Basement membrane

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The basement membrane, schematic representation. Under the light microscope, em can only be seen under an electron microscope

A basement membrane is a layer of reticular , argyrophilic fibrils and fibers that can be recognized by light microscopy and which is PAS-positive due to its glycoprotein content . Epithelia , endothelia and glial cell associations are separated from the connective tissue stroma with a basement membrane and at the same time anchored to it; Fat and muscle cells are completely surrounded by it.

The basement membrane can be divided into layers using an electron microscope :

• The basal lamina lies directly next to the cell and is further subdivided according to its electron permeability into
  • the lamina rara and
  • the lamina densa .
• The lamina fibroreticularis merges into the connective tissue without a clear border.

The basal lamina is about 20 nm thick  , the entire basement membrane about 1–2 μm thick  . In different tissues there are structural differences in the composition of the basement membranes, which are adapted to the respective requirements of the epithelium. Above all, the composition of the laminins can vary greatly here .

The malignancy of many carcinomas is also characterized, among other things, by the fact that the degenerate cells are able to enzymatically break down basement membranes - primarily through matrix metalloproteases (MMP). This is often what makes it possible for the tumor cells to invade and metastasize into neighboring tissues.

Basal lamina

Electron microscope image of the basal lamina

The basal lamina is a homogeneous protein layer under the base of the epithelial cells. It connects the epithelial cells with the fibroreticular, connective tissue basement membrane. It is a specialized extracellular matrix in which the dense network of reticular fibrils (lamina fibroreticularis) of the basement membrane is anchored. It influences cell metabolism , cell polarity and cell differentiation and is a guiding rail for cell migration. The basal lamina is composed of a lamina rara (also called lamina lucida ) that appears light in electron microscope images and a lamina densa that appears dark . Lamina rara and the lamina densa below it mainly consist of the proteins collagen type IV and laminin , which is linked to the extracellular domains of the integrins of the epithelial cell membranes, as well as entactin ( nidogen ) and proteoglycans such as perlecan . In addition, other proteins can be associated with the matrix components mentioned.

Lamina fibroreticularis

Below the basal lamina is the network of reticular fibrils that form the lamina fibroreticularis . In contrast to the basal lamina, it is mainly formed by the cells of the connective tissue.

Basement membranes without lamina fibroreticularis

In some organs, the epithelia and endothelia come so close that the lamina fibroreticularis falls away and the two laminae densae merge into one layer. The lamina rara facing the epithelium is then called Lamina rara externa , the lamina rara interna facing the endothelium . Examples are the blood-air barrier in the lungs or the blood-urine barrier in the kidneys.

Individual evidence

1. ↑ Renate Lüllmann-Rauch: Pocket textbook histology. Thieme, Stuttgart 2012, ISBN 978-3-13-129244-5 , p. 132.