A cell invasion assay is a laboratory method used to measure how well cells move through a dense, tissue-like barrier. This technique helps researchers understand cellular movement in various biological processes. By observing cells in a controlled environment, scientists gain insights into behaviors that are otherwise difficult to study in living organisms.
This assay allows for the investigation of cellular mechanics and their roles in both healthy body functions and disease progression. It provides a simplified model to observe how cells interact with their surroundings and penetrate physical obstacles, contributing to a broader understanding of cell biology.
The Purpose of Studying Cell Invasion
Studying cell invasion is important in cancer research, as it directly relates to metastasis. Metastasis occurs when cancer cells detach from a primary tumor and spread to other parts of the body, forming new tumors. For cancer to spread, cells must invade surrounding healthy tissues, enter blood vessels (intravasation), travel through the bloodstream, exit vessels at a distant site (extravasation), and colonize a new organ.
Cell invasion assays help researchers understand how cancer cells break through physical barriers like the extracellular matrix, a network of molecules providing structural support to tissues. These assays allow scientists to test potential new drugs or therapies that might block cancer cell invasion, potentially preventing metastasis. Beyond cancer, cell invasion is also observed in the movement of immune cells to sites of infection or inflammation, in normal tissue development, and in wound healing.
How the Assay Works
The most widely used method for studying cell invasion is the Transwell, also known as the Boyden Chamber assay. This setup uses a small plastic insert with a porous membrane that divides a well into two separate compartments: an upper and a lower chamber.
For an invasion assay, the porous membrane is coated with an extracellular matrix (ECM) material, often a gelatinous protein mixture called Matrigel. This Matrigel layer acts as a synthetic barrier that cells must actively degrade and move through, mimicking the dense tissue environment. Cells are placed in the upper chamber, on top of this ECM layer. A chemical attractant is placed in the lower chamber, creating a gradient that encourages cells to move downwards. Over typically 4-24 hours, invasive cells degrade the Matrigel and pass through the membrane pores to reach the lower chamber.
A key distinction exists between cell migration and invasion assays, though they use similar equipment. In a migration assay, the membrane lacks the dense ECM coating, so cells only move through pores in response to a chemical signal. An invasion assay, conversely, requires cells to not only migrate but also actively break down and move through the complex ECM barrier. This setup allows researchers to specifically study the enzymatic capabilities cells use to degrade tissue, a characteristic of aggressive cellular behavior.
Variations of the Assay
While the Transwell assay is widely used, scientists have developed several variations to better mimic complex conditions within the body.
Spheroid Invasion Assay
In this technique, cells are grown as a three-dimensional (3D) spherical cluster before being embedded in a matrix like collagen or Matrigel. This 3D structure provides a more realistic model of a tumor or tissue, allowing researchers to observe how cells invade from a compact mass into the surrounding environment.
Microfluidic Assays
These miniature systems, often called “lab-on-a-chip” devices, create highly controlled microenvironments, including precise chemical gradients and flow conditions that resemble physiological settings. Microfluidic platforms allow for real-time monitoring of cell invasion and can integrate various cell types to study their interactions within a simulated tissue environment. These models offer higher-throughput capabilities and a more nuanced understanding of cell invasion compared to traditional two-dimensional methods.
Interpreting the Results
After cells have had time to invade, scientists quantify the results. This involves removing non-invading cells from the upper membrane surface. Successfully invaded cells on the underside are then fixed and stained with a dye like crystal violet, making them visible under a microscope.
Researchers use a microscope to count the stained cells on the lower side of the membrane, often in multiple fields of view. The total number of invading cells provides a numerical measure of their invasive capacity. For example, a lower count in a drug-treated group compared to an untreated group suggests the drug inhibits invasion. Conversely, a high cell count in a cancer cell line might indicate a more aggressive form of cancer. These quantitative results allow scientists to draw conclusions about factors influencing cell invasion and their implications for health and disease.