Establishment of the circulatory system occurs very early in development to support the rapid growth of the embryo. Therefore, the heart is the first functional organ to be formed during both avian and mammalian development. Historically, cardiac development has been considered to occur only during embryogenesis from cell sources located within the primordial structures that generate the myocardium and associated coronary vascular endothelium and smooth muscle and cardiac fibroblasts. Recently, however, contribution to the cardiac structures has been demonstrated to occur during embryonic development from extracardiac sources, like the anterior heart field, raising questions as to whether cardiogenesis may be an ongoing process that extends into adult life. In this brief article, we describe the contribution of circulating adult bone marrow hematopoietic stem cells to the cardiac cell populations and the potential regulation of their differentiation by the extracellular matrix protein, periostin.