Basement membrane

The basement membrane is a thin, fibrous, extracellular matrix of tissue that separates the lining of an internal or external body surface from underlying connective tissue in metazoans. This surface may be epithelium (skin, respiratory tract, gastrointestinal tract, etc.), mesothelium (pleural cavity, peritoneal cavity, pericardial cavity, etc.) and endothelium (blood vessels, lymph vessels, etc.) The basement membrane is a thin, fibrous, extracellular matrix of tissue that separates the lining of an internal or external body surface from underlying connective tissue in metazoans. This surface may be epithelium (skin, respiratory tract, gastrointestinal tract, etc.), mesothelium (pleural cavity, peritoneal cavity, pericardial cavity, etc.) and endothelium (blood vessels, lymph vessels, etc.) As seen with the electron microscope, the basement membrane is composed of two layers, the basal lamina and the underlying layer of reticular connective tissue. The underlying connective tissue attaches to the basal lamina with collagen VII anchoring fibrils and fibrillin microfibrils. The two layers together are collectively referred to as the basement membrane. The basal lamina layer can further be divided into two layers. The clear layer closer to the epithelium is called the lamina lucida, while the dense layer closer to the connective tissue is called the lamina densa. The electron-dense lamina densa membrane is about 30–70 nanometers thick, and consists of an underlying network of reticular collagen IV fibrils which average 30 nanometers in diameter and 0.1–2 micrometers in thickness. In addition to collagen, this supportive matrix contains intrinsic macromolecular components. The lamina densa, whose collagen IV fibers are coated with the heparan sulfate-rich proteoglycan perlecan, and the lamina lucida (made up of laminin, integrins, entactins, and dystroglycans) together make up the basal lamina. Integrins are not part of the basal lamina, they are part of desmosomes which are in the basement membrane but not the basal lamina.