Myocardial mesostructure and mesofunction.

Area of Science:

• Cardiovascular Science
• Cardiac Mechanics
• Biophysics

Background:

• The human heart comprises approximately five billion cardiomyocytes, whose organized structure enables coordinated electrical and mechanical function.
• Cardiomyocyte orientation changes during contraction, resulting in base-to-apex shortening, circumferential shortening, and left ventricular torsion.
• The left ventricular wall thickens significantly (30-40%) during contraction, necessitating myocardial structural rearrangement.

Purpose of the Study:

• To review histological and physiological studies supporting the sheetlet shear model of myocardial wall thickening.
• To discuss recent advancements in imaging techniques for visualizing cardiomyocyte organization.
• To examine the impact of cardiac remodeling on myocardial mesostructure and function.

Main Methods:

• Histological examination of myocardial mesostructure.
• Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) for mesoscale imaging.
• Cardiac Diffusion Tensor Imaging (DTI) to assess cardiomyocyte orientation and reorientation in vivo.

Main Results:

• The sheetlet shear model, where cardiomyocyte collections (sheetlets) rearrange via shear, is established as the primary mechanism for myocardial wall thickening.
• Mesostructural sheetlets have been confirmed in both animal and human hearts using histology and DTI.
• In vivo cardiac DTI has demonstrated dynamic reorientation of sheetlets during the cardiac cycle.

Conclusions:

• Myocardial sheetlet shear is fundamental to cardiac contraction and wall thickening.
• Advanced imaging techniques like DTI are crucial for understanding cardiac mesostructure and function.
• Pathological remodeling significantly affects sheetlet organization, impacting ventricular function and leading to dysfunction.