Heart Development Summary

Development of the Heart

Essential Facts

  • Cardiovascular system is the first to function in the embryo.
  • The heart is the first organ to function (end of 3rd week).
  • Blood flow begins during the 4th week.
  • The entire cardiovascular system is of mesodermal origin.
  • Cardiac wall layers (inside to outside):
    • Endocardium
    • Myocardium
    • Epicardium
  • Endocardium forms from the primitive heart tube.
  • The primitive heart tube forms from mesenchyme in the cardiogenic area.
  • Myocardium and epicardium form from splanchnic mesoderm.

Stages of Heart Development

  • A) Appearance of the cardiogenic field
  • B) Formation of two endocardial tubes
  • C) Formation of the primary heart tube
  • D) Formation of five dilatations
  • E) Formation of the cardiac loop
  • F) Differentiation of dilatations of the cardiac loop
  • G) Development of various chambers (septation)

Cardiogenic Field

  • Heart development starts before folding.
  • Cardiac progenitors migrate from the epiblast to caudal to the buccopharyngeal membrane.
  • These cells differentiate into cardiac myoblasts.
  • The union of islands forms a horseshoe-shaped endocardial tube.

Endocardial Tubes

  • On the 19th day, the horseshoe-shaped tube forms a pair of endocardial tubes.
  • These tubes connect with two primitive aortae.
  • The pericardial cavity develops dorsal to the tubes from the intraembryonic coelom.

Primary Heart Tube

  • At the beginning of the 4th week, cephalic and lateral folding occurs.
  • Two endocardial tubes move towards the thoracic region and fuse into a single primary heart tube.
  • Cephalic and caudal ends remain separate (arterial and venous ends).
  • Each end has two horns.
  • Blood passes through cephalic horns to two primitive aortae.
  • Blood returns via six veins: common cardinal, vitelline, and umbilical.

Five Dilatations

  • Occur after 21 days.

Cardiac Loop

  • The bulbus cordis and primitive ventricle grow rapidly.
  • The heart tube invaginates into the pericardial cavity forming a U-shaped cardiac loop.
  • Bulbus cordis bends caudally, ventrally, and to the right.
  • The primitive ventricle is displaced to the left.
  • The primitive atrium is displaced dorsally and cephalically.

Septation of the Heart

  • The primitive heart tube is divided into four chambers by four septa:
    • Atrioventricular septum
    • Interatrial septum
    • Interventricular septum
    • Aorticopulmonary septum

Atrioventricular Septum

  • Divides the AV canal into right and left AV canals.
  • Formed by the fusion of AV cushions (endocardial cushions).
  • Two thickenings appear on the dorsal and ventral walls of the AV canal.
  • These fuse to form the AV septum (septum intermedium).

Interatrial Septum

  • Formed by two septa: septum primum and septum secundum.
  • Septum primum:
    • Grows from the roof of the primitive atrium towards the AV cushions.
    • The gap between the septum primum and septum intermedium is the foramen primum.
    • The upper part of the septum primum breaks down forming the foramen secundum (ostium secundum).
  • Septum secundum:
    • Forms to the right of the septum primum.
    • An oblique passage(foramen ovale) is created between the upper margin of the septum primum and lower margin of the septum secundum, shunting blood from right to left atrium during embryonic life.
    • After birth, functional closure is facilitated by decreased right atrial pressure and increased left atrial pressure.
    • Later, septum primum and secundum fuse to complete the atrial septum.
    • The lower part of interatrial septum forms Fossa ovalis is derived from septum primum. The upper part of interatrial septum annulus ovalis (Limbus) forms from the septum secundum.

Interventricular Septum

  • Muscular IV septum:
    • A median muscular ridge grows upward from the floor of the primitive ventricle towards AV cushions.
  • IV foramen:
    • Located between the free edge of the muscular IV septum and the fused AV cushions.
    • Closed by the membranous IV septum.
  • Membranous IV septum:
    • Forms by the proliferation and fusion of tissue from the right bulbar ridge, left bulbar ridge, and AV cushions.

Aorticopulmonary Septum

  • AP spiral septum divides the truncus arteriosus into the ascending aorta and pulmonary trunk.
  • Neural crest cells migrate and invade the truncal and bulbar ridges.
  • These ridges grow and twist spirally, eventually fusing to form the AP septum.

Fate of Sinus Venosus

  • Most blood flow shifts to the right horn.
  • The left horn regresses to become the coronary sinus.
  • The right venous valve forms the valve of IVC (Eustachian valve) and valve of the coronary sinus (Thebesian valve).
  • The crista terminalis forms (ridge between smooth and rough parts of the right atrium).
  • The left venous valve and septum spurium fuse with the interatrial septum and mostly disappear.
  • The right horn enlarges to become part of the posterior wall of the right atrium (sinus venarum).

Congenital Anomalies

  • Defects in position
  • Defects in the Interatrial septum (ASD)
  • Defects in the Atrio-ventricular canal
  • Defects in the Bulbus cordis and truncus arteriosus

Defects in Position

  • Dextrocardia:
    • Reversal of chambers and blood vessels.
    • May be part of situs inversus.
  • Ectopia cordis:
    • Defect in sternum fusion.
    • Heart is not enclosed and is presented outwards.

Defects in the Interatrial Septum

  • Patent foramen ovale: Incomplete fusion of septum primum and secundum, but remains closed functionally.
  • Ostium primum defect:
    • Failure of septum primum to reach endocardial cushions.
    • Persistent foramen primum.
  • Ostium secundum defect:
    • Failure of septum secundum development.
    • Left-to-right shunt, increased load on the right side of the heart.

Defects in the Atrio-Ventricular Canal

  • Common atrioventricular canal:
    • Defective AV cushions.
    • Communication between all four chambers.
  • Tricuspid atresia:
    • Insufficient AV cushion tissue.
    • Fusion of tricuspid valve cusps.
    • No communication between right atrium and right ventricle.

Defects in Bulbus Cordis and Truncus Arteriosus

  • Fallot’s Tetralogy:
    • Pulmonary stenosis
    • Overriding aorta
    • IV septal defect
    • Right ventricular hypertrophy
    • Right-to-left shunting leads to cyanosis.
  • Defects in Spiral Septum:
    • Persistent truncus arteriosus: Lack of spiral septum development.
    • Communication between ascending aorta and pulmonary trunk.
    • Transposition of great vessels: Aorta from right ventricle, pulmonary trunk from left ventricle due to nonspiral development of aorticopulmonary septum.

Development of Pericardium

  • Two components: serous and fibrous pericardium.
  • Serous pericardium: visceral and parietal layers.
  • Embryological source:
    • Visceral layer: splanchnopleuric mesoderm.
    • Parietal layer and fibrous pericardium: somatopleuric mesoderm.