|Basal lamina lining outer surface of cell membrane|
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.
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 best known macromolecular components of basal laminae are laminin, type IV collagen, entactin and perlecan. All these components are secreted at the basal poles of the epithelial cells. Their precise proportions in basal laminae vary between and within tissues. Basal laminae are attached to reticular fibers made of type III collagen in the underlying connective tissues by anchoring fibrils of type VII collagen. These proteins, that are produced by cells of the connective tissue, form a layer below the basal lamina called the reticular lamina.
Basal laminae have many functions. In addition to simple structural and filtering functions, they are also able to influence cell polarity; regulate cell proliferation and differentiation by binding and concentrating growth factors, influence cell metabolism and survival; organize proteins in the adjacent plasma membrane; and serve as pathways for cell migration.
The layers of the basal lamina ("BL") and those of the basement membrane ("BM") are described below:
|Name||Part of BL?||Part of BM?||Notes|
|lamina lucida / lamina rara interna||yes||yes||electron-lucid layer containing the glycoprotein laminin|
|lamina densa||yes||yes||electron-dense layer composed of type IV collagen|
|lamina lucida / lamina rara externa||yes||yes||Similar composition to lamina rara interna. Some sources do not consider this a distinct layer.|
|lamina reticularis||no||yes||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.
Basal lamina vs. basement membrane
The basement membrane is visible with light microscopy. Electron microscopy reveals to us that the basement membrane actually consists three layers: the lamina lucida (electron-lucent), lamina densa (electron-dense), and lamina fibroreticularis (electron-lucent).
The lamina densa was fomerly 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. 
The term "basal lamina" is usually used with electron microscopy, while the term "basement membrane" is usually used with light microscopy. Basement membrane is used to specify a periodic acid-Schiff (PAS)-positive layer, visible with the light microscope beneath epithelia. The basement membrane is formed by the combination of a basal lamina and a reticular lamina and is therefore thicker.
Examples of basement membranes include:
- Alveolar-capillary barrier
- Basolateral membrane
- Glomerular basement membrane
- Lamina propria
- Reticular lamina
- Anthony L. Merscher (2010). Junqueira's Basic Histology (12th ed.). p. 66.
- BU Histology Learning System: 22403loa
- UIUC Histology Subject 500
- UIUC Histology Subject 499
- BU Histology Learning System: 20904loa
- BU Histology Learning System: 22203loa
- Wheater's Functional Histology 4th edition (2000)
- Chan F, Inoue S (1994). "Lamina lucida of basement membrane: an artefact". Microsc Res Tech 28 (1): 48–59. doi:10.1002/jemt.1070280106. PMID 8061357.