Observations on the development of the human atrioventricular node and bundle

Area of Science:

• Embryology
• Cardiovascular Science
• Developmental Biology

Background:

• The cardiac conduction system, including the atrioventricular (AV) node and bundle, is crucial for coordinated heart function.
• Understanding the embryonic origins of the AV node is essential for explaining congenital heart anomalies.

Purpose of the Study:

• To investigate the developmental origins of the human atrioventricular (AV) node and AV bundle using light microscopy.
• To elucidate the cellular sources and formation process of the AV nodal subdivisions and their relationship with the AV bundle.

Main Methods:

• Light microscopic examination of cardiac tissues from 27 human embryos, fetuses, and postnatal hearts.
• Tracing the development of AV nodal primordia and the AV bundle from early embryonic stages.

Main Results:

• The superficial and deep portions of the postnatal AV node originate from two distinct cellular primordia in the posterior atrial wall at the 6-mm stage.
• A small right primordium forms the superficial AV node, while a larger left primordium forms the deep AV node subdivision.
• The AV bundle, identified at 13 mm, arises from the posterior AV canal and connects to the nodal primordia by 25 mm.

Conclusions:

• The adult AV node typically forms from the fusion of two embryonic nodal subdivisions.
• Failure of fusion between AV nodal primordia or with the AV bundle can explain the presence of dual AV nodal masses and congenital heart block.