Cellular recruitment and the development of the myocardium

The vertebrate embryo experiences very rapid growth following fertilization. This necessitates the establishment of blood circulation, which is initiated during the early somite stages of development when the embryo begins to exhibit three-dimensional tissue organization. Accordingly, the contractile heart is the first functional organ that develops in both the bird and mammalian embryo. The vertebrate heart is quickly assembled as a simple two-layer tube consisting of an outer myocardium and inner endocardium. During embryogenesis, the heart undergoes substantial growth and remodeling to meet the increased circulatory requirements of an adult organism. Until recently, it was thought that all the cells that comprise the muscle of the mature heart could trace their roots back to two bilaterally distributed mesodermal fields within the early gastrula. It is now known that the cellular components that give rise to the myocardium have multiple ancestries and that de novo addition of cardiac myocytes to the developing heart occurs at various points during embryogenesis. In this article, we review what is presently known about the source of the cells that contribute to the myocardium and explore reasons why multiple myocardial cell sources exist.