 days of gestation, and decreased to 2% on days 2 and 7 after birth. Asymmetrically dividing cells belonged to immature lymphoid cells of larger size. The biological implications of the present phenomenon are discussed.