This micrograph of the field shows portions of the atrial (A) and ventricular (V) walls at the level of the atrioventricular septum and the root of the mitral valve (MV). Both chambers and the valve are lined with the squamous endothelium of the endocardium (En). Purkinje fibers (PF) of the cardiac conduction system are seen in the atrial wall between the relatively thin subendocardial connective tissue (CT) and the underlying modified cardiac muscle cells (CM) of the atrioventricular node (AVN). Dense fibrous connective tissue (DCT) that is continuous with that of the septum and the subendocardial layers of the atrium and ventricle extends from the root of the valve into the leaflet. Thin cardiac muscle fibers can also be seen extending from the wall of the atrium into the upper portion of the valve. Inset. This higher-magnification view of the field outlined by the rectangle (turned ~90°) shows more clearly the endothelial layer of the endocardium (En) and the dense fibrous connective tissue of the endocardium (DCT) and subendocardial layer. A thin layer of smooth muscle (SM) appears between the more densely packed fibrous tissue immediately subjacent to the endothelium and the more loosely packed dense fibrous tissue of the subendocardium. Particularly evident are the longitudinally sectioned Purkinje fibers (PF) of the cardiac conduction system. These modified cardiac muscle cells contain the same fibrillar contractile system as their smaller counterparts in the myocardium, but the fibrils are fewer, are more loosely packed, and often surround what appear to be vacuolated areas. Intercalated discs (ID), typical of cardiac muscle cell organization, are evident in some areas.
Coronary artery and cardiac vein, heart, human, H&E ×30.
This micrograph shows cross sections of a coronary artery and cardiac vein in the coronary sulcus. The surrounding adipose tissue (AT) serves to cushion the blood vessels that run in the coronary sulcus. The coronary artery (CA) in the lower left of this micrograph is surrounded by small bundles of small cardiac muscle cells (CM) that are part of the atrioventricular node (AVN). A loop of the conduction bundle (CB) containing Purkinje fibers is evident to the right of the artery. The darkly stained tunica intima (TI) is delimited by an internal elastic membrane (IEM) that is easily distinguished even at this relatively low magnification. The thick muscular tunica media (TM) is also easily distinguished from the thinner, fibrous tunica adventitia (TA). A smaller arterial vessel (A ).
KEY A, atrium A, small artery AT, adipose tissue AVN, atrioventricular node B, blood CA, coronary artery CB, conduction bundle CM, cardiac muscle CT, connective tissue CV, cardiac vein DCT, dense connective tissue En, endothelium ID, intercalated disc IEM, internal elastic membrane LN, lymph node MV, mitral valve PF, Purkinje fibers SM, smooth muscle TA, tunica adventitia TM, tunica media TI, tunica intima V, ventricle