Heart
General
Embryology

Author: R. Amita, M.D.(see Authors page)

Revised: 7 February 2016, last major update February 2016

Copyright: (c) 2015-2016, PathologyOutlines.com, Inc.

PubMed Search: Embryology [title] heart
Cite this page: Embryology. PathologyOutlines.com website. http://pathologyoutlines.com/topic/heartembryology.html. Accessed February 20th, 2017.
Definition / General
  • Heart develops from the embryonic mesodermal germ layer cells that differentiates into mesothelium, endothelium and myocardium
  • Mesothelial pericardium forms the outer lining of the heart
  • The inner lining of the heart develops from endothelium

Embryonic heart:
  • On either side of the neural plate, a horseshoe shaped area develops as the cardiogenic region
  • This is formed from cardiac myoblasts and blood islands
  • By day 19, an endocardial tube begins to develop in each side of this region
  • The two endocardial merge to form the tubular heart, also called the primitive heart tube, that loops and septates into the four chambers and paired arterial trunks that form the adult, human heart
  • The tubular heart quickly differentiates into the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium and the sinus venosus
  • The truncus arteriosus splits into the ascending aorta and pulmonary artery
  • The bulbus cordis forms part of the ventricles
  • The sinus venosus connects to the fetal circulation

Cardiac looping:
  • As the heart tube loops, the cephalic end of the heart tube bends ventrally, caudally and slightly to the right
  • The bulboventricular sulcus becomes visible from the outside, and from the inside a primitive interventricular foramen forms
  • The internal fold formed by the bulboventricular sulcus is known as the bulboventricular fold
  • The bulboventricular segment of the heart is now U shaped
  • The bulbus cordis forms the right arm of the U shaped heart tube and the primitive ventricle forms the left arm
  • This looping of the bulboventricular segment causes the atrium and sinus venosus to become dorsal to the heart loop
  • At this stage, the paired sinus venosus extends laterally and gives rise to the sinus horns

  • The following veins drain into the sinus venosus on each side:
    • The common cardinal vein, which drains from the anterior cardinal vein (draining the cranial part of the embryo)
    • The posterior cardinal vein (draining the caudal part of the embryo)
    • The umbilical vein (connecting the heart to the primitive placenta)
    • The vitelline vein (draining the yolk sac, gastrointestinal system and the portal circulation)

  • As the cardiac looping progresses, the paired atria form a common chamber and move into the pericardial sac
  • The atrium now occupies a more dorsal and cranial position and the common atrioventricular junction becomes the atrioventricular canal, connecting the left side of the common atrium to the primitive ventricle
  • The primitive ventricle will develop into the left ventricle and the proximal portion of the bulbus cordis will form the right ventricle
  • The distal part of the bulbus cordis will form the outflow tract of both ventricles, and the truncus arteriosus will form the roots of both great vessels

Atrioventricular canal:
  • Atrioventricular valves form during the 5 to 8 week of development
  • Initially, endocardial cushion tissue forms bulges at the atrioventricular junction
  • These two masses will meet in the middle, thus dividing the common atrioventricular canal into right and left atrioventricular orifices
  • The formation of the atrioventricular valve starts when the atria and inlet portion of the ventricle enlarge, but the atrioventricular junction (or canal) lags behind
  • This process causes the sulcus tissue to invaginate into the ventricular cavity, forming a hanging flap
  • The endocardial cushion tissue is located at the tip of this flap, which is formed from three layers—the outer layer from atrial tissue, the inner layer from ventricular tissue and the middle layer from invaginated sulcus tissue

Atria and atrial septum:
  • The trabeculated portions (appendages) of the right and left atria are from the primitive atria, whereas the smooth walled posterior portions of the left and right atria originate from the incorporation of venous blood vessels
  • The posterior aspect of the left atrium is formed by the incorporation of the pulmonary veins, but the posterior smooth portion of the right atrium is derived from the sinus venosus
  • The two sinus horns are initially paired structures; later, they fuse to give a transverse sinus venosus
  • The sinoatrial junction will become guarded by two valve-like structures, resulting from the invagination of the atrial wall at the right and left sinoatrial junction
  • The right and left sinoatrial valves join at the top, forming the septum spurium
  • Atrial septation starts when the common atrium becomes indented externally by the bulbus cordis and truncus arteriosus
  • This indentation will correspond internally with a thin sickle shaped membrane developing in the common atrium on day 35, which divides the atrium into right and left chambers, called the septum primum
  • The orifice connecting the two atria is called the ostium primum
  • As the superior and inferior endocardial cushions fuse, dividing the atrioventricular canal into a right and left orifice, the concave lower edge of the septum primum fuses with it, obliterating the ostium primum
  • However, just before this happens, fenestrations appear in the posterosuperior part of the septum forming the ostium secundum, thus maintaining a communication between the two atria

Ventricles

    Muscular interventricular septum:
    • On approximately day 30, a muscular fold extending from the anterior wall of the ventricles to the floor appears at the middle of the ventricle near the apex and grows toward the atrioventricular valves with a concave ridge
    • In addition, trabeculations from the inlet region coalesce to form a septum, which grows into the ventricular cavity at a slightly different plane than that of the primary septum
    • This is the inlet interventricular septum, which is in the same plane of that of the atrial septum
    • The point of contact between these two septa will cause the edge of the primary septum to protrude slightly into the right ventricular cavity, forming the trabecular septomarginalis
    • The fusion of these two septa forms the bulk of the muscular interventricular septum

    Cardiac outflow tract:
    • In 1942, Kramer suggested that there are three embryological areas: the conus, the truncus, and the pulmonary arterial segments
    • The aortopulmonary septum is formed by ridges separating the fourth (future aortic arch) and the sixth (future pulmonary arteries) aortic arches
    • The truncus ridges are formed in the area where the semilunar valves are destined to be formed, thus forming the septum between the ascending aorta and the main pulmonary artery
    • The conus ridges form just below the semilunar valves and from the septation between the right and left ventricular outflow tracts

    Systemic venous system:
    • At first, the sinus venosus communicates with the common atrium
    • At 8 weeks, the distal end of the left cardinal vein degenerates, and the more proximal portion forms the common connection between left brachiocephalic vein and right anterior cardinal vein or right brachiocephalic vein, thus forming the superior vena cava
    • The left posterior cardinal vein also degenerates, and the left sinus horn becomes the coronary sinus
    • The right vitelline vein becomes the inferior vena cava, and the right posterior cardinal vein becomes the azygos vein
    • The left umbilical vein degenerates and the right umbilical vein connects to the vitelline system through the ductus venosus
Diagrams / Tables

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Development of heart

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Formation of AV canal

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Formation of atrial septum

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Development of systemic venous system

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Additional References