Definition of "endothelium" [en•do•the•li•um]

• A thin layer of flat epithelial cells that lines serous cavities, lymph vessels, and blood vessels. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "A single layer of cells called the endothelium lines the walls of every blood vessel in the body."
• "The endothelium is the thin layer of cells that line the interior surface of blood vessels."
• "This inner lining called the endothelium is only a single cell thick yet it controls the activity of the entire artery."