Chapter 117. Atrioventricular and Interatrial Septation and Development - Review of Medical Embryology Book

Introduction: the cardiac tube is folded in the pericardial cavity by day 28 and consists of

THE SINUS VENOSUS, into which enter the vitelline veins, the umbilical veins, and the common cardinal veins
THE ATRIAL REGION, which communicates with the ventricle via the atrioventricular canal
THE VENTRICULAR REGION
THE BULBUS CORDIS, which is a prolongation of the ventricle and is continuous with the truncus arteriosus and gives rise to the aortic roots
PARTITIONING of the atrioventricular canal, the atrium, and the ventricle begins about the middle of week 4 and is essentially complete by the end of week 5 (days 27 to 37) when the embryo grows in length from 5 mm to about 16 or 17 mm. Although described separately, the processes take place concurrently

Atrioventricular canal septation: by day 28, the atrial region forms a large cavity dorsal (behind) to the ventricular region and becomes divided into the right and left atria. Thus, at this time, the ventricle is bounded ventrally by the bulbus cordis and the atria dorsally. The fold between the ventricle and the bulbus rapidly disappears

THE SEPARATION BETWEEN ATRIA AND VENTRICLE increases, resulting in a shrinkage of the atrioventricular canal. On the ventral and dorsal walls of the canal, thickenings of subendocardial tissue now appear, the 2 endocardial cushions, move toward each other, and finally fuse (between days 35 and 40) to form the primitive interventricular septum

By day 40, the atrioventricular canal is divided into right and left atrioventricular canals. The mesenchyme around each canal proliferates and forms the atrioventricular valves (mitral valve at left and tricuspid valve at right)

Primitive interatrial septation begins during week 5. At this stage, one sees a single atrium, common cardinal, vitelline, and umbilical veins

A THIN, SICKLE-SHAPED MEMBRANE, the septum primum, appears on the posterosuperior wall of the primitive atrial chamber and grows toward the endocardial cushions. A large, temporary opening exists between the lower free edge of the septum primum and the endocardial cushions called the foramen primum, which rapidly gets smaller
BEFORE CLOSURE OF THE FORAMEN PRIMUM, small openings or perforations appear in the upper central part of the septum primum, which merge to form another opening, the foramen secundum. At the same time, the free edge of the septum primum fuses with the left side of the fused endocardial cushions to obliterate the foramen primum

Thus, when the foramen (ostium) primum is closed, the foramen (ostium) secundum remains patent and affords free access between the 2 atria

A NEW CRESCENTERIC MEMBRANE appears to the right of the "delicate" septum primum, on the antero-superior wall of the atrium, near the end of week 5. It converges toward the endocardial cushions as the septum secundum

The septum secundum enlarges, covers the foramen secundum in the septum primum, but remains as an incomplete partition which results in an oval-shaped passageway, the foramen ovale, in the interatrial septum directly in the path of the blood coming from the inferior vena cava
The upper portion of the septum primum, which is attached to the roof of the left atrium, gradually disappears, but the rest of the septum becomes the valve of the foramen ovale

COMPLETE FUSION OF THE SEPTUM PRIMUM to the septum secundum forms the definitive interatrial septum, obliterating the foramen ovale

Traces of the former passage are often seen
A depression, the fossa ovalis, is evident on the right side of the interatrial septum
The crest of the septum secundum becomes the limbus of the fossa ovalis