Cardiac Muscle Histology

• The lecture focuses on the histology of cardiac muscle fibers and the wall of the heart.
• It aims to enable students to describe the microscopic structure of cardiac muscle fibers, correlate structure to function, differentiate between muscle fiber types, and detail the heart wall's histological structure.
• Professor Nasra Ayuob is the Head of Medical Histology & Cell Biology.

Cardiac Muscle Fibers

• Cardiac muscle fibers are involuntary and found in the heart wall (myocardium).
• The cardiac muscle fiber is not a syncytium.
• Individual muscle cells are joined end to end via cell junctions known as intercalated discs.

Light Microscopy (LM) of Cardiac Muscle

• Cardiac muscle cells have a small caliber, measuring about 25 um.
• They exhibit variable lengths with extensive branching and anastomosing.
• Connective tissue (endomysium) surrounds the muscle and contains blood vessels and lymphatics.
• Irregular striations are present.
• Nuclei are large, oval, central, and can be mono or binucleated.
• The cytoplasm is granular and acidophilic.

Electron Microscopy (EM) of Cardiac Muscle Fibers

• The sarcoplasm of cardiac muscle fibers contains glycogen granules, mitochondria, and Golgi saccules.
• Lipid droplets and lipochrome pigment are also present.
• Lipochrome accumulation increases with age, potentially leading to brown atrophy of the heart.
• Myofibrils are rich, traversed by dark staining intercalated discs.
• The myofibrils exhibit branching and anastomosing.
• They show less regular arrangement compared to light microscopy.
• Myofibrils are variable in diameter.
• Tubules and sarcoplasmic reticulum in cardiac muscle are less developed.
• Cardiac muscle fibers form diads instead of triads composed of a T-tubule and terminal cisterna.
• T-tubules are wider and surround the myofibrils at the Z-line.
• Terminal cisternae are small and incomplete.

Intercalated Discs

• Intercalated discs are specialized interdigitating junctions between cardiac muscle cells.
• They always coincide with Z lines and bind the cells.
• Under light microscopy, intercalated discs appear as straight or step-like structures.
• Specific staining can be achieved using Iron Hematoxylin or Silver.
• Intercalated discs transmit forces of contraction.
• Three types of membrane-to-membrane contact comprise the structure: fascia adherence, desmosomes, and gap junctions
• Fascia adherence performs intracellular adhesion and anchors myofibrils, representing half a Z-line.
• Desmosomes prevent cell separation during contraction.
• Gap junctions, with a 2 nm gap, enable direct communication via interconnecting channels that allow free and rapid transmission of nerve impulses resulting in the fiber contracting as a single unit physiologically.
• The growth of cardiac muscle occurs through hypertrophy, involving an increase in the size of cardiac muscle cells due to the new formation of myofibrils.
• There is no cell division or regeneration.
• Repair occurs with no satellite cells. When muscle fibers are lost, they are replaced with fibrous C.T. resulting in healing by fibrosis.

Structure of the Heart Wall

• The heart wall consists of 2 thin atria and 2 thick ventricles.
• Three layers comprise the heart wall: epicardium, myocardium, and the endocardium,.
• Visceral layer of the double layered pericardium. It invests the heart from outside.
• Epicardium is formed of simple squamous epithelium (outer) and a subserous layer of C.T. containing coronary blood vessels and fat cells.
• The C.T. blends with the endomysium of the myocardium.
• The myocardium or main bulk are cardiac muscle fibers running in various directions and separated by loose C.T. endomysium containing blood vessels and lymphatics.
• Covered internally by the endocardium and externally by the epicardium.
• The endocardium lines the chambers and covers the valves.
• It consists of four layers from inside to outside: simple squamous endothelial layer (most inner), subendothelial dense C.T. layer, a layer of dense elastic fibers, and a layer of loose C.T which contains blood vessels and purkinji fibers (outer).
• Valves consist of a fold of endocardium with a supporting middle layer of dense collagen fibers, elastic fibers, phagocytic cells, and thickened at the base (mitral and tricuspid valves) from rings of C.T. around the orifice, and blood vessels, smooth muscle fibers and capillaries at the base.
• The impulse conducting system contains S-A Node (the pace maker), A-V Node, A-V bundle of His, and 2 branches and their ramification including the moderator band.

Moderator Band

• Cardiac muscle fibers form a bundle through which the right branches of the A-V bundle of His traverse into the lateral wall of the right ventricle under the endocardium.
• It appears pale, formed mainly of Purkinje fibers separated by loose C.T. endomysium, and contains fat cells with blood capillaries.
• Paler color is due to large amount of glycogen that dissolve during preparation and few myofibrils arranged peripherally.
• Larger in diameter, with faster conduction due to the gap junction.
• There are no intercalary discs but numerous gap junctions are present for rapid conduction.
• Desmosomes, no t-tubules, and cytoplasm is granulated with bundles of C.T. sheath.