Polarization of Xenopus embryos during cleavage. (A) Deposition of new (basolateral) membrane during cleavage. A dividing one-cell embryo is shown, with membrane inherited from the egg and new membrane trafficked to the furrow. In the inset, concentrations of furrow MTs are seen at the base of the ingressing cleavage furrow. Separate astral MTs are also present. (B) Formation of superficial (outer) and deep (inner) cell layers. All cells have a superficial surface at the 32-cell stage. Asymmetric divisions over the next several cleavage cycles produce a population of deep cells that lie in the interior of the embryo, as shown in a 128-cell embryo. Membranes of superficial cells are polarized, with aPKC and Par6B at apical surfaces and Par1 and Lrp6 at basolateral surfaces. Notch signaling, which is inhibited by Par1, is high in superficial cells (dark shading) and low in deep cells (light shading). Lrp6 asymmetry also creates differences in Wnt signaling between superficial and deep cells (not depicted).