Heart Development: From Tube to Four Chambers

The heart is the first organ to function in the developing embryo, beating by day 22–23 of gestation — before the embryo has even completed neurulation. This early onset of cardiac function is necessary because the rapidly growing embryo exceeds the limits of diffusion-based nutrition and requires a circulatory system to deliver oxygen and nutrients. The transformation of a simple tubular heart into the complex four-chambered organ is a process of extraordinary morphological complexity, and its disruption at any stage can produce the spectrum of congenital heart defects (CHD), which occur in approximately 8 per 1,000 live births. This guide is for educational purposes only.

## Cardiac Crescent and Heart Tube Formation

Cardiac progenitor cells — the lateral plate mesoderm cells destined to form the heart — are induced during gastrulation and migrate cranially and laterally to form the horseshoe-shaped cardiac crescent (primary heart field) in the splanchnic lateral plate mesoderm cranial to the oropharyngeal membrane. By day 18–19 these cells coalesce bilaterally and begin differentiating into cardiomyocytes.

As the embryo undergoes cranial and lateral folding during weeks 3–4, the two lateral cardiac primordia fuse in the midline to form a single primitive heart tube — the earliest functional heart. The heart tube is invested with cardiac jelly (a gelatinous extracellular matrix) and covered by a layer of epicardial cells. A secondary heart field (from pharyngeal mesoderm) contributes cells to the arterial pole (outflow tract) and venous pole of the heart, expanding the length and complexity of the original tube.

## Heart Tube Looping

The straight heart tube consists of (from caudal to cranial, corresponding to future definitive heart regions): the sinus venosus (receives systemic venous return), the primitive atrium, the primitive ventricle, the bulbus cordis (which gives rise to the right ventricle and the conus cordis/outflow tract), and the truncus arteriosus (aorta and pulmonary trunk). The heart tube is connected cranially to the aortic sac and caudally to the vitelline veins.

By day 23 the elongating heart tube undergoes dextral (D) looping — bending to the right — so that the originally cranial outflow tract (bulbus cordis) moves anterior and rightward while the future ventricles form the cardiac apex. This looping is controlled by left-right signalling (Nodal pathway) and is the first morphological sign of left-right asymmetry in the embryo. Situs inversus totalis (mirror-image looping) or situs ambiguus/dextrocardia may result from defects in laterality determination.

## Septation of the Heart

Division of the single-chambered heart into four chambers occurs through a series of overlapping septation events primarily during weeks 4–7.

Atrial septation: The septum primum grows downward from the roof of the primitive atrium towards the endocardial cushions (atrioventricular septum) leaving an opening — the ostium primum — which allows right-to-left shunting. Before the ostium primum closes, fenestrations coalesce in the upper part of the septum primum to form the ostium secundum. A second septum — the septum secundum — then grows to the right of the septum primum, incompletely covering the ostium secundum and leaving an oblique gap — the foramen ovale. In fetal life, the foramen ovale allows oxygenated placental blood to shunt from the right atrium to the left atrium, bypassing the non-functional fetal lung. After birth, increased left atrial pressure presses the septum primum against the septum secundum, functionally closing the foramen ovale; anatomical closure (fusion) is usually complete by age 3 months, though a patent foramen ovale (PFO) persists in approximately 25% of adults and may allow paradoxical embolism.

Ventricular septation: The muscular interventricular septum grows upward from the apex of the primitive ventricle, leaving an interventricular foramen. This foramen is ultimately closed by the membranous interventricular septum, formed from contributions of the endocardial cushions and conotruncal ridges. Ventricular septal defect (VSD) — the most common individual congenital heart defect — most commonly involves the membranous portion and results from failure of complete closure.

## Valve Formation

The endocardial cushions — swellings of cardiac jelly colonised by mesenchymal cells derived partly from endocardial cells that undergo epithelial-to-mesenchymal transition — form in the atrioventricular canal and the outflow tract. In the AV canal they fuse to divide it into the tricuspid (right) and mitral (left) orifices and contribute to formation of the AV valves and the membranous septum. The tricuspid and mitral valve leaflets form from the endocardial cushion tissue and from proliferations of the ventricular wall.

The semilunar valves (aortic and pulmonary) form from three swellings of cardiac jelly in the truncus arteriosus that become excavated — creating the three cusps of each valve — as the conotruncal ridges spiral and partition the truncus into the aorta and pulmonary trunk.

## Conotruncal Development

The outflow tract of the heart (conus cordis and truncus arteriosus) must be partitioned and realigned so that the aorta connects to the left ventricle and the pulmonary trunk connects to the right ventricle. This is achieved by the conotruncal ridges — paired masses of cardiac jelly and neural crest cells that grow into the lumen of the outflow tract and spiral around each other as they fuse, creating a helical (spiral) septum that divides the truncus into the aorta (left and posterior) and pulmonary trunk (right and anterior). Neural crest cells are essential contributors to this process; disruption of cardiac neural crest migration (as in DiGeorge syndrome, from 22q11.2 deletion) produces conotruncal defects including tetralogy of Fallot, interrupted aortic arch, and persistent truncus arteriosus.

## Common Congenital Heart Defects

Congenital heart defects are the most common birth defect. Ventricular septal defect (VSD) accounts for ~30% of all CHD; small VSDs may close spontaneously, while large ones cause pulmonary overcirculation and heart failure. Atrial septal defect (ASD) — most commonly in the position of the foramen ovale (secundum ASD) — allows left-to-right shunting and, if large, causes right heart dilatation and atrial arrhythmias. Patent ductus arteriosus (PDA) — persistence of the fetal vascular connection between the pulmonary artery and aorta — is common in premature infants and can be closed medically with indomethacin (which reduces prostaglandin E2, needed to keep the ductus open) or surgically. Tetralogy of Fallot — the most common cyanotic CHD — comprises VSD, overriding aorta, pulmonary stenosis, and right ventricular hypertrophy; it results from anterosuperior displacement of the infundibular septum and is associated with neural crest cell abnormalities. Transposition of the great arteries — the aorta arising from the right ventricle and the pulmonary trunk from the left ventricle — results from failure of the conotruncal spiral, producing parallel rather than crossing circulations incompatible with postnatal life without intervention.