Cell Polarity and Asymmetry

Cell polarity is the organized asymmetry of a cell, in which its components, including the plasma membrane, cytoskeleton, and organelles, are distributed unevenly to create distinct domains and a defined directionality. This asymmetry lets epithelial cells separate an apical from a basal surface, lets migrating cells distinguish front from rear, and lets dividing cells segregate fate determinants, making polarity fundamental to how cells build tissues and perform directional functions.

Definition

Cell polarity is the establishment and maintenance of an asymmetric organization of a cell's membrane domains, cytoskeleton, and contents, producing functionally distinct regions and a defined axis of orientation.

Scope

This entry covers the concept of cell polarity, the conserved protein systems that establish and maintain it, and its main forms, including apical-basal polarity of epithelia and front-rear polarity of migrating cells. It is a reference and educational topic in cell biology; tissue morphogenesis and migration as processes are treated in related entries, and no clinical guidance is provided.

Core questions

• What does it mean for a cell to be polarized?
• Which conserved protein complexes establish and maintain polarity?
• How do epithelial apical-basal and migratory front-rear polarity differ and relate?
• How is polarity coupled to the cytoskeleton and membrane traffic?

Key concepts

• Apical-basal polarity
• Front-rear (planar/migratory) polarity
• Par, Crumbs, and Scribble polarity complexes
• Mutual antagonism of cortical domains
• Polarized membrane trafficking
• Cytoskeletal asymmetry
• Asymmetric cell division

Key theories

Conserved polarity complexes

St Johnston and Ahringer describe how a small set of conserved protein modules, including the Par, Crumbs, and Scribble complexes, interact through mutual antagonism to partition the cell cortex into distinct domains across diverse cell types from eggs to epithelia.

Interconvertible polarity states

Nelson describes apical-basal and front-rear polarity as related, interconvertible organizations of the same underlying machinery, allowing epithelial cells to switch between a stationary, tissue-building state and a migratory state.

Mechanisms

Polarity is established when symmetry-breaking cues, such as cell-cell or cell-matrix contacts or external gradients, are amplified by conserved cortical protein complexes that mutually exclude one another to define separate membrane domains. The Par complex, Crumbs complex, and Scribble complex partition the apical and basolateral surfaces of epithelial cells, while related signalling polarizes the front and rear of migrating cells. These cortical domains organize the actin and microtubule cytoskeleton and direct polarized vesicle trafficking so that specific proteins are delivered to particular surfaces, and in dividing cells this asymmetry can be used to segregate fate determinants between daughters.

Clinical relevance

Polarity underlies the barrier and transport functions of epithelial tissues, and loss of normal polarity is a feature of disorganized and neoplastic tissue, so the concept is relevant to histology and pathology. This entry describes normal cell polarity for reference and education and is not a basis for diagnosis or treatment.

Evidence & guidelines

The account here is grounded in authoritative reviews of cell polarity and in standard cell biology textbooks; it is descriptive basic science rather than clinical guideline content.

History

The recognition that cells are spatially asymmetric is long-standing in the study of epithelia and fertilized eggs, but the molecular basis emerged from genetic studies in model organisms such as Caenorhabditis elegans and Drosophila, which identified the Par genes and other conserved polarity regulators. Subsequent work, synthesized in reviews by St Johnston and Ahringer and by Nelson, showed that the same modules act across very different cell types and that distinct polarity states are interconvertible.

Debates

How unified is the polarity machinery across cell types?

While core complexes such as Par are broadly conserved, how far apical-basal and front-rear polarity share a single mechanism versus deploying distinct context-specific programmes remains an active question.

Key figures

• Daniel St Johnston
• Julie Ahringer
• W. James Nelson

Seminal works

• stjohnston-ahringer-2010
• nelson-2009

Frequently asked questions

What is the difference between apical-basal and front-rear polarity?

Apical-basal polarity organizes a stationary epithelial cell into a top (apical) surface facing a lumen or outside and a bottom (basal) surface attached to other cells and matrix; front-rear polarity organizes a migrating cell along its direction of movement. The two use overlapping machinery and can interconvert.

Why does cell polarity matter for tissues?

Polarity lets epithelial cells form ordered sheets with distinct surfaces, enabling directional transport and barrier function; without it, tissues lose their organized architecture.