Compaction and radial polarization in the eight-cell mouse embryo. During the eight-cell stage, rounded nonpolar blastomeres compact with one another and also polarize along their radial axis. A 16-cell embryo is shown on the right. Par3, Par6B, and aPKC are restricted to contact-free surfaces, whereas Par1/EMK1 and E-cadherin are found at cell contacts.