Development of the Heart: Part 1 - The Heart Tube

Introduction

• Focus on the development of the heart.
• Explores the formation and role of the heart tube.

Germ Layers and Mesoderm

• Three germ layers: ectoderm, endoderm, mesoderm.
• Mesoderm divides into:
  • Paraxial mesoderm
  • Intermediate mesoderm
  • Lateral plate mesoderm
• Lateral plate mesoderm forms an intra-embryonic cavity, splitting into:
  • Somatic layer
  • Splanchnic layer (heart develops here)

Formation of Heart Tube

• Begins as a cluster of cells stimulated by the endoderm.
• Differentiation into blood islands-vessels(tubes); process called vasculogenesis.
• Two heart tubes form (paired heart tubes).
• Heart lies in the pericardial cavity.

Embryo Folding(day20)

• Embryo folds in two directions:
  • Lateral folding - sides come together to form a single heart tube.
  • Cephalocaudal folding - head and tail ends bend.
• Folding positions the heart tube into the thoracic region.
• Heart beat day -22

Heart Tube Orientation

• Heart tube has a venous end and an arterial end.
• Layers of the heart tube:
  • Endocardium (endothelial lining)
  • Myocardium (surrounding myoblasts)
  • Cardiac jelly (connective tissue) b/w endo myo
  • Epicardial layer (visceral pericardium)

Heart Structure basics

• Four chambers: right atrium, left atrium, right ventricle, left ventricle.
• Atria:
  • Right atrium receives blood from superior/inferior vena cava and coronary sinus.
  • Left atrium receives blood from four pulmonary veins.
  • Smooth posterior walls; rough anterior walls (muscle pectinati).
• Atrioventricular Valves:
  • Tricuspid valve (right), mitral valve (left).
  • Valve leaflets attached to chordae tendinae and papillary muscles.
• Ventricles:
  • Trabeculated walls (muscular ridges - trabeculae carneae).
  • Right ventricle pumps blood into pulmonary trunk, left into ascending aorta.
  • Smooth outflow tracts: infundibulum (right), aortic vestibule (left).
• Semilunar Valves:
  • Pulmonary valve (right), aortic valve (left).

Developmental Process(2 months)

• Heart tube with alternating dilations/constrictions forms different chambers.
• Sections of the Heart Tube:(arterial end to venous end)
  • Truncus arteriosus :forming ascending aorta and pulmonary trunk.
  • Bulbus cordis (divided into distal, middle, proximal thirds):
    - Distal third is truncus arteriosus.
    - Middle third (conus cordis) forms smooth outflow tracts.
    - Proximal third forms trabeculated right ventricle.
  • Primitive ventricle forms trabeculated left ventricle.
  • Primitive atrium forms atria’s rough walls.
  • Sinus venosus forms venous end.(smooth wall of ra and coronary sinus,sa node

Cardiac Looping(28th day)

• Truncus arteriosus, bulbus cordis, primitive ventricle bend ventrally.
• Primitive atrium and sinus venosus shift dorsally and upwards.
• Arrangement enhances heart structure resemblance.

Key points:

• Between bulbous cordis (prox)and primitive ventricle bulboventricular sulcus -site for iv septum
• B/w Pa-pv is av canal(splited)
• B/w pa-sinous venosus is sinoatrial orifice.
• Blood flow from venous end to arterial end.
• Neural crest cells migrate into area where artria meets ventriclesand contributes to valves and to the bases of pulmonary trunk and ascending aorta.
• Foramen ovale (ia septum till birth) then closes to form fossa ovalis
• Most common defect in development is ventricular septal defect (superior iv septum hole)