Comparative stereology of mouse atria

Area of Science:

• Cardiovascular Biology
• Cellular Ultrastructure
• Comparative Anatomy

Background:

• Cardiac muscle cells possess specialized structures like sarcoplasmic reticulum and transverse tubules crucial for excitation-contraction coupling.
• Understanding the ultrastructural differences between cardiac chambers is essential for comprehending variations in electrical and mechanical function.

Purpose of the Study:

• To quantitatively compare the stereologic features of the sarcoplasmic reticulum, transverse tubules, and mitochondria in mouse left and right atria.
• To compare atrial ultrastructure with that of the mouse left ventricle.

Main Methods:

• Stereologic techniques were employed to analyze the volume fraction (Vv) and surface area per unit cell volume (Sv) of subcellular components.
• Comparative analysis was performed between the left atrium, right atrium, and left ventricle of the mouse heart.

Main Results:

• Left atrium exhibited greater volume and surface area of interior junctional sarcoplasmic reticulum (IJSR), total JSR, and extended JSR compared to the right atrium.
• Volume and surface area of free SR, transverse tubules, and mitochondria were similar between the two atria.
• Left ventricle showed greater Vv and Sv for transverse tubules, mitochondria, and most SR components (except free SR) compared to both atria.
• Atria had a greater volume of free SR than the left ventricle, potentially linked to higher calcium content and sensitivity.

Conclusions:

• Differences in IJSR between atria may arise from varying distributions of cell types analogous to ventricular working and conducting fibers.
• The observed differences in SR and T-tubule volumes between atria and ventricles correlate with functional roles in calcium handling and excitation-contraction coupling.
• Findings support the concept of a constant surface area to cell volume ratio in cardiac cells, as evidenced by plasmalemma surface areas.