The basal lamina (often erroneously called basement membrane) is a layer on which epithelium sits. This layer is composed of an electron-dense layer (lamina densa) between two electron-lucid layers (lamina lucida), and is approximately 40-50 nm thick (with exceptions such as the 100-200 nm glomerular basement membrane). The lamina densa is composed of type IV collagen. The lamina lucida is adjacent to the epithelial cells and contains the glycoprotein laminin.

Structure

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The two layers of the basal lamina typically sit on top of the lamina reticularis, which is synthesized by cells from the underlying connective tissue and contains type IV collagen. 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 lamina reticularis and loop around collagen bundles. Although found beneath all basal laminae, they are especially numerous in stratified squamous cells of the skin.

Basement membrane

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The combination of the basal lamina and lamina reticularis or of two basal laminae is called the basement membrane. The term basement membrane is often confused with basal lamina. But whereas the basement membrane is visible under light microscopy, the basal lamina and lamina reticularis are only visible via electron microscopy.

The basal lamina is actually linked to the underlying type IV collagen, which makes up the basement membrane.

References

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• 1 Kierszenbaum, AL. Histology and Cell Biology: An Introduction to Pathology. Mosby, Inc, MO: 2002. Chapter 4.

Histology

Basallamina | Lame basale