endothelium

Examples of endothelium in the following topics:

• Lymphatic Vessel Structure

  • The endothelium, a general term for the inner layer of a vessel, is composed of an inner lining of single, flattened epithelial cells (simple squamous epithelium).
  • It sits on a highly permeable basement membrane made out of extracellular matrix that separates the endothelium from the other layers.
  • The endothelium is designed with junctions between cells that allow interstitial fluid to flow into the lumen when pressure becomes high enough (such as from blood capillary hydrostatic pressure), but does not normally allow lymph fluid to leak back out into the interstitial space.
  • The next layer is smooth muscles arranged in a circular fashion around the endothelium that alters the pressure inside the lumen (space) inside the vessel by contracting and relaxing.
  • One of the main structural features of lymph vessels is their valves, which are semilunar structures attached to opposite sides of the lymphatic endothelium.

• Sinusoid Development

  • A sinusoid is a small blood vessel that is a type of capillary similar to a fenestrated endothelium.
  • Sinusoids found within terminal villi of the placenta are not comparable to these because they possess a continuous endothelium and complete basal lamina.

• Hepatic Portal Circulation

  • Blood from either source passes into cavities between the hepatocytes of the liver called sinusoids, which feature a fenestrated, discontinuous endothelium allowing for the effecient transfer and processing of nutrients in the liver.
  • Note the microvilli of hepatocytes in the space of Disse external to the endothelium.

• Venules

  • Venule walls have three layers: an inner endothelium composed of squamous endothelial cells that act as a membrane, a middle layer of muscle and elastic tissue, and an outer layer of fibrous connective tissue.

• Platelet Plug Formation

  • Platelets may also bind to subendothelial VWF to anchor them to the damaged endothelium.
  • The completed plug will cover the damaged components of the endothelium and will stop blood from flowing out of it, but if the wound is large enough, blood will not coagulate until the fibrin mesh from the coagulation cascade is produced, which strengthens the platelet plug.

• Clot Retraction and Repair

  • As the healing process occurs following blood clot formation, the clot must be destroyed in order to prevent thromboembolic events, in which clots break off from the endothelium and cause ischemic damage elsewhere in the body.
  • During this process, the edges of the endothelium at the point of injury are slowly brought together again to repair the damage.

• Veins of the Head and Neck

  • Dural sinuses are composed of dural mater lined with endothelium, making them distinct from arteries, veins, and capillaries.

• Inflammation

  • Together, rolling and adhesion are referred to as margination, the accumulation of leukocytes on the endothelium.
  • The next step is for neutrophils to squeeze through the gaps in the endothelium into the tissues through binding with PECAM-1 expressed on the endothelium, a process called extravasion.

• Lymphatic Capillaries

  • Lymphatic capillaries are primarily made out of an endothelium layer that sits on a permeable basement membrane.
  • The flap-like minivalves, located at gap-like junctions in the endothelium, are formed from the overlap of endothelial cells and are normally closed.
  • They extend out from the lymphatic capillary, attaching the endothelium to fibroblast cells in the connective tissue.

• Arteries, Veins, and Capillaries

  • Veins and arteries both have two further tunics that surround the endothelium: the middle, tunica media is composed of smooth muscle, while the outer tunica externa is connective tissue (collagen and elastic fibers).
  • Capillaries consist of a single layer of epithelial cells, the endothelium tunic (tunica intima).