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The basal lamina is a strong, fibrous, and permeable matrix that acts primarily as a base on which cells in the human body can grow. It also links cells together and to underlying connective tissue. This matrix is composed of collagen, glycoproteins, and cell receptor molecules. It is made up primarily of two sub-layers, the lamina densa and lamina lucida. The basal lamina and the reticular lamina make up the basement membrane.
There was wide confusion about the structures within the basement membrane, and the existence of the basal lamina, until the basement membrane was viewed with a transmission electron microscope (TEM). TEMs use a beam of electrons, focused and concentrated by electro-magnetic lenses, to penetrate and view minuscule details in an ultra-thin section of tissue. TEMs can work on structures as small as 0.2 nanometer (nm) while traditional light microscopes can only work to about 200 nm.
The basal lamina is approximately 50.8 nm thick. It is impossible to see the fine details of the basement membrane and resolve the lamina's eistence within the basement membrane with light microscopy. TEM not only confirmed the existence of the basal lamina, but it also revealed that it could be divided further into two layers, the lamina densa and the lamina lucida.
The lamina densa and lamina lucida derive their names from the staining method used to visualize structures in transmission electron microscopy. In TEM, heavy metal stains are used to provide tissue contrast. The heavy metal stains bind to the lamina densa and, when viewed in the TEM, this structure absorbs the electrons, or is electron dense. The lamina densa name is, thus, derived from the dense or dark characteristic it takes on in the TEM. Lucida means bright and, since the lamina lucida is electron transparent and appears bright in TEM, it was given the opposite descriptive name.
The lamina densa is composed of type IV collagen. Type IV collagen is a structure that looks like posts and balls linked together. This architecture produces a strong but open framework to support the adjacent cells and components of the lamina lucida.
The lamina lucida is composed of the cell receptor molecule, integrin and the glycoproteins laminin and entactin. Integrin cross-links cell surfaces, binding cells together. Laminin and entactin act both as pillows for the neighboring cells and to bind molecules outside the cell. Entactin, laminin and integrin molecules of the lamina lucida are embedded within and protrude from the open spaces in the collagen matrix of the lamina densa.
The alveolar-capillary barrier is the important blood-air layer in the lungs. The alveolar-capillary barrier is partially composed of the basal lamina. In this barrier, the basal lamina provides structural support for the lungs, while it also allows for the exchange of gases such as oxygen and carbon dioxide.
The basal lamina is also an integral part of the glomerular basement membrane, the outer lining of the glomerulus in the kidneys. The glomerulus is responsible for filtering waste from the blood and turning it into urine. In the glomerular basement membrane, the lamina provides architectural integrity to this filtering structure while allowing the exchange of blood waste across its surface.
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