83. Striated cardiac muscle tissue. Types of myocytes

Striated Cardiac Muscle Tissue - Origin and Characteristics

• Origin: Cardiac muscle tissue originates from the mesoderm, specifically from a specialized region called the mesenchyme during embryonic development.

• Involuntary Control: Cardiac muscle operates involuntarily, controlled by the autonomic nervous system (ANS). However, the heart's intrinsic conduction system primarily drives the heartbeats, allowing for automatic and rhythmic contractions without conscious control.

• Striated Muscle: Similar to skeletal muscle, cardiac muscle is striated due to the regular arrangement of actin and myosin filaments.

• Location: Found exclusively in the myocardium, the muscular layer of the heart wall, and in the walls of some large vessels entering and exiting the heart (e.g., the aorta and pulmonary veins).

key features

Intercalated Discs: Specialized cell junctions that connect individual cardiac muscle cells (cardiomyocytes) end-to-end. These structures facilitate synchronized contraction and efficient transmission of electrical impulses throughout the heart. They consist of:

• Fascia Adhaerens: Anchors actin filaments of the sarcomere to the plasma membrane and connects adjacent cells.

• Macula Adhaerens (Desmosomes): Provide strong adhesion between cardiomyocytes, preventing them from separating during contraction.

• Gap Junctions: Allow for electrical and ionic continuity between cells, ensuring coordinated contraction.

Structure of Cardiomyocytes (Cardiac Muscle Cells)

• Shape and Size: Elongated, cylindrical cells that are often bifurcated (branched), allowing for a three-dimensional network that supports synchronized contraction.

  • Length: 85-100 µm

  • Diameter: 15-20 µm

• Nucleus: Typically, each cardiomyocyte has a single, centrally located nucleus that appears pale-staining due to the presence of euchromatin, which is less densely packed and active in gene expression.

• Sarcoplasmic Reticulum (SR) and T-Tubules: In cardiac muscle, the SR is less extensive compared to skeletal muscle. The T-tubules are larger and form a diad (a T-tubule associated with a single SR cisterna) instead of the triadseen in skeletal muscle.

• Mitochondria: Abundant, occupying approximately 40% of the cytoplasmic volume, providing the necessary ATP for continuous contraction. The high energy demand is due to the heart's requirement for sustained rhythmic contractions.

• Other Inclusions:

  • Lipid Droplets and Glycogen Granules: Serve as energy reserves.

  • Lipofuscin: Pigmented granules that accumulate with age, reflecting the long lifespan and ongoing metabolic activity of cardiomyocytes.

  • Atrial Granules: Found mainly in atrial cells, these granules store vasoactive substances such as Atrial Natriuretic Factor (ANF) and Brain Natriuretic Factor (BNF), which are involved in regulating blood pressure and fluid balance.

Types of Cardiac Myocytes

• Contractile Myocytes:

  • Function: Responsible for the contractile activity of the heart. These cells make up the bulk of the myocardium and are involved in the rhythmic pumping action.

• Conductive Myocytes:

  • Location: Found in the heart's conduction system, including the Sinoatrial (SA) Node, Atrioventricular (AV) Node, Bundle of His, and Purkinje Fibers.

  • Function: Specialized for generating and conducting electrical impulses rapidly, ensuring the coordinated contraction of the heart muscle.

• Secretory Myocytes:

  • Location: Primarily located in the atria, particularly the right atrium.

  • Function: Contain secretory granules that store and release vasoactive peptides like ANF and BNF, which help regulate blood volume and blood pressure.

Regeneration of Cardiac Muscle Tissue

• Limited Regenerative Capacity: Cardiac muscle tissue has very limited ability to regenerate. After early childhood, the potential for regeneration is almost nonexistent.

• Lack of Cell Division: Mature cardiomyocytes do not typically undergo cell division. Instead, damage to the myocardium (e.g., from a myocardial infarction) results in the replacement of dead cells with fibrous connective tissue, leading to the formation of myocardial scars. This scar tissue lacks the contractile properties of healthy myocardium, which can compromise heart function.