Histology for Pathology Cardiac System Theresa Kristopaitis MD

Histology for Pathology Cardiac System Theresa Kristopaitis, MD Associate Professor Director of Mechanisms of Human Disease Kelli A. Hutchens, MD, FCAP Assistant Professor Assistant Director of Mechanisms of Human Disease Loyola Stritch School of Medicine

Objectives 1. Identify the 3 layers of the heart – endocardium, myocardium, epicardium 2. Differentiate cardiac, skeletal and smooth muscle on H&E stained slides citing the key distinguishing morphologic features 3. Define “intercalated disc” 4. Name the layer of the heart in which Purkinje fibers are found and describe their function 5. Explain which chamber of the heart has the thickest layer of myocardium vs the thinnest 6. Explain the general histologic structure of a cardiac valve

Heart Wall Myocardium Endocardium Epicardium

Endocardium • Innermost layer • Composed of: – Simple squamous epithelium (endothelium) – Connective Tissue – Subendocardium: in contact with cardiac muscle and contains small vessels, nerves, and Purkinje Fibers.

Purkinje Fibers • Impulse conducting fibers • Large modified muscle cells – Cluster in groups together – 1 -2 nuclei and stain pale due to fewer myofibrils • Terminal branches of the AV bundle branches located in the subendocardial connective tissue

Myocardium • Thickest layer of the heart • Thickest in left ventricle because must pump hard to overcome high pressure of systemic circulation • Right atrium the thinnest because of low resistance to back flow • Consist of cardiac muscle cells = myocytes – Different from smooth or skeletal muscle cells due to placement of nuclei, cross striations, and intercalated disks • Intercalated disks – Junctional complexes that contain fascia adherens, desmosomes, and gap junction to provide connection and communication. – Bind myocytes and allow ion exchange to facilitate electrical impulses to pass

Cardiac Myocytes Branching myocytes Central nuclei Fibers with Cross Striations

Smooth Muscle Long, slender central nuclei, lying within narrow, fusiform cells. No cross striations Skeletal muscle Fibers with cross-striations and peripheral nuclei.

Epicardium • Outermost layer of the heart • Composed of connective tissue with nerves, vessels, adipocytes and an outer layer of mesothelium • Mesothelium secretes pericardial fluid • Covers and protects the heart

Cardiac Valves • 4 valves – 2 AV (mitral and tricuspid) in the chambers – 2 semilunar (aortic /pulmonary) • Composed of connective tissue layers covered by endothelium on each side; 3 layers – Spongiosa: loose collagen – Fibrosa: dense core of connective tissue – Ventricularis: dense connective tissue with many elastic and collagen fibers