Chapter 121. The Cardiac Valves and Conducting System - Review of Medical Embryology Book

THE SEMILUNAR VALVES form from 3 valve swellings or ridges of subendocardial connective tissue at the openings of the aorta and pulmonary trunk

The swellings hollow out and reshape themselves to form 3 thin cusps, which are covered by endocardium

THE ATRIOVENTRICULAR VALVES [the tricuspid (right AV valve) and mitral (left AV valve)] develop simultaneously from localized proliferation of subendocardial tissue around the atrioventricular canals

Some muscle remains as the trabeculae carneae of the ventricular walls
Other muscles form the papillary muscles and modify to form the chordae tendineae which connect the ventricular walls with the atrioventricular valves

THE MUSCULAR LAYERS (myocardium) of the atrium and ventricle are initially continuous, and the primitive atrium serves as a pacemaker for the primitive heart until the sinus venosus takes over that function

The sinoatrial (SA) node is originally in the right wall of the sinus venosus, but as it is incorporated with the sinus venosus into the wall of the right atrium, it comes to lie near where the superior vena cava enters the right atrium
After the sinus venosus is incorporated into the right atrium, cells from its left wall, which are found in the base of the interatrial septum just anterior to the opening of the coronary sinus, and cells along the atrioventricular canal make up the atrioventricular (AV) node and the bundle of His
As the 4 heart chambers form, a band of connective tissue grows in from the epicardium and separates the atrial muscle from the ventricular muscle to form the cardiac skeleton. Thus, the conducting system remains as the only pathway from the atria to the ventricles
The sinoatrial (SA) node, the atrioventricular (AV) node, and the atrioventricular bundle of His are soon supplied by external nerves (the parasympathetic vagus and sympathetic fibers). Histologic differentiation of these specialized tissues is continued up to, and after, birth