Development of the human heart.

Molecular and genetic studies around the turn of this century have revolutionized the field of cardiac development. We now know that the primary heart tube, as seen in the early embryo contains little more than the precursors for the left ventricle, whereas the precursor cells for the remainder of the cardiac components are continuously added, to both the venous and arterial pole of the heart tube, from a single center of growth outside the heart. While the primary heart tube is growing by addition of cells, it does not show significant cell proliferation, until chamber differentiation and expansion starts locally in the tube, by which the chambers balloon from the primary heart tube. The transcriptional repressors Tbx2 and Tbx3 locally repress the chamber-specific program of gene expression, by which these regions are allowed to differentiate into the distinct components of the conduction system. Molecular genetic lineage analyses have been extremely valuable to assess the distinct developmental origin of the various component parts of the heart, which currently can be unambiguously identified by their unique molecular phenotype. Despite the enormous advances in our knowledge on cardiac development, even the most common congenital cardiac malformations are only poorly understood. The challenge of the newly developed molecular genetic techniques is to unveil the basic gene regulatory networks underlying cardiac morphogenesis.