1. Introduction
The heart is not only a mechanical pump but also a highly specialized electrical organ. Its rhythmic contraction depends on the synchronized propagation of action potentials through specialized conduction structures. These include the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, bundle branches, and Purkinje fibers.
While the ventricular myocardium consists of contractile cardiomyocytes, specialized conduction fibers have unique structural and histological characteristics that enable rapid electrical conduction with minimal contractile force. Histological identification of these structures is essential for understanding cardiac electrophysiology, conduction abnormalities, and arrhythmogenesis.
This post focuses on Purkinje fibers, bundle branches, and related conduction pathways, highlighting their structural features, histological appearance, and functional relevance.
2. Purkinje Fibers
2.1 Definition and Function
- Purkinje fibers are modified myocardial cells specialized for rapid conduction of action potentials within the ventricles.
- They form the terminal portion of the His–Purkinje system, distributing electrical impulses to ventricular myocardium.
- Conduction velocity in Purkinje fibers is significantly higher (~2–4 m/s) than in ordinary ventricular myocytes (~0.5–1 m/s), enabling synchronous ventricular contraction.
2.2 Structural Features
- Cell Size:
- Large diameter (~40–50 µm vs. 10–20 µm for typical cardiomyocytes)
- Length: shorter than contractile ventricular myocytes
- Cytoplasm:
- Rich in glycogen, giving a pale appearance on hematoxylin and eosin (H&E) staining
- Fewer myofibrils compared to contractile cells, reflecting their electrical rather than contractile function
- Nucleus:
- Centrally located, often oval
- Euchromatic, reflecting active transcription for ion channel proteins
- Intercalated Discs:
- Present but less prominent than in working myocardium
- Gap junctions are abundant, facilitating rapid electrical coupling
- Mitochondria:
- Numerous to support energy demands of action potential propagation
2.3 Histological Appearance
- On light microscopy (H&E):
- Large, pale-staining cells embedded among darker ventricular myocytes
- Central nucleus, scant myofibrils
- On special stains (Periodic acid–Schiff, PAS):
- Glycogen-rich cytoplasm stains strongly
- Immunohistochemistry:
- Connexin 40 and Connexin 43 expression, key for gap junction communication
3. Bundle Branches and the His–Purkinje System
3.1 Overview
- The bundle of His originates from the AV node and splits into right and left bundle branches, which traverse the interventricular septum.
- Left bundle further divides into anterior and posterior fascicles, delivering impulses to corresponding regions of the left ventricle.
- These branches ensure rapid and organized conduction to Purkinje fibers and ultimately the ventricular myocardium.
3.2 Structural Features
- Cell Type:
- Similar to Purkinje fibers, but smaller in diameter than terminal Purkinje cells
- Fewer glycogen deposits than distal Purkinje fibers
- Histological Identification:
- Lightly staining fibers with central nuclei
- Located within fibrous tissue of the interventricular septum, distinguishable from surrounding myocardium
- Function:
- Rapid propagation of action potentials
- Minimizes conduction delay between right and left ventricles
4. Distinguishing Features of Conduction Pathways
| Feature | Purkinje Fibers | Bundle Branches | Ventricular Myocytes |
|---|---|---|---|
| Diameter | Very large | Intermediate | Small |
| Cytoplasm | Glycogen-rich, pale | Less glycogen | Abundant myofibrils, eosinophilic |
| Nucleus | Central, large, oval | Central, smaller | Central or slightly eccentric |
| Intercalated Discs | Present but less prominent | Present | Prominent |
| Contraction | Minimal | Moderate | Strong |
| Gap junctions | Abundant | Present | Present |
| Location | Subendocardial, papillary muscles, ventricular septum | Interventricular septum | Entire ventricular wall |
5. Specialized Conduction Structures Beyond Purkinje Fibers
5.1 SA Node
- Pacemaker of the heart
- Histology: small, pale-staining nodal cells, fewer myofibrils, rich in glycogen
- Surrounded by fibrous tissue for insulation
5.2 AV Node
- Delays conduction from atria to ventricles (~0.1 s)
- Histology: small, tightly packed nodal cells, sparse myofibrils
- Low conduction velocity prevents atrial impulses from arriving too early in ventricles
5.3 Transitional Cells
- Connect nodal tissue to working myocardium
- Gradual change in cell size, myofibril content, and conduction velocity
- Prevents abrupt change in conduction properties
6. Functional Correlation
- Rapid conduction: Purkinje fibers ensure nearly simultaneous depolarization of ventricular walls.
- Coordination: Bundle branches transmit action potentials from the AV node to apex and lateral walls.
- Histological adaptation: Large glycogen-rich fibers with abundant gap junctions enable high-speed conduction without requiring strong contraction.
- Clinical relevance:
- Bundle branch blocks result from damage to these structures, prolonging QRS interval on ECG.
- Purkinje fiber abnormalities can lead to ventricular arrhythmias.
7. Techniques for Histological Identification
7.1 Routine Staining
- Hematoxylin & Eosin (H&E): pale cytoplasm, central nuclei
- Masson’s trichrome: distinguishes fibers from connective tissue
7.2 Special Stains
- Periodic acid–Schiff (PAS): highlights glycogen in Purkinje fibers
- Silver or gold staining: for nervous tissue and conduction pathways
7.3 Immunohistochemistry
- Connexins (Cx40, Cx43): identify gap junction distribution
- Ion channel markers: voltage-gated sodium and potassium channels
7.4 Electron Microscopy
- Reveals:
- Sparse myofibrils
- Abundant glycogen granules
- Gap junctions between cells
- Central nuclei
8. Clinical Correlation
- Bundle Branch Block (BBB):
- Right or left bundle branch damage → delayed ventricular activation
- Histology shows fibrosis or degeneration in conduction fibers
- Purkinje Fiber-Related Arrhythmias:
- Abnormal automaticity or re-entry can trigger ventricular tachycardia or fibrillation
- Seen in myocardial infarction or inherited conduction disorders
- Aging and Fibrosis:
- Conduction tissue is sensitive to fibrotic changes → slower conduction, predisposition to arrhythmias
9. Developmental Histology
- Purkinje fibers and bundle branches differentiate from subendocardial cardiomyocytes during embryogenesis.
- Markers of conduction tissue differentiation: Cx40, Nkx2.5, and GATA4
- Histological appearance: initially similar to working myocardium, later develops pale glycogen-rich cytoplasm
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