Development of the Heart

Heart development begins on the 18th or 19th day after fertilization from mesodermal cells in the cardiogenic area at the embryo's head end. These cells form cardiogenic cords that transform into hollow-centered endocardial tubes. By day 21, the paired tubes fuse into a single primitive heart tube, which differentiates into five distinct regions and begins pumping blood by day 22.

The five regions, in blood flow order, are the sinus venosus, primitive atrium, primitive ventricle, bulbus cordis, and truncus arteriosus. The sinus venosus forms the right atrium's smooth wall and SA node. The primitive atrium becomes the pectinate walls of both atria. The primitive ventricle develops into the trabeculated ventricle walls. The bulbus cordis forms the smooth ventricular outflow regions, while the truncus arteriosus becomes the ascending aorta and pulmonary trunk.

Starting around day 23, the primitive heart tube elongates and initially takes a U-shape, then progresses to an S-shape. By day 28, these looping movements reorient the primitive atria and ventricles into their final positions. This reorientation is critical for establishing proper blood flow through the developing chambers and preparing the heart for its four-chambered adult configuration.

Around day 28, endocardial cushions form as thickenings of the inner heart wall's mesoderm. These cushions fuse together to divide the single atrioventricular canal into separate left and right canals. They also contribute to growth of the interatrial septum, splitting the atrial region into right and left atria, and support formation of the interventricular septum separating the ventricles.

By the end of the fifth week, heart partitioning is essentially complete, with both the muscular and membranous interventricular septa fully formed. The atrioventricular valves develop between weeks five and eight, followed by semilunar valve formation between weeks five and nine. By week nine, the heart becomes fully four-chambered with mature AV and semilunar valves functional.

The foramen ovale is a small opening connecting the fetal atria, allowing blood to bypass the nonfunctional fetal lungs during development. This opening closes shortly after birth when the newborn begins breathing and pulmonary circulation becomes functional. The closure of the foramen ovale marks a critical transition from fetal to postnatal circulation patterns.

The primitive ventricle develops into the trabeculated walls of both left and right ventricles, creating the irregular muscular ridges characteristic of mature ventricular tissue. The bulbus cordis gives rise to the smooth part of the right and left ventricles. This differentiation creates distinct functional regions: trabeculated areas for forceful contraction and smooth regions for efficient outflow.