Staging of the commitment of murine cardiac cell progenitors

The staging of murine cardiomyocyte specification and determination was inv estigated in cultures of tissue explants from pre- and postgastrulated embr yos and after transplantation of cardiac or cardiogenic tissues from mouse embryos into 2-day-old chick embryos in different locations. The developmen t of transplanted and cultured cells in cardiomyocytes was evaluated by tes ting the expression of several cardiac transcription factor genes (Nkx 2.5, eHAND, dHAND, GATA-4), alpha-cardiac actin mRNA, and beta-myosin heavy cha in 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, as indicated by the expression of Nkx 2.5, eHAND, dHAND, and GATA-4 cardiac transcription factors; desmin transgene; alpha-cardiac a ctin; and beta-myosin heavy chain. Conversely, epiblasts transplanted into the chicken semitic environment did not exhibit full cardiogenic cell diffe rentiation. It was determined that chick host axial structures did not infl uence cardiogenesis in transplants. Mesoderm from late streak explants was capable of differentiating into the cardiac phenotype in the avian heteroto pic environment, indicating that the specification of cardiac precursors (u nder way by 6.5 dpc) became irreversible at around the late streak stage in mouse embryo. Although in vitro analyses showed that interaction with endo derm is not required for the specification of murine cardiac cells, the pre sence of endoderm in explant cultures between mid- and late streak stages s timulated emerging mesodermal cells to adopt a myocardial pathway, whereas ectoderm had no influence on cardiomyogenesis. (C) 2000 Academic Press.