Intermediate - Primordial Heart Tube

From Embryology
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Begin Intermediate: Primordial Heart Tube  Heart Tube Looping  Atrial Ventricular Septation  Outflow Tract  Heart Valves  Cardiac Abnormalities  Vascular Overview


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The heart primordium arises predominantly from splanchnic mesoderm in the cardiogenic region of the trilaminar embryo. The cardiogenic region can be thought of as bilateral fields that merge cranially to form a horseshoe-shaped field. During the third week of development (approximately day 18) angioblastic cords develop in this cardiogenic mesoderm and canalise to form bilateral endocardial heart tubes.

Dorsal view of 18 day embryo
Lateral view of 18 day embryo

Lateral folding of the embryo brings the heart tubes into the ventral midline, allowing them to fuse to form a single primordial heart tube. Fusion of the heart tubes begins cranially and extends caudally and is facilitated by apoptosis. The animation below shows a cross section of the embryo and the development of the endocardial heart tubes as well as their migration and fusion in the midline.


Brightfield images showing fusion of the heart tubes and the earliest appearance of segments of the fused heart tube
Early divisions of the heart tube


After fusion, constrictions and dilations appear in the heart tube, forming the following divisions (listed from cranial to caudal position):

  • Truncus arteriosus
  • Bulbus cordis
  • Primordial ventricle
  • Primordial atrium
  • Sinus venosus

The sinus venosus is also divided into two parts: the right horn of the sinus venosus and the left horn of the sinus venosus.


By day 22, coordinated contractions of the heart tube are present and push blood cranially from the sinus venosus.


In the previous animation you saw that the dorsal aortae develop concurrently with the endocardial heart tubes and form a cranial connection with the endocardial heart tubes prior to folding. As the embryo folds, the cranial ends of the dorsal aortae are pulled ventrally until they form a dorsoventral loop: the first aortic arch arteries. The embryonic vascular system is discussed in further detail here.

Heart Layers

Myocardium: forms from splanchnic mesoderm surrounding the pericardial coelom. Additional myocardial cells are added to the outflow tract during heart looping.

Cardiac Jelly: gelatinous connective tissue separating the myocardium and heart tube endothelium.

Endocardium: forms from the endothelium of the heart tube.

Epicardium: develops from mesothelial cells arising from the sinus venosus which spread cranially over the myocardium.


Cross-section through the ventricular section of the heart tube


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Heart terms

Angioblastic cords: Groups or ‘columns’ of embryonic precursor cells which will form the walls of both arteries and veins.

Aortic arch arteries: (Or pharyngeal arch arteries.) Each early developing pharyngeal arch contains a lateral pair of arteries arising from the aortic sac, above the heart, and running into the dorsal aorta. Later in development these arch arteries are extensively remodelled to form specific components of the vascular system.

Bulbus cordis: A region of the early developing heart tube forming the common outflow tract, will differentiate to form three regions of the heart.

Cardiac jelly: Term used in early heart development to describe the initial gelatinous or sponge-like connective tissue separating the myocardium and heart tube endothelium.

Cardiogenic region: The area in the embryo where the precursor cells for heart development lie.

Connective tissue: Fibrous tissue that acts to support body structures or bind other forms of tissue.

Dorsal aortae: Two largest arteries either side of the midline which later fuse to form the descending portion of the aorta.

Endocardial heart tubes: Two tubes formed from the cardiogenic plate in the developing embryo. These form the primordium of the truncus arteriosus, the atrium and the ventricles; later invested with myocardium.

Endocardium: The epithelial membrane lining the inside surface of heart, which along with the endothelial layer forms a continuous lining of the entire cardiovascular system. The endocardium, like the majority of the heart is mesoderm in origin.

Endothelium: A simple squamous epithelium lining blood vessels.

Epicardium: The outer layer of heart tissue.

Left horn of sinus venosus: The left side of the sinus venosus (initially symmetrical with the right) collecting blood from half of the paired veins: common cardinal veins, umbilical veins and vitelline veins. Later the left horn diminishes and becomes the small coronary sinus.

Myocardium: The middle layer of the heart wall composed of cardiac muscle.

Pericardial coelom: The anatomical body cavity in which the heart lies. The pericardial cavity forms in the lateral plate mesoderm above the buccopharyngeal membrane, as part of the early intraembryonic coelom. This cavity is initially continuous with the two early pleural cavities. Note the single intraembryonic coelom forms all three major body cavities: pericardial cavity, pleural cavity, peritoneal cavity.

Primordial atrium: Common cavity in the upper portion of the developing heart. Later divides to form the left and right atria.

Primordial ventricle: Common cavity in the lower portion of the developing heart. Later divides to form the left and right ventricles.

Right horn of sinus venosus: The right side of the sinus venosus (initially symmetrical with the left) collecting blood from half of the paired veins: common cardinal veins, umbilical veins and vitelline veins. Later the right horn dilates, receiving all the veins, and becomes the sinus venarum of the right atrium.

Sinus venosus: An early developmental cardiovascular structure, thin walled cavity, forming the input to developing heart which has 3 venous inputs (vitelline vein, umbilical vein, common cardinal vein). Later in heart development this structure gets incorporated into the wall of the future right atrium.

Splanchnic mesoderm: Gastrointestinal tract (endoderm) associated mesoderm formed by the separation of the lateral plate mesoderm into two separate components by a cavity, the intraembryonic coelom. Splanchnic mesoderm is the embryonic origin of the gastrointestinal tract connective tissue, smooth muscle, blood vessels and contribute to organ development (pancreas, spleen, liver). The intraembryonic coelom will form the three major body cavities including the space surrounding the gut, the peritoneal cavity. The other half of the lateral plate mesoderm (somatic mesoderm) is associated with the ectoderm of the body wall.

Truncus arteriosus: An embryological heart outflow structure, that forms in early cardiac development and will later divides into the pulmonary artery and aorta. Term is also used clinically to describe the malformation of the cardiac outflow pattern, where only one artery arises from the heart and forms the aorta and pulmonary artery.