Asymmetric (differential) cell division of thymic lymphocytes by means of cytoplasmic polarization: possible biological meanings.

A series of morphological studies at the light- and electron-microscopical levels have led to the conclusion that a significant proportion of immature thymic lymphocytes would undergo asymmetric (differential) cell division: at the telophase of such dividing cells one of two daughter cells retains morphological characteristics very similar to those of its parent cell, whereas the counterpart daughter cell looks much differentiated. Polarization of the cytoplasm is considered to be the mechanism whereby the asymmetry is generated. The phenomenon is observable in both prenatal and postnatal life of all the animal species examined, that is chicken, guinea pig, rat and mouse, meaning that it is a general phenomenon among the animals possessing the thymus. The frequency of asymmetrically-dividing cells is relatively high in large-sized cell fractions (more than 40% of total dividing cells). The conclusions leading from the above-mentioned morphological studies are supported by the cell kinetic studies performed by Metcalf in the mid-1960s. Asymmetric cell division may be primarily involved in the maintenance of immature 'stem cells' while concomitantly producing differentiated cells of smaller sizes. The phenomenon might also play an important role in the differentiation of thymocytes into distinct lineage of thymic lymphocytes such as cortex and medulla lymphocytes. Furthermore, the present findings raises the idea that hematopoietic cells including lymphoid cells would possess, in general, intracellular mechanisms to undergo asymmetric cell division.