Developmental patterning of the myocardium

The heart in higher vertebrates develops from a simple tube into a complex organ with four chambers specialized for efficient pumping at pressure. Dur ing this period, there is a concomitant change in the level of myocardial o rganization. One important event is the emergence of trabeculations in the luminal layers of the ventricles, a feature which enables the myocardium to increase its mass in the absence of any discrete coronary circulation. In subsequent development, this trabecular layer becomes solidified in its dee per part, thus increasing the compact component of the ventricular myocardi um. The remaining layer adjacent to the ventricular lumen retains its trabe culations, with patterns which are both ventricle- and species-specific. Du ring ontogenesis, the compact layer is initially only a few cells thick, bu t gradually develops a multilayered spiral architecture. A similar process can be charted in the atrial myocardium, where the luminal trabeculations b ecome the pectinate muscles. Their extent then provides the best guide for distinguishing intrinsically the morphologically right from the left atrium . We review the variations of these processes during the development of the human heart and hearts from commonly used laboratory species (chick, mouse , and rat). Comparison with hearts from lower vertebrates is also provided. Despite some variations, such as the final pattern of papillary or pectina te muscles, the hearts observe the same biomechanical rules, and thus share many common points. The functional importance of myocardial organization i s demonstrated by lethality of mouse mutants with perturbed myocardial arch itecture. We conclude that experimental studies uncovering the rules of myo cardial assembly are relevant for the full understanding of development of the human heart. Anat Rec 258:319-337, 2000. (C) 2000 Wiley-Liss, Inc.