Steps in embryonic heart formation.
A. Myocardial progenitor cells originating in the primitive streak (PS) migrate to the anterior part of the embryo at approximately day E6.5 of gestation, to form the 'Cardiogenic region'.
B. These cells come to lie under the head folds (HF) and form the cardiac crescent, where differentiated myocardial cells can be observed by day E7.5.
C. The early cardiac tube then forms through fusion of the cardiac crescent at the midline (ML) by E8 of gestation.
D. Subsequently, the cardiac tube undergoes looping (E8.5).
E. By E10.5, a well-defined chamber is observed, however the heart is still a tube-shaped structure.
F. By day E14.5, the chambers of the fetal heart are separated as a result of septation and are connected to the pulmonary trunk (PT) and aorta (Ao), which ensure separate pulmonary and systemic circulation of the blood, respectively, after birth.