Background Myocardial infarction (MI) is a common cause of death, with approximately 175000 inpatient episodes of acute myocardial infarction in the UK in 2012. Following an MI the loss of cardiac myocytes triggers a remodelling process depositing extracellular matrix in the infarct region. Hypocontractile, damaged cardiac myocytes surround the infarct, forming a border zone. One of the key structural components regulating excitation-contraction coupling in the heart is the t-tubule network. Conventional confocal microscopy (resolution ˜100 nm) of isolated cardiac myocytes from the border zone post-MI has shown a loss of t-tubules. Here we apply, for the first time to our knowledge, serial block face scanning electron microscopy (SBF-SEM), to investigate the morphology of the t-tubule network within the infarct border zone cardiac myocytes to provide nano-scale structural details of the remodelling process.

Methods A porcine model of MI was employed for this study. All animal work was approved by the University of Manchester local ethics committee and was covered by the necessary UK Home Office project and personal licences. Tissue (˜0.5 mm³) was collected from the border and remote zones of the MI pigs and corresponding regions were also taken from control animals and processed for SBF-SEM. Blocks were imaged using an FEI Quanta 250 FEG SEM equipped with a Gatan 3View system. Serial images were collected at different magnifications ranging from 5.4 to 90.0 nm per pixel in the X-Y plane, while the cutting depth along the Z-axis was fixed at 50 nm for all the datasets. Images were segmented and rendered in Fiji or IMOD.

Results 3D reconstruction of the t-tubule network from the left ventricle of control animals revealed a spoke-like arrangement, similar to that observed in human cardiac myocytes and other large mammals e.g. the sheep. Cardiac myocytes within the remote region of the infarcted heart have a t-tubule network indistinguishable from that of the control myocytes. In contrast, border zone myocytes showed large areas that were devoid of t-tubules. 3D modelling revealed that the surviving transverse tubules presented gross deformations, appearing to be the result of t-tubules fusing with each other to form a large complex that adopts a variety of orientations within the cardiac myocyte.

Conclusion Employing SBF-SEM we have collected 3D datasets of cardiac myocytes in situ within the left ventricle of infarcted pigs at magnifications corresponding to ˜5 nm per pixel in the X-Y plane. This has allowed the resolution of nano-scale details of the remodelled t-tubules, including features such as the basal lamina. Together the loss of t-tubules within parts of the cell, coupled with the formation of super-tubule networks, provide novel structural insights towards unravelling the hypocontractile properties of the border zone cardiac myocytes.