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.
Takahashi
H
H
Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan.
Shimizu
T
T
D000818:Animals; D017173:Caenorhabditis elegans / Q000196:embryology; D002448:Cell Adhesion; D002455:Cell Division; D002469:Cell Separation; D048429:Cell Size; D015638:Cytochalasin D / Q000494:pharmacology; D003599:Cytoskeleton / Q000187:drug effects / Q000648:ultrastructure; D007365:Intercellular Junctions / Q000648:ultrastructure; D009835:Oligochaeta / Q000166:cytology / Q000196:embryology* / Q000648:ultrastructure; D013045:Species Specificity; D008941:Spindle Apparatus / Q000187:drug effects / Q000648:ultrastructure
D015638:Cytochalasin D