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 cells. 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.
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.
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.