Heart Histology: Structure and Function of the Heart Wall

Heart Histology: Structure and Function of the Heart Wall

1. Heart Wall Layers

The heart wall consists of three distinct layers: the epicardium, myocardium, and endocardium, with the presence of visceral and parietal pericardium surrounding the heart.

Epicardium

  • Composition: Mesothelium (simple squamous/cuboidal epithelium) + connective tissue (CT)
  • Components: Adipose tissue, blood vessels, and parasympathetic ganglia (not always visible)
  • Function: The mesothelium secretes serous fluid to reduce friction

Myocardium

  • Composition: Cardiac muscle tissue + connective tissue
  • Characteristics:
    • Single central nucleus
    • Branched myocytes
    • Intercalated discs
    • Striated myofibrils (sarcomeres)
    • Absence of mitosis (no hyperplasia)

Endocardium

  • Composition: Endothelium (simple squamous epithelium) + areolar connective tissue
  • Components: Purkinje fibers (not always visible), blood inside the heart

2. Specialized Heart Cells

Atrial Myocytes
  • Endocrine Function: Production and secretion of atrial natriuretic peptide (ANP)
  • Location: Principally in the atria
  • Trigger for Secretion: Increased atrial pressure
  • Function of ANP: Lowers blood pressure by promoting sodium excretion (natriuresis) and reducing blood volume, contrary to aldosterone

Purkinje Fibers

  • Histological Differences from Regular Myocytes:
    • Larger size
    • Located in the subendocardium
    • Glycogen-rich cytoplasm pushes myofibrils to the periphery
    • Connected by desmosomes and gap junctions (no intercalated discs)
  • Electrical Function: Rapid conduction of electrical impulses through the AV node, bundle branches, and into the ventricles

3. Heart Valves

[h3]Structure
  • Layers: Lined by endocardium and have a core of fibrous connective tissue
  • Components: Chordae tendineae and papillary muscles

Histological Composition

  • AV Valves (Tricuspid and Mitral): Dense collagen core with endocardium lining
  • Semilunar Valves (Aortic and Pulmonary): Similar structure to AV valves
  • Clinical Relevance: Pathological conditions such as endocarditis, often linked to rheumatic fever (streptococcal infection), can cause valve dysfunction

4. Clinical Correlations

Pericarditis

  • Causes: Infection, autoimmune disorders, and other inflammations leading to a swollen pericardial cavity
  • Diagnostic Imaging: MRI can reveal pericardial effusion and inflammation
Myocardial Infarction (MI)
  • Pathology: Death of cardiac tissue due to ischemia, identified by the release of cardiac troponins
  • Histology: Presence of granulation tissue, fibrosis, and disarrayed muscle cells
Endocarditis[
  • Etiology: Infection (bacterial or fungal) or autoimmune response leading to inflammation of heart valves
  • Histology: Inflammatory infiltrate, Anitschkow cells in rheumatic heart disease, and vegetations seen on echocardiograms
Hypertrophy
  • Normal: Adaptive response to increased workload as seen in athletes
  • Abnormal: Pathological in hypertrophic cardiomyopathy (HCM) and linked with genetic mutations leading to disorganized myocyte structure
Other Relevant Terms
  • Myopathy: General muscle tissue disorder
  • Myocarditis: Inflammation of the myocardium
  • LVH (Left Ventricular Hypertrophy)[/strong]: Result of systemic hypertension, increased size of myofibers
 Great Vessels
  • Aorta: Thick tunica media with robust elastic lamina, prone to aneurysm in disorders like Marfan's syndrome
  • Vena Cava: Prominent tunica adventitia with longitudinal muscle bundles
Capillaries
  • Types: Continuous, fenestrated, and sinusoidal, each adapted to specific tissue needs
  • Function: Facilitating exchange of gases, nutrients, and waste between blood and tissues