CARDIAC MYOSIN HEAVY-CHAIN EXPRESSION DURING HEART DEVELOPMENT IN XENOPUS-LAEVIS

Muscle-specific gene expression in the heart during Xenopus developmen t was investigated using reverse transcription-polymerase chain reacti on (RT-PCR) and whole-mount in situ hybridization to detect transcript s of the gene for the cardiac myosin heavy chain (CMHC). RT-PCR analys is determined that CMHC transripts are present in the cardiac mesoderm at stage 13, demonstrating that muscle-specific gene expression in th e primitive myocardium has begun by the early neurula stage, approxima tely 30 h before the heart beat begins. Xenopus, therefore, is similar to amniotes and mammals in that cardiac precursor cells begin to expr ess muscle-specific gene transcripts soon after commitment to the card iac myocyte lineage. The earliest CMHC gene transcripts can be detecte d in the heart using wholemount in situ hybridization is early tailbud stage 28, which coincides with the onset of heart tube morphogenesis. CMHC gene expression was also detected in skele tal muscle: RT-PCR an alysis determined that CMHC transcripts are transiently expressed in t he somite during the initial phases of skeletal muscle differentiation . Furthermore, CMHC mRNAs are expressed in a subset of head muscles of the feeding tadpole. CMHC gene expression is induced in ectodermal ce lls of the animal cap in blastula-stage embryos injected with syntheti c MyoD or Myf5 RNA, suggesting that the CMHC gene contains regulatory elements that are responsive to the activity of those skeletal-muscle- specific transcription factors.