Cell Polarity and Establishment of Asymmetry

Core Idea

Epithelial cells establish distinct apical (exposed to lumen) and basolateral (facing blood) domains with different lipid and protein compositions. PAR proteins (Par3, Par6, PKCζ, Par1) create an apical domain and exclude basolateral proteins; tight junctions (claudins, occludin, JAMs) seal the epithelium and maintain domain separation. Distinct delivery of vesicles to each domain via motility, and selective retention of domain-specific proteins, maintains asymmetry. Cell polarity is essential for proper tissue function; loss of polarity is associated with developmental defects and cancer progression.

Explainer

From your understanding of plasma membrane organization, you know that the membrane is a dynamic mosaic of lipids and proteins that can be laterally organized into distinct regions. Cell polarity takes this concept to its functional extreme: an epithelial cell doesn't just have a membrane — it has *two fundamentally different* membrane domains, each with its own lipid composition, protein repertoire, and functional identity. The apical surface faces the lumen (the inside of a tube, like your intestine), while the basolateral surface contacts neighboring cells and the underlying tissue. These two domains are as different from each other as two different cell types might be.

The establishment of polarity begins with a conserved set of proteins called the PAR complex (Par3, Par6, and atypical protein kinase C, or aPKC). Think of the PAR system as a molecular "this end up" label. Par3/Par6/aPKC accumulate at what will become the apical domain and actively exclude basolateral-specifying proteins (like Par1 and Lgl) through phosphorylation — Par1, when phosphorylated by aPKC, is kicked out of the apical zone and confined to the basolateral domain. Reciprocally, Par1 phosphorylates Par3 to exclude it from the basolateral side. This mutual antagonism creates a sharp, self-reinforcing boundary between the two domains, much like two rival gangs enforcing territory lines.

Tight junctions serve as the physical fence that maintains this separation. Located at the boundary between apical and basolateral domains, tight junctions are composed of transmembrane proteins (claudins, occludin, and JAMs) that stitch adjacent cells together so tightly that even small molecules cannot pass between them. This paracellular barrier forces substances to cross the epithelium *through* the cells (transcellularly), giving the epithelium control over what passes. Equally important, tight junctions act as a membrane fence that prevents apical membrane proteins from drifting into the basolateral domain and vice versa — without this fence, the two domains would mix and polarity would collapse.

Maintaining polarity also requires polarized vesicle trafficking. The cell's secretory pathway sorts newly synthesized proteins into different vesicle populations destined for either the apical or basolateral surface. Motor proteins carry these vesicles along cytoskeletal tracks to the correct domain. When polarity breaks down — through disruption of PAR signaling, loss of tight junctions, or trafficking defects — epithelial cells lose their organized architecture. This is a hallmark of epithelial-to-mesenchymal transition (EMT), a process central to both embryonic development and cancer metastasis. Cancer cells that lose polarity can detach from their tissue, invade surrounding structures, and spread to distant sites, which is why understanding polarity is not just a cell biology exercise but a window into disease mechanisms.