Professor Alfred Cuschieri, Department of Anatomy, University of Malta
The cardiovascular system begins to develop in the third week of gestation. Blood islands (angiocysts) develop in the newly formed mesoderm, and consist of (a) a central group of haemoblasts, the embryonic precursors of blood cells; (b) endothelial cells.
Blood islands coalesce to form a vascular plexus. Preferential channels form arteries and veins .
Day 17 - Blood islands form first in the extra-embryonic mesoderm
Day 18 - Blood islands form next in the intra-embryonic mesoderm
Day 19 - Blood islands form in the cardiogenic mesoderm and coalesce to form a pair of endothelial heart tubes
The endothelial heart tubes fuse to form a single primitive heart tube with a cranial (arterial) end and a caudal (venous) end.
The heart tubes are derived from the cardiogenic mesoderm situated next to the pericardial cavity, the cranial-most end of the intra-embryonic coelom.
Initially, at 18 days, the cardiogenic mesoderm lies at the most cranial end of the trilaminar embryo.
After the formation of the head fold (at 20 days) the cardiogenic mesoderm is shifted ventrally and comes to lie ventral to the primitive pharynx.
The primitive heart tube is divided into a number of primitive chambers separated by grooves.
The truncus arteriosus divides into a pair of aortic arches.
The sinus venosus consists of right and left horns
The pharyngeal endoderm induces the cardiogenic mesoderm to differentiate into four layers, surrounded by the pericardial cavity.
Development of a circulation
A circulation is established during the 4th week after the myocardium is differentiated.
The cranial end communicates with the paired branchial arches that open into paired dorsal aortae. These fuse into a single dorsal aorta. At this stage three main pairs of arteries are present (i) to the head, (ii) vitelline arteries to the yolk sac and (iii) paired umbilical arteries to the placenta . Three corresponding veins drain into the sinus venosus.
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 of elongation of the heart tube, which causes it to become asymmetrical.
As a result of folding of the heart tube:
1. The atrium lies dorsal to the ventricle, bulbus cordis and truncus arteriosus, and bulges on either side of the truncus
2. The bulbus cordis lies to the right of the ventricle
3. The ventricular septum lies between the bulbus cordis and ventricle
4. The A-V opening overhangs both chambers
Formation of the transverse sinus of the pericardium
The heart is suspended in the pericardial cavity by a mesocardium, a double fold of coelomic epithelium situated in the midline.
The mesocardium breaks down 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 to the heart tube between the arterial and venous ends, and communicates the two sides of the pericardial cavity. It maintains the same relationship in the adult heart.
Initially the veins entering the sinus venosus are symmetrical. During the fourth week the venous system becomes asymmetrical causing extensive remodelling of the sinus venosus.
Initially three sets of paired veins enter the sinus venosus:
1. The common cardical veins enter the sinus venosus laterally. They receive:
a. the anterior cardinal veins from the cranial half of the body (head, neck and upper limbs)
b. the posterior cardinal veins from the caudal half of the body (abdomen and lower limbs)
2. The umbilical veins receiving oxygenated blood from the placenta
3. The vitelline veins drasining the gut and yolk sac.
These veins all pass through the septum transversum before entering the sinus venosus. At the same time the liver begins to develop within the septum transversum from cells derived from the foregut. A venous plexus of sinusoids develops between the liver cells and communicates with the umbilical and vitelline veins.
Venous symmetry is radically altered by:
1. establishment of left to right shunts in the venous system, and
2. obliteration of some veins draining into the sinus venosus
Three left to right shunts are formed:
1. A left to right shunt between the two anterior cardinal veins. This will form the left brachiocephalic vein.
On the left, the common cardinal, the posterior cardinal and most of the anterior cardinal veins are largely obliterated.
2. The ductus venosus - a preferential channel from the left umbilical to the right vitelline veins, bypassing the liver sinusoids.
The left umbilical vein loses its direct communication with the sinus venosus and the right umbilical vein is obliterated
3. The vitelline veins communicate by three anastomoses.
Consequences of rearrangement of the venous system
a) The cardinal veins
* The shunt between the two anterior cardinal veins forms the left brachiocephalic vein.
* On the left, the common cardinal, the posterior cardinal and most of the anterior cardinal veins are largely obliterated. Their remnants form the oblique vein of the left atrium
* All the blood from the cardinal veins now drains into the right horn of the sinus venosus
b) The umbilical veins
* The left umbilical vein drains into the right horn of the sinus venosus via the ductus venosus, and loses its direct communication with the sinus venosus
* The right umbilical vein is obliterated
c) The vitelline veins lose their direct communication with the sinus venosus. A preferential channel fromed of parts of the right and left vitelline veins and the three anastomoses between them forms the portal vein, which drains into the hepatic sinusoids.
d) The right horn of the sinus venosus dilates considerably as it receives all the veins. It forms the sinus venarum part of the right atrium. The left horn of the sinus venosus becomes small. It forms the coronary sinus.
Asymmetry of the sinus venosus shifts the sinu-atrial opening to the right of the common atrium.
Two flaps of endothelium project into the atrium from the sides of the SA opening forming transient right and left venous valves. They unite near the roof of the atrium to form the septum spurium.