Evidence for the expression of neonatal skeletal myosin heavy chain in primary myocardium and cardiac conduction tissue in the developing chick heart

We isolated a neonatal skeletal myosin heavy chain (MHC) cDNA clone, CV11E1 , from a cDNA library of embryonic chick ventricle, At early cardiogenesis, diffuse expression of neonatal skeletal MHC mRNA was first detected in the heart tube at stage 10, During subsequent embryonic stages, the expression of the mRNA in the atrium was upregulated until shortly after birth, It th en diminished, dramatically, and disappeared in the adult, On the other han d, in the ventricle, only a trace of the expression was detected throughout embryonic life and in the adult. However, transient expression of mRNA in the ventricle was observed, post-hatching. At the protein level, during the embryonic stage, the atrial myocardium was stained diffusely with monoclon al antibody 2E9, specific for chick neonatal skeletal MHC, whereas the vent ricles showed weak reactivity with 2E9, At the late embryonic and newly hat ched stages, 2E9-positive cells were located clearly in the subendocardial layer, and around the blood vessels of the atrial and ventricular myocardiu m. These results provide the first evidence that the neonatal skeletal MHC gene is expressed in developing chick hearts. This MHC appears during early cardiogenesis and is then localized in cardiac conduction cells. (C) 2000 Wiley-Liss, Inc.