The heart muscle cells of this transgenic fish expresses GCaMP, a genetically encoded fluorescent calcium indicator. Visible is the wave of cardiac conduction, traveling over the heart from atrium (bottom) to ventricle (top left). The heart has a size of about 250 um and beats at a rate of about two times per second. It was imaged in 3D inside the living zebrafish using Selective Plane Illumination Microscopy (SPIM) (Huisken et al. 2004).