Determination of relative fiber orientation in heart muscle: Methodological problems

Knowledge of the muscle pattern in the heart is important to understanding cardiac contraction and propagation of the electrical stimulus. Most work on this pattern has been carried out by blunt gross dissection, whereby fiber bundles are easily visible on the peeled heart wall. However, it has never been shown, to our knowledge, that the orientation of macroscopic fiber bundles seen in a peeled heart corresponds to that of the constituent myofibers (muscle cells). For this purpose, one needs to carry out a three‐dimensional microscopic reconstruction within a documented macroscopic reference frame. To draw valid conclusions in such a coordinated macroscopic and microscopic study, one must estimate the (slice) angle between the long axis of a muscle cell and the plane of section. Otherwise any alleged differences between the macroscopic and microscopic orientations may be just an artifact of sectioning. In this study we have shown that, provided the images of the myofibers meet simple criteria, one can be reasonably confident that the potential error incurred by sectioning is small. On this basis, we demonstrated that while there is a general correspondence between the macroscopic fiber and the microscopic myofiber orientations, there are significant differences in detail.