Fig. 2.7.

Model for the formation of asymmetries during maternal Xenopus development. During oogenesis specific mRNAs are localized to the vegetal cortex, while others are concentrated in the animal hemisphere. These mRNAs and potentially other components (e.g., proteins and metabolites) form the molecular basis of the animal-vegetal polarity of the fully grown stage 6 oocyte. The translation of some localized mRNAs into proteins during oocyte maturation creates additional animal-vegetal asymmetries present in unfertilized eggs. After fertilization the asymmetries of the egg are inherited by specific blastomeres to generate the initial animal-vegetal polarity of the embryo. During embryogenesis the activities of the animal and vegetal localized mRNAs and proteins create additional asymmetries. For example, the Bic-C repressor blocks translation of the Cripto-1 mRNA in vegetal cells, and as a result the Cripto-1 protein accumulated in animal cells. This is an example of a secondary asymmetry that results from the activity of the molecules localized in oocytes and eggs. In this way mRNA localization differences in the animal-vegetal hemispheres that begin in oocytes are carried into the embryo and elaborated upon both temporally and spatially into secondary and tertiary gradients