Design and physiology of the heart | Cellular Ultrastructure of Cardiac Cells in Fishes

Contraction and relaxation of the working fish myocardium is a result of the complex interaction of many individual cells. The fundamental contractile cell of the working fish myocardium is called a cardiomyocyte. Each cardiomyocyte is connected to each other via specialized structures called fascia adheren junctions, desmosomes, and gap junctions. Fish cardiomyocytes are long, thin, and spindle shaped. The interior and exterior environments of the cardiomyocyte are separated by a plasma membrane, the sarcolemmal membrane. The myofibrils, which are the contractile elements of the cardiomyocyte, are located peripherally, forming a single cylindrical-shaped layer directly below the sarcolemmal membrane. The mitochondria, the powerhouse of the cardiomyocyte, are centrally located just under the myofibrils. The sarcoplasmic reticulum, an intracellular storage site for Ca2+, is usually sparse in fish cardiomyocytes, but can be well developed in highly active fish species. Most fish cardiomyocytes contain one (some have two) centrally located nucleus, with a variable amount of fat and glycogen droplets surrounding it. The percentage of cardiomyocyte volume that each cell structure occupies differs between species, cell type, and tissue type, and appears to be adapted to suit the function of the cardiomyocyte. Thus, the fish cardiomyocyte is an excellent example of how cellular structure can fit perfectly to its function.