(227al) Centering Mechanisms of the Pronuclear Complex

The proper positioning and orientation of the mitotic spindle is crucial for the asymmetric cell division and generating cell diversity during development. We present the first systematic study of the effect of interactions of O(1000) microtubules (MTs), the pronuclear complex (PNC), and the cell cortex with the cytoplasmic fluid on the pronuclear migration of the first cell division of C-elegans embryo. This is made possible through developing a highly efficient and parallelized numerical framework that explicitly accounts for long range hydrodynamic interactions (HI) between the MTs in stokesian fluids in addition to modeling their flexibility, dynamic instability and interactions with the molecular motors. Through direct simulations we show that different choices of the previous approximations of the drag coefficient of the PNC can lead to over-prediction and under-prediction of the active forces or the time scale of the PNC migration by an order of magnitude. To study the dynamics of the PNC migration we consider three centering mechanisms: (1) cortical pushing in which growing astral MTs are pushed away from the cortex by polymerization forces; and (2) cortical pulling where pulling forces are applied by cortically- bound dyneins upon MTs; and (3) cytoplasmic pulling where minus-end directed dynein motors carrying cytoplasmic organelles bind and apply a pulling force on MTs. We consider two variations of cortical pushing mechanism, (a) free sliding where the MTs grow and slide tangential to the cortex boundaries and (b) no sliding where the MTs cannot grow tangential and orthogonal to the boundary. We find that except for free sliding model, where the PNC is only centered and fails to rotate, the other mechanisms can properly center the PNC and align it with the Anterior-Posterior axis of the cell. Finally we show that the force transduction by the cortex in cortical pushing and pulling mechanisms vs moving cargos in cytoplasmic pulling mechanism as well as the conformation of the MTs in each mechanism produce specific fingerprints in the generated cytoplasmic flows. These fingerprints can be used to differentiate between the mechanisms and determine their contribution to the migration process. Considering that the flow signatures are generic features of each model, they can used to investigate the presence and role of these active processes in other stages of the cell division or possibly other organisms.