Atrioventricular Canal and Valve Development

Atrioventricular Canal and Valve Development refers to the series of morphogenetic events and cellular interactions that give rise to the atrioventricular (AV) canal, the partitioning of this canal, and the eventual formation and remodeling of the mitral and tricuspid heart valves. This complex developmental sequence ensures proper separation and function of the heart’s right and left sides, establishing unidirectional blood flow between the atria and ventricles.

Embryonic Origins of the Atrioventricular Canal

The primitive heart tube, formed during early embryogenesis, initially consists of a single atrioventricular canal connecting the common atrium to the common ventricle. This canal is the initial pathway for blood flow between the upper and lower chambers of the developing heart. The walls of the primitive AV canal are lined with endocardial cells, beneath which a layer of cardiac jelly supports subsequent morphogenesis.

Key features at this stage include:

• A single, undivided AV canal.
• Endocardial lining and abundant extracellular matrix (cardiac jelly).
• Myocardial and endocardial layers surrounding the canal.

Formation and Fusion of Endocardial Cushions

Superior and Inferior Endocardial Cushions

Around the fourth week of human development, localized thickenings of the extracellular matrix, termed endocardial cushions, appear on the dorsal (superior) and ventral (inferior) walls of the AV canal. These cushions are populated by mesenchymal cells derived from endothelial-to-mesenchymal transition (EndMT) of endocardial cells.

These cushions grow toward one another and eventually fuse, partitioning the AV canal into right and left channels.

Endocardial Cushion Fusion

Fusion of the superior and inferior endocardial cushions begins centrally and proceeds to seal the canal into two distinct orifices. This process is crucial for preventing abnormal communications (such as AV septal defects) between the atria and ventricles.

Partitioning of the Right and Left AV Canals

Once the endocardial cushions have fused, the AV canal is divided into right and left channels. Additional small lateral cushions contribute to further refinement of the orifices. The right AV canal will become the site of the tricuspid valve, while the left AV canal gives rise to the mitral valve.

Key steps in partitioning:

• Lateral cushions support further separation.
• Muscular and fibrous tissues remodel the region to form definitive AV orifices.
• Correct alignment with the atrial and ventricular septa ensures precise separation of blood flow.

Valve Leaflet Formation and Delamination

AV Leaflet Delamination

The valve leaflets arise from the endocardial cushions through a process called delamination, where specific regions of the cushion tissue separate from the ventricular wall and thin into flexible leaflets. This transformation involves cellular proliferation, apoptosis, and remodeling of the extracellular matrix.

Remodeling of the Tricuspid and Mitral Valves

Tricuspid Leaflet Remodeling

The right AV canal forms the tricuspid valve, composed of three leaflets (anterior, posterior, and septal). As development proceeds, the leaflets undergo further thinning, elongation, and sculpting, ultimately attaching to the right ventricular wall via chordae tendineae.

Mitral Leaflet Remodeling

The left AV canal forms the mitral valve, which has two primary leaflets (anterior and posterior). These leaflets are similarly remodeled through delamination, thinning, and matrix reorganization, ensuring a tight, flexible seal during ventricular contraction.

Chordal Apparatus and Papillary Muscle Development

The leaflets of both AV valves are anchored by chordae tendineae, which are fibrous cords that connect their free edges to the papillary muscles. These structures originate from muscular trabeculations in the ventricular wall, which transform into fibrous structures as development progresses. The papillary muscles themselves differentiate from the inner myocardial layer and provide the necessary tension to keep the valves closed during systole.

Adult Derivatives of the AV Junction

After birth, the fully developed AV canal and valve structures maintain the separation between the right and left sides of the heart. The tricuspid valve (right AV) and mitral valve (left AV) ensure unidirectional flow from the atria to the ventricles, preventing backflow during systole. The fibrous skeleton of the heart, formed from the remodeled endocardial cushions and surrounding connective tissue, provides structural support and electrical insulation between atria and ventricles.

Clinical Correlations

Defects in AV canal and valve development can result in a spectrum of congenital heart diseases:

• AV septal defects (endocardial cushion defects)
• Mitral or tricuspid valve malformations (e.g., cleft, stenosis, atresia)
• Abnormalities of chordae or papillary muscles contributing to regurgitation

These anomalies can compromise cardiac function and often require surgical intervention.

Summary

Atrioventricular canal and valve development is a multi-step process beginning with the formation of the primitive AV canal, followed by cushion formation and fusion, canal partitioning, leaflet delamination, and remodeling into mature valve structures. The resulting valves and supporting apparatus are crucial for proper cardiac function and efficient blood flow.