Traditionally the endothelium was considered to be an inert stratum of cells. More recently, however, it has been realized that the tissue lining the lumen of all blood vessels is more dynamic than previously thought. Not only does the endothelium allow the passage of molecules of water and certain other materials across the walls of blood vessels, but it also secretes and modifies various vasoactive substances. Moreover, the endothelium plays a key role in blood flow mechanics, the contraction and relaxation of smooth muscle located in vessel walls, and the regulation of coagulation.

The bovine pulmonary artery endothelial cells that appear in the digital image above were resident in a BPAE cell culture that was stained with MitoTracker Red CMXRos, Alexa Fluor 488 conjugated to phalloidin, and Hoechst 33342, which target the mitochondrial network, filamentous actin, and DNA in the nucleus, respectively. Images were recorded in grayscale with a 12-bit digital camera coupled to a Nikon Eclipse 80i microscope equipped with bandpass emission fluorescence filter optical blocks. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.