Expression of the smooth‐muscle proteins α‐smooth‐muscle actin and calponin, and of the intermediate filament protein desmin are parameters of cardiomyocyte maturation in the prenatal rat heart

Background Coexpression of α‐ and β‐myosin heavy chain (MHC) is a characteristic of the primary myocardial tube. To establish if the smooth‐muscle proteins α‐smooth‐muscle actin (α‐SMA) and calponin, and the intermediate filament protein, desmin, contribute to the specific functional properties of these early cardiomyocytes, we studied their spatiotemporal expression pattern. Methods Sections of prenatal and neonatal Wistar rats were stained with antibodies against α‐ and β‐MHC, α‐SMA, calponin, and desmin. Results The expression of α‐SMA and calponin in embryonic cardiomyocytes increases to reach its highest level at ED14. Subsequently, these proteins gradually disappear, beginning in the interventricular septum (IVS) and followed successively by the compact myocardium of the left ventricle, the right ventricle, and the central atrium. Expression of α‐SMA persists longer in the ventricular conduction system, making it a convenient marker for the ventricular conduction system of the fetal rat. Desmin becomes expressed one day later than α‐SMA, but also reaches its peak at ED14, suggesting that a relatively high concentration is required to form mature sarcomeres. Conclusions The results indicate that α‐SMA, calponin, and desmin are involved in the myofibrillar development in rat heart. The presence of spatiotemporal differences in the expression of these proteins reveals regional differences in the developmental timing of cardiomyocyte maturation. The maturation process extends from the compact myocardium in the IVS to the left and right ventricular free walls, whereas the atrio‐ventricular junction, the ventricular trabeculae, and developing ventricular conduction system show a relatively slow maturation. Smoothmuscle proteins may contribute to the slow shortening speed that is characteristic of the embryonic myocardium. Anat. Rec. 249:495–505, 1997. © 1997 Wiley‐Liss, Inc.