Cell polarity refers to the asymmetric organization of cellular components that allows a cell to carry out specialized and directional functions. Rather than being uniform spheres, many cells exhibit spatial differences in protein distribution, membrane composition, cytoskeletal organization, and signaling activity. This asymmetry is essential for processes such as migration, division, differentiation, and tissue organization.
Among all cell types, epithelial cells provide one of the clearest and most studied examples of polarity. These cells line body surfaces and internal cavities, forming protective barriers and mediating selective transport between compartments. Their polarity is described as apical–basal polarity, meaning that the plasma membrane is divided into two functionally and structurally distinct domains:
- The apical domain, facing the lumen or external environment
- The basolateral domain, contacting neighboring cells and the extracellular matrix
This organized architecture enables directional absorption, secretion, and communication while preserving tissue integrity.
In this article, we explore the fundamental principles of apical–basal polarity, the molecular mechanisms that establish it, the role of cell junctions, and its importance in tissue organization.
Cell Polarity in Epithelial Cells
What Is Cell Polarity?
Cell polarity is defined as the uneven distribution of cellular components along one or more axes. In epithelial cells, polarity is typically oriented along the apical–basal axis. However, it is important to distinguish between:
- Apical–basal polarity, which defines vertical asymmetry
- Planar cell polarity, which refers to coordinated orientation within the plane of the tissue
Apical–basal polarity is critical for epithelial function because it ensures that different regions of the cell membrane perform distinct roles.
Apical vs. Basolateral Domains
The apical membrane faces the lumen or external surface. Depending on the tissue type, it may contain:
- Microvilli (in absorptive epithelia)
- Cilia (in respiratory or reproductive epithelia)
- Specialized transporters and enzymes
The apical surface is often adapted for absorption, secretion, or sensing environmental signals.
The basolateral membrane includes:
- The lateral surface (contacting adjacent cells)
- The basal surface (interacting with the extracellular matrix)
This domain contains adhesion molecules, receptors, and transporters distinct from those on the apical surface.
Crucially, these two domains differ in:
- Protein composition
- Lipid organization
- Cytoskeletal attachments
- Signaling complexes
The maintenance of this separation ensures that transport and communication occur in a controlled, directional manner.
Why Polarity Is Essential
Apical–basal polarity enables:
- Directional transport of ions and nutrients
- Vectorial secretion of signaling molecules
- Barrier formation to regulate permeability
- Functional compartmentalization of tissues
Without polarity, epithelial tissues would lose their ability to control internal environments effectively.
Molecular Determinants of Apical–Basal Polarity
The establishment of polarity is not spontaneous; it requires coordinated molecular mechanisms involving conserved protein complexes, cytoskeletal organization, and targeted membrane trafficking.
Polarity Protein Complexes
Three evolutionarily conserved protein complexes play central roles in defining membrane domains:
1. The PAR Complex
Composed of:
- PAR3
- PAR6
- Atypical protein kinase C (aPKC)
The PAR complex localizes near the apical region and helps specify apical identity. It regulates cytoskeletal dynamics and junction assembly.
2. The Crumbs Complex
This complex contributes to apical membrane specification and stabilization. It reinforces apical domain identity and works cooperatively with the PAR complex.
3. The Scribble Complex
Located in the basolateral domain, the Scribble complex promotes basolateral identity and prevents apical proteins from spreading into inappropriate regions.
Mutual Antagonism and Domain Segregation
These complexes exhibit mutual antagonism. Apical complexes inhibit basolateral determinants in their domain, and vice versa. This reciprocal inhibition sharpens boundaries between membrane regions and ensures stable polarity.
Role of the Cytoskeleton
The cytoskeleton provides structural support and directional trafficking pathways:
- Actin filaments form a dense cortical network beneath the apical membrane.
- Microtubules are often oriented with minus ends apical and plus ends basal, guiding vesicle transport.
- Intermediate filaments contribute to mechanical stability.
Through motor proteins such as kinesins and dyneins, vesicles carrying membrane proteins are delivered to specific domains.
Membrane Trafficking and Protein Sorting
Polarized cells rely on sophisticated trafficking mechanisms:
- Proteins contain sorting signals in their cytoplasmic tails.
- Vesicles are selectively targeted to apical or basolateral membranes.
- Endocytosis and recycling pathways maintain domain composition.
These processes prevent random distribution of membrane proteins and preserve domain specificity.
Role of Cell Junctions in Establishing and Maintaining Polarity
Cell junctions are essential structural and functional components that reinforce apical–basal organization.
Tight Junctions as Polarity Barriers
Tight junctions are located at the boundary between apical and basolateral domains. They serve two critical roles:
- Barrier (Gate) Function
Regulating paracellular transport between cells. - Fence Function
Preventing lateral diffusion of membrane proteins between domains.
By acting as diffusion barriers, tight junctions preserve membrane asymmetry.
Adherens Junctions and Polarity Signaling
Adherens junctions, located just below tight junctions, mediate cell–cell adhesion through cadherins. Beyond adhesion, they:
- Initiate cell–cell contact
- Recruit polarity complexes
- Coordinate cytoskeletal organization
The formation of adherens junctions often precedes tight junction assembly, highlighting their role in polarity establishment.
Junctional Complex as an Integrated System
In epithelial tissues, tight junctions, adherens junctions, and desmosomes form a coordinated structure known as the junctional complex. This arrangement:
- Provides mechanical stability
- Anchors cytoskeletal networks
- Coordinates signaling pathways
- Supports dynamic remodeling during development
Polarity is therefore reinforced not only by molecular signaling but also by physical cellular architecture.
Functional Consequences of Apical–Basal Polarity in Tissue Organization
Apical–basal polarity is essential for the physiological functions of epithelial tissues.
Directional Transport and Vectorial Secretion
Polarized transport allows substances to move in a specific direction across the epithelium.
For example:
- Nutrients may enter through the apical surface and exit via basolateral transporters.
- Secreted molecules may be released selectively into the lumen.
This vectorial movement depends on differential localization of channels, pumps, and receptors.
Lumen Formation and Tissue Morphogenesis
During development, polarity guides tissue architecture:
- Polarized cells organize around a central lumen.
- Oriented cell divisions contribute to tissue shape.
- Coordinated polarity across cells ensures structural symmetry.
In three-dimensional culture systems, epithelial cells can self-organize into cyst-like structures with a central lumen, demonstrating the intrinsic capacity of cells to establish polarity.
Maintenance of Tissue Homeostasis
Polarity contributes to long-term stability of tissues by:
- Maintaining ordered cell layers
- Controlling cell positioning
- Regulating interactions with the extracellular matrix
References
Textbooks
- Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., & Walter, P. (2022). Molecular biology of the cell (7th ed.). W. W. Norton & Company.
- Cooper, G. M., & Hausman, R. E. (2019). The cell: A molecular approach (8th ed.). Sinauer Associates.
- Karp, G. (2020). Cell and molecular biology: Concepts and experiments (9th ed.). Wiley.
- Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., Ploegh, H., & Amon, A. (2021). Molecular cell biology (9th ed.). W. H. Freeman.
- Ross, M. H., & Pawlina, W. (2020). Histology: A text and atlas (8th ed.). Wolters Kluwer.
Review Articles
- Goldstein, B., & Macara, I. G. (2007). The PAR proteins: Fundamental players in animal cell polarization. Developmental Cell, 13(5), 609–622. https://doi.org/10.1016/j.devcel.2007.10.007
- Rodriguez-Boulan, E., & Macara, I. G. (2014). Organization and execution of the epithelial polarity programme. Nature Reviews Molecular Cell Biology, 15(4), 225–242. https://doi.org/10.1038/nrm3775
- St Johnston, D., & Ahringer, J. (2010). Cell polarity in eggs and epithelia: Parallels and diversity. Cell, 141(5), 757–774. https://doi.org/10.1016/j.cell.2010.05.011
- Tepass, U. (2012). The apical polarity protein network in epithelial cells: Regulation of junctions and morphogenesis. Annual Review of Cell and Developmental Biology, 28, 655–685. https://doi.org/10.1146/annurev-cellbio-092910-154033
- Bryant, D. M., & Mostov, K. E. (2008). From cells to organs: Building polarized tissue. Nature Reviews Molecular Cell Biology, 9(11), 887–901. https://doi.org/10.1038/nrm2523
Apical–basal polarity refers to the asymmetric organization of epithelial cells, where the apical surface faces the lumen or external environment and the basal surface interacts with the basement membrane.
Apical–basal polarity allows epithelial cells to perform specialized functions such as selective transport, absorption, secretion, and barrier formation.
Cell polarity is established through polarity protein complexes (such as Par, Crumbs, and Scribble) and maintained by cell–cell junctions and interactions with the cytoskeleton.
Loss of apical–basal polarity disrupts tissue organization and can contribute to diseases such as cancer, where cells lose normal structure and controlled growth.
