• Cardiac development begins in the cardiogenic mesoderm.

• Two endocardial tubes form → fuse into a single primitive heart tube.

• Regions of the heart tube (cranial → caudal):

  1. Truncus arteriosus → outflow tracts, aorta, pulmonary artery.

  2. Bulbus cordis → right ventricle and outflow tracts.

  3. Primitive ventricle → left ventricle.

  4. Primitive atrium → trabeculated atria.

  5. Sinus venosus → smooth part of right atrium, coronary sinus.

• By day 22–23, the primitive heart starts beating.

• Looping (day 23–28): heart tube bends into its adult spatial configuration.

• Septation (weeks 4–8): partitions the atria, ventricles, and great vessels.

 

 

 

 

The heart and blood vessels are created from the mesoderm that forms blood islands (isolated cell masses) around which the endothelial tubes are formed.

Vascular smooth muscle cells and connective tissue derived from the local mesoderm then surround the endothelial tubes.

After arterial and venous ECs are specified, the complex blood vasculature is formed via extensive remodeling. At embryonic day (E) 9.5, a subset of ECs of the cardinal vein acquires a lymphatic endothelial cell (LEC) fate and develops into lymphatic vessels.¹