Physiology in Phylogeny: Modeling of Mechanical Driving Forces in Cardiac Development

Abstract

In pursuing the evolutionary history, or phylogeny, of the heart one looks for common characteristics between the embryonic and adult hearts. One pronounced similarity is in the helical organization of the muscle fibers. The simple helical organization of the fibers has long been observed histologically [1]. Moreover, there is some evidence that the muscle fibers, unlike the collection of fibers that includes collagen, are in a double helical bundle. This has been observed in a controversial illustration where the collagen is removed off the heart by boiling [2,3]. Although the idea that such a band exists was formed on the basis of disputed histologic evidence, in recent diffusion tensor MR imaging studies done by Helm and colleagues [4] one can easily identify the band architecture (Fig. 1). This is significant, because this method images the actin, which is present in muscle fibers, but not in the collagen matrix or any other part of heart tissue. By Ernst Haeckel's premise that ontogeny follows phylogeny, researchers use embryonic fish heart as a biologic model for the human embryonic heart [5,6]. By keeping track of the fluorescent myocardial cells, it is possible to create a picture of their movement during a heartbeat. The resulting looped movement is not symmetric, forming an oval for each cell. The long axis of these ovals has to be aligned with the direction of the fiber, because that is the major direction of the contraction. The directionality of the oval paths of the myocardial cells is not constant throughout the heart tube. The only way that a muscle fiber could have such a nonconstant direction as seen in the images is if it was helically arranged [7,8]. These fibers are not only helically shaped but they seem to be bundled into a band. Both the adult heart and the embryonic heart seem to be comprised of helical muscle bands. How are these two observations related?

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