Blastomeres and epithelial cells. (A and B) A generic zygote, morula stage embryos containing unpolarized then polarized blastomeres (A), and a blastocyst stage embryo containing polarized epithelial cells (B) illustrate the concepts of blastomere polarization and subsequent epithelial cell formation. (A) The zygote undergoes cleavage to first produce unpolarized blastomeres. Blastomeres subsequently develop radial polarity by differentiating their contacted (red) and contact-free (green) surfaces. (B) Polarized blastomeres eventually develop into fully polarized epithelial cells. Cell surface domains of generic mammalian epithelial cells, along with representative polarity and junction proteins found within these domains, are shown. The developmental stage when blastomeres polarize radially and the time required for cells to transition to mature epithelia vary considerably among species, as exemplified by the three model systems that are the focus of this review. Crb, Crumbs; Lgl, Lethal giant larvae.