Fig. 2.2.

Formation of asymmetries and specific cell types during Xenopus early development. (a) Animal-vegetal polarity is established during oogenesis by the partitioning of molecules to animal and vegetal hemispheres. These partitioned molecules are exemplified by specific mRNAs that become localized during oogenesis to vegetal cortex and the animal hemisphere (Medioni et al. 2012; King et al. 2005; Houston 2013). After fertilization the cells of the embryo inherit these molecular asymmetries when they contribute to the formation of the ectoderm, the mesoderm, and the endoderm germ layers. (b) After fertilization asymmetry in the dorso/anterior to ventral/posterior dimension is established as a result of cortical rotation (Gerhart et al. 1989; Houston 2012). Wnt11 is translocated from the vegetal pole of the egg during cortical rotation to create the embryonic asymmetry in the dorso/anterior to ventral/posterior dimension (Schroeder et al. 1999). Wnt11 activates Wnt signaling to direct the accumulation of beta-catenin protein. The high levels of beta-catenin cause the dorso/anterior cells to induce and give rise to the organizer (Heasman et al. 1994). At the gastrula stage, the organizer produces extracellular signals that influence and pattern the adjacent cells of each germ layer (Gerhart et al. 1991; Harland and Gerhart 1997)