The cardiovascular system is one of the first systems to form in an embryo, and the heart is the first functional organ. This video with Dr. Carr is, once again, worth watching to either give you an introduction or consolidate what you will have read here:
Dr. Carr explains the embryology of the heart
The human heart, derived from cardiogenic mesoderm, begins its existence as two simple endothelial tubes that quickly fuse to form a single chamber or heart tube that is busily pumping blood by the 23rd day (3rd week) of gestation.
The two endothelial heart tubes are derived from the cardiogenic mesoderm situated next to the pericardial cavity, the cranial most end of the intra-embryonic coelom.
The single fused primitive heart tube has a cranial (arterial) end and a caudal (venous) end. As the heart tubes fuse, an external layer of the embryonic heart - primordial myocardium - is formed from splanchnic mesoderm surrounding the pericardial coelom. At this stage, the developing heart is composed of a thin endothelial tube seperated from a thick muscular tube, the primordial myocardium, by geletinous connective tissue called cardiac jelly.
In adult form, the endothelial tube becomes the internal endothelial lining of the heart or endocardium, and the primordial myocardium becomes the muscular wall of the heart or myocardium. The visceral pericardium or epicardium is derived from mesothelial cells that arise from the external surface of the sinus venosus and spread over the myocardium.
As folding of the head region in the embryo occurs, the heart and pericardial cavity come to lie ventral to (in front of) the foregut (pharynx). Concurrently, the tubular heart elongates and develops alternate dilations and constrictions. In a cranial (head) to caudal (tail) orders these are:
- Truncus arteriosus, that gives rise to the Pulmonary trunk and Ascending Aorta in future
- Bulbus cordis, that gives rise to the vestibules of the pulmonary trunk and aorta as well as the adult form of right ventricle
- Ventricle, the strongest pumping chamber of the primitive heart. It gives rise to the left ventricle of adult heart
- Atrium, this primitive chamber eventually becomes the ridged parts (auricles) of the right and left atria
- Sinus venosus, this chamber receives all the blood from the body of the developing embryo and becomes the smooth-walled part (sinus venerum) of the right atrium and the coronary sinus in the adult heart.
The truncus arteriosus is continuous cranially with the aortic sac, from which the paired aortic arches arise.
The sinus venosus receives three paired sets of veins, the umbilical from chorion, the vitelline draining the yolk sac, and the common cardinal veins draining the body wall of the embryo.
The arterial and venous ends of the heart tube are fixed by the pharyngeal arches and septum transversum, respectively.
Folding of the Heart Tube
Folding of the heart tube occurs on days 23-28 at two sites: (i) the bulboventricular sulcus (bv), and (ii) the atrio-ventricular groove (av). As a result the heart tube becomes S-shaped. Folding occurs because the bulbus cordis and ventricle grow more rapidly than the other parts of the tube and because the arterial and venous ends of the tube are confined by the pericardium, the tube begins to loop and fold as it is unable to elongate further within the confines of the pericardial sac which causes it to become asymmetrical. As a result of folding of the heart tube:
- The atrium lies dorsal to the ventricle, bulbus cordis and truncus arteriosus, and bulges on either side of the truncus
- The bulbus cordis lies to the right of the ventricle
- The ventricular septum lies between the bulbus cordis and ventricle
- The A-V opening overhangs both chambers
Formation of the transverse sinus of the pericardium
As the heart elongates and bends, it gradually invaginates into the pericardial cavity.The heart is initially suspended from the dorsal body wall of the embryo by a mesentery, the dorsal mesocardium, a double fold of coelomic epithelium situated in the midline.The central part of this mesocardium soon degenerates forming the transverse sinus of the pericardium. The heart tube remains attached to the pericardium at its cranial (arterial) and caudal (venous) ends. The transverse sinus lies dorsal (posterior) to the heart tube and forms a communication between the right and left sides of the pericardial cavity. It maintains the same relationship in the adult heart.
Circulation through the Primordial Heart
The initial contractions of the heart originate in the muscle (myocardium) i.e., they are of myogenic origin. The muscle layers of the atrium and ventricles are continuous, and contractions occur in paristalsis-like waves that begin in the sinus venosus. Blood enters the sinus venosus through the three sets of veins, umbilical, vitelline, and common cardinal. From sinus venosus, blood enters the primordial atrium; flow from it is controlled by sinuatrial valves. The blood then passes through the atrioventricluar canal into the primordial ventricle. When the ventricle contracts, blood is pumped through the bulbus cordis and truncus arteriosus into the aortic sac, from which it is distributed to the aortic arches (arterial channels) in the pharyngeal arches. The blood then passes into the dorsal aortae for distribution to the embryo, yolk sac, and placenta.
Partitioning of the Atrioventricular Canal
Towards the end of the fourth week of development, endocardial cushions form on the dorsal and ventral walls of the primitive atrioventricular canal. The endocarial cushions develop from a specialized extracellular matrix or cardiac jelly. As these masses of tissue are invaded by mesenchymal cells, the AV endocardial cushions approach each other and fuse, dividing the AV canal into right and left AV canals.
Partitioning of the Primitive Atrium/Formation of the Interatrial Septum
The primordial atrium is divided into right and left atria by the formation and subsequent modification and fusion of two septa, the septum primum and septum secundum.
The septum primum, a thin crescent shaped membrane, grows towards the fusing endocardial cushions from the roof of the primitive atrium partially dividing the common atrium into right and left halves. As this curtainlike septum grows, a large opening, the foramen primum, forms between the crescentric free edge of the growning septum and the endocardial cushions. This foramen primum serves as a shunt, enabling the oxygenated blood to pass from the right to the left atrium. The foramen primum becomes progressively smaller and finally disappears as the septum primum completely fuses with the endocardial cushions. Before the foramen primum disappears, perforations, produced by programmed cell death - appear in the central part of the septum primum. As the septum fuses with the endocardial cushions, the perforations coalesce to form another opening known as foramen secundum.
After the fusion of septum primum with the fused endocardial cushions, foramen secundum ensues a continuous flow of oxygenated blood from the right to the left atrium.
The septum secundum, another crescentric muscular membrane, grows from the ventrocranial (anterosuperior) wall of the atrium, immediatelly to the right of the septum primum. As this thick septum grows during the fifth and sixth weeks, it gradually overlaps the foramen secundum in the septum primum. The septum secundum forms an incomplete partition between the atria; consequently, an oval foramen (foramen ovale) forms. The cranial part of the septum primum, initially attached to the roof of the left atrium, gradually disappears. The remaining part of the septum primum attached to the endocardial cushions, forms the flaplike valve of the oval foramen.
Partitioning of the Primitive Ventricle
Division of the primitive ventricle is first indicated by a median muscular ridge, the primitive interventricular (IV) septum - in the floor of the ventricle near its apex. This thick crescentric fold has a concave free edge. Until the seventh week of development there is a crescent shaped interventricular foramen between the free edge of the IV septum and the fused endocardial cushions. This IV foramen closes down by the formation of the membranous part of the IV septum. The membranous part of the septum is derived from an extension of tissue from the right side of the endocardial cushion to the muscular part of the IV septum. After closure of the IV foramen and formation of the membranous part of IV septum, the pulmonary trunk is in communication with the right ventricular cavity and the aorta is in communication with the left ventricular cavity.
Partitioning of Bulbus Cordis and Truncus Arteriosus
During the fifth week of development, active proliferation of mesenchymal cells in the walls of the bulbus cordis results in the formation of bulbar ridges. Similar ridges form in the truncus arteriosus that are continuous with the bulbar ridges. As the neural crest cells migrate through the primitive pharynx and reach these ridges, the bulbar and truncal ridges undergo a 180 degree spiraling. The spiral orientation of these ridges, possibly caused by the streaming of blood from the ventricles, results in the formation of a spiral aorticopulmonary septum when the ridges fuse. This septum divides the bulbus cordis and truncus arteriosus into two seperate arterial channels, the Aorta and Pulmonary trunk Because of the spiraling of the aorticopulmonary septum, the pulmonary trunk twists around the ascending aorta and comes to lie in front of the ascending aorta.
Development of Atrioventricular and Semilunar Valve
The semilunar (aortic and pulmonary) valves begin to develop from three swellings of the subendocardial tissue around the orifices of aorta and pulmonary trunk. These swellings are hollowed out and reshaped to form three thin-walled cusps. The atrioventricular (mitral and tricuspid) valves develop similarly from localized proliferations of tissue around the atriventricular canal.
Fate of the Sinus Venosus
Initially the sinus venosus is a seperate chamber of the fetal heart and opens into the dorsal (posterior) wall of the primitive atrium and its right and left horns are of the same size due to the symmetry of venous pattern. During the fourth week venous pattern becomes asymmetrical resulting in progressive enlargement of the right horn of sinus venosus. The venous symmetry is radically altered by two left to right shunts in the venous system, and the obliteration of some veins draining into the left horn of sinus venosus.
The first shunt results from transformation of the vitelline and umbilical veins. With the development of liver in septum transversum the left vitelline vein completely disintegrates while the right vitelline vein maintains its connection with the right horn of sinus venosus. The left umbilical vein loses its direct connection with the left horn of sinus venosus and drains into the right vitelline vein through a preferential channel, the ductus venosus and the right umbilical vein completely degenerates.
The second shunt occurs when the anterior cardinal veins become connected by an anastomosis. This communication shunts blood from the left to the right anterior cardinal vein. the shunt eventually becomes the left brachiocephalic vein. On the left side, the common cardinal, posterior cardinal, and most of the anterior cardinal veins are largely obliterated The right anterior cardinal and right common cardinal veins will eventually become the superior vena cava (SVC).
As a consequence of all this rearrangement in venous pattern, the blood from the entire body of the embryo is eventually draining into the right horn of sinus venosus. Therefore, by the end of the fourth week, the right horn is noticeably larger than the left horn. As this occurs the sinuatrial orifice (opening) moves to the right and opens in the part of primitive atrium that will become the adult right atrium. As the development proceeds, the left horn of the sinus venosus becomes the coronary sinus, and the right horn becomes incorporated into the wall of the right atrium.
Primitive Pulmonary Vein and Formation of the Left Atrium
Most of the wall of the adult left atrium is smooth-walled because it is formed by incorporation of the primitive pulmonary vein. This vein develops as an outgrowth of the dorsal (posterior) atrial wall, just to the left of septum primum. As the atrium expands, the primitive pulmonary vein and its main branches are gradually incorcporated into the wall of the left atrium.
Why is the cardiovascular system one of the first 'organ systems' to develop?
proceed to Heart 9: Congenital Heart Defects