The endothelial cells form a one-cell thick walled layer called endothelium that lines all of our blood vessels such as arteries, arterioles, venules, veins and capillaries. Smooth muscle cells layer beneath the endothelial cells to form the blood vessel.

The largest blood vessels are arteries and veins, which have a thick, tough wall of connective tissue and many layers of smooth muscle cells1. The wall is lined by an exceedingly thin single sheet of endothelial cells separated from the surrounding outer layers by a basal lamina.

The amounts of connective tissue and smooth muscle in the vessel wall vary according to the vessel's diameter and function, but the endothelial lining is always present. In the finest branches of the vascular tree—the capillaries and sinusoids—the walls consist of nothing but endothelial cells and a basal lamina, together with a few scattered—but functionally important pericytes. 

 
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Endothelium is classified as continuous, fenestrated or discontinuous. Capillaries with a continuous endothelium are found in the lungs, muscle and central nervous system. Within continuous endothelium, the endothelial cells are joined by tight junctions,anchored to a continuous basal membrane and contain negatively charged complexes that regulate the passage of molecules out of tissue to the blood stream.

 

Fenestrated endothelium is characterized by the presence of perforations of 50 – 60nm (fenestrae), which make them more permeable than continuous endothelium. Fenestrated endothelium is usually found in kidney and villi of intestine. The pores expand and contract in response to stimuli which include hormones, neurotransmitters, nicotine and alcohol. Similar to continuous endothelium, fenestrated endothelium is secured to a continuous basal membrane. These blood vessels allow for passive transport across the endothelium, which makes them a key component in the filtration role, within the gastrointestinal tract and kidney glomeruli.

 

Discontinuous endothelium also contains pores however these are considerably larger than those of fenestrated endothelium and do not have a diaphragm. The basal membrane is discontinuous. This form of endothelium exists in the blood vessels of the spleen, liver and bone marrow and has greater permeability than continuous or fenestrated endothelium. 

 

Functions of Endothelial Cells

The endothelium serves as a permeable barrier for the blood vessel and is involved in the regulation of blood flow. Within basic research, endothelial cells are pivotal to applications related to wound healing, angiogenesis, inflammatory processes, blood brain barriers, diabetes and other cardiovascular diseases. Since the circulatory system lines the entire body, endothelial research is tied to many diseases, which are top-funded research areas. For this reason, dysfunction of endothelial cells has implications in diabetes, pulmonary diseases, inflammatory diseases, cardiovascular diseases and immune diseases to name a few.

 

In addition, blocking angiogenesis has important applications in cancer.  Cancerous tissue is as dependent on a blood supply as is normal tissue, and this has led to a surge of interest in endothelial cell biology. Studies are focused on understanding interactions between blood vessel formation and tumor expansion. Endothelial cells also have important applications in tissue engineering studies especially in the development of artificial vascular grafts as alternatives to donor-sourced grafts or supporting organ or tissue engineering with a vascular network. Endothelial cells have a remarkable capacity to adjust their number and arrangement to create an adaptable life-support system, extending by cell migration into almost every region of the body. If it were not for endothelial cells extending and remodeling the network of blood vessels, tissue growth and repair would be impossible.

 

 

Barrier Function

The endothelium acts as a barrier between the blood and the rest of the body tissue while being selectively permeable to certain chemicals and white blood cells to move across from blood to tissue or for waste and carbon-dioxide to move from tissue to blood. This property of endothelial cells is especially investigated in the blood-brain-barrier system. In certain neuro-degenerative diseases, it is difficult to develop drugs that can cross the endothelial barrier efficiently. Research is focused on better mimicking and understanding the functions of blood brain barrier systems to increase the efficacy of drug development.

 

Illustration of blood brain barrier model

Image courtesy of Pharmaceutics 2014, 6(4), 557-583; doi:10.3390/pharmaceutics6040557 

 

Regulation of Blood Flow

Vasodilation and vasoconstriction, the widening and narrowing of blood vessels respectively, are essential processes to controlling blood flow. The endothelium responds to various vasoactive factors to maintain the vascular tone of arteries and veins and achieves this via the contraction or relaxation of the smooth muscle cells which underlie the basal membrane of these vessels. Nitric oxide, produced by many cells in the body, is a vasodilator. Norepinephrine, which functions as a hormone and a neurotransmitter, is a vasoconstrictor. Endothelial cells generate an anti-thrombotic surface that facilitates transit of plasma and cellular constituents throughout the vasculature and maintenance of homeostasis.

 

 

Wound Healing

Hemostasis, the process by which bleeding is stopped at the site of an injury while normal blood flow is maintained throughout the circulatory system, is one of the first stages of wound healing. The endothelium is pivotal to this, for example by controlling platelet adhesion and activation and by synthesizing the essential blood clotting protein Factor VIII. Endothelial cells are also involved in fibrinolysis, the dissolution of blood clots once the wound has healed. They can achieve this by secreting tissue-type plasminogen activator (t-PA).

 

 

Prevention of Thrombosis

Thrombosis is the abnormal formation of a blood clot within the vasculature which can obstruct blood flow. Although blood clotting is an essential stage of the wound healing process, it can be fatal if allowed to occur in an uncontrolled manner. The endothelium has various anti-thrombotic properties. These include the secretion of prostacyclin which inhibits platelet aggregation, and the storage and release of von Willebrand factor which binds to the clotting protein Factor VIII.

 

Angiogenesis

Angiogenesis, the formation of new blood vessels, occurs during early embryonic development and continues throughout the entire human lifespan. It is essential to the process of wound healing but has particularly been a subject of interest to researchers who study tumorigenesis. The proliferation and metastasis of cancer cells is dependent on an adequate supply of oxygen and nutrients. Tumor growth is supported by formation of new blood vessels that provide nutrients for these cells to expand. Many cancer therapeutic strategies have targeted tumor angiogenesis in hopes of limiting cancer progression.

 

 

The Inflammatory Response

Endothelial cells are key regulators of the inflammatory response since they form a crucial line of defense against infection. Dysregulation of the inflammatory response is implicated in a wide variety of diseases, and researchers have studied many different signaling pathways to advance understanding of the role of endothelial cells in conditions which include atherosclerosis, rheumatoid arthritis and inflammatory bowel diseases.

 

Endothelial Cells In Vitro 

While endothelial cells show heterogeneity among different tissue origins in terms of genes they express, they are typically similar in morphology. Endothelial cells consist of a "cobblestone" morphology, stain positive for Factors VIII (an essential blood-clotting protein synthesized by endothelial cells) and take up acetylated low-density lipoprotein. HUVECs stain positive for CD-31.

Primary endothelial cell diversity revealed by global expression profiling of 53 different lots isolated from 14 distinct tissue origins. The data illustrates varying gene expression profile of endothelial cells based on different vessel sizes and origin of cells (veins or arteries). The differences were conserved over a few passages. Chi JT. et al. (2003) PNAS 100:10623-8. By permission of the author. Copyright 2003 National Academy of Sciences. 

References

1 Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Blood Vessels and Endothelial Cells.