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).
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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 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.
Examples of basement membranes include:
- Alveolar-capillary barrier
- Basolateral membrane
- Glomerular basement membrane
- Lamina propria
- Reticular lamina
- Histology image:22403loa from Vaughan, Deborah (2002). A Learning System in Histology: CD-ROM and Guide. Oxford University Press. ISBN 978-0195151732.
- UIUC Histology Subject 500
- UIUC Histology Subject 499
- Histology image:20904loa from Vaughan, Deborah (2002). A Learning System in Histology: CD-ROM and Guide. Oxford University Press. ISBN 978-0195151732.
- Histology image:22203loa from Vaughan, Deborah (2002). A Learning System in Histology: CD-ROM and Guide. Oxford University Press. ISBN 978-0195151732.
- 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.