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This structure is visible only with the electron microscope, where it appears as an electron-dense layer, 20-100 nm thick (with exceptions such as the basal laminae that compose the 100-200 nanometre thick glomerular basement membrane).
The three above layers of the basal lamina 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 fibrils composed of type VII collagen extend from the basal lamina into the underlying reticular lamina and loop around collagen bundles. Although found beneath all basal laminae, they are especially numerous in stratified squamous cells of the skin.
These layers should not be confused with the lamina propria, which is found outside the basal lamina.
The basement membrane is visible with light microscopy. Electron microscopy reveals to us that the basement membrane actually consists of three layers: the lamina lucida (electron-lucent), lamina densa (electron-dense), and lamina fibroreticularis (electron-lucent).
The lamina densa was formerly known as the basal lamina. The terms basal lamina and basement membrane were often used interchangeably, until it was realised that all three layers seen with the electron microscope represent the single layer seen with the light microscope. This has led to considerable terminological confusion and, if used, the term basal lamina should be confined to its meaning as lamina densa.
Some theorize that the lamina lucida is an artifact created when preparing the tissue, and that the basement membrane is therefore equal to the lamina densa in vivo.