[Cellular architecture of the sheep myocardium and heart conduction system as revealed with silver-impregnation...

Morphological studies were carried out to delineate the cellular architecture of the myocardium and heart conduction system in adult sheep using the silver-impregnation method. Reticular fibers were heavily stained in a deep black color, while collagen fibers were less intensely silver-stained in a brownish color. Individual working myocardial cells were ensheathed by thin reticular fibers and showed a polygonal form, and were connected with adjacent cells mainly end-to-end and sometimes side-to-side. Thick collagen septa were distributed between masses of myocardial cells. The nodal cells in both the sinoatrial (SA) node and atrioventricular (AV) node were spindle shaped, smaller in size than myocardial cells, and surrounded by thin reticular fibers. The bundles of SA node cells were linearly oriented, while those of AV node cells formed a reticular pattern. The cells in the His bundle and Purkinje system were oval in shape, larger in size than myocardial cells and formed the strands composed of 4-8 cells. Each stand was surrounded by thick reticular fibers. These cells had both end-to-end and side-to-side contacts. Purkinje cells were followed by transitional cells which were in contact with myocardial cells. On the other hand, the reticular fibers and collagen fibers showed characteristic structures at the different portion of heart. The present study discusses the topographical relationship between cardiac muscle cells containing the conduction system and the connective tissue sheaths surrounding muscle cells.