Characterization of cell cycle and morphology of avian heart from gastrulation through looping and chamber outgrowth

Throughout the past century, much has been learned about vertebrate heart development, from the formation of myocardial and endocardial precursors during gastrulation to the fusion, looping and remodeling of the primitive heart tube. However, some uncertainty remains regarding basic facets of early heart development. My experiments in the chick have addressed the length of the cell cycle of gastrulating tissues and of the heart-forming mesoderm in particular, the timing of the separation of the myocardial and endocardial lineages and cellular changes which enable looping and chamber outgrowth. My own calculations of cell cycle length in ovo have yielded an estimate of over 10.5 hours for all tissues during gastrulation. Ectoderm had the shortest cycle, primitive streak cells showed intermediate values and ingressed tissues had the longest cycles. My retroviral lineage analysis, combined with an estimate of the length of the cell cycle in precardiac mesoderm, indicate that the myocardial and endocardial lineages may separate as late as HH stage 7. This calls into question previous reports regarding the timing of this cell fate decision. My analyses of cellular aspects of early heart morphogenesis find roles for differential proliferation in looping, for changes in myocardial wall thickness in chamber outgrowth and for cellular shape changes in looping and chamber outgrowth. Apoptosis does not appear to be a factor in early heart development. Finally, a comprehensive estimation of the morphological changes enabled by differential proliferation, wall thickness and cell shape differences is presented to ascertain the contribution of each of these factors to the developing heart.