Endothelial Cells: Definition

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Endothelial cells are thin, flat cells that line the inside of every blood vessel and lymphatic vessel in the body. They also line the inside of the heart. Together, these cells form a single, smooth layer called the endothelium, which sits between the blood and the vessel wall.

Endothelial cells are a normal part of almost every tissue sample, because almost every tissue contains blood vessels. Seeing them in a pathology report is not a diagnosis. This article explains what endothelial cells do, what they look like under the microscope, and why a pathologist may mention them in your report.

What do endothelial cells do?

Endothelial cells are much more than a simple lining. They have several important jobs:

Forming a barrier. Endothelial cells control what passes between the blood and the surrounding tissue, including fluid, nutrients, and waste.
Keeping blood flowing smoothly. A healthy endothelium prevents blood from clotting inside the vessel. When a vessel is injured, the endothelial cells help start the clotting process to stop bleeding.
Controlling blood flow. Endothelial cells release chemicals that tell the muscle in the vessel wall to relax or tighten. This helps control blood pressure and how much blood reaches each organ.
Guiding immune cells. During infection or injury, endothelial cells help white blood cells leave the bloodstream and enter the damaged tissue.
Building new vessels. Endothelial cells can divide and form new blood vessels. This process helps wounds heal, and it is also how tumors develop their own blood supply.

What do endothelial cells look like under the microscope?

Under the microscope, normal endothelial cells are very thin and flat. They form a single layer lining the open space inside the vessel, called the lumen. Each cell has a long, narrow nucleus that may bulge slightly into the lumen. In the smallest vessels, called capillaries, one or two endothelial cells can wrap around the vessel.

Endothelial cells can change their appearance. When a tissue is inflamed or injured, they often become larger and rounder. Pathologists call these cells “plump” or “reactive” endothelial cells. This is a normal response and does not mean cancer.

Why are endothelial cells mentioned in a pathology report?

Because endothelial cells are part of every blood vessel, your report may mention them for several different reasons:

To describe a blood vessel. Your report may describe a space as “endothelial-lined,” meaning it is a blood or lymphatic vessel. This matters when the pathologist is looking for lymphovascular invasion, meaning cancer cells have entered a vessel.
To describe a vascular tumor. Some tumors are made of endothelial cells. These include noncancerous tumors such as a hemangioma and pyogenic granuloma, and cancers such as Kaposi sarcoma and angiosarcoma.
To describe injury to blood vessels. Some conditions directly damage endothelial cells. Examples include vasculitis, which is inflammation of blood vessels, and thrombotic microangiopathy, in which damaged small vessels fill with tiny clots.
To describe transplant rejection. In a transplanted kidney, the immune system can attack the new organ’s endothelial cells. This finding is part of the diagnosis of antibody-mediated rejection.

What words might describe endothelial cells in a report?

Pathologists use several terms to describe how endothelial cells look. Common examples include:

Bland. The cells look normal or near normal, with no worrying features. This wording is common in noncancerous vascular tumors.
Plump or reactive. The cells are larger than usual because of inflammation or injury. This is usually a normal response.
Hobnail. The cells bulge into the lumen, like the rounded head of a nail. This appearance is seen in some specific vascular tumors.
Atypical. The cells look abnormal. Atypia in endothelial cells may be reactive, but it can also indicate vascular cancer. The pathologist considers it together with all the other findings.

How do pathologists identify endothelial cells?

In most cases, a pathologist can recognize endothelial cells by their shape and position lining a vessel. When the cells are harder to recognize, such as in a tumor, the pathologist may use immunohistochemistry (IHC). This test uses antibodies to detect proteins that endothelial cells make.

Common markers for endothelial cells include CD31, ERG, CD34, and FLI1. A different marker, D2-40, is present in the endothelial cells of lymphatic vessels. It helps the pathologist tell lymphatic vessels apart from blood vessels.

Questions to ask your doctor

Why does my report mention endothelial cells?
Does my report describe a tumor made of endothelial cells?
Were the endothelial cells described as normal, reactive, or atypical?
Were any cancer cells found inside blood or lymphatic vessels?
Did they use special stains such as CD31 or ERG on my sample?
Is there any sign of damage or inflammation in my blood vessels?
Does this finding change my treatment or follow-up?