Cardiac Wall Histology

Overview
  • From a histological perspective the muscular heart wall is a giantly expanded blood vessel and possesses the same three basic histological layers as do all blood vessels (See: Cardiovascular Histology). Special features of the muscular heart wall include a highly expanded tunica media, composing the bulk of the heart's muscle, and a highly elaborate vaso vasorum which compose the coronary arteries.
Basic Components
  • Tunica Intima: (AKA: Endocardium)
    • The Endocardium is composed of a continuous endothelial lining, with attendant basement membrane, that lies on top of a layer of collagen. The endothelium of the heart is continuous with that of the aorta, vena cava, pulmonary artery, and pulmonary veins, allowing for in an un-interrupted layer of endothelium between the heart and the systemic/pulmonary vasculature.
  • Tunica Media: (AKA: Myocardium)
    • The myocardium is a variably-sized layer of cardiomyocytes that lies between the endocardium and epicardium and makes up the bulk of the heart's mass. Cardiomyocytes are arranged end to end, in a branching network of fibers and are attached to one another by specialized sections of membrane known as intercalated discs. The membrane at intercalated discs possess a large number of gap junctions which allow nearly free passage of ions between cardiomyocytes which is critical for cardiac action potential propagation. It should be noted that cardiomyocytes are not completely homogeneous and display nuanced histological differences depending on their specialization in either generating contractile force or rapidly propagating the cardiac action potential.
  • Tunica Adventitia: (AKA: Epicardium)
    • The Epicardium of the heart is lined by a layer of flat mesothelial cells that lie on top of a layer of collagenous tissue. The epicardium makes up the visceral pericardium and serves as a lubricated surface which allows for the free movement of the heart within the pericardium. The outer, parietal pericardium also possesses a layer of mesothelium but is not considered a histological component of the heart itself.
  • Vaso Vasorum: (AKA: Coronary Arteries)
    • The coronary arteries are perhaps the most elaborate and largest manifestation of the vaso vasorum and provide the myocardium with sufficient blood flow to maintain the energy-intensive process of contraction.