Investigations in Early Polarity in the Sea Urchin Embryo

Abstract

Establishment and maintenance of cell polarity has become an increasingly interesting biological question in a diversity of cell types and has been found to play a role in a variety of biological functions. Previously, it was thought that the echinoderm embryo remained relatively unpolarized until the first asymmetric division at the 16 cell stage of development. However, there is mounting evidence to suggest that polarity is established much earlier. I analyzed roles of the cell polarity regulators, the PAR complex proteins, and how their disruption in early development affects later developmental milestones such as blastula formation. I found that PAR6 along with aPKC and CDC42 localize to the apical cortex (free surface) as early as the 2 cell stage of development and this localization is retained through the gastrula stage. Interestingly, PAR1 also colocalizes with these apical markers through the gastrula stage, despite the formation of a polarized epithelium and a series of asymmetric divisions. Additionally, PAR1 was found to be in complex with aPKC, but not PAR6, during these developmental stages. PAR6, aPKC, and CDC42 are anchored in the cortex by assembled myosin; however, a clear role for myosin assembly in PAR1 localization could not be determined. Furthermore, myosin assembly was found to be necessary to maintain proper PAR6 localization through subsequent cleavage divisions. Interference with myosin assembly prevented the embryos from reaching the blastula stage, while transient disruptions of either actin or microtubules did not have this effect. Similarly, inhibition of aPKC activity during early cleavage stages impeded blastula formation; however, aPKC is not involved in the regulation of the first asymmetric division at the 16 cell stage in sea urchin embryos. These observations suggest that disruptions of the polarity complex in the early embryo can have a significant impact on the ability of the embryo to reach later critical stages in development.