endothelium A single layer of thin platelike cells that line the inner surfaces of blood and lymph vessels and the heart. Endothelium is derived from the mesoderm. Compare epithelium; mesothelium.

endothelium (en-doh-th'ee-li-um) n. the single layer of cells that lines the heart, blood vessels, and lymphatic vessels. It is derived from embryonic mesoderm. Compare epithelium.
—endothelial adj.