Overview of the Major Embryonic Regions

The heart is the human body's first functioning organ. Its development is a complex process that consists of consecutive growth and alignment stages. The four major embryonic regions that comprise this process in vertebrates include the: 1) first heart field, 2) second heart field, 3) cardiac neural crest, and 4) proepicardial organ.

By day 15 of human development, the newly gastrulating embryo undergoes a mesodermal cell migration through the primitive streak forming an anterior-lateral first heart field and an anterior-medial second heart field. These mesodermal cells become the myocytes or heart cells. Cells from the first heart field are involved in creating the linear heart tube, as well as the development of the left ventricles and parts of atria.

The second heart field develops into the outflow tract, the right ventricle, and atria. During looping of the linear heart tube, progenitor cells from this field play an essential role in lengthening and growth, giving rise to myocardium, smooth muscle, and endothelial cells. Clinical relevance of congenital heart defects is correlated with abnormal development particularly in this heart field.

After correct positioning and looping, septation and complete remodeling of primitive vasculature can proceed. Cells from the neural tube migrate through the aortic arches into the developing outflow tract. These cardiac neural crest cells are necessary for complete septation of the outflow tract and ventricles, as well as the formation of the anterior parasympatic plexus. It is known that these cells also play a role in the development of the leaflets of the atrioventricular valves.

Cells from the proepicardial organ surround the primary heart tube prior to looping with the epicardium, which will then turn into the coronary vasculature. Recently, it has been shown that the epicardium provides a source of cells forming the interstitial myocardium and cushion mesenchyme which later contribute to myocardial regeneration after myocardial infarction. The following chapters of this tutorial explore the timeline of cardiac development and its associated regions of maturation in greater depth.