Embryology

Venous

The anterior & posterior cardinal veins (ac & pc) drain blood from the neural tube to the heart. They coalesce with the vitelline (yolk sac) vessels initially & then with the umbilical vessels (when the placenta has developed) at the sinus venosus. The pulmonary veins form later draining the lung bud to the heart.
14 days plus. Diagram represents variety of stages.

Arterial

The blood flows from the heart tube around the foregut to the dorsal aorta to supply the developing nervous system & onwards, initially to the yolk sac and later to the placenta via the umbilical arteries Diagram 14 days plus.

Cardiac Segments

Pulsation occurs by 22 days. Segments are separated by constrictions (endocardial cushions) which act as primitive valves.

Looping of the tube occurs at 28 days and is normally to the left. Abnormal looping is associated with isomeric cardiac lesions.

Atrial Septum

32 Days. The septum primum initially grows downwards to meet the endocardial cushion. As it reaches the cushion a perforation (ostium secundum) is formed. Recent studies by Anderson et al however suggest that the atrial septum development is actually more complicated than this.

Atrial septum

Subsequently the septum secundum grows to form the flap valve that will postnatally seal the ostium secundum.

Ventricular Septum

42 Days.

The septum is formed from three parts:

Inlet septum is derived from coalescence of the muscular trabeculations.
Main body of septum is formed from ingrowth of anterior myocardial wall
The outlet septum divides the arterial trunk

Note the area where the 3 elements join (star) is the perimembranous region of the heart

Valve Formation

The endocardial cushions are resorbed early & the true atrio-ventricular valves are formed from invagination & then dehiscence of myocardium to form valve & tensor apparatus. The semilunar valves are formed by ingrowth from the truncal wall.

This page was last edited 16/2/2004

Acknowledgements