Development of the Cardiovascular System
The cardiovascular system consisting of the heart and blood vessels is mostly mesodermal in origin but with significant contributions made by neural crest cells to the development of the heart.
Cardiac development begins in the 3rd week of embryonic life with the onset of gastrulation and migration of epiblast cells into the cranial region of the embryonic disc to form a horseshoe shaped mass of mesoderm called the primary heart field or cardiogenic area.
Primary and secondary heart fields develop under the influence of factors secreted by the endoderm of the .pharynx. Each heart field is subdivided into zones committed to form specific sections of the primitive heart tube. Formation of the heart fields coincides with the establishment of right/left asymmetry further specifying right and left sides of the heart.
Fig. 1: Primary and secondary heart fields form within the right and left sides of the horseshoe shaped cariogenic area. These cells become committed to develop specific components of the right and left sides of the heart. A atrium, LV left ventricle, RV right ventricle, C conus, T truncus).
Mesoderm of the cardiogenic region forms a pair of endocardial tubes that fuse in the midline to form a primitive heart tube, as shown in the animation.
Craniocaudal folding of the embryo in the fourth week displaces the heart tube caudally to its final location within the thoracic cavity.
Fig. 2: Animation showing the horseshoe shaped cariogenic area cranial to the developing forebrain. After the endocardial tubes fuse in the ventral midline, they begin to form the chambers of the primitive heart tube.
Layers of the Heart Tube Wall and their Derivitives
Fig. 3: Diagram of a cross section of the primitive heart tube and the concentric arrangement of its layers and its relationship to the pericardium and pericardial sac.
Initially, the heart tube is suspended within the pericardial sac by a dorsal mesocardium, which later degenerates to create the transverse pericardial sinus.
Developmentally, the heart tube consists of 4 layers:
- endocardium - innermost lining, analogous to tunica intima of blood vessels
- cardiac jelly - forms the cardiac skeleton (annulus fibrosus - attachments for the cusps of the cardiac valves and the associated trigone
- myocardium - layer that forms cardiac muscle, analogous to tunica media of blood vessels
- epicardium - outermost covering of the heart; analogous to the tunica adventitia of blood vessels
Folding of the Heart Tube
Fig. 4: Animation depicting how the primitive heart tube begins to fold to accommodate to the space limits of the pericardial sac, first as a 'U" and then an "S" shaped configuration.
In the animation, the caudal end of the heart tube is at the bottom of the frame and the cranial end at the top of the frame. Observe that the heart folds first as a "U" shape and then as an "S" shape. The yellow arrows indicate the direction of blood flow through the developing heart tube.
As development of the heart proceeds, the hear tube folds in order to accommodate to the space allotted within the pericardial cavity.
By day 23, the heart folds in to a "U"-shaped configuration and by day 24 it is "S"-shaped.
Blood enters the heart at the caudal end and leaves the heart at the cranial end.
Expansion of the ventricles causes the heart to project to the left within the pericardial sac.
Fig. 5: Progressive folding of the heart tube in day 22, 23 and 24. The red arrows indicate the direction of blood flow through the embryonic heart.
Segmentation of the Heart Tube
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Start Slide Show
At day 22 of development, the heart tube is linear and can be subdivided into several regions from caudal to cranial.
- sinus venosus - venous inflow tract
- common atrium
- common ventricle
- bulbus cordis - outflow tract
- primitive right ventricle
- conus cordis
- truncus arteriosus
Day 22
Fig. 6: Blood enters the sinus venous through veins (arrows) and flows through the heart tube in a cranial direction through the common atrium, common ventricle and into bulbus cordis, the outflow track, consisting of right ventricle, conus cordis and truncus arteriosus.
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Day 24
Fig. 7: As the heart begins to fold, the sinus venosus moves posterior to the common atrium and a little more cranial. The trunks arteriosus moves anterior and a bit more caudal.
By day 24, the folding of the heart tube moves sinus venosus slightly to the right and the common ventricle shifts to the left.
The primitive atrium begins its division into right and left atria.
The bulbus cordis moves to the right and anterior to the developing atria. -
In this anterior view of a heart, around day 30, note the locations of each of the chambers.
The original three subdivisions of the bulbus cordis are labelled, right ventricle, conus cordis and truncus arteriosus.
The sinus venous is not visible because of its posterior location.Day 30
Anterior View
Fig. 8: Anterior View; Heart is closer in appearance to its adult orientation during week 5. The subsequent development of the heart involves production of septaa to separate the atria and ventricle and to form shunts through the heart for bypassing the pulmonary circuit during development of the lungs.
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In this posterior view of a heart, around day 30, the sinus venosus is visible
It is subdivided into right and left sinus horns.
The right sinus horn is incorporated into the right atrium and receives the superior and inferior vena cavae. The left sinus horn becomes the great cardiac vein and coronary sinus.Day 30
Posterior View
Fig. 9: Posterior View; Sinus venous is partially incorporated into the interatrial septum. The right sinus horn will contribute to the venae cave, while the left sits horn forms the coronary sinus and great cardiac vein.
In the subsequent weeks, completion of partitions in the interior of the heart will be result in a four chambered mammalian heart with separate outflow channels for the two ventricles and separate inflow channels for the two atria.