Endothelium

Depending on its embryonic origin and its position in the adult body, simple squamous epithelium is referred to by a variety of names. The lining of blood and lymphatic compartments is called endothelium. Endothelial cells are long and polygonal-shaped, and form leaky barriers that partially restrict the diffusion of macromolecules. Notice how the cells are so flattened that their nuclei, which bulge into the vessel lumen, are their only recognizable feature. The basement membrane here is thin and stains similarly to the cytoplasm, making it difficult to recognize. Also visible are red blood cells in the lumen of the blood vessel.