This thesis focuses primarily on three aspects on cleavage: the relationship between failure of cleavage and aneuploidy; disruption of the cleavage process by hydrophobic chemicals; and the positioning of the spindle and, thus, the cleavage plane. The established Chinese V79 hamster fibroblast cell line was used as the experimental system.
Time-lapse video recording of the division of binucleated cells revealed that the daughter cells are often aneuploid, indicating that disturbance of cytokinesis may give rise to aneuploidy.
Exposure to a number of alcohols and chlorinated hydrocarbons with different degrees of lipophilicity revealed that cytokinesis can be disrupted by unspecific hydrophobic interactions. This must be taken into account before postulating a specific effect on cleavage. Furthermore, the ratio between multinucleated and binucleated cells allows rough discrimination between mechanisms disturbing the spindle or cleavage, and this ratio might be one valuable endpoint in a test system for aneuploidy.
The mechanism underlying positioning of the spindle was investigated using time-lapse video filming, immunostaining and image analyses. Misplaced spindles were found to move to a central position during anaphase, and this translocation involved outgrowth of astral microtubules. Localisation of dynactin suggested that the dynein/dynactin system interact with astral microtubules and the basal layer of the cell to move the spindle. Thus, unpolarised cells appear to contain a system for positioning the spindle similar to that in polarised cells.
Finally, even a minor decrease in the level of reduced glutathione (GSH) in primary, non-transformed human fibroblasts by oxidation or conjugation interfered with alignment of the chromosomes in metaphase, suggesting an increased risk for aneuploidy. Although this misalignment did not seem to involve disturbance of cytoskeletal elements such as actin and tubulin, the dynactin/dynein system was altered when the level of GSH was lowered sufficiently to give rise to misarranged metaphase chromosomes. More extensive oxidation of GSH lead to rearrangement of actin and inhibition of cleavage.
The present findings increase our understanding of factors which disrupt cleavage and the possible consequences of such disruption. This information will aid in interpreting result obtained with systems designed to screen for agents which induce aneuploidy. Furthermore, the demonstration that unpolarised cells possess a system for positioning the spindle similar to that in polarised cells might facilitate the development of a simple test for screening agents that interfere with this process, which is of fundamental importance for development.
Stockholm: Stockholm University, 2001. , 42 p.