Endothelium

The endothelium is a vital, single layer of squamous cells. These are thin, flat cells that line the interior of blood and lymphatic vessels. This layer serves as a crucial interface between the circulating fluids and the vessel wall. The fluid inside the vessel is called the lumen. In the circulatory system, these are known as vascular endothelial cells. When they line the lymphatic system, they are called lymphatic endothelial cells. In the chambers of the heart, this lining is called the endocardium.

This tissue functions as a highly controlled barrier. It manages the passage of substances and fluids into and out of tissues. The endothelium also regulates the transit of white blood cells. This movement is essential for the body's immune response. If the endothelium becomes too permeable, it can lead to edema, which is tissue swelling. This increased permeability is also linked to cancer extravasation.

Endothelial cells perform several complex mechanical roles. They control blood pressure through vasoconstriction and vasodilation. Vasoconstriction is the narrowing of a vessel, while vasodilation is its enlargement. The cells also manage hemostasis, which is the process of stopping blood flow. Under normal conditions, the endothelium prevents unnecessary clotting. It expresses substances like heparan sulfate. This molecule acts as a cofactor for antithrombin, a protein that inactivates coagulation factors. This ensures blood flows smoothly without forming dangerous clots.

Another major process is angiogenesis, or the formation of new blood vessels. This is vital for growing embryos and repairing damaged tissues. Angiogenesis often begins when tissues experience hypoxia, which is a lack of oxygen. Specific signals like angiopoietins and VEGF trigger this growth. The process follows a specific sequence of steps. First, activating signals bind to receptors on the endothelial cells. Then, the cells release proteases to degrade the basement membrane. This allows the cells to migrate and proliferate toward the area needing oxygen.

Beyond physical structure, the endothelium provides what is called structural immunity. These cells express immune genes in an organ-specific way. They use these genes to communicate with hematopoietic immune cells. This allows the endothelium to respond quickly to immunological challenges. This is different from the way epithelial cells work. While they look similar, endothelial cells develop from the mesoderm. They are also distinguished by the presence of vimentin filaments rather than keratin.

When these cells fail to function properly, it is called endothelial dysfunction. This is a key early event in atherosclerosis, a disease where fats build up in artery walls. Dysfunction can lead to hypertension, or high blood pressure, and thrombosis. It is often seen in patients with diabetes, coronary artery disease, or high cholesterol. One mechanism of this failure is the loss of nitric oxide. This can happen due to high levels of asymmetric dimethylarginine. Another cause is an increase in reactive oxygen species, which impair nitric oxide activity.

History shows how our understanding of these cells has grown. The term "endothelium" was first coined in 1865 by Wilhelm His Sr., a Swiss anatomist. In 1958, A. S. Todd at the University of St Andrews demonstrated that these cells have fibrinolytic activity. This means they help break down blood clots. Modern research also looks at how diet affects these cells. A Mediterranean diet and eating walnuts can improve function. Conversely, diets high in processed sugar or fried foods can increase cell adhesion.

Serious medical conditions can also involve the endothelium. Angiosarcoma is a rare cancer of these cells, with about 300 cases per year in the US. It has a five-year survival rate of 35 percent. Additionally, researchers at the University of Zurich and Harvard Medical School studied COVID-19. In 2020, they found viral elements in the endothelial cells of deceased patients. They suggested this could lead to endotheliitis, an inflammatory response that contributes to multi-organ failure.