23.19: Development of the Heart
The development of the human heart, a crucial organ, commences from the mesoderm on the 18th or 19th day after fertilization. This process initiates in the cardiogenic area, a group of mesodermal cells at the embryo's head end, which evolves into elongated strands known as cardiogenic cords. These cords undergo a transformation to form hollow-centered endocardial tubes.
As the embryo undergoes lateral folding, these paired tubes approach each other, merging into a single primitive heart tube by the 21st day. This tube soon differentiates into five distinct regions by the 22nd day and begins to pump blood. These regions, listed in the order of blood flow direction, include the sinus venosus, primitive atrium, primitive ventricle, bulbus cordis, and truncus arteriosus.
Each region has a specific fate. The sinus venosus forms the smooth posterior wall of the right atrium, coronary sinus, and the sinoatrial (SA) node. The primitive atrium transforms into the pectinate walls of the right and left atria and their respective auricles. The primitive ventricle develops into the trabeculated walls of the left and right ventricles, while the bulbus cordis gives rise to the smooth part of the right and left ventricles. Lastly, the truncus arteriosus evolves into the ascending aorta and pulmonary trunk.
From day 23, the primitive heart tube starts to elongate, initially taking a U-shape and subsequently an S-shape. By the 28th day, these movements reorient the primitive atria and ventricles into their final postnatal positions. Around the same time, endocardial cushions, thickenings of the inner heart wall's mesoderm, emerge. These cushions fuse together, dividing the single atrioventricular canal into separate left and right canals. They also contribute to the growth of the interatrial septum, which eventually splits the atrial region into the right and left atria. Simultaneously, the interventricular septum begins to form. This structure develops from the floor of the primitive ventricle, growing upwards toward the endocardial cushions. The muscular part of the interventricular septum initially forms, followed by the membranous part, which completes the separation between the left and right ventricles.
By the end of the fifth week, the partitioning of the heart is essentially complete. The atrioventricular valves form between the fifth and eighth weeks, followed by the formation of the semilunar valves between the fifth and ninth weeks.
