Ultrastructure of Cardiac Muscle and Blood Vessels

This chapter summarizes the fine structure of three of the essential cell types found in the heart, which include cardiac muscle cells, also called myocardial cells or cardiomyocytes, vascular smooth muscle cells, and endothelial cells. The first cell category constitutes most of the heart's mass, whereas the remaining two make up the walls of the heart's extensive system of blood vessels. The shape of a cardiac muscle cell is largely the product of its internal construction. The cell is a fasces-like collection of rod-like myofibrils with which an external covering, the sarcolemma, is wrapped. The enveloped myofibrils are often of different lengths, and thus, form staggered cell ends that are the basis for the step-like profiles of the intercalated discs. Endothelial cells appear either as rounded polygonal bodies arranged in a cobblestone pattern or as more elongated profiles whose long dimensions are aligned with the direction of blood current flow. Both shapes may be found along stretches of the same vessel and their distributions appear to be related to hemodynamics. Within the extensive vasculature of the heart, endothelial cells generally appear to be of the more elongate variety.