Genesis of Epithelial Polarity in Early Mammalian Embryos

Epithelial cells comprise the surface layers that cover tissues and organs, and by definition, exhibit an asymmetric surface domain and hence apical-basal polarity.The development of early mammalian embryos, from a fertilized oocyte to a blastocyst and implanted embryo, provides an excellent system to observe the formation and morphogenesis of several epithelia.From our studies of early mouse embryonic development and morphogenesis, we propose that there are at the least three distinct types of mechanisms for polarization of epithelial cells: the classical tight junction and Par complexes; cell autonomous polarity established by endocytosis; and the subtle polarization caused by the formation of an apical actin cap of adjacent cells.Here, we describe the understanding of the genesis of four embryonic epithelial structures, the trophectoderm, inner cell mass, primitive endoderm, and epiblast, and the genes that are critical for their epithelial polarity and associated morphogenesis.