Basal lamina

Structure

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.

Basement membrane

The basement membrane is visible under light microscopy. Electron microscopy shows that the basement membrane consists of three layers: the lamina lucida (electron-lucent), lamina densa (electron-dense), and lamina fibro-reticularis (electron-lucent).

The lamina densa was formerly called 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 constituted the single layer seen with the light microscope. This has led to considerable terminological confusion; 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 lamina lucida is therefore equal to the lamina densa in vivo.

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

Examples of basement membranes include:

• Basilar membrane
• Bruch's membrane
• Descemet's membrane
• Glomerular basement membrane

The glomerular basement membrane is a special case, consisting of a fusion of the podocyte and endothelial basal laminas, and lacking a lamina reticularis. Thus, it consists of an especially thick lamina densa, sandwiched on its inside and outside by layers of lamina lucida / rara (one from each cell type). These two enveloping layers are often referred to as lamina rara externa and lamina rara interna.