DEVELOPMENT OF HEART

Development of fetal heart (Fig. 17.1) begins from mesoderm in 3^rd week of intrauterine life with:

• Formation of the cardiac tube (18-22 days) on ventral surface of embryo by angiogenic cell clusters with multiple folding and differentiation,

• Cardiac looping (22-24 days), i.e.

• Morphological segmentation (24-28 days), i.e. appearance of internal horizontal bulges within the primitive cardiac tube to form four chambers: (i) Common atrium in continuation of sinus venosus, (ii) Primitive ventricle, connected to common atria by common atrioventricular canal, (iii) Bulbous cordis, connected to primitive ventricle by bulboventricular foramen, and (iv) Truncus areteriosus.

• Formation of mature heart. (26-30 days) as follows:

- Common atrium divides vertically in left atrium (LA) and right atrium (RA) by two incomplete septa-left-sided septum primum and right-sided osteum secundum, with a left-over communication via foramen ovale.

- Common atrioventricular canal divides horizontally by fusion of endocardial cushions to produce atrioventricular septum and atrioventricular valves, i.e. mitral valve (MV) and tricuspid valve (TV).

Fig. 17.1: Development of the heart.

TA: Truncus arteriosus; BC: Bulbous cordis; PV: Primitive ventricle; CA: Common atria; RV: Right ventricle; LV: Left ventricle; RA: Right atrium; LA: Left atrium; AO: Aorta; PA: Pulmonary artery.

- Primitive ventricle develops into left ventricle (LV), while bulbus cordis develops into right ventricle (RV).

- Bulboventricular foramen elongates with septation to form interventricular septum.

- Truncus arteriosus divides length-wise to form ascending aorta connected with LV and pulmonary trunk connected to RV.

Remaining aorta, head and neck vessels and proximal pulmonary vessels develop from elongation of embryonic aortic sac and multiple arterial arches developing from it.

17.1.2