Only cardiac muscle has this particular appearance, which is given to it by something called intercalated discs. Cardiac muscle also consists of single muscle cells, called Cardiomyocytes; where something such as skeletal muscle cells are MULTInucleated (more than one cell nucleus).
mitochondria packed between myofibrils in cardiac muscle the crystalline appearance of the numerous mitochondria is a result of densely packed cristae or infoldings of the inner membrane this design dramatically increases the surface area of the inner mitochondrial membrane in which ATP-producing machinery is embedded colored TEM credit: Science Photo Library
Growth of the zebrafish heart from embryo to adult is tracked using colored cardiac muscle clones, each containing many cellular progeny of a single cardiac muscle cell. Here, a large clone of green cardiac muscle cells (top) expands over the surface of many smaller clones in a growing heart. Photo credit: Vikas Gupta
Heart muscle cells (shown in green), regress to a more youthful state after injury, start dividing again (indicated by a red marker) to replenish lost cells and then mature a second time into cardiomyocytes.