Development of myocardial fiber organization in the rat heart

Abstract

Confocal laser-scanning microscopy of phalloidine-stained actin fibers is a relatively new tool for studying the development of myocardial fiber organization. It seems to show orientation of myocytes in rather early embryonic stages. To further evaluate the differentiation of the myocardium, this technique was compared with transmission electron microscopy in rat embryos aged between 11 and 18 days. Although the confocal images of actin filament patterns pointed to early myocyte orientation, the electron micrographs revealed that even at 17 days the ventricular myocardium was far from mature. Myofibrils never completely filled the myocytes, and lack of organization was the rule rather than the exception. The organized structure as revealed by confocal microscopy was based on cell-to-cell continuity, whereas electron microscopy showed crossing and disarray within individual myocytes. Exceptions were in the ventricular trabeculations, which showed precocious myofiber differentiation. The trabeculations probably support ventricular systole in those stages in which the free walls do not yet provide efficient contractions. The other exception was the wall of the outflow tract, which showed well-oriented myofibrils from early stages onwards. Apparently, the outflow tract has a different function in these stages. The differences found between confocal microscopy and electron microscopy suggest that some caution is indicated in the interpretation of fluorescent images of relatively low magnification.

A retrospective clonal analysis of the myocardium reveals two phases of clonal growth in the developing mouse heart

“…Our results in the mouse imply that a myofibre is polyclonal and that architecture is prefigured in the embryonic heart as early as E10.5, by the orientation of cell division, reflected by the arrays of nuclei seen in a cluster. This is in contrast to the later appearance of a supracellular organisation of the sarcomeres at E15 described for the rat embryo (Wenink et al, 1996). A possible signal for the orientation of the rows of cells may originate from mechanical forces, created by tissue contraction and blood pressure, that are thought to influence myocardial architecture (see Taber, 1998;Sedmera et al, 2000).…”

Developmental Changes in Ventricular Diastolic Function Correlate With Changes in Ventricular Myoarchitecture in Normal Mouse Embryos

“…Compared with trabecular myocardium, the compact myocardium expresses higher levels of alkali ventricular myosin light chain (MLC1V), regulatory ventricular myosin light chain (MLC2V), connexin 43, sarcoplasmic Ca 2ϩ -ATPase 2A (SERCA2A), and phospholamban (PLB), suggesting that the compact myocardium is better adapted to relaxation than trabecular myocardium. 11,12 The exponential correlation between active relaxation and compact region may be due to developmental changes in the properties of the compact myocardium, including changes in both myofilament and nonmyofilament structures and processes, such as the extramyofilament cytoskeleton 33,37,38 and calcium metabolism. 39,40 Myofibril never completely fills the myocyte in early embryonic stage and the lack of myofibrillar organization might affect systolic and diastolic myocardial function.…”

“…39,40 Myofibril never completely fills the myocyte in early embryonic stage and the lack of myofibrillar organization might affect systolic and diastolic myocardial function. 33,37,38 The genes encoding the calcium regulatory proteins, SERCA2A, PLB, and sarcolemmal Na ϩ -Ca 2ϩ exchanger (NCX1), are expressed at barely detectable levels before ED12.5, and increase during mid and late gestation. 39,40 Both developmental organization of myofibrils and developmentally regulated increases in the expression of these calcium regulatory protein genes likely result in progressive increases in the rate of relaxation of compact myocardium.…”

Developmental changes in the myocardial architecture of the chick

“…In contrast, the radial arrangement of secondary trabeculation and the ellipsoidal shape of the intertrabecular spaces probably reflects the direction of stresses during contractions by analogy with the orientation of collagen fibrils in the aortic valve leaflets, i.e., perpendicular to the wall tangent at any point (Peskin and McQueen, 1994). Together with Wenink et al (1996) we speculate that the trabeculae are the main contractile element at these stages. The arrangement of secondary trabeculation supports its role in ventricular contractility.…”

“…The arrangement of secondary trabeculation supports its role in ventricular contractility. It should also be noted that the cells in trabeculae form most of the myocardial mass during this period (Sedmera et al, 1995), and are more differentiated than the cells in the compact layer (Markwald, 1969;Thompson et al, 1995;Wenink et al, 1996).…”