Embryonic Heart

The cells that will develop into the heart are some of the first cell lineages to be established in the vertebrate embryo (10). Most blood vessels arise within the embryonic mesoderm almost simultaneously with the establishment of the germ layer. Small clusters of mesodermal cells known as blood islands develop into both the blood vessels and the blood cells. Th embryonic blood islands yield both blood vessels and blood cells, so they are involved in angiogenesis. and hemopoiesis. Blood islands merge, forming a connected vascular network that eventually links part of the embryo to each other and other connect it to nutrient supply and respiratory organs.

Figure 1. Fetal red blood cells (rbc) can also be identified by the presence of a nucleus that is absent in the adult red blood cell. Fetal red blood cells also contain a fetal haemoglobin which has different oxygen/carbon dioxide binding characteristics to adult red blood cell haemoglobin. Image Credit: http://embryology.med.unsw.edu.au/Notes/heart20.htm

The embryonic heart is tubular and has autonomous rhythmic beats. These rhythmic constructions is what drives the blood through the developing vascular network. The cardiovascular system of the embryo, just like that of the adult, becomes an essential part of the respiration, metabolism, growth, development and excretion.

Figure 2 Figure 1. Stage 11 chick embryo. Image credit: Swarthmore

When first formed, the embryonic vertebrate heart, is already contractile and includes four major adjoining chambers (2). The sinus venosus is the first chamber to receive returning blood. Blood flows next into the atrium, then into the ventricle, and finally into the fourth chamber, the bulbus cordis. 

From the bulus cordis, blood leaves the heart to enter arteries departing for the body of the embryo.In most tetrapods, splanchnic mesoderm forms the basic four-chambered, tubular heart. Development of the heart begins when cells leave the splanchnic mesoderm to form a medial pair of endocardial tubes.  Cells remaining in the splanchnic mesoderm proliferate, producing a thickened lateral region, the paired epimyocardium. Cells of the endocardial tube and emimyocardium grow toward the mid-line and fuse into the single, centrally-located tubular heart. Specifically, the fused endocardial tubes form the epithelial lining of the heart, called the endocardium, and the epimyocardium gives rise to the extensive cardiac muscle of the heart wall, the myocardium, together with the thin visceral peritoneum covering the heart's surface. With these fusions, the basic four-chambered embryonic heart is established. 

Figure 3. (a) Formation of the cardiac tube. Transverse sections of the chick embryo show bilateral epithelial tubes forming and merging along the centerline of the embryo. (b) Schematic of the structure of the early heart tube, which is composed of three layers: myocardium (MY), cardiac jelly (CJ), and endocardium (EN). The dorsal mesocardium (DM) connects the tube to the embryo. Image credit: Science Direct

Flexions and Expansions of the tubular heart twist the heart into different configurations, but the internal path of the blood flow remains the same (20).  In most fishes, adults retain this basic embryonic heart. However, in lungfishes and tetrapods, varying degrees of internal subdivisions cordon off additional compartments within the heart, and some of the original chambers may become reduced or appropriated by by other parts of the adult vascular system.   

Fig. 4. Schematic of cardiac looping in the chick embryo. Chick embryo in the successive stages of incubation. The four-chambered heart consists of sinus venosus, atrium, and baldus cordis. Once it forms, subsequent folding and enlargement shift the relative positions of these chambers. This process does not alter the rout of blood flow through the functioning embryonic heart. Image Credit: Science Direct.