Heart

The heart starts to develop very early during 3rd week of gestation. It begins to beat at about 22-23 days! The heart is therefore one of the earliest differentiating and functioning organs.

The mesoderm layer gives rise to the heart by a process called gastrulation. Mesoderm is one of the three primary germ layers that differentiates early in development that collectively gives rise to all subsequent tissues and organs.

The heart begins to develop near the head of the embryo in a region known as the cardiogenic area.

Another germ layer – the endoderm gives chemical signals to the mesoderm which results in the cardiogenic area to form two strands called the cardiogenic cords. As the cardiogenic cords develop, a lumen rapidly develops within them. At this point, they are referred to as endocardial tubes.

The two tubes migrate together and fuse to form a single primitive heart tube. The primitive heart tube quickly forms five distinct regions. From head to tail, these include the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus.

Note that all these structures of the primitive heart tube will develop into structures of the adult human heart:

1. Truncus arteriosus will eventually divide and give rise to the ascending aorta and pulmonary trunk.
2. Bulbus cordis develops into the right ventricle.
3. Primitive ventricle forms the left ventricle.
4. Primitive atrium becomes the anterior portions of both the right and left atria, and the two auricles.
5. Sinus venosus develops into the posterior portion of the right atrium, the SA node, and the coronary sinus.

Initially, all venous blood flows into the sinus venosus, and contractions propel the blood from tail to head, or from the sinus venosus to the truncus arteriosus.

As the primitive heart tube elongates, it begins to fold within the pericardium, eventually forming an S shape. This places the chambers and major vessels into an alignment similar to the adult heart. This process occurs between days 23 and 28.

In the middle of the fourth week, the atrioventricular canal, primordial atrium and ventricle start to partition, and this process is completed by the end of week eight. It begins with the formation of the endocardial cushions, specialized extracellular matrix tissue related to myocardial tissue. At the end of the fourth week, these cushions appear on the ventral and dorsal walls of the AV canal and start to grow toward each other. They eventually fuse, separating the AV canal into left and right components, partially separating the atrium and ventricle and acting as AV valves.

Within the primitive atrium forms the septum primum, which separates the right from the left. This septum extends down toward the endocardial cushions. A space in this septum, known as the foramen primum, maintains blood flow within the heart. As the foramen primum decreases in size, the foramen secundum forms. To the right of the septum primum, the septum secundum forms and eventually expands to cover a majority of the foramen secundum. The remaining foramen is maintained as the foramen ovale in the fetus to maintain a right to left shunt. The atrial septum is then formed by a fusion of the septum primum and secundum. A flap covering the foramen ovale is formed from the septum primum. Shortly after birth, the increase in left atrial pressure causes the flap to close. A patent foramen ovale is able to form if there is a failure of the septa to fuse.