Development of the Cardiovascular System

The cardiovascular system consists of the heart and blood vessels. All of these structures are mesodermal in origin with significant contributions made by neural crest cells to the development of the heart.

Cardiac development begins in the 3rd week with the migration of epiblast cells, into the splanchnic layer of lateral plate mesoderm to form a horseshoe shaped region called the primary heart field or cardiogenic area, rostral to the developing forebrain. Primary and secondary heart fields form under the influence of factors secreted by the endoderm of the developing pharynx

These cells begin to differentiate into a pair of endocardial tubes which will eventually fuse to form a primitive heart tube. Initially the center of the cardiogenic area is rostral to the developing brain, but later is pushed caudally by folding of the embryo.

As the embryo folds in laterally, the two endocardial tubes are pushed toward each other in the midline where they fuse.

Layers of the Heart Tube Wall and their Derivitives

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
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

Segmentation of the Heart Tube

At day 22 of development, the heart tube is a linear structure that 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 arteriosis

Blood enters the sinus venosus through three pairs of veins. It flows into the common atrium and through the atrioventricular canal into the primitive ventricle.

The bulbus cordis is the beginning of the outflow tract and can be divided into three parts:

proximal 1/3 will contribute to formation of the right ventricle
middle 1/3 will become the conus cordis, the common outflow for both ventricles
distal 1/3 will become the truncus arteriosus which will form the roots of the aorta and pulmonary artery.

Folding of the Heart Tube

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.

Day 24

By day 24 in the fourth week, the primitive atrium begins to develop into two bulges, the right and left atria. The bulbus cordis moves to the right of the primitive ventricle. Subsequent development will involve separation of primitive atrium into right and left atria and the common ventricle into right and left ventricles.

Day 30

In this anterior view of a day 30 heart, 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 no longer visible because it has been incorporated into the right artrium.

In the subsequent weeks, the interior of the heart will be partitioned to form the four chambered mammalian heart with separate outflow channels for the two ventricles and separate inflow channels for the two atria.