Myofiber orientation in the weanling mouse heart

This study provides a quantitative description at the cellular level of myofiber orientation throughout the ventricles of the mouse heart. We employed computer-based methods of three-dimensional reconstruction from 3 microns plastic-embedded serial sections. Registration marks were introduced by drilling minute holes into each plastic block. Subfields of selected sections were photographed at 20 x magnification, using a computer-controlled microscope. The 35-mm film frames were projected onto a digitizer tablet and the epi- and endocardial boundaries were digitized manually. The "heads" and "tails" of linear segments of a representative myofiber sample present in each projected image were digitized in point mode. The many x-, y-, z-coordinate tables generated by digitization were reassembled automatically, giving a numerical description of the myofiber pattern. This pattern was studied interactively on a high-performance graphics workstation. We find that the heart wall is, to a first approximation, a "sandwich," in which the myofibers in the middle layer run mainly circumferentially, whereas those in the inner and outer layers run parallel or oblique to the apical-basal axis, a variant of the classical model of the myofiber pattern. We observed a "sleeve" in the interventricular septum, formed by longitudinal and oblique myofibers, a feature which apparently has not been described previously. Myofibers not running parallel to the transverse or longitudinal planes were not resolved in this study. We conclude that three-dimensional reconstruction of the cardiac myofiber pattern at the light-microscopic level, while laborious, is technically feasible and scientifically worthwhile.