Abstract
Cortical contractions are observable in cell divisions in many species, from meiotic divisions in Ascidian and starfish eggs to the early mitotic divisions of the Xenopus embryo. There have been many hypotheses concerning the function of these contractions, from generating pressure gradients to localizing mRNAs. Importantly, the physical and molecular mechanisms underlying these global contractions remain unknown, but would be essential in providing insights into their functions.As a system well-suited for live imaging and physical manipulations, we are studying the prominent contraction that occurs at the highly asymmetric meiotic division in starfish oocytes. In this system, the contraction occurs immediately before the division but its importance for the division remains controversial. Therefore, we set out to understand the molecular details and mechanics of the cortical contraction.Detailed analysis of the oocytes' shape and cytoplasmic flows revealed that the contraction wave is a flattening of the cell cortex which progresses across the whole oocyte towards the site of division with increasing intensity. This results in a strong and varied cytoplasmic flow. Visualizing the localization of components of the cytoskeleton indicates that F-actin and non-muscle myosin II are recruited to the cortical area currently undergoing contraction.To test the involvement of myosin II in the contraction, we applied the myosin II inhibitor blebbistatin that strongly hindered the contraction wave. Our analysis of the inhibition phenotype will reveal important details of the mechanisms of the contraction wave and allow us to determine the contraction waves' function. We will search for additional factors controlling the contraction wave for which an important clue is that changing the nuclear position prior to contraction repositions the axis of contraction, suggesting the involvement of nuclear factors.
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