Basal lamina

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The basal lamina is a component of the basement membrane that separates epithelium from the underlying connective tissue.

Electron micrograph showing basal lamina lining the outer surface of the cell membrane.

The basal lamina (often confused with the basement membrane, see below) is a layer on which epithelium sits and which is secreted by the epithelial cells. This layer is composed of an electron-dense layer (the lamina densa) composed of type IV collagen between two electron-lucid layers (the lamina lucida) containing the glycoprotein laminin. It is typically about 40-50 nm thick (with exceptions such as the basal laminae that comprise the 100-200 nm glomerular basement membrane).

1 Structure
2 Basement membrane
3 See also
4 References
5 External links

[edit] Structure

The three layers of the basal lamina (lamina densa sandwiched between two layers of lamina lucida) typically sit on top of the reticular lamina, which is synthesized by cells from the underlying connective tissue and contains fibronectin. The exception is when two epithelial layers abut one another as in the alveoli of the lungs and glomeruli of the kidneys, in which the basal lamina of one epithelial layer fuses with that of the other.

Anchoring fibers composed of type VII collagen extend from the basal lamina into the underlying reticluar lamina and loop around collagen bundles. Although found beneath all basal laminae, they are especially numerous in stratified squamous cells of the skin.

[edit] Basement membrane

The structure known as the basement membrane in light microscopy refers to the stained structure anchoring an epithelial layer. This encompasses the basal lamina secreted by epithelial cells and typically a reticular lamina secreted by other cells. Note that the basal lamina cannot be distinguished under the light microscope. The combination of the basal lamina and reticular lamina (or of two basal laminae) is called the basement membrane, a light microscopy term that predated the invention of the electron microscope. Under the higher magnification of an electron microscope, the basal lamina and lamina reticularis are visibly distinct structures. The term basement membrane is often confused with basal lamina, and the terms are used inconsistently in the literature.