Chapter 115. Development of The Heart: Formation of The Heart Loop - Review of Medical Embryology Book

Formation of the heart loop: since the cardiac tube grows more rapidly than the pericardial cavity, it must undergo a series of complex foldings to be accommodated

The intrapericardial part consists of the future bulboventricular portion (bulbus cordis plus ventricle), whereas the atrial part and sinus venosus are still paired and lie outside the pericardium in mesenchyme of the septum transversum

The bulbus cordis, ventricle, and atrium appear first (day 22?1). The truncus arteriosus and sinus venosus are seen a day later (day 23?1). The truncus is continuous caudally with the bulbus cordis and cranially with the aortic sac and aortic arches
The sinus venosus (a large venous sinus) receives the umbilical, common cardinal, and vitelline veins from the primitive placenta, body of the embryo proper, and the yolk sac, respectively

During development, the bulboventricular part grows faster than the pericardial cavity, and because its 2 ends are fixed to the surrounding tissue outside the pericardial cavity (arterial end to the branchial arches, venous end to the septum transversum), elongation cannot take place in a longitudinal direction

THE CEPHALIC END OF THE LOOP bends ventral and caudal and slightly to the right. As a result, the bulboventricular sulcus (between bulbus cordis and ventricle) becomes visible on the outside. Internally, it remains narrow as the primary interventricular foramen, with a fold being formed, the bulboventricular fold

Because the bulbus cordis and ventricles grow faster than the other regions, the heart tube bends on itself, forming a U-shaped bulboventricular loop
As the heart bends, the atrium and sinus venosus come to lie dorsal to the bulbus cordis, truncus arteriosus, and ventricl In addition, the sinus venosus at this stage also develops lateral expansions, the right and left sinus horns

AS A SECONDARY SEQUENCE OF BENDING AND TORSION, the atrioventricular junction comes to lie on the left side of the pericardial cavity, while the right side is occupied by the greatly elongated bulbus cordis

The cardiac loop thus consists of a cephalic or ascending limb (the bulbus) and a descending limb formed by the embryonic ventricle
The bulbus cordis is narrow except for its proximal one-third which will form the trabeculated part of the right ventricle. Its midportion, the conus cordis, forms the outflow tracts of both ventricles. Its distal part, the truncus arteriosus, forms the roots and proximal parts of the aorta and pulmonary artery

WHILE THE CARDIAC LOOP IS FORMING, changes occur throughout the length of the tube

The atrial part, a paired structure outside the pericardial cavity, forms a common atrium by fusion of the right and left sides. During fusion, it is incorporated into the pericardial cavity and moves in a dorsocranial direction. The atrioventricular junction assumes a cranial position and forms the atrioventricular canal, connecting the left side of the common atrium with the embryonic ventricle

WHEN LOOP FORMATION ENDS, the smooth-walled heart tube forms primitive trabeculae in 2 distinct areas: just proximal and distal to the primary interventricular foramen. The atrial part and other parts of the bulbus remain temporarily smooth-walled
THE PRIMITIVE VENTRICLE, now trabeculated, is called the primitive left ventricle since it will form the major part of that definitive structure. The trabeculated proximal third of the bulbus cordis is called the primitive right ventricle
THE CONUS-TRUNCUS PART of the heart tube, initially on the right side of the pericardial cavity, shifts to a medial position as a result of the formation of 2 transverse dilatations of the atrium, which bulge on each side of the bulbus cordis

As a result, the truncus is found in a depression between the right and left atria, and the conus takes an oblique position between the roof of the primitive left ventricle and the anteromedial wall of the atrium