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Figure 3 (A) Myocardial fiber and sheet orientation within a block of muscle excised from the anterior wall of the left ventricle. Myocytes lie in planes parallel to the long axis of the heart. Within these planes, myocyte orientation is circumferential at the mid-wall, but rotates clockwise (when viewed from the outside) to form a –60 degree left-handed helix in the epicardium (Epi) and counterclockwise to a +60° right-handed helix in endocardium (Endo). Myocytes are organized into sheets 4 cells thick that are stacked like shingles. (B) Myocardial sheet movement during cardiac cycle. During systole, due to rearrangement of myocytes, sheets are becoming more perpendicular toward the endocardium, thus helping to transform a 14% decrease in myocyte length to 40% thickening of the left ventricular wall.