Development of Heart

The heart is an important muscular pump that propels blood through the systemic and pulmonary circulations before and after birth. The requirements of developing tissues for oxygen and nutrients make early development of a functional cardiovascular system necessary. The heart is well formed before the end of the embryonic period. During the fetal period, the foramen ovale in the interatrial septum shunts blood away from the pulmonary circulation into the systemic circulation. After birth, the foramen ovale closes, establishing the adult pattern of blood flow through the heart.

Scheme of Development of Heart. Source: Illustration by Paul E. Neumann, MD

Formation of the Tubular Heart (weeks 3 and 4)

Cardiogenic zone

In the trilaminar embryonic disk, the cardiogenic zone (or plate) is splanchnic (visceral) mesoderm located rostral to the presumptive oropharyngeal membrane, and ventral to the intraembryonic coelom (the part that is the presumptive pericardial cavity). The myocardium forms from the cardiogenic zone; whereas the endothelium (endocardial heart tube) forms from mesenchyme between the cardiogenic zone and the endoderm.

Midsagittal section of trilaminar embryo. 1. Intraembryonic coelom, 2. endocardial heart tube 3. Prechrodal plate, 4. Presumptive clocal membrane, 5. Presumptive septum transversum, 6. Amnion. Source: Wikimedia commons

The cardiovascular system in the trilaminar embryonic disc prior to embryo folding (4th week). The endocardial heart tubes (Blue) are located at the anterior end of the embryonic disc. Gray's Anatomy (1918) figure 458 (originally from Eternod). Source: Wikimedia Commons

Transverse section of rabbit embryo through the pharynx (ph) and heart tubes. The embryo has folded bringing the heart tubes together prior to their fusion. The heart tubes, consisting of the endocardium (ih) and myocardial mantle (ah), are located within the pericardium (p). The dorsal aortas (ao) are located dorsal to the pharynx. Gray's Anatomy (1918) figure 457 (originally from Kölliker). Source: Wikimedia Commons

Endocardial heart tube

While the embryo begins to fold in the fourth week, the endocardial heart tubes form and take on a myocardial mantle as they enter the pericardial cavity. Cardiac jelly, an extracellular matrix secreted by myocardial precursor cells, appears between the endocardium and myocardium.

Formation of tubular heart

During the 4th week, the primitive heart progresses through three phases: plexifom, straight tubular and looped tubular. Heart contractions and cardiac tube segments (sinus venosus, atrium, ventricle, bulbus cordis, and truncus arteriosus) are recognizable in the looped heart at the end of the 4th week.

The straight tubular heart is formed by fusion of the right and left endocardial heart tubes. In this figure the tubular heart is beginning to loop. Gray's Anatomy (1918) figure 461 (from Ecker-Ziegler model). Source: Wikimedia Commons

Looped tubular heart begins to display segments. The first bulges are the ventricle and bulbus cordis. Gray's Anatomy (1918) figure 462 (from model by His). Source: Wikimedia Commons

Conversion of the Tubular Heart into a Four-Chambered Heart (Weeks 5-7)

During the second month of the embryonic period the number of cardiac tube segments increases and the "in series" sequence of the segments changes to an "in parallel" arrangement of left and right chambers. This conversion process involves development of six septae: septum primum, septum secundum, atrioventricular septum (endocardial cushions), muscular and membranous parts of the interventricular septum, and the aortico-pulmonary (conotruncal) septum.

Septum primum and Muscular interventricular septum

During the 5th week, continued looping of the heart drops the ventricles inferior to the atria, circulation of blood becomes unidirectional, and septation of the atria and ventricles begins. Oxygenated blood from the umbilical vein moves through the heart to the aorta and umbilical arteries. The transformation from "in series" to "in parallel" starts with a shift of the atrioventricular canal to the right so that both atria are adjacent to the AV canal, and establishment of separate pulmonary and aortic streams in the conotruncus arising from the right and left ventricles, respectively. The sinus venosus opens exclusively into the right atrium through the sino-atrial orifice. Atrial septum primum and the muscular part of the interventricular septum are formed.

Interior of dorsal half of heart at about 5 weeks showing early septation of the heart: septum primum, muscular interventricular septum ("septum inferius") and endocardial cushions. Gray's Anatomy (1918) figure 465 (from model by His). Source: Wikimedia Commons

Endocardial cushions and Conotruncal ridges

At about 5 weeks, the endocardial cushions of the atrioventricular canal and the conotruncal ridges appear. The endocardial cushions are derived from cardiac jelly and the conotruncal ridges from ectomesenchyme from the cranial neural crest. The endocardial cushions will divide the AV canal and contribute to formation of the AV (mitral and tricuspid) valves. The conotruncal ridges will divide the cardiac outflow tract and contribute to formation of the semilunar (aortic and pulmonary) valves.

Septum secundum and Membranous interventricular septum

In the 6th and 7th week, septation of the atria, AV canal, ventricles and conotruncus continue to complete the form of the fetal heart. In the 6th week, the foramen secundum appears, the foramen primum is obliterated, the AV canal is divided, and the conotruncal septum forms. In the 7th week, the atrial septum secundum and foramen ovale appear and the membranous part of the interventricular septum obliterates the secondary interventricular foramen. The membranous interventricular septum is formed by fusion of the conotruncal ridges and the endocardial cushions.

The heart in the 6th week showing enlargement of the atria and ventricles. The part labelled "bulbus cordis" is best described as conus cordis. Gray's Anatomy (1918) figure 466 (from Ecker-Zeigler model). Source: Wikimedia Commons

Interior of dorsal half of heart in the 6th week. Note the septum secundum and atrioventricular septum ("septum intermedium"). Gray's Anatomy (1918) figure 467 (from model by His). Source: Wikimedia Commons

Interior of right side of the heart in the 6th week, viewed from the right. Note conotruncal ("aortic") septum. Gray's Anatomy (1918) figure 468 (from model by His). Source: Wikimedia Commons

Diagrams showing the division of the atrioventricular canal, and formation of the aorticopulmonary septum and the membranous part of the interventricular septum. Gray's Anatomy (1918) figure 470 (originally from Born). Source: Wikimedia Commons

Blood Flow Through the Fetal Heart

Blood is shunted "right-to-left" around the pulmonary circulation where there is high resistance until the lungs expand at birth. Two shunts exist: the foramen ovale, an intracardiac shunt, and the ductus arteriosus, an extracardiac shunt. The foramen ovale allows passage of blood from the right atrium to the left atrium. The ductus arteriosus shunts blood from the pulmonary artery to the aorta.

Venous return to the right atrium includes oxygenated blood from the inferior vena cava (supplied by the umbilical vein), and a smaller amount of deoxygenated blood from the superior vena cava. About half of the oxygenated stream passes through the foramen ovale, the rest mixes with the deoxygenated blood in the right ventricle.
Most of the outflow into the pulmonary artery is shunted through the ductus arteriosus into the descending aorta, and then on to the trunk, lower limbs, and umbilical arteries. Only a small amount of the right ventricular output goes into the pulmonary circulation.
Venous return to the left atrium adds a modest amount of mixed blood to the oxygenated blood that has passed through the foramen ovale. This well-oxygenated blood passes through the right ventricle into the ascending aorta, chiefly supplying the head and upper limbs.

Fetal circulation showing direction of blood flow and quality of the blood (Red - oxygenated; Blue - deoxygenated; Purple - mixed). Gray's Anatomy (1918) figure 502. Source: Wikimedia Commons

Changes at Birth

Interruption of placental circulation and the beginning of pulmonary respiration cause functional and then structural changes in the prenatal shunts. Expansion of the chest and aeration of the lungs reduces resistance to pulmonary circulation. Increased pressure in the left atrium, caused by increased pulmonary vein blood flow closes the foramen ovale. Probe patency usually lasts for several months, but may still be found in 20-25% of adults. The ductus arteriosus usually closes 1-4 days after birth.

Clarification on terms

Bulbus cordis

The bulbus cordis is the more cranial of the two bulges in the early heart tube. (The caudal bulge is usually called either "the ventricle" or the "primitive left ventricle".)
It is unclear what the bulbus cordis develops into later in development. Several theories exist. The most common being:

• Conus cordis, and the trabeculated part of the right ventricle;
• Conus cordis;
• Conus cordis, trabeculated part of the right ventricle, and truncus arteriosus.
  

During later developmental stages, the term "bulbus cordis" should be avoided because of the ambiguity. Similarly, "conal ridges" is preferable to "bulbar ridges".

Conus cordis

The conus cordis is the part of the outflow tract of the developing heart that develops into the infundibulum (the non-trabeculated part) of the right ventricle and the outflow region of the left ventricle.

Conotruncus

The conotruncus is the outflow region or tract of the developing heart. It consists of the conus cordis and the truncus arteriosis. The conus is the part that will be inferior to the aortic and pulmonary valves, and the truncus is the part superior to the valves that is continuous with the ventral aorta (aortic sac).
The term conotruncal ridges refers to the conal and truncal ridges.

External Links

Human Developmental Anatomy Center (HDAC) of the National Museum of Health & Medicine

• O'Rahilly R & Müller F. Developmental Stages in Humans. Carnegie Institution of Washington, 1987.
  

Embryology.ch - an online course in embryology for medical students

Other Resources

O'Rahilly R and Müller F. Human Embryology & Teratology, 3rd Ed. New York: Wiley-Liss, 2001.