Oral abstract session240Cardiogenesis: clonal relationships within the second myocardial cell lineage and the contribution of sub-lineages, via the second heart field, to the poles of the heart

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We had previously shown that two myocardial cell lineages, which segregate very early during development, contribute to the embryonic mouse heart. The parts of the heart which form from the first or second lineage correlate with the derivatives of the first or second heart fields, respectively. We have now examined clonal relationships within the second lineage and the contribution of sub-lineages to the arterial and venous poles of the heart. The results are based on retrospective clonal analysis using the a-cardiac actinnlaacZ/ + line or the Rosa26 conditional reporter line, with a CreERT2 allele, induced with low levels of tamoxifen. In both cases, labelling of cells as a result of a very rare recombination event permits clonal analysis. At the arterial pole, myocardium of the pulmonary trunk or aorta derives from distinct sub-lineages, and shares a common progenitor with a subgroup of skeletal muscles in the head derived from second branchial arch mesoderm on the left or right side of the embryo, respectively. Unexpectedly, head skeletal muscles derived from the first branchial arches are clonally related to right ventricular myocardium. Certain skeletal muscles in the neck also have a non-somitic origin, and, like head muscles, their progenitors express markers of the second heart field. These neck muscles also show clonality with myocardium, notably on the left side with pulmonary trunk myocardium, but also with myocardium at the venous pole. Analysis of sub-lineages that contribute to the venous pole does not distinguish caval vein myocardium, which had been proposed to have a distinct origin based on genetic tracing experiments. Rather, left caval vein, pulmonary vein and left atrial myocardium come from the same sub-lineage, in contrast to that of the right atrium and right caval vein. This analysis also reveals clonality between pulmonary trunk myocardium and myocardium on the left side of the venous pole. The resulting lineage tree will be presented and the implications discussed in the context of the second heart field, cell movement and gene expression patterns. In particular, the difference between genetic tracing and lineage analysis will be emphasised.

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