Cellular basis for volume related wall thickness changes in the rat left ventricle

Nine arrested rat hearts fixed by glutaraldehyde perfusion of the coronary arteries were studied in vertical cross-sections of the left ventricle by phase contrast microscopy. The number of fibers per unit area in the wall and the number of fibers between the epicardium and endocardium were measured and correlated with ventricular volume and wall thickness. Average center to center fiber separation was derived from these measurements.Increasing ventricular volume is associated with a decrease in wall thickness and fiber diameter which follows theoretical predictions. The relationship between these measurements, however, presents a dimensional paradox in that the decreases in wall thickness are 3 to 4 times greater than the decrease observed in separation of fiber centers. Thus, an increase in fixation volume from 0.03 to 0.60 ml results in a 68% decrease in wall thickness, while center to center fiber separation decreases only 20%. Changes in wall thickness are therefore mainly accounted for by internal rearrangement of the disposition of fibers composing the ventricular wall. A mechanism for such rearrangements is illustrated by low magnification cross-sections of the ventricular wall which demonstrate open spaces or sliding planes between groups of muscle fibers. A volume related alteration in the orientation of these sliding planes is noted.