Within nearly every animal cell, a force known as cortical tension shapes the cell’s structure and guides its actions. It is generated in the cell cortex, a specialized layer of proteins just inside the cell membrane that creates tension, allowing the cell to alter its shape. The cortex provides physical integrity, much like the surface tension of a water droplet maintains its spherical form.
This internal tension allows cells to resist external forces, move, and divide. The cell cortex itself is a thin layer, between 100 and 1000 nanometers thick, that is both strong and adaptable. The proteins making up this layer are in a constant state of flux, enabling the cortex to be mechanically sturdy yet highly plastic.
The Molecular Machinery of Cortical Tension
Cortical tension arises from an interplay between two main protein types: actin and non-muscle myosin II. The cell cortex is rich with actin filaments, long fibers that form a mesh-like network providing structural support to the cell membrane. Interspersed within this actin mesh are myosin II motors, the molecular machines that generate cortical tension.
Generating tension is an active process requiring energy in the form of adenosine triphosphate (ATP). Myosin II motors use this energy to move along actin filaments, causing adjacent filaments to slide past one another. Because the actin filaments are interconnected, this sliding action generates a contractile force that pulls the entire network inward, creating tension.
This mechanism is analogous to muscle contraction on a microscopic scale. The actin-myosin network is attached to the cell membrane through specific anchoring proteins. This connection ensures that the forces generated within the cortex are effectively transmitted to the cell surface, influencing its overall mechanics.
Fundamental Cellular Processes Driven by Cortical Tension
One of the primary roles of cortical tension is the maintenance and alteration of cell shape. The tension in the cortex provides the cell with a default spherical shape when in suspension and allows it to resist deformation. When a cell needs to change its shape, it can locally adjust this tension, causing specific parts of its surface to contract or expand.
Cell division, or cytokinesis, is a clear example of cortical tension at work. As a cell prepares to divide, a specialized structure called the contractile ring forms at the cell’s equator. This ring is highly enriched in actin and myosin II, and its intense contractile force constricts the cell membrane, ultimately pinching the cell in two.
Cortical tension is also a participant in cell migration. While forward protrusion is driven by other mechanisms, the retraction of the cell’s rear is influenced by cortical contractility. The tension helps to pull the trailing edge of the cell forward as it moves. The overall stiffness of the cortex also affects how the cell interacts with its environment and navigates through tissues.
Dynamic Regulation and Scientific Measurement
Cortical tension is not a fixed property but a dynamic feature regulated in both space and time. Cells can rapidly increase or decrease tension in response to internal and external signals. This regulation is controlled by signaling pathways, with key regulators being proteins from the Rho family of GTPases, which act as molecular switches.
Activated signaling pathways can modify the cortical machinery. They can promote the assembly of new actin filaments or increase myosin II motor activity to raise tension. Conversely, they can trigger the disassembly of the actin network or inhibit myosin activity to reduce tension. This allows a cell to make localized adjustments to drive processes like migration.
Scientists use several techniques to quantify this cellular force. A common method is micropipette aspiration, where a tiny glass needle with a hollow tip applies controlled suction to a cell’s surface. By measuring the force required to pull the cell surface into the pipette, researchers can calculate the tension within the cortex.
Another technique is laser ablation, which uses a focused laser to sever actin filaments in a small region of the cortex. By observing the speed and extent to which the severed network recoils, scientists can infer the amount of tension it was under. A faster recoil indicates higher tension, and these methods reveal how tension changes during cellular activities.
Significance of Cortical Tension in Development and Disease
The control of cortical tension is important for the health and development of an organism. During embryonic development, changes in cell shape and movement that form tissues and organs depend on cortical tension. Processes like gastrulation, where the body plan is established, and neural tube formation rely on coordinated contractions in large groups of cells driven by their internal tension.
When the regulation of cortical tension goes awry, it can contribute to various diseases, such as cancer. Alterations in cortical tension are frequently observed in cancer cells and influence their behavior. For instance, a decrease in cortical stiffness can make it easier for cancer cells to deform and squeeze through dense tissues, a process that aids tumor invasion and metastasis. The migration of cancer cells is also linked to the contractility of the cell cortex.
The implications of dysregulated cortical tension extend beyond cancer. Aberrations may be involved in developmental disorders where tissue formation is impaired. It also plays a role in processes like wound healing, which requires the coordinated movement of cells to close a gap. Understanding the molecular basis of cortical tension and its control provides insight into these processes and may offer new avenues for therapeutic intervention.