Basal lamina

The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below.

It is typically about 40-50 nanometers thick (with exceptions such as the basal laminae that compose the 100-200 nanometre thick glomerular basement membrane).

It includes proteoglycans.

Layers
The layers of the basal lamina ("BL") and those of the basement membrane ("BM") are described below:

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.

Basal lamina vs. basement membrane
The term "basal lamina" is usually used with electron microscopy, while the term "basement membrane" is usually used with light microscopy. 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.

The basal lamina cannot be distinguished under the light microscope, but under the higher magnification of an electron microscope, the basal lamina and lamina reticularis are visibly distinct structures.

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.

Examples of basement membranes include:
 * Basilar membrane
 * Bruch's membrane
 * Descemet's membrane
 * Glomerular basement membrane