Role of intercellular contacts in generating an asymmetric mitotic apparatus in the Tubifex embryo

The 2‐cell stage embryo of Tubifex is composed of a smaller cell, AB, and a larger cell, CD. At the second cleavage, the CD‐cell divides unequally. The mitotic apparatus (MA) involved in this division is organized asymmetrically: the MA pole to be segregated to a smaller cell is flattened and truncated, and associated with the anterior cortex facing the AB‐cell, while the other pole is symmetric and located more centrally. The present study was undertaken to elucidate the mechanism that generates asymmetry in the MA organization in CD‐cells. When CD‐cell nuclei, which are normally located near the anterior cortex, were displaced toward the posterior end of the cell (i.e. opposite AB‐cells) by centrifugation, MA assembled ectopically there, and were bilaterally symmetric in organization. Similar symmetric MA were formed in isolated CD‐cells, which divided more equally than intact cells. This equality of cell division was dramatically reduced if the anterior surface of isolated CD‐cells formed contact with other cells, such as AB‐, C‐ and 4D‐cells. The MA that formed in these reconstituted embryos were asymmetric in organization; one MA pole was always found to be truncated and apposed to the cortical site at the cell contact. Symmetric MA were also observed in cytochalasin‐treated embryos. Together with the finding that one of the MA poles is physically attached to the anterior cortex of the intact CD‐cell, these results suggest that factors generating asymmetry in the spatial organization of MA poles reside at the anterior cortex of the CD‐cell and that this cortical mechanism is dependent upon cell contacts.