Figure 2 (below). Most blood vessel walls have three tunics or layers, including the artery (left micrograph) and vein (right micrograph) below. Lining the vessel, in contact with the blood, is the Tunica Intima (TI). It consists of a simple squamous epithelium with a small amount of underlying connective tissue (both too thin to be seen very clearly here). The middle layer, the Tunica Media, contains dark-staining smooth muscle cells and more connective tissue, especially elastic tissue in arteries. Finally, on the outside, is the lighter-staining Tunica Adventitia (TA). It is mostly collagen. Note how the relative thicknesses of the TM's and TA's differ between arteries and veins.