The tricuspid and mitral valves (atrioventricular valves)
Very early in cardiac development the atria, which have formed in the rear upper part, are separated from the ventricles by the sulcus atrio-ventricularis.
In the interior of the heart, endocardial cushions form that grow first both dorsally and ventrally into the lumen.
Somewhat later, these two cushions fuse in the middle of the lumen (stage 18) and thereby divide the av-canal into right and left openings.
- Right ventricle
- Ventral av-septum
- Left ventricle
- Right av-canal
- Dorsal av-septum
- Septum interventriculare
- Left av-canal
The lower part of the heart has been cut away and one sees the av-plane from below.
The dorsal cushion has approached the ventral one.<br>Left and right av-canals exist already, however. They create the connections of the left and right atria to the left and right ventricles, respectively.
- Right ventricle
- Ventral av-septum
- Left ventricle
- Right av-canal
- Dorsal av-septum
- Septum interventriculare
- Left av-canal
The two cushions come closer and closer. The posterior process of the interventricular septum (IVS) connects up with the dorsal av-septum. Both av-canals can be easily seen.
- Right ventricle
- Ventral av-septum
- Left ventricle
- Right av-canal
- Dorsal av-septum
- Septum interventriculare
- Left av-canal
The two cushions now lie against each other, fuse and thus definitively separate the atria from the ventricles.
- Right ventricle
- Ventral av-septum
- Left ventricle
- Right av-canal
- Dorsal av-septum
- Septum interventriculare
- Left av-canal
In this stage one no longer sees where the fusion took place. The passage between the right and left ventricles is now the foramen interventriculare (pars membranacea).
This part will soon be closed off by the conus septum.
One can first rightly understand the relationships at the valve level when one takes into consideration all the septa that contribute to the separation of the cardiac chambers in this region. This is done best using a section of a ventricle.
In the cushion area the cardiac jelly looks like a thick membrane basale that protrudes into the lumen and thus fulfills a primitive valve-like function.
In this region, induced by molecules that are given off by the myocardium cells leave the endocardium, assume a mesenchymal form and migrate into the cardiac jelly. It can be experimentally shown that only cells in the atrioventricular region and near the outflow tract can react in this way to an induction of the myocardium.