Development of the inlet portion of the right ventricle in the embryonic rat heart: the basis for tricuspid valve development.

BACKGROUND

Before septation the entire atrioventricular canal is connected with the ventricular inlet segment (primitive left ventricle) whereas the mature heart exhibits an exclusive connection of the right atrium to the right ventricle. The process which is responsible for this change is controversial.

METHODS

Graphic reconstructions of serially sectioned embryonic rat hearts as well as scanning electron micrographs of similar specimens were made.

RESULTS

The first indication of a right atrioventricular connection was seen as a groove in the atrioventricular junctional myocardium to the right of the inferior endocardial cushion. This groove expanded to form the right ventricular inlet portion. The right, inferior, and superior walls of this newly formed cavity were formed from junctional myocardium, which demarcated it from the trabeculated right ventricular portion in all developmental stages. The left wall equally developed from this junctional myocardium and formed the ventricular inlet septum. The junctional myocardium between right ventricular inlet and trabeculated portions was seen to develop into the tricuspid valve and its tension apparatus.

CONCLUSIONS

The preseptation embryonic heart has no inlet portion to the right ventricle. This new cavity is created by remodelling of atrioventricular junctional myocardium. This myocardium also provides the material contribution to the tricuspid valve and its tension apparatus. Malformations of the right ventricular inlet portion and of the tricuspid valve are indissolubly linked.