Basal Lamina is an extracellular structure that is closely related to the plasma membrane on the epithelial and endothelial cell bases and surrounds muscle and adipose tissue. Basal Lamina is a composite extracellular structure located on the basal surface of epithelial cells and the membranous structure of connective tissue. It has a supporting connection and is also a semi-permeable membrane that is transparent to the substance. It is a specific region of the extracellular matrix. A typical base film is about 50 nm thick, and some base films have a thickness of 200 nm.
The basement membrane is a network composed of different protein fibers. The main components are laminin (LN), type IV collagen, entactin, tenascin, and thorombospondin. Heparin sulfate glycoprotein, fibronectin (FN) and so on. LN plays an important role in the formation of basement membranes because LN has many different domains with specific sites for binding to other protein complexes. In addition, LN binds to cell surface receptors and can bind to other LN molecules as well as other glycoproteins, including other components of the basement membrane. More importantly, LN binds to type IV collagen to form a separated cross-linked network structure, and the base film formed is tightly bound to the cell surface receptor by LN, and the base film is tightly bound to the covered cells. Therefore, LN is not only the main component of the base film, but also the organizer of the base film, and the IV type glue principle is the mesh steel frame of the base film. It is a film formed between the basal surface of epithelial cells and deep connective tissue. Because it is very thin, it is generally not distinguishable on HE-stained sections, but the basement membrane of pseudo-stratified ciliated columnar epithelium and stratified squamous epithelium is thicker and can be seen in pink. It is stained with silver plating and the basement membrane is black. Under the electron microscope, the basement membrane is divided into two parts, the part close to the epithelium is the substrate, which is secreted by the epithelial cells; the part that is connected with the connective tissue is the stencil.
The basement membrane not only supports the tissue structure, but also acts as a barrier to permeability, regulating the movement of molecules and cells. In kidney cells, the basement membrane acts as a filter that allows small molecules to enter the urine while deducting macromolecular proteins. The basement membrane under the epidermal cells prevents the connective tissue cells from entering the epidermis on the one hand, and allows the "defense fighter" on the other hand: the movement of white blood cells.
Major basal lamina components
Studies have shown that Laminin, Type IV Collagen, Perlecan, and Nidogen are major components of invertebrate basal lamina. The Drosophila genome contains four Laminin genes, which encode two α, one β, one γ subunits, two Type IV Collagen genes, which encode the α1(IV) and α2(IV) subunits, one Nidogen and one Perlecan gene.
Molecular and genetics studies in Drosophila have shown that mutations that prevent the assembly of Laminin-1 (α3, α5, β1, γ1) molecules lead to the disruption or absence of basal laminae during embryogenesis resulting in embryonic lethality. Furthermore, mutations in Perlecan and Nidogen do not affect the assembly and stability of basal laminae during embryogenesis, raising the possibility that other components contribute to the assembly and stability of basal laminae.
Basal lamina secretion and assembly
There are two major sources of basal laminae components in Drosophila: the hemocytes and the fat body. With the exception of the hemocytes, all Drosophila tissues are associated with a basal lamina. Biochemical and genetic studies conducted have provided the following model of basal lamina assembly. Laminin is the first component of the basal lamina to be secreted and assembled into a network and in its absence no basal lamina formation occurs. Secreted Laminin binds to the ECM cell surface receptors β1-integrin and α-dystroglycan via its C-terminal LG domain. Binding to cell surface receptors facilitates its polymerization through the N-terminal short arms. Following Laminin polymerization into a network, Collagen IV is secreted and polymerized into a network that is tethered to the Laminin network. Linkage of the Collagen IV network to the Laminin network is believed to be mediated through the small ECM glycoprotein Nidogen that can bind to both Type IV Collagen and Laminin. Perlecan does not polymerize like Collagen IV and Laminin but can bind to both of these network forming molecules and Nidogen.
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2. Slavkin HC.; et al. Basal lamina persistence during epithelial-mesenchymal interactions in murine tooth development in vitro. Journal of Craniofac Genet Dev Biol. 1983;3(4):387-407.