(Class Video) Heart Development and Embryonic Folding

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Heart Development Overview

• The heart develops from the mesoderm germ layer, specifically the splanchnic layer of the lateral plate mesoderm.
• Initial heart development begins as a cluster of cells stimulated by the underlying endoderm to differentiate into blood islands.
• Blood islands form vessels through vasculogenesis, leading to paired heart tubes.
• The heart sits in the pericardial cavity, which originates from the intra-embryonic cavity.

Embryonic Folding

• The embryo folds in two directions: lateral and cephalocaudal.
• Lateral folding involves the sides of the embryo coming together, closing the ventral body wall, resulting in the fusion of the two heart tubes into a single heart tube within the pericardial cavity.
• Cephalocaudal folding involves the bending of the head and tail ends, positioning the heart from the cephalic region to the thoracic region.
• Initially, the heart tube and pericardial coelom are separate, but their relationship changes as folding occurs, eventually pushing the heart tube into the pericardial cavity.

Heart Tube Structure

• The heart tube has a venous end and an arterial end and consists of three layers:
  • Endocardium: derived from the endothelial lining of the heart tube.
  • Myocardium: formed by cardiac myoblasts, which surround the endocardium.
  • Epicardium: the outer layer formed by migrated mesothelial cells, also known as the visceral layer of the pericardium.
• Cardiac jelly, a specialized connective tissue, separates the endocardium and myocardium.

Adult Heart Structures

• The adult heart has four chambers: the right atrium, left atrium, right ventricle, and left ventricle.
• Atria:
  • The right atrium receives blood from the superior vena cava, inferior vena cava, and coronary sinus.
  • The left atrium receives blood from the four pulmonary veins.
  • Both atria have a smooth posterior wall where these openings are located.
  • The anterior wall is rough, featuring muscular ridges called muscle pectinati.
• Atrioventricular Valves:
  • The tricuspid valve is located on the right side and the mitral valve is located on the left side.
  • Valve leaflets are attached to chordae tendinae, which are connected to papillary muscles on the ventricular walls.
• Ventricles:
  • The ventricular walls have muscular ridges and bridges called trabeculae carneae, making them rough or trabeculated.
  • The right ventricle pumps blood into the pulmonary trunk, and the left ventricle pumps blood into the ascending aorta.
  • Outflow tracts leading to these vessels are smooth.
• Ventricular Outflow Tracts:
  • The outflow tract of the right ventricle is called the infundibulum or conus arteriosus.
  • The outflow tract of the left ventricle is called the aortic vestibule.
• Semilunar Valves:
  • The pulmonary valve is located on the right, and the aortic valve is on the left.

Heart Tube Regions and Development

• The heart tube has alternating dilations and constrictions, which will form the different chambers of the heart.
• From arterial to venous end, the regions are:
  • Truncus arteriosus: forms the ascending aorta and pulmonary trunk, separating via the aorticopulmonary septum.
  • Bulbus cordis: divided into distal, middle (conus cordis), and proximal thirds.
    • The distal third forms the truncus arteriosus.
    • The middle third (conus cordis) forms the smooth outflow tracts (infundibulum and aortic vestibule) of both ventricles.
    • The proximal third forms the trabeculated part of the right ventricle.
  • Primitive ventricle: forms the trabeculated part of the left ventricle.
  • Primitive atrium: forms the rough anterior walls of both atria.
  • Sinus venosus: related to the smooth posterior walls of the atria (discussed in part two).
• Bulboventricular sulcus: located between the bulbus cordis and primitive ventricle, marking the site of the interventricular foramen and eventual interventricular septum formation.
• Atrioventricular canal: separates the atria and ventricles, splitting to form the right and left sides.

Blood Flow Through the Heart Tube

• Blood flows from the sinus venosus into the primitive atrium, then to the primitive ventricle, bulbus cordis, and finally to the truncus arteriosus, exiting via the arterial end.

Cardiac Looping

• Involves the truncus arteriosus, bulbus cordis, and primitive ventricle bending ventrally, enlarging the ventricles, and shifting this part of the heart tube forward and lower.
• Simultaneously, the primitive atrium and sinus venosus shift backward (dorsally) and upwards, positioning them behind the ventricles.
• After looping:
  • The truncus arteriosus forms the ascending aorta and pulmonary trunk.
  • The conus cordis forms the smooth outflow tracts of both ventricles.
  • The proximal third of the bulbus cordis forms the trabeculated part of the right ventricle.
  • The primitive ventricle forms the trabeculated part of the left ventricle.
  • The primitive atrium forms the rough parts of both the right and left atria.