Patterns of expression in the developing myocardium: towards a morphologically integrated transcriptional model

The heart is the first embryonic organ to function. Early in development, the heart shows autorhythmycity and peristaltoid contraction waves 1, 2. Contraction requires the expression of a specific set of proteins that form the contractile apparatus, i.e. the sarcomere. The contraction–relaxation cycle of the sarcomeric apparatus is mediated by changing local concentrations of free calcium. This function is achieved by another set of specific proteins, located in the sarcoplasmic reticulum and in the sarcolemma. Fascinating questions that are still poorly understood are how the cardiogenic lineage becomes established to form the peristaltoid contracting tube without valves and how this tube becomes transformed into the synchronous-contracting four-chambered heart with unidirectional valves. It is well documented that the expression of the different isoforms of contractile proteins changes considerably during these stages (for a review see 3). However, a detailed analysis of the changes in the patterns of gene expression in relation to cardiac morphogenesis is lacking. In the present review we try to fill this gap. We have centred our attention on gene products (mRNA and protein) expressed in the working myocardium of mammals and birds. No distinction has been made when mRNA and protein display the same pattern of expression, however we have highlighted those cases where the pattern of expression differs between mRNA and protein. The development and expression pattern of genes of the conduction system of the heart merits an independent review 4(Moorman et al., Circ. Res., in press). Data referring to other experimental models as Drosophila , Xenopus or zebrafish ( Danio rerio ) are included only if they are helpful for our general understanding. Often only gene expression in the presumptive atria and/or presumptive ventricles is mentioned, whereas understanding the functional significance of the patterns of gene expression requires knowledge of the entire pattern including the … * Corresponding author. Tel.: +31 (20) 5664928; fax: +31 (20) 6976177; e-mail: a.f.moorman@amc.uva.nl