1. From the cardiogenic fields to the heart tube

The heart is the first organ to acquire coordinated function, but it does not arise as a miniature four-chambered structure. Cardiogenic mesoderm forms bilateral fields that merge in the midline as the embryo folds. The resulting tube contains an endocardial lining, an intervening extracellular matrix traditionally called cardiac jelly, and an external myocardial mantle. The tube is initially continuous with paired venous inflows at its caudal pole and with the aortic sac and pharyngeal arch arteries at its cranial pole [1,2].

The early tube is not composed of fixed segments that simply enlarge into adult chambers. Cardiomyocytes and progenitor cells are added progressively at both poles, particularly from the second heart field. Addition at the arterial pole lengthens the outflow tract and contributes to the right ventricle and intrapericardial arterial trunks; addition and remodelling at the venous pole contribute to the atrial component and systemic venous junctions. Adult topology is therefore produced by growth, addition, bending, differential expansion, and tissue transformation rather than by straight enlargement of a linear template [1-3].

Human development is best described by Carnegie stage and post-conceptional age rather than by a rigid day-by-day schedule. Around Carnegie stage 10, approximately four weeks after conception, the heart has a solitary lumen. By stages 11 and 12, the tube has lengthened, bent, and begun to show the ventricular loop and atrial component. Three-dimensional reconstructions of human embryos are essential because a single histological plane can make structures that are continuous appear separate, or can conceal the oblique relationships that determine later chamber alignment [1,2].

2. Looping and formation of the ventricular chambers

The ventricular loop forms as the growing tube bends and changes its relationships within the pericardial cavity. The inlet and outlet are not initially aligned as in the postnatal heart. The inlet component of the loop provides the primordium of the left ventricle, whereas the apical right ventricle expands from the outlet component. Trabeculations appear along the outer curvature as the apical ventricular components expand. This chamber formation has been described as ballooning, but the term denotes regional expansion of the tube rather than inflation of pre-existing cavities [1,2].

The inner curvature remains relatively unexpanded and later contributes to the ventricular inlets, the subaortic outlet, and the base of the heart. The right ventricle does not obtain its inlet by transfer of an entire chamber segment. Instead, rightward expansion of the atrioventricular canal establishes direct continuity between the developing right atrium and right ventricle. At Carnegie stage 13, the atrioventricular canal still empties predominantly into the left ventricle; by stage 14, expansion of its rightward margin has created the right atrioventricular connection [1,2,4].

The adult right and left ventricles thus reflect different contributions from the loop. Their trabecular patterns, inlet components, apical portions, and outflow regions are not interchangeable. The muscular ventricular septum develops between the expanding ventricular chambers, with its apical trabecular part forming before closure of the embryonic interventricular communication. Development explains why the definitive septum is curved and why the ventricular cavities spiral relative to one another in attitudinal orientation [1,2,5].

3. Development of the atrioventricular junctions

The initial atrioventricular canal is supported by myocardium and contains superior and inferior cushions. These mesenchymal structures approach and fuse while the canal expands rightward. Fusion helps separate the right and left atrioventricular valvar orifices, but the adult junctions are not created by cushion fusion alone. Myocardial remodelling, formation of the atrial vestibules, development of the valve leaflets, and establishment of the fibrous insulating planes all contribute [4,6].

The right and left atrioventricular junctions are unions between atrial and ventricular myocardium, interrupted in the normal heart by variably developed fibrous tissue. They should not be represented as two uniform fibrous rings. The mural leaflets acquire their hinge lines within the developing junctions, while the inferior atrioventricular cushion contributes to the septal components and to the fibrous continuity between the atrioventricular valves. The distinctive offset between the tricuspid and mitral hinges emerges as the junctions remodel around the ventricular septal crest [4,6,7].

Failure of normal cushion, vestibular, or septal remodelling can produce a common atrioventricular junction or abnormal valvar attachments. Such malformations also alter the position of the atrioventricular conduction axis. Developmental anatomy is therefore clinically useful only when it predicts the morphology of the malformed heart; embryological labels should not replace description of the junctions and valves actually present.

4. Systemic venous return and formation of the pulmonary vein

The early systemic venous tributaries enter paired right and left sinus horns. Remodelling of the cardinal, vitelline, and umbilical systems shifts systemic venous return toward the right. The right sinus horn becomes incorporated into the right atrial venous component, contributing the smooth-walled region between the caval veins. The left sinus horn retains its own walls and becomes the coronary sinus and the vein of Marshall. The terminal crest marks the internal boundary between the incorporated venous component and the trabeculated appendage [1,2].

The pulmonary vein develops as a channel in the dorsal mesenchyme and becomes connected to the developing left atrium. Progressive incorporation of the common pulmonary venous component produces separate venoatrial junctions, while the left atrial appendage retains the trabeculated myocardium derived from the original atrial chamber. This distinction explains the marked architectural difference between the smooth posterior left atrial wall and the pectinated appendage [1,2].

The systemic and pulmonary venous poles remain important in electrophysiology. Their myocardial sleeves and junctional architecture are products of incorporation and remodelling rather than extensions added to a finished atrium. Development also explains normal variants such as a persistent left superior caval vein draining to an enlarged coronary sinus and provides the framework for interpreting anomalous pulmonary venous connections.

5. Atrial septation and the oval fossa

Atrial septation is produced by the convergence of myocardial and mesenchymal components. The primary atrial septum grows from the atrial roof to the left of the systemic venous sinus. It carries a mesenchymal cap on its leading edge, which approaches the atrioventricular cushions and the vestibular spine. Fusion closes the primary atrial foramen, while perforations in the upper part of the primary septum coalesce to maintain the secondary foramen [1,8].

The structure often called the septum secundum is largely an infolding of the atrial roof rather than a second parallel muscular sheet. Its inferior rim, together with muscularised mesenchymal tissues derived from the cap and vestibular spine, forms the rims of the oval fossa. The primary septum becomes the flap valve. After birth, increased left atrial pressure apposes this flap to the surrounding rim. Anatomical fusion is incomplete in a substantial minority of individuals, producing a probe-patent oval foramen without loss of atrial septal tissue [8].

Only the flap valve and its immediate muscular rim form the true interatrial septum, because only these tissues can be removed without exiting the heart. Defects in the flap valve are true atrial septal defects. Sinus venosus and coronary sinus communications lie outside the confines of the oval fossa and should be described as interatrial communications. This developmental distinction is directly relevant to transseptal puncture and device closure [8].

6. Ventricular septation and commitment of the aortic root

The primary interventricular communication initially connects the inlet of the ventricular loop with its outlet. Once the right atrioventricular junction forms, the persisting communication becomes the outlet of the developing left ventricle and can be termed the secondary interventricular communication. It is bounded by the muscular ventricular septum and the inner curvature and remains open while the aortic root is still supported predominantly above the right ventricle [1,2,5].

Closure depends on remodelling of the proximal outflow cushions and their myocardialisation. This process builds a shelf beneath the aortic root and creates the subaortic outflow tract, committing the aorta to the left ventricle. The membranous septum is formed as fibrous tissues complete the closure at the junction of the ventricular septal crest, atrioventricular cushions, and aortic root. Its atrioventricular and interventricular components are defined later by the hinge of the septal tricuspid leaflet [1,5,7].

The mechanism explains why perimembranous ventricular septal defects occupy the region beside the central fibrous body and conduction axis. It also explains why a ventricular septal defect should be classified by its borders and relationships rather than by an assumed embryological failure. Comparable holes can have different margins and therefore different risks to the conduction tissues.

7. Outflow tract separation and formation of the arterial roots

The early outflow tract has myocardial walls and a single lumen. Addition of second-heart-field cells lengthens the tract, while neural-crest-derived and other mesenchymal populations contribute to its separation and to the walls of the intrapericardial arterial trunks. The distal outflow is divided by remodelling of the aortic sac and fusion of the aortopulmonary septum with the distal outflow cushions. Proximal fusion of the cushions separates the subaortic and subpulmonary channels [1,2].

The arterial roots form within the middle part of the outflow tract. The distal margins of the major cushions and the intercalated valvar swellings excavate to form the three leaflets of each arterial valve. Their semilunar hinge lines create crown-shaped roots; they are not supported by simple circular annuli. As distal myocardium regresses and non-myocardial tissues form the arterial walls, the pulmonary and aortic trunks acquire their definitive intrapericardial relationships [1,12].

The aortic root becomes wedged between the mitral valve and the ventricular septum as it is committed to the left ventricle. This remodelling creates aortic-mitral fibrous continuity, the inferoseptal recess, and the relationships of the membranous septum and atrioventricular conduction axis. Abnormal rotation, separation, or ventricular commitment can produce discordant ventriculoarterial connections, common arterial trunk, or double-outlet patterns. Again, the final anatomy should be described directly rather than inferred solely from a proposed developmental mechanism.

8. Formation of the atrioventricular valves and ventricular walls

The atrioventricular valve leaflets arise through remodelling of cushion-derived mesenchyme together with adjacent ventricular and junctional tissues. Delamination and excavation create mobile leaflets while preserving attachments to papillary muscles through the tendinous cords. The mural leaflets are related to the atrial vestibules; the septal tricuspid leaflet acquires attachments along the septal surface, whereas the anterior mitral leaflet becomes continuous with the fibrous tissues of the aortic root [4,6].

Ventricular walls initially contain prominent trabeculations. Growth of the compact wall and remodelling of trabecular myocardium proceed together; the mature compact and trabecular layers should not be imagined as products of a simple global compaction wave. Papillary muscles and tendinous cords emerge from remodelling of ventricular myocardial and mesenchymal structures. The right ventricular septomarginal trabeculation and moderator band retain the trajectory of the right bundle branch toward the anterior papillary muscle [1,2].

Developmental arrest is often invoked to explain excessive trabeculation, but morphology alone cannot establish a failed embryological process. Normal trabecular complexity varies, and the diagnosis of cardiomyopathy requires integration with ventricular size, function, genetics, family history, and clinical context.

9. Development of the atrioventricular conduction axis

The primordium of the atrioventricular conduction axis is related to the primary interventricular ring, a population of cardiomyocytes surrounding the early interventricular communication. Expansion of the atrioventricular canal bends and repositions this ring. Part remains on the crest of the muscular ventricular septum and provides the ventricular axis, while another component encircles the developing right atrioventricular junction [3,9,10].

The compact atrioventricular node is not positioned definitively until the inferior pyramidal space and atrial septal buttress form. Muscularisation of the vestibular spine and mesenchymal cap establishes atrial septal connections to the node. Fibrous continuity between the atrioventricular valves insulates the penetrating bundle from atrial myocardium. Distally, the axis branches on the ventricular septal crest, giving rise to the left bundle branch and then the right bundle branch [3,9,10].

Remodelling continues during foetal life. Connections between nodal, fascicular, and ventricular cardiomyocytes can persist as normal microscopic variants and may provide substrates for unusual pre-excitation. Development also explains the altered conduction-axis course in atrioventricular septal defect, Ebstein malformation, and other congenital lesions. The axis must therefore be located from the definitive septal and junctional anatomy of each heart, not from a universal diagram [3,9-11].

10. Transition to the postnatal circulation

Before birth, the placenta supplies oxygenated blood and the fetal circulation uses the oval foramen and arterial duct as physiological bypasses. At birth, lung expansion lowers pulmonary vascular resistance, pulmonary venous return raises left atrial pressure, and interruption of placental flow increases systemic resistance. The flap valve of the oval fossa closes functionally against its rim. Constriction of the arterial duct separates the pulmonary and systemic arterial circuits [8].

These changes do not create the four-chambered anatomy; they change pressure, flow, and the functional state of structures already formed. The arterial duct becomes the ligamentum arteriosum, the umbilical vein becomes the ligamentum teres, and the ductus venosus closes. Persistence of a communication should be distinguished from absence of the tissue required to close it. This principle is especially important when interpreting a patent oval foramen versus a true atrial septal defect.

11. Development as a guide to clinical anatomy

Developmental anatomy is most valuable when it explains relationships visible in the postnatal heart: why the right ventricle acquires a separate inlet, why the aortic root is wedged between the mitral valve and septum, why the oval fossa has a flap valve, and why the conduction axis lies beside the membranous septum. It should not be used to substitute hypothetical labels for observed morphology.

Modern three-dimensional datasets from human embryos provide a stronger basis than flat schematic diagrams. They show that several traditional concepts—including a simple downward growth of septa, a uniformly fibrous valvar skeleton, or a single conotruncal septum—do not capture the actual remodelling. For the atlas, developmental sequences should preserve one attitudinal orientation and allow the reader to follow how each adult landmark is produced [1,2].