ABSTRACT
Cell polarity is defined by asymmetric distribution of structural and other macromolecules (proteins, lipids and carbohydrates) within the cell. Epithelial cell polarity is based on compartmentalization of the cell and its membrane into two major domains: apical, facing an open space or lumen; basolateral, taking part in cell-cell and cell-matrix interactions. At the boundary between these two domains, called the subapical zone, epithelial cells form tight junctions that seal the membranes of adjacent cells together to prevent the admixing of apical and basolateral membrane proteins and free diffusion through the intercellular space (Figure 15.1). Tight junctions are complex multiprotein structures that, apart from the described function in mediating selective diffusion across the epithelial sheet, also play an important role in regulating cell polarity, proliferation and differentiation. The role of tight junctions in epithelial polarity has recently been extensively reviewed.1,2
We will focus here on just a few key issues pertinent to our discussion of the establishment of polarity in mammalian embryo and the role of the polarized phenotype in human embryonic stem cells (hESCs).
