Cardiac myofibers are organized into sheet architectures, which contribute to up to 40% of the heart wall thickening for ejection of blood for circulation. It is important to delineate the sheet architecture for a better understanding of cardiac mechanisms. However, current sheet imaging technologies are limited by fixation-induced dehydration/deformation and low spatial resolution. Here we implemented high-resolution label-free photoacoustic microscopy (PAM) of the myocardial sheet architecture. With high endogenous optical-absorption contrast originating mainly from cytochrome, myoglobin, and melanin, PAM can image the unfixed, unstained and unsliced heart without introducing deformation artifacts. A fresh blood-free mouse heart was imaged by PAM ex vivo. The three-dimensional branching sheets were clearly identified within 150 µm depth. Various morphological parameters were derived from the PAM image. The sheet thickness (80±10 μm) and the cleavage height (11±1 μm) were derived from an undehydrated heart for the first time. Therefore, PAM has the potential for the functional imaging of sheet architecture in ex vivo perfused and viable hearts.
The laminar myocardial sheet architecture and its dynamic change play a key role in myocardial wall thickening.
Histology, confocal optical microscopy (COM), and diffusion tensor MRI (DTI) have been used to unveil the structures
and functions of the myocardial sheets. However, histology and COM require fixation, sectioning, and staining
processes, which dehydrate and deform the sheet architecture. Although DTI can delineate sheet architecture
nondestructively in viable hearts, it cannot provide cellular-level resolution. Here we show that photoacoustic
microscopy (PAM), with high resolution (~1 μm) and label-free detection, is appropriate for imaging 3D myocardial
architecture. Perfused half-split mouse hearts were also imaged by PAM in vitro without fixation, dehydration, nor
staining. The laminar myocardial sheet architecture was clearly visualized within a 0.15 mm depth range. Two
populations of oppositely signed sheet angles were observed. Therefore, PAM promises to access dynamic changes of
myocardial architectures in ex vivo perfused-viable hearts.