Corpus: Endothelium

The endothelium is a thin layer of endothelial cells that lines the inside (lumen) of blood vessels. It serves as a barrier to the tissue, but also produces e.g. nitric oxide, which is used for regulation in the cardiovascular system.

Continuous endothelium[Bearbeiten]

The continuous endothelium is a particularly impermeable form of the inner lining of blood-conducting vessels. An important function is the formation of a (diffusion) barrier between blood and tissue - especially at the capillary level.

Continuous capillaries have a uniformly thin endothelial cell layer and have no interruptions or pores. They are characterised by the formation of particularly dense cell-cell contacts (tight junctions) between the endothelial cells.

Highly selective transcellular transport mechanisms are used for the exchange of substances through the endothelium, which is impermeable to most substances and cells in the blood. In addition, the endothelial cells - with the exception of capillaries of the central nervous system - are rich in so-called caveolae.

Occurrence:

• Lungs
• Skeletal muscles
• Thymus
• Adipose tissue
• skin
• exocrine glands
• Nervous system (blood-brain barrier)
• heart

Fenestrated endothelium[Bearbeiten]

The fenestrated endothelium (from Latin: fenestra - window) is, in contrast to the continuous endothelium, permeable to water and smaller hydrophilic substances. As a special feature, this endothelium has 20-100 nm large fenestrations, which, however, have no discontinuity in the basement membrane and are only closed by so-called diaphragms. These diaphragms are about 4 nm thick, consist of extracellular matrix (heparan sulphate proteoglycan) and have a negative electrical charge.

The fenestrated endothelium of the glomeruli in the kidneys is said to be an exception: According to general doctrine, it has no diaphragms so that filtration is possible. However, this hypothesis is disputed by some authors.^[1]

Occurrence[Bearbeiten]

• peritubular in the kidney
• endocrine organs (e.g. endocrine pancreas)
• Intestinal mucosa
• nasal mucosa
• renal glomerulus

Discontinuous endothelium[Bearbeiten]

The discontinuous endothelium is a particularly permeable form of endothelium that is characteristic of the so-called sinusoids. The permeability is based on the wide perforated spaces (10 to 40 µm) between most endothelial cells. There are also transcellular pores, which in the case of so-called migration pores are even accessible to blood cells. This special architecture of coarse-meshed cell contacts is mainly used for paracellular transport. Capillaries with discontinuous endothelium in the liver sinusoids enable unhindered transport of all blood components. Some authors further subdivide discontinuous capillaries into perforated capillaries with open pores without a diaphragm (bone marrow, liver) and disjunct capillaries with true endothelial gaps (spleen).

Occurrence:

• Liver sinusoids
• Spleen sinusoids
• Bone marrow

The surface area of the endothelium in adults varies between 1,000 and 7,000 m², with a weight of approx. 1 to 1.5 kg. It is made up of an estimated one trillion cells. These figures are not measurements, but model calculations, which is why they are only of limited significance.

The endothelium is not just a simple boundary of the vessel wall, but fulfils a variety of physiological functions. These include, among others:

• Regulation of blood pressure, e.g. through the formation of nitric oxide (NO)
• Regulation of the exchange of substances between tissues and the vascular system
• Maintaining the fluidity of the blood
• Inhibition or activation of coagulation processes
• Involvement in angioneogenesis
• Involvement in inflammatory processes

see also: Endothelin system