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