High resolution imaging of the human cardiac conduction system using reflectance confocal microscopy.

Rhythmical contraction of the heart is controlled by the cardiac conduction system (CCS) that consists of the three main parts: the sino-atrial node, the atrioventricular node and the His-Purkinje system. A heartbeat signal, originated from CCS, spreads through its branches to the different parts of the heart, initiating depolarization of the ventricles. However, this highly important system could not be distinguished visually from the surrounding heart tissues: myocardium (MC) and connective tissue (CT). Thus, during surgical procedures, CCS could be easily damaged; namely, the reliable method for identification of CCS either in vivo or ex vivo does not exist. Accordingly, there is a definite need for developing a CCS imaging method. Reflection confocal microscopy (RCM) offers non-destructive imaging of the tissue at depths of up to 0.35 mm with the capability of identification of a single cell. During the visualization procedure, a given tissue is illuminated with infrared laser light and the image is obtained because of different reflections from the tissue structures. However, the reflective structures in the heart tissues are still not identified. In the present study, for the first time we investigated cardiac tissues by RCM. The resolution of the method allowed us to distinguish MC cells and CCS cells. The method also allowed us to distinguish the network-like structures that are main components of CT. The ability to visualize different tissue components indicates a great potential for RCM to be used in non-destructive cardiac investigations and for imaging CCS.