Regional elastic modulus measurements of the ex-vivo murine heart using atomic force microscopy

Stiffness is an important mechanical index for characterisation of human and animal cells, tissues and intact organs. This study presents efforts for passive mechanical characterisation of the ex-vivo C57BL/6 murine heart using the technique of nano-indentation with atomic force microscopy. It also investigates regional stiffness variability in sectioned left ventricular (LV) samples, and the existence of epicardial and endocardial stiffness differences. Experimental results demonstrate that no spatial differentiation, or variability in stiffness was observed in the various epicardial areas of the normal intact LV myocardium (spanning apical [27.4 +/- 2.3 kPa], middle [25.5 +/- 1.9 kPa], and basal areas [21.4 +/- 1.1 kPa], p = NS). However, a transmural stiffness variation was noted in sectioned myocardial tissue (25.2 +/- 1.1 kPa for intact vs. 14.7 +/- 0.8 kPa for sectioned tissue, p<0.0001) with epicardial muscle stiffness shown to be significantly decreased compared to corresponding endocardial values (14.7 +/- 0.8 kPa vs. 25.9 +/- 3.5 kPa, p<0.0001). The study can be extended to identify relative passive force contributions in intact tissue or organs from cellular constituents and the extracellular matrix, investigate morphogenesis and development, and assess epicardial tissue stiffness changes post-stem cell therapy.