Atrioventricular valves (embryology)

The atrioventricular valves of the heart develop in the region of the atrioventricular orifice by:

• the growth of subendocardial mesenchymal tissue that is contiguous with the lateral and inferior endocardial cushions
• the action of the flow of blood on the ventricular surface:
  • it acts to 'hollow out' the mesenchyme so that just flaps - early valve leaflets - are left in contact with the atrioventricular orifices
  • the edges of the leaflets are initially connected to the ventricular wall by muscular fibres, but gradually the muscle becomes replaced from the valve edges by connective tissue - this leads to the adult arrangement in which papillary muscles are connected to the valves by the fibrous chordae tendineae.

This process occurs on both sides of the dividing ventricle to produce three leaflets for the tricuspid valve on the right side and two leaflets for the mitral valve on the left side.