# Tubular heart

The tubular heart or primitive heart tube is the earliest stage of heart development. The heart is the first organ to develop during human embryonic development.
From the inflow to the outflow, the tubular heart consists of sinus venosus, primitive atrium, the primitive ventricle, the bulbus cordis, and truncus arteriosus. The sinus venosus will become part of the right atrium and contain the primary cardiac pacemaker. The primitive atrium and primitive ventricle will develop into the upper and lower chambers of the heart. The bulbus cordis will form part of the right ventricle, while the truncus arteriosis split into pulmonary and aortic vessels that carry blood away from the heart. Blood flow is driven by contractions and is different compared to that of an adult heart.
The tubular heart forms primarily from splanchnic mesoderm, an embryonic tissue that develops into several key structures in the body. It consists of three layers essential for proper heart function, corresponding to those in the adult human heart: the endothelial lining, the muscular bulk, and the external surface. The endothelial lining acts as a barrier between the blood and surrounding tissues, the muscular bulk contains cardiac muscle that contracts to pump blood, and the external surface provides a protective covering for the heart.

## Development

The development of the human heart begins during the third week of embryonic life, in the cardiogenic area of the chest. At around day 18, this area develops into the splanchnic mesoderm, where stem cells form two endocardial tubes, one on each side of the embryo's midline. As the embryo folds, these tubes are brought together in the future chest cavity. By day 21-22, the endocardial tubes fuse to form a single primitive heart tube structure known as tubular heart.
After this stage is the formation of the main chambers of the tubular heart, which are the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and sinus venosus. The heart tube is suspended within the heart cavity by the dorsal mesocardium, which is a temporary layer of tissue that connects to the developing heart tube, and later degenerates to allow further growth. Finally on day 23, the heart tube begins to elongate and bend, initiating the process of cardiac looping.

### Cardiac looping

The process of cardiac looping rearranges the regions of the primitive heart tube so that all regions are in the correct positions for features of the mature heart to develop. This looping process is mediated by a few key laterality genes including inversin and lefty genes. There are three stages to cardiac looping: the C-shaped, S-shaped and terminal stage. During the C-shaped phase, the primitive tube bends into a loop that points towards the right, initiating the looping process. In the subsequent S-shaped phase, the dorsal mesocardium, a structure that initially supported the primitive tube, begins to break down. This allows for the primitive atrium to loop upwards, towards the right and the back of the heart. Finally, during the terminal looping stage, the primitive atrium is moved closer to the head with respect to the primitive ventricle and this last movement helps rearrange the segments of the tube into their final positions.
At the end of cardiac looping, all primitive segments of the heart are formed and in the correct positions they will occupy in the mature heart’s structure. These segments then continue to remodel in order to develop into the final structures that are needed for the mature heart to function.