Molecular aspects of myocardial differentiation

The embryonic heart can pump blood in a single direction without one-way valves. With the development of molecular cell markers specific for contraction and relaxation, functional aspects of myocardial differentiation have been addressed through the use of in situ hybridization. In this study, we report how expression of the cardiac sarcoplasmic reticulum calcium-adenosine triphosphatase (SERCA2) and phospholamban (PLB) in the rat may partly explain why the embryonic atrium and ventricle function essentially as they do in the adult. SERCA2 is expressed in a craniocaudal gradient from as early as 10 embryonic days (ED) of development. PLB is the first expressed at 12 ED but in a gradient essentially opposite to that seen for SERCA2. This spatial pattern of expression is maintained throughout much of fetal development. The spatial distribution of skeletal a-actin in the developing human heart indicates that a-actin isoform gradients or switching are not important in the establishment of unidirectional blood flow in the absence of valves, but it may serve as a marker for cardiac maturation.