There are five major stages of heart development. The first is the development of the primitive heart tube, in which the heart begins to beat. The second is the heart looping stage, where the form of the heart starts to take shape. The third marks the development of two separate heart chambers. The fourth and fifth stages occur when the third and fourth heart chambers are formed, respectively. The entire development of the heart takes about eight weeks.
The first stage of heart development involves the primitive heart tube. It is made up of a single heart tube, which is formed when cardiac precursor cells fuse together. At this early stage, even though it appears to be just a tube, the heart consists of different regions and layers. The regions are the cranial, caudal, and bulbs ordis, which develop to form the aorta, the largest artery in the human body, and ventricles. The layers are the cardiac jelly, and cardiac mantle layers will form the myometrium and epicardium of the heart.
The heart begins to beat during the first stage of heart development. Usually during week five of pregnancy, about 22 or 23 days after conception, a tiny flickering heart beat can be seen from a vaginal ultrasound. This very primitive but functional tube-shaped heart is a close resemblance to a fish heart.
The second stage of heart development occurs rapidly, within 24 hours. It is called heart looping. As it grows, the tube-shaped heart contorts into an S shape and bends to the right, which is called d-looping. This new shape creates a primitive region for the ventricle to grow. This stage of development is triggered by activated heart-specific proteins.
The next stage of heart development is called the two-chambered stage. The two chambers consist of one atrium and one ventricle. During this stage, the heart continues to grow rapidly. The cardiac jelly serves as a valve between the ventricular areas and the atria. The two-chambered stage of the growing human heart resembles the heart of a frog.
The fourth stage of heart development is triggered by the atria dividing. This stage is called the three-chambered heart. The three chambers include the two atria, which have split, that rest on top of the ventricle, the third chamber. This stage of the developing heart looks like a three-chambered snake or turtle heart.
The final stage of heart development occurs by the end of the tenth week of pregnancy. It is marked by the development of the fourth heart chamber. The heart is now fully formed with two atria, two ventricles, and two large blood vessels to carry blood to and from the heart.