[Implications of environmental cues in cardiomyogenesis]

The staging of murine cardiomyocyte specification and determination was investigated in cultures of tissue explants from pre- and postgastrulation embryos and after transplantation of cardiac or cardiogenic tissues from mouse embryos into chick embryos. The development of cultured and transplanted cells in cardiomyocytes was evaluated by testing the expression of several cardiac transcription factor genes (Nkx 2.5, eHAND, dHAND, GATA 4), alpha cardiac actin, and beta myosin heavy chain protein. In vitro analyses showed that cells with the potential to form cardiac muscle were present prior to gastrulation in 6.5-day postconception (dpc) epiblasts. Although, as shown by in vivo experiments, neurectodermal derived structures did not influence cardiogenesis in epiblast transplants, these transplants did not exhibit full cardiogenic cell differentiation in the chicken environment. In in vitro culture, the neurectoderm also had no effect on murine cardiomyogenesis. In contrast, the presence of endoderm in explants between mid- and late streak stages stimulated emerging mesodermal cells to adopt a myocardial pathway. Mesoderm from late streak explants (7.5 dpc) was capable of differentiating into a cardiac phenotype in the avian heterotopic environment, indicating that the specification of cardiac precursors became irreversible around the late streak stage in mouse embryo.