What is the function of the basement membrane?

The main function of basement membranes is to separate the epithelium from underlying connective tissue or lamina propria and protect them from mechanical stress. It also assists cells in binding and anchoring to these tissues. The basement membrane produces specific supporting structure and scaffolding for epithelial tissue, to allow for a continuous layer of cells. It protects tissues from mechanical stress by acting as a barrier. For example, it prevents cells from malignant cells from reaching the deeper tissue layers. It may also surround muscle cells, peripheral nerves, or fat cells throughout the body. Additionally, it also functions to regulate the behavior of epithelial cells by modulating shape, migration, adhesion, proliferation, and apoptosis. For example, the process of angiogenesis is correlated with the degradation and reformation of the basement membrane.  

The basement membrane also has functions within tissues. For example, in the kidney glomerulus, it behaves as a filter by only permitting water and small molecules to pass into the urine. In the lungs, the basement membrane is found in the alveoli as a fused membrane between the endothelium and epithelium which permits gas exchange. At the neuromuscular junction, the basement membrane facilitates the separation of nerve and muscle cells, as well as the generation of presynaptic and postsynaptic elements.