Transmission electron micrograph of a section of the right atrium of a guinea pig heart. A portion of nucleus is seen at far left, with two centrioles close to the nuclear envelope. A region of myofibrils is seen at upper right. Between the nucleus and myofibrils are several homogeneously staining atrial granules and numerous small, darly staining glycogen granules. Image made available by James D. Jamieson and the Department of Cell Biology, Yale University School of Medicine.