Intracellular molecular gradients within Xenopus laevis oocytes prepare the cell for asymmetric division

Abstract
Starting from a single fertilized oocyte; the original egg cell, through manifold of divisions a complex organism is developed that has distinct head-tail (bottom-up), left-right and dorsal-ventral (back-belly) asymmetries. One of the main challenges in developmental biology is to understand how and when these asymmetries are generated and how they are controlled. The African clawed frog (Xenopus laevis) ideal model for studies of early development thanks to their very large oocytes. We have developed a unique molecular tomography platform based on RT-qPCR, RNA-seq and iTraq UPLC-ESI-MS/MS to measure asymmetric localization of fate determining mRNAs, non-coding RNAs and proteins within the oocyte and among the early stage blastomeres. We find the first axis, called animal-vegetal, is formed during oogenesis by mRNA and microRNA gradients and determines head – tail organization. First cell division following fertilization producing 2-cell stage embryo forms the left-right, and second cleavage generating 4-cell embryos specifies the dorso-ventral axis.