Cardiac muscle. Light micrograph of a section through muscle tissue from a heart, showing the cardiomyocyte cells. Heart muscle cells show central nuclei but the dominant feature is the abundance of deeply-stained mitochondria (dark spots) that supply the energy required for life-long cell contractions. Cardiomyocytes are branched and the heart tissue has a rich vascular supply seen here as many capillaries. Magnification: x150 when printed 10 centimetres tall.