Transmission electron micrograph of glycogen in the ventricular papillary muscle of a cat heart. Glycogen is an important energy source for cardiac muscle. In this tissue, it does not aggregate to form the alpha particles but occurs as individual particles distributed throughout the sarcoplasm in close association with the mitochondria and the sarcoplasmic reticulum. Pink tint. (Reference: MICROCOSMOS, figure 2.28, page 29.)