The cardiac tissues that make up the heart of vertebrates are composed of several different cell types. Cardiac fibroblasts are the most prevalent cell type and make up over 50% of the heart. The muscular walls of the heart, or the myocardium, are made up of cardiomyocytes. The interior lining or the heart, or endocardium, are composed of endothelial cells. Arteries connected to the heart contain smooth muscle cells. Finally, a small amount of specialized cells include pacemaker cells and purkinje fibers, which have neuron-like functions. A variety of primary cardiac cells are available, isolated from human, mouse, or rat sources.