27.9: Basal Lamina are the Specialized Form of ECM

The basal lamina is a thin extracellular layer that lies underneath the cells and separates them from other tissues. The three layers of the basal lamina are lamina lucida, lamina densa and lamina reticularis. The basal lamina, a mixture of glycoproteins and collagen, provides an attachment site for the epithelium, separating it from underlying connective tissue. The framework of basal lamina has other essential proteins such as laminins mesh, perlecan, entactin, and type IV collagen.

Proteins like integrins and dystroglycan in the cell membranes attach to proteins like laminins and fibronectin in the basal lamina, associating the cell with the extracellular matrix of the connective tissue. The basal lamina present in the kidney, serves as a molecular filter, whereas, at the neuromuscular junction, it surrounds the muscle cells, separating them from the nerve cells at the synapse. The basal lamina further helps regenerate the synapse after an injury and aids in localizing acetylcholine receptors.

The basement membrane or basal lamina is a specialized form of extracellular matrix or ECM. Its organization and function depend on the cell type to which it is attached.

For example, at the neuromuscular junction, the basal lamina surrounds the muscle cells, separating them from the nerve cells. In contrast, the basal lamina in the epithelium is placed below the cells, separating them from the underlying connective tissue.

In the kidney glomerulus, it is present between sheets of endothelial and epithelial cells, acting as a molecular filter preventing the passage of macromolecules from the blood into the urine.

The basal lamina is mainly composed of type IV collagen, and glycoproteins such as  laminins, entactin and perlecan.

These proteins together act as a bridge by linking the cells to the ECM of the connective tissue.