Development of the heart

Primordium of the heart

Cells migrate from the medial and cranial sections of the primitive streak to settle between the prechordal plate and the cranial end of the embryonic disc, forming a mesodermal layer between ectoderm and endoderm. This is the cardiogenic region (also termed cardiogenic plate). Due to the formation of the intra-embryonic coelom, the mesodermal layer in the remaining lateral plate (lateral mesoderm) splits into a visceral layer (splanchnopleural mesoderm) and a parietal layer (somatopleural mesoderm). The visceral layer of the coelomic epithelium of the cardiogenic region thickens and develops into the myocardium, in which cardiomyocytes mature. At S8, angiogenic cell masses arise between the myocardium and the endoderm to form blood islands, which increase rapidly in number and size. These blood islands develop into endothelium-lined vesicles and contain primitive blood cells. The vesicles coalesce to form a network of small blood vessels, the medially located endocardial plexus. Endothelial tubes arise more laterally, connecting to each other by means of the endocardial plexus. Masses of blood islands also develop near the midline, the cavities of which coalesce to form the paired dorsal aortae. These aortae become connected dorso-caudally with the endocardial plexus. The caudal ends of the endothelial tubes join with the two omphalomesenteric veins and later with the umbilical veins (animation: development of the endocardial tube).
The intra-embryonic coelom in the cranial region of the embryo is horseshoe-shaped and separated from the extra-embryonic coelom. The middle part of the intra-embryonic coelom, situated in the cardiogenic region, becomes the pericardial cavity.