The basement membrane is a thin sheet of extracellular matrix that provides foundational support to tissues. This layer is for organizing and maintaining the structural integrity of different tissue types. It serves as a boundary, separating cells from the underlying connective tissue and facilitating communication between different tissue layers.
Composition and Location of the Basement Membrane
The basement membrane is located at the junction between epithelial tissues and the connective tissue below. This includes the lining of the skin, blood vessels, and internal organs. It also encases individual cells, such as muscle fibers and fat cells, providing a specialized microenvironment for them.
Its primary components are laminins and type IV collagen, which form a scaffold-like network. Other proteins, including nidogens and perlecan, help crosslink these main components, adding to the structure’s stability and function. These proteins are synthesized by the epithelial cells that rest upon the membrane. On an electron microscope, it often appears as two layers: a denser layer closer to the cells called the basal lamina and a more diffuse layer, the reticular lamina, which merges with the connective tissue.
Physical Support and Selective Filtration
A primary function of the basement membrane is to provide a stable foundation for epithelial cells to anchor upon. This attachment is achieved through specific adhesion molecules that connect the cells to the membrane, establishing cellular polarity and preventing the cells from detaching. The membrane acts as a mechanical barrier, confining epithelial cells to their designated layer.
The basement membrane also functions as a highly selective filter, regulating the passage of molecules between tissue compartments. An example is within the kidney’s glomerulus, a structure responsible for filtering blood. Here, the basement membrane permits water and small waste products to pass through to form urine while retaining larger molecules like proteins and red blood cells in the bloodstream.
Guiding Cell Behavior and Tissue Repair
The basement membrane is a dynamic layer that actively influences the behavior of adjacent cells. It contains molecules that can bind to receptors on the cell surface, triggering signaling pathways that regulate cell growth, differentiation, and survival. The composition and stiffness of the membrane can change, which in turn affects how tissues are sculpted during development.
In the event of injury, the basement membrane plays a role in the healing process. It serves as a scaffold that guides the migration of new cells to regenerate the damaged area, acting as a blueprint to ensure the tissue repairs itself in an organized fashion. For instance, at the junction between nerves and muscles, the basement membrane helps to regenerate the synapse after an injury.