Clonal analysis of cardiac morphogenesis in the chicken embryo using a replication‐defective retrovirus. III: Polyclonal origin of adjacent ventricular myocytes

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Abstract
Replication-incompetent variants of the avian spleen necrosis virus (SNV) encoding cytoplasmic or nuclear-directed β-galactosidase (β-gal) have been used to trace the clonal growth of myocytes during left ventricular free-wall formation. Tubular-stage hearts were infected with a mixed suspension of both retroviruses and, after hatching, the progeny of marked cells in the ventricular wall were examined by X-gal histochemistry. When a small number of virions was introduced individual blue patches contained myocytes with only one label type (cytoplasmic or nuclear). These results confirmed our previous conclusion that each cluster or patch represents a single clone (Mikawa et al., 1992, Dev. Dynamics, 193:11–23). Each of these clones formed a cone-shaped patch which often extended through the entire thickness of the ventricular myocardium, but typically each patch was heterogeneous, containing a mixture of labeled and unlabeled cells. We then asked whether the two populations of myocytes in each patch were clonally related or generated from more than one progenitor. When hearts were infected with high titer viral suspensions many patches were observed in which cytoplasmic-tagged myocytes were intermingled with nuclear-tagged myocytes. Thus, the cone-shaped myocyte patches in the ventricular wall are polyclones derived from separate progenitors in the precardiac mesoderm. This finding led us to examine the separation of clonally related ventricular myocytes in the developing hearts. Embryos were infected with retroviral suspensions at varying stages of development and the resulting colonies examined after hatching. Infections on embryonic days one or two (E1, E2) gave rise to transmural colonies, whereas infection at stages later than E3 resulted in smaller clusters which were not transmural, i.e., the patches filled only a layer of each cone. This result is most simply explained by myocyte migration until E2 but with progressively restricted myocyte movements after E3.