Direct measurement of transmural laminar architecture in the anterolateral wall of the ovine left ventricle: New implications for wall thickening mechanics

Laminar, or sheet, architecture of the left ventricle (LV) is a structural basis for normal systolic and diastolic LV dynamics, but transmural sheet orientations remain incompletely characterized. We directly measured the transmural distribution of sheet angles in the ovine anterolateral LV wall. Ten Dorsett-hybrid sheep hearts were perfusion fixed in situ with 5% buffered glutaraldehyde at end diastole and stored in 10% formalin. Transmural blocks of myocardial tissue were excised, with the edges cut parallel to local circumferential, longitudinal, and radial axes, and sliced into 1-mm-thick sections parallel to the epicardial tangent plane from epicardium to endocardium. Mean fiber directions were determined in each section from five measurements of fiber angles. Each section was then cut transverse to the fiber direction, and five sheet angles (β) were measured and averaged. Mean fiber angles progressed nearly linearly from -41° (SD 11) at the epicardium to +42° (SD 16) at the endocardium. Two families of sheets were identified at approximately +45° (β⁺) and -45° (β^-). In the lateral region (n = 5), near the epicardium, sheets belonged to the β⁺ family; in the midwall, to the β^- family; and near the endocardium, to the β^- family. This pattern was reversed in the basal anterior region (n = 4). Sheets were uniformly β^- over the anterior papillary muscle (n = 2). These direct measurements of sheet angles reveal, for the first time, alternating transmural families of predominant sheet angles. This may have important implications in understanding wall mechanics in the normal and the failing heart.