| Abstract |
Cell division is well known to be accomplished by the constriction of a contractile ring. However, many exceptions exist in which a cell is divided with an incomplete ring or contractile band that has loose ends and generates contractile forces while extending around the cell. This type of cell division is present in unilaterally cleaving embryos and therefore underlies development in many different species, from jellyfish, over lizards to platypus. In the early embryos of these species, the cells are attached to yolk, which prevents the formation of a complete ring. However, the physical mechanisms of how a contractile band with loose ends can be stable and ingress, rather than collapse under tension, remains elusive. In this thesis work, I investigate the division mechanism by combining laser severing of the contractile band with rheological measurements of the cytoplasm in vivo. I show that gelation of the bulk cytoplasm during the presence of an interphase microtubule network is an essential anchoring mechanism that allows the band to be stable during growth. Moreover, when the cell progresses to the M-phase of the second cycle, the cytoplasm fluidises, thereby reducing the anchoring of the band to the cytoplasm, causing the band to shrink but also allowing it to ingress. This balance between stability and growth versus instability and ingression repeats for several cell cycles until the division is complete, resulting in a mechanical ratchet that drives cell division without a complete actin ring. This study underscores the role of temporal control over cytoplasm rheology as a fundamental factor driving unilateral cytokinesis. |