S19 hearts, a stage that is characterized by the presence of both ostia and coronary stems. A, this coronal section shows a coronary artery (CA) with a single layer of VSMCs at the point of emergence from the aorta (Ao) into the left ventricle (LV). B. a coronary artery with a complete layer of smooth is seen at the aortic root, but lacks VSMCs more distally, (azure-Mallory stain). C, downstream development of the tunica media of the left anterior descending coronary artery at the apical atrioventricular groove is preceded by a blood island (BI). D, with the establishment of coronary flow by S19, perfusion of the preformed capillary bed occurs as capillaries (arrows) appear dilated in the compact region (between arrowheads) of the ventricle. Magnification bar = 100 μm.