Heart muscle cells, typically arising from progenitor cells of both the first and second heart fields, undergo spatially and temporally controlled maturational steps, which yield their fully differentiated adult phenotypes. The myocardium is composed of myocardial cells, which are striated muscle cells featuring myofibrils. The myofibrils, built of numerous myofilaments,, are visible threads that run parallel to the long axis of the cells and provide contractive capacities to these cells. Early myocardial development is characterized by subdivision of the myocardial wall into the outer, highly mitotic compact zone and an inner trabecular zone, which has much less proliferative activity. The compact zone (recognizable at E11.5) is essential for myocardial growth, while the trabeculated myocardium functions in myocardial perfusion (E9.5-13.5). During cardiac looping, cardiac jelly, or the extracellular matrix, is largely secreted between the endocardium and myocardium. The cardiac jelly region is populated by cushion cells and is important for propagation, integration, regionalization and/or stabilization of endocardial signals.