Sinusoids: Definition, Structure, Types and Functions

It is a blood vessel that is found in the parenchyma of certain organs.

Organs such as the liver, spleen, and bone marrow contain blood vessel structures called sinusoids instead of capillaries. Like capillaries, sinusoids are composed of endothelium.

Individual endothelial cells, however, do not overlap as in capillaries and spread.

The fenestrated sinusoidal endothelium contains pores to allow the exchange of small molecules such as oxygen, carbon dioxide, nutrients, proteins and debris through the thin walls of the sinusoids.

This type of endothelium is found in the intestines, kidneys and in the organs and glands of the endocrine system .

Structure

The sinusoids vary in size from approximately 30-40 microns in diameter. In comparison, the capillaries measure in size from approximately 5-10 microns in diameter.

The discontinuous sinusoidal endothelium contains even larger pores that allow blood cells and larger proteins to pass between the vessels and the surrounding tissue. This type of endothelium is found in the sinusoids of the liver, spleen, and bone marrow.

A sinusoid is a small blood vessel that is a type of capillary similar to a fenestrated endothelium. Sinusoids are actually classified as a type of open pore (or discontinuous) capillary as opposed to continuous and fenestrated types.

The fenestrated capillaries have diaphragms that cover the pores, while the open pore capillaries lack a diaphragm and only have an open pore. The open pores of endothelial cells greatly increase their permeability.

In addition, the permeability increases with large intercellular grooves and fewer narrow junctions. The level of permeability is such that it allows small and medium proteins such as albumin to enter and leave the bloodstream easily.

Types

Sinusoids are found in the liver, lymphoid tissue, endocrine organs and hematopoietic organs, such as bone marrow and spleen.

The sinusoids that are inside the terminal villi of the placenta are not comparable to these because they have a continuous endothelium and a complete basal lamina. This word was used for the first time in 1893.

A hepatic sinusoid is a type of sinusoidal blood vessel (with discontinuous fenestrated endothelium) that serves as a place to mix the oxygen-rich blood from the hepatic artery and the nutrient-rich blood from the portal vein.

The hepatocytes are separated from the sinusoids by the Disse space. The Kupffer cells are within the sinusoids and can absorb and destroy foreign material like bacteria.

Functions

The hepatic stellate cells are present in the space of Disse and participate in the formation of scars in response to liver damage.

Sinusoidal endothelial cells are cultured for a variety of research purposes. The usefulness of these cells is of particular interest.

One problem to overcome is the reversal of cell differentiation that has made these cells highly specialized phenotypically in vitro.

Sinusoids are composed of specialized non-parenchymal cells and also exhibit structural and functional heterogeneity.

Near its origins from portal veins and hepatic arterioles, sinusoids are slightly narrower than tortuous and anastomotic forming interconnected polygonal networks.

Further from the venules of the portal, the sinusoids are organized as parallel vessels that end in central venules ( terminal hepatic venules ).

Short intersinusoidal sinusoids connect adjacent parallel sinusoids. The tortuous and anastomotic periportal network of sinusoids becomes almost non-existent at the point of the vascular septum formed by sinusoids that intervene between the terminal venules of the portal.

As a result, a sickle-shaped area of ​​tortuous anastomotic sinusoids is formed, which is believed to provide an equipotential line hemodynamically at its edge where the parallel sinusoids begin.

The latter converge radially to end in the central venule. The fact that there is a sickle-shaped entry front has also been demonstrated by in vivo microscopy monitoring the inflow of dye injected into the portal vein.

In the periportal area, the volume of liver occupied by sinusoids is greater than that of the surrounding central venules.

However, due to the smaller size and anastomotic nature of periportal sinusoids, the surface available for exchange in this area (surface / volume ratio) is greater than in centrilobular sinusoids.

The size and distribution pattern of endothelial fenestra differ throughout the sinusoid. At the end of the portal, the fenestrae are larger but comprise less of the endothelial surface area than in the pericentral region.

The functional significance of these regional differences is not clear, but they are related to the functional metabolic heterogeneity that has been demonstrated for hepatocytes in different regions of the lobe.

This in turn may depend on the recognized intralobular portal-to-central oxygen gradient.

Although it is tempting to attribute regional differences in sinusoidal porosity to differences in permeability, no significant differences were found between periportal and centrilobular sinusoids in the transport of molecules ranging in size between 35 and 160 kDa.

However, selective “screening” probably exists for larger solutes, such as chylomicron remnants.

Differences in the organization and diameters of the sinusoids between the periportal and centrilobular regions probably explain the higher linear velocity of blood flow in centrilobular sinusoids.

However, it has not been measured whether the functional hematocrit is altered or not due to the porosity of this endothelium.