The last major contributor to vertebrate heart development discussed in this chapter is the proepicardium. Prior to heart looping, the primary heart tube consists of endocardium, cardiac jelly, and myocardium. It is not until the start of heart looping that the epicardium surrounds the myocardium, forming the fourth layer of the primary heart tube (Fig. 3) (22). This population of cells will eventually give rise to the coronary vasculature. A neural crest origin of the coronary vessels was originally hypothesized, but lineage tracing studies have shown that the neural crest gives rise to cells of the tunica media of the aortic and pulmonary trunks, but not to the coronary arteries (13,23). These investigators also concluded that the coronary vasculature is derived from the proepicardial organ, a nest of cells in the dorsal mesocardium of the sinus venosus or septum transversum. These cells, which are derived from an independent population of splanchnopleuric mesoderm cells, migrate onto the primary heart tube between day 22 and day 28 of human development (Fig. 3), just as the heart begins its looping (8,14). Prior to migration, these cells are collectively called the proepicardium or the proepicardial organ.
Interestingly, three lineages of the coronary vessel cells (smooth muscle, endothelial, and connective tissue cells) are segregated in the proepicardium prior to migration into the heart tube (13,24). These cells will coalesce to form coronary vessels de novo via the process of vasculogenesis (25). It has also been shown that the epicardium provides an intrinsic factor needed for normal myocardial development and is a source of cells needed for forming the interstitial myocardium and cushion mesenchyme (14,26). It should be noted that an understanding of the embryological origin of the vascular system and its molecular regulation is thought to be important in helping to explain the varying susceptibility of different components of the vascular system to atherosclerosis (13,27).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.