Cells are the fundamental building blocks of all living organisms. Within these microscopic structures, a remarkable principle of organization exists known as cell polarity. This concept describes the inherent asymmetry within a cell, where different regions possess distinct molecular compositions, structures, and specialized functions. This internal, directional property allows cells to perform their roles precisely and efficiently.
Understanding Cell Polarity
Cell polarity signifies a cell’s intrinsic asymmetry, where internal components are unevenly distributed, leading to specialized regions or “poles” with unique molecular makeup and functions. Unlike a non-polarized cell, a polarized cell has a defined top and bottom, or front and back, enabling specific directional tasks.
Cell polarity manifests in various forms, each tailored to a cell’s particular role.
Apical-basal polarity is common in epithelial cells, which form linings and barriers. These cells have a distinct apical surface facing an open space and a basolateral surface adhering to underlying tissues. This arrangement enables directional transport, such as nutrient absorption in the intestine or filtration in the kidney.
Planar cell polarity (PCP) coordinates cell orientation within a tissue, perpendicular to the apical-basal axis. This is evident in the organized arrangement of hair cells in the inner ear, ensuring sound transduction, or the uniform direction of hairs on mammalian skin. PCP also guides processes like convergent extension during embryonic development.
Front-rear polarity is characteristic of migrating cells, such as immune cells or fibroblasts. These cells establish a distinct leading edge that extends in the direction of movement and a trailing rear that retracts. This directional asymmetry is essential for coordinated movement, allowing cells to navigate their environment.
How Cells Establish Polarity
Cells establish and maintain their polarized state through coordination of internal structures and external signals.
The cytoskeleton, a dynamic network of protein filaments like actin and microtubules, acts as an internal scaffolding system, guiding the asymmetric distribution of cellular components. Actin filaments often drive initial symmetry breaking, while microtubules stabilize the polarized organization. These filaments possess intrinsic polarity, which motor proteins use to transport cargo to specific cellular regions.
Specific protein complexes organize polarity. For instance, PAR, Crumbs, and Scribble complexes are conserved and establish distinct membrane domains. In epithelial cells, PAR and Crumbs localize to the apical membrane, while Scribble is found at the basolateral membrane. These complexes interact with signaling molecules, including Rho GTPases, to regulate protein and lipid localization.
External signals also influence cell polarity. Cues from the extracellular environment, such as contact with neighboring cells or the extracellular matrix, trigger internal reorganization. For example, binding of extracellular matrix components to integrin receptors can initiate polarized actin cytoskeleton remodeling. Similarly, gradients of soluble chemical signals can induce asymmetric activation of cell-membrane receptors, leading to a leading edge in migrating cells.
The Vital Roles of Cell Polarity
Cell polarity enables cells to perform their functions with precision and direction across many biological processes.
In tissue formation, cell polarity is important for creating and maintaining complex structures. Epithelial cells rely on apical-basal polarity to form tight barriers and glandular structures, ensuring fluid and nutrient transport across tissues like the intestine or kidney. This arrangement allows for selective exchange of molecules and protects internal environments.
Cell polarity also governs asymmetric cell division. In this process, a parent cell divides into two daughter cells that differ in size, fate, or molecular content. This asymmetry, guided by polarized distribution of cellular components and mitotic spindle orientation, generates cellular diversity during development and maintains stem cell populations.
Cell polarity is important for cell migration, directing movement during physiological events. During embryonic development, wound healing, or immune responses, cells must move in a coordinated, directional manner. Front-rear polarity, with a distinct leading edge and retracting rear, facilitates this directed movement, allowing cells to navigate environments and repair damaged tissues.
Specialized cells rely on their inherent polarity. Neurons, for instance, exhibit a polarized structure with distinct dendrites for receiving signals and a long axon for transmitting them. This polarity ensures unidirectional flow of electrical and biochemical signals, important for nervous system communication. Kidney cells use their polarity to filter waste and reabsorb nutrients.
Consequences of Polarity Disruption
When cell polarity is compromised, it can lead to cellular dysfunctions and contribute to diseases.
Loss of cell polarity is a feature in cancer progression. Cancer cells often lose their organized structure and apical-basal polarity, which enables them to detach from their original tissue, invade surrounding areas, and metastasize. This altered polarity promotes uncontrolled proliferation and invasive behavior.
Disruptions in cell polarity can also manifest as developmental disorders. The establishment of polarity is important for the formation and patterning of tissues and organs during embryonic development. When polarity cues are misregulated, organs fail to form correctly, leading to birth defects or impaired organ function. For example, defects in planar cell polarity have been linked to neural tube defects and polycystic kidney disease.
Maintaining cell polarity is important to overall health. Impaired tissue function, such as kidney filtration or neurological signaling issues, can arise from cells’ inability to maintain their specialized, directional properties. The integrity of cellular organization, dictated by cell polarity, underpins the healthy functioning of individual cells and complex body systems.