Heart Development Overview

Establishment of Cardiogenic Field

• The cardiogenic field is horseshoe-shaped, located in front of the buccopharyngeal membrane and neural plate.
• Composed of cardiac myoblasts and blood islands called angioblastic cords. These develop into blood cells and vessels.

Formation of Single Heart Tube

• The cardiogenic field is situated anterior to the buccopharyngeal membrane and neural plate, within the thoracic region.
• Cephalocaudal folding of the embryo causes the buccopharyngeal membrane to be pulled forward, moving the heart and pericardial cavity to the thorax.
• Lateral folding during embryo development merges the caudal region of the paired cardiac primordia. This results in a continuous tube except for the caudal-most end.

Results of Folding

• The heart ends up located within the thoracic cavity.
• A single, expanded tube forms, lined by an inner endothelial lining and outer myocardial layer.
• The caudal pole receives venous drainage, while the cranial pole pumps blood out.

Formation of Cardiac Loop (Looping)

• The heart tube elongates and bends on day 23.
• The cephalic portion of the tube bends ventrally, caudally, and to the right.
• The caudal (atrial) portion bends dorsally, cranially, and to the left.
• This looping process is complete by day 28.

Formation of Heart Compartments

• As the cardiac loop forms, local expansions become visible:
  • Sinus venosus
  • Atrium
  • Ventricle
  • Bulbus cordis (conus cordis)
  • Truncus arteriosus

Fate of Sinus Venosus

• The sinus venosus becomes incorporated into the right atrium.

Fate of Bulbus Cordis

• The bulbus cordis elongates and forms three parts:
  • Proximal part forms the trabeculated portion of the right ventricle.
  • Middle part forms the conus cordis (outflow tract).
  • Truncus arteriosus, which develops into the ascending aorta and pulmonary trunk.

Abnormalities of Cardiac Looping

• Dextrocardia: The heart lies in the right side of the thorax.

Formation of Cardiac Septa (Septation)

• Cardiac septa are major structures that form between days 27-37, separating:
  • Common atrium
  • Atrioventricular canal
  • Truncus arteriosus
  • Conus cordis
  • Ventricle

Formation of Cardiac Septa

• Cells in the heart tube proliferate to form the endocardial cushions.
• Endocardial cushions are located at:
  • Atrioventricular junction (between atrium and ventricle)
  • Truncoconal junction (between conus cordis and trunk)

Formation of Cardiac Septa

• The endocardial cushion at the atrioventricular junction develops into:
  • Interatrial septum
  • Atrioventricular canal
  • Membranous part of the interventricular septum
• The endocardial cushion at the truncoconal junction develops into:
  • Aortic and pulmonary channels

Formation of Interatrial Septum

• Endocardial cushions at the atrioventricular junction proliferate.
• Septum primum (sickle-shaped crest) grows from the roof of the atria.
• The opening between the septum primum and endocardial cushion is called the ostium (foramen) primum.
• The septum primum and endocardial cushion grow to close the ostium (foramen) primum.
• Some cells in the septum primum die, creating a new opening known as the ostium (foramen) secundum.
• These foramina allow free blood flow from right to left due to low left atrial pressure.
• A crescent-shaped fold appears on the right side of the septum primum, called the septum secundum.