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Introduction Normal Cardiac Development Part 1 Normal Cardiac Development Part 2 Septal Defects Right Heart Lesions Left Heart Lesions Anomalies of Arteries and Veins Cardiac Transplantation References
Differentiation and Septation Development of the Arteries and the Aortic Arch Coronary Vasculature Conduction System Fetal and Postnatal Circulation Cardiac Maturation Normal Anatomy and its Relationships at Birth

Differentiation and Septation

The transition from the linear heart tube through looping and accretion to a chambered organ requires proper alignment of the looped tube and septation. During weeks 5 to 8 in human development, endocardial cushion tissues in superior (dorsal) and inferior (ventral) regions fuse in the midline, dividing the common atrioventricular canal into right and left atrioventricular orifices. Endocardial cells from both the atrioventricular canal and the outflow tract migrate into the cardiac jelly to form bulges and cushions. The sulcus atrioventricularis forms an indentation between the atrium and ventricle segments. Eventually, the ventricular septum, endocardial cushion tissue from the proximal outflow tract, and the atrioventricular cushion merge into each other during the process of septation.

Atrioventricular valve formation occurs due to the growth of atria and apical inlet portion of the ventricles as well as the lagging of the atrioventricular canal region. Note the importance of neural crest cell influence for this developmental process. The sulcus tissue invaginates into the ventricular cavity; the tissue layer consists of atrial, ventricular, and sulcus tissue at the tip. The inlet portion of the ventricles forms tethering cords (chordae tendinae), which hold the valve leaflets. The sulcus tissue comes in contact with endocardial cushion tissue at the tip of the valve, interrupting muscular continuity between atria and ventricle.

The valve developing between the right atrium and ventricle consists of three leaflets, named the tricuspid valve. The valve on the left side of the heart develops only two leaflets and is called the mitral valve.

Atrial chambers

Formation and septation of the atria

During weeks 4 and 5, the right and left atria undergo remodeling. The bilateral sinus horns fuse to form the transverse sinus venosus, which then shifts from a midline position to drain exclusively into the right atrium through a slit-like orifice. The orifice from the sinus venosus is incorporated into the wall of the right atrium to form the sinus venarum, creating the smooth walled portion of the posterior atrial chamber. The smooth sinus venarum is clearly distinguishable from the atrial appendage (auricle) and the remainder of the right atrium, which has pectinate trabeculations that are functionally contractile. A ridge of tissue called the crista terminalis demarcates the line between the sinus venarum and the remainder of right atrium. This tissue contains fibers that will later carry impulses from the sinoatrial node to the atrioventricular node, allowing for the spread of depolarizing electrical current and contraction of the heart. The orifice continues to enlarge to allow the inferior vena cava and coronary sinus to drain into the right atrium. Tissue flaps develop on the right and left sides of these orifices to form the venous valve opening to the inferior vena cava and coronary sinus.

A single pulmonary vein originates from the dorsal mesocardium and initially lies in the midline, connecting the future lungs to the dorsal aspect of the common atrium. This pulmonary vein shifts toward the left and commits itself to the definitive left atrium. The single large orifice is later divided into the four orifices of the four pulmonary veins, then bifurcating twice to form the two right and two left pulmonary veins. The first two pulmonary vein branches are incorporated into the left atrium, forming the smooth walled region of the left atrium. The trabeculated and contractile portion of left atrium is shifted toward the left and ventrally forms the appendage (auricle).

Atrial septation

Between days 28 and 37 the major events in heart septation occur via differential growth and remodeling (Larsen). The process of atrial septation is characterized by creating a series of shunts, ducts, and foramen. On day 26 of human development approximately, atrial septation begins when the bulbus cordis and truncus arteriosus push on the superior and exterior aspect of the common atrium, creating a depression along the midline.

Complete septation requires formation of 2 separate structures, the septum primum and the septum secundum. On day 28, a sickle-shaped tissue wedge called septum primum begins to grow from the posterosuperior wall, extending toward the atrioventricular canal and growing up to meet and contribute to septal formation. Cells originate from the dorsal mesocardium at the inferior pole.

A foramen in the septum primum, called the ostium primum, is located near the atrioventricular septum and leaves a connection between the right and left atria. This right-to-left shunt, acting as a one-way flutter valve, allows oxygenated blood to bypass the lungs and go directly from the right atrium into the left atrium, since the pulmonary system is not used prior to birth. While the septum primum forms and lengthens, an additional sickle-shaped and thicker structure, the septum secundum, develops from the anterosuperior wall of right ventricle. Programmed cell death near the superior edge of the septum primum creates a new foramen, the ostium secundum. The septum secundum grows toward the atrioventricular region, overlapping the ostium secundum. However, growth stops before reaching the atrioventricular septum leaving a hole near the floor of the right atrium, referred to as foramen ovale. The septum itself fuses with the atrioventricular cushions.

After birth, there is normally no shunting from the left atrium to the right. Decreased blood flow from the placenta and increased pulmonary blood flow, as well as an increased pulmonary venous return to left heart, cause the pressure in the left atrium to be higher than in the right atrium. The increased left atrial pressure then closes the foramen ovale against the septum secundum. The septum primum collapses back against the thicker septum secundum. With the first breath, the output from the right ventricle now flows entirely into the pulmonary circulation.

Realignment of primitive chambers

Cardiac looping leaves a direct pathway between the primitive atrium and future left ventricle via the atrioventricular canal. A further connection consists between the future right ventricle and outflow tract that later forms the aorta and pulmonary trunk. Realignment between the atrioventricular canals, ventricles, and cardiac outflow tract must occur before final septation of the atria, ventricles, and the outflow tract. The development of valves, coronary vasculature, and the conduction system occur afterwards. During the process of myocardialization, dorsal endocardial cushion tissue is replaced by invading myocardial cells and the outflow tract shifts toward the left, eventually lying over the atrioventricular canal. The dorsal and ventral endocardial cushion tissue simultaneously grows to divide the atrioventricular canal into right and left orifices which can then align with the right and left atria and ventricles, respectively.

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