[The mouse as a model of heart morphogenesis in mammals: origin and lineage of myocytes]

In order to follow cardiac precursor cells, we have adopted a retrospective clonal approach, based on the nlaacZ genetic label. Random clones were generated and observed at different developmental stages in murine myocardium. The distribution of these clones in clusters suggest for the first time that cells fated to form myocardium proliferate in two steps. The first growth phase, before E8.5, is dispersive and polarised along the axis of the primitive cardiac tube, contributing to its elongation. The second growth phase is coherent and polarised differentially in different cardiac subregions. Interestingly, this can be correlated with production of geometrical forms (dilatation of a sphere, enlargement of a tube), showing the relation between heart morphogenesis and the controlled proliferation of myocardial cells. The restricted distribution of clones to the right or left ventricule was also investigated with the goal of establishing the time at which cardiac chamber identity emerges. Right and left ventricular lineages appear to segregate early, in agreement with the existence of two populations of cardiac precursors, the so-called primary (or posterior) and secondary (or anterior) heart fields.