Question

What is the basement membrane?

Answer 1

The extracellular matrix (ECM) is a three-dimensional network structure consisting of extracellular macromolecules and minerals eg collagen, enzymes, glycoproteins and hydroxyapatite that provide structural and biochemical support to surrounding cells, as multicellularity evolved independently in different multicellular lineages, the composition of ECM varies between multicellular structures.

Complete answer:
The basement membrane is a structure which is thin, pliable sheet-like. It is a type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between epithelial tissues such as mesothelium and endothelium, and the underlying connective.
Electron microscopic studies reveal that the basement membrane is composed of two layers, the basal lamina and the reticular lamina. The underlying connective tissue attaches to the basal lamina with collagen anchoring fibrils and fibrillin microfibrils.
The basal lamina layer can further be subdivided into two layers based on their visual appearance as seen in electron microscopy. The light colored layer closer to the epithelium is called the lamina lucida, while the denser dark colored layer closer to the connective tissue is called the lamina densa, it is about 30-70 nanometer thick,and consists of an underlying network of reticular collagen fibrils. Other than collagen, this supportive matrix contains intrinsic macromolecular components.
The lamina lucida layer is composed of laminin, integrins, entactins, and dystroglycan. Integrins is the major component of hemidesmosomes which serve to anchor the epithelium to the underlying basement membrane.
The main function of the basement membrane is to anchor down the epithelium to its loose connective tissue (the dermis or lamina propria) underneath, this is achieved by cell-matrix adhesions through substrate adhesion molecules (SAMs).
The basement membrane acts as a mechanical barrier, preventing malignant cells from invading the deeper tissues; it is also essential for angiogenesis( development of new blood vessels).

Examples of basement membranes-
1. The glomerular basement membrane of the kidney, by the fusion of the basal lamina from the endothelium of glomerular capillaries and
2. The podocyte basal lamina, and between lung alveoli and pulmonary capillaries, by the fusion of the basal lamina of the lung alveoli and of the basal lamina of the lung capillaries, which is where oxygen and $CO_2$
diffusion happens (gas exchange).

Note:
Due to genetic defects, injuries by the body's own immune system, or other mechanisms there are some diseases resulting from a poorly functioning basement membrane. These diseases include- Genetic defects in the collagen fibers of the basement membrane which includes Alport syndrome and Knobloch syndrome. Some diseases stemming from improper function of the basement membrane zone are united under the name epidermolysis bullosa.