Functional morphology of the conduction system and the myocardium in the sheep heart as revealed by scanning and transmission electron microscopic analyses.

The pacemaker, Purkinje system and myocardium of the sheep heart were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the case of SEM, the heart tissues were subjected to chemical digestion procedures. The nodal cells in both the sinoatrial (SA) node and atrioventricular (AV) node were small in size and contained few nexuses with poor sarcoplasmic reticulum and myofibrillar development. These nodal cells were spindle-shaped and their ends often showed ramifications. In addition, the strands of nodal cells in the central part of the AV node were considerably compact and connected with neighboring strands to form a complicated three dimensional architecture. The muscle cells in the common bundle and Purkinje system were cuboidal or oval in shape and were broader and shorter than the working cardiac muscle cells. They had abundant nexuses, but exhibited poor sarcoplasmic reticulum and myofibrillar development. Three-dimensionally, the Purkinje strands formed a delicate network resembling a fishing-net. The atrial and ventricular myocardium consisted of long cylindrical muscle cells which often bifurcated and connected with neighboring cells. These cells had abundant nexuses, rich sarcoplasmic reticulum and well-developed myofibrils. This report discusses such morphological findings in correlation with their physiological properties.