Transmission electron micrograph showing capillary mouse myocardium continuous endothelium using a hemoglobin tracer in caveolae. Hemoglobin, used as a tracer, is seen in the lumen of a capillary lined by a continuous endothelium, in endothelial caveolae, and in the extracellular space. Tracers such as hemoglobin or small heme-peptides were used by these investigators to demonstrate the role of caveolae in transcytosis of molecules across continuous capillary endothelial cells. Image made available by James D. Jamieson and the Department of Cell Biology, Yale University School of Medicine.