CVS Embryology (Amoud University)

Cardiovascular System School of heath and science A.U

CVS is the first system to function in the embryo. As the embryo grows the development of the cardiovascular system become essential to the developing embryo so cardiovascular system develops during the third week. The cardiac progenitor cells lie in the epiblast. From there they migrate through the streak. Cells destined to form the cranial segments of the heart migrate first, and cells forming more caudal portions ( right ventricle, left ventricle sinus venosus respectively migrate in a sequential order.
Establishment of the cardiogenic field

Here they reside in the splanchnic layer of the lateral plate mesoderm ultimately they form a cardiac myoblasts . Blood islands also appear in the mesoderm where they will form a blood vessels by the process of vasculogenesis . With time the islands unite and form a horse shoe-shaped endothelial lined tube surrounded by a myoblast this region is known as cardiogenic field.
Cont……..

Formation and position of the heart tube
Initially the central portion of the cardiogenic area is anterior the buccopharyngeal membrane and the neural plate with closure of the neural tube and the formation of the brain vesicles. As a result of the growth of the brain and cephalic folding of the embryo the buccopharyngeal membrane is pulled forward, while the heart and the pericardial cavity move first to the cervical region and finally to the thorax.

CONT.. The crescent part of the horse shoe shaped area expands to form the out flow track of ventricular regions, thus the heart becomes continuous expanding tube consisting of an inner endothelial lining and outer myocardial layer. A mesothelial cells from the regions of the sinus vensus migrate over the heart to form epicardium thus the heart tube consist of three layers. Endocardium: forming internal endothelial lining of the heart. Myocardium: forming the muscular walls. Epicardium: covering the outside of the tube. This outer layer is responsible for formation of the coronary arteries including their endothelial lining and smooth muscles.

Formation of the cardiac loops
The heart tube continue to enlongate and bend on day 23. The cephalic portion of the tube bends ventrally, caudally and to right and to left. This bending creates the cardiac loop . It is a complete by a day 28. while the cardiac loop is forming, local expansions become a visible throughout the length of the tube. The atrial portion forms a common atrium. At the end of loop formation, the smooth walled heart tube begins to form a primitive trabeculea ( a cord of tissue that serves as a supporting structure).

Cont….
The bulbus cordis has three parts:
Proximal part which forms the trabeculated part of the right ventricle.
Middle part known as cornus cordis forms out flow track of both ventricles.
Distal part known as truncus arterioses forms the roots and proximal portions of the aorta and the pulmonary artery

Is the most caudal portion of the primitive heart tube.
In the middle of the 4 th week it consists of
small transverse portion
Right and left sinus horns

It receives all veins of the body
Each horn receives blood from 3 important veins
vitteline vein from the yolk sac .
umbilical vein from the placenta .
common cardinal vein from the body of the embryo.

The left sinus horn
The left sinus horn losses its importance rapidly because of the left to right shunts.
Blood which is from the embryo itself and the one from the placenta change their direction to the right side of the sinus venosus .

The left umbilical vein is obliterated at 5mm.
The left vitteline vein is obliterated at 7mm.
The left common cardinal vein is obliterated at 60mm. The remaining parts of the left sinus horn forms.
coronary sinus and.
Oblique vein of the left atrium.

Right sinus horn
The right sinus horn becomes incorporate into the right atrium and forms the smooth walled part of the right atrium.
RIGHT AND LEFT VENOUS VALVES.
Entrance of sinus venosus into the atrium is called sino-atrial orifice.
On each side of it is guided by a valve called venous valves

The two valves meet and fuse above to form aridge known as septum spurium.
The fate of these valves are as follows :
.The left venous valve degenerate rapidly and fuse into the interatrial septum .
The upper part of right venous valve will later form the crista terminals which is a ridge in the wall of the right atrium extending from the front of the opening of the s.v.c. to the front front opening of the i.v.c.
The lower part of the right venous valve is divided into two part .

The valve of the inferior vena cava and .
The valve of the coronary sinus.

Formation of the cardiac septa
It is formed between 27 th and 37 th days of the development .
When the embryo grows in length from 5mm to 17mm. 1). Two actively growing masses of tissue approach each other until they fuse . There by dividing the lumen into two separate channels. 2). Growth of the single mass of tissue which continue expanding till it reaches the opposite sides of the lumen .

Masses known as endocardial cushion develop in the atrioventricular . In these locations they assist the formation of the atrial and ventricular septum, atrio-ventricular canals and the aortic and pulmonary canals.

Septum formation of common atrium
Two septa ( the septum primum and septum secondum)are essentials to divide the cavity of primitive single atrium into two channels. The septum primum
Grows at the 4 th week as asickle shaped crest from the roof of the comman atrium into the lumen .
The two limbs of this septum extend into the direction of the endocardial cushions in the AV canal .
The opening bt septum primum and the endocardial cushions is the ostium primum .

During further development the endocardial cushions grows towards the septum primum thereby closing the ostium primum.
Before the closure is completed perforations appear in the septum primum which fuse to form
ostium secundum

The septum secundum
When the lumen of the right atrium expands as a result of incorporation of the sinus horn , anew crescent –shaped fold appear .this new fold is called septum secundum. T he septum secundum never forms a complete partition in the cavity of the atrium.
The lower free border of septum secundum and osteum secundum form Oval foramen.
Which allows free passege of blood bt the two atria .

Further differentiation of the atria
Primitive atrium enlarges by incorporation of the right sinus horn.
Right atrium has two parts :
Smooth posterior part { sinus venarum } and
Rough anterior part { pactinate part } containing the pactinate muscles..

Septum formation in the atrioventricular canal
The major septa of the heart are formed between the 27 th and 37 th days of development.
Two actively growing masses of tissue or endocardial cushions located on opposite sides of the atrioventricular canal begin to grow toward each other to form the septum intermedium .
They continue to grow toward each other until they fuse, thereby dividing the lumen into two separate canals.
This septum now divides the common atrioventricular canal into left and right atrioventricular canals.

Atrioventricular Valve Formation:
After the atrioventricular endocardial cushions fuse, each AV orifice is surrounded by localized proliferations of mesodermal tissue.
When tissue located on the ventricular surface of these proliferations becomes hollowed out and thinned by the bloodstream, valves are formed that remain attached to the papillary muscles of the ventricular wall by muscular cords ( chordae tendineae ).
Finally, muscular tissue in the cords degenerates and is replaced by dense connective tissue.
two valve leaflets are formed in the left AV canal -- mitral ( bicuspid ) valve.
three valve leaflets are formed in the right AV canal – tricuspid valve.

SEPTUM FORMATION IN THE TRUNCUS ARTERIOSUSAND CONUS CORDIS
• During the fifth week, pairs of opposing ridges appear in the truncus.
• These ridges, the truncus swellings, or cushions, lie on the right superior wall ( rightsuperior truncus swelling ) and on the left inferior wall ( left inferior truncus swelling ) .
• The right superior truncus swelling grows distally and to the left
• and the left inferior truncus swelling grows distally
and to the right.

• Hence, while growing toward the aortic sac, the swellings twist around each other, foreshadowing the spiral course of the future septum .
• After complete fusion, the ridges form the aorticopulmonary septum, dividing the truncus into an aortic and a pulmonary channel .

Septum formation in the ventricles

It is the end of the fourth week when the primitive ventricles begin to expand. The medial walls of the expanding ventricles become apposed and gradually merge, forming the muscular interventricular septum.
The interventricular foramen which is above the muscular portion of the interventricular septum, shrinks on completion of the conus septum. The IV foramen is closed by the conal ridges , outgrowth of the inferior endocardial cushion, and connective tissue from the muscular interventricular septum. This portion of the I.V. septum is called the membranous part of the interventricular septum

Semilunar Valves
When partitioning of the truncus is almost complete, primordia of the semilunar valves become visible as small tubercles found on the main truncus swellings.
• One of each pair is assigned to the pulmonary and aortic channels, respectively.
• Gradually the tubercles hollow out at their upper surface, forming the semilunar valves.
• Recent evidence shows that neural crest cells contribute to formation of these valves.

Longitudinal sections through the semilunar valves
6 th week 7 th week 9 th week

The membranous portion
Of the septum is the most
common congenital cardiac
malformation
Isolated defect in the membranous portion of the interventricular septum. Blood from the left ventricle flows to the right through the interventricular foramen ( arrows).

Formation of the Conducting System of the Heart
• Initially the pacemaker for the heart lies in the caudal part of the left cardiac tube.
• Later the sinus venosus assumes this function, and as the sinus is incorporated into the right atrium,
• pacemaker tissue lies near the opening of the superior vena cava. Thus, the sinuatrial node is formed.
• The atrioventricular node and bundle (bundle of His) are derived from two sources:
– ( a) cells in the left wall of the sinus venosus,
– (b) cells from the atrioventricular canal.
 Once the sinus venosus is incorporated into the right atrium, these cells lie in their final position at the base of the interatrial septum.

There are two main parts of vascular system:
Arterial system:
Aortic arches
Vitelline arteries
Umbilical Arteries
Venous System:
Vitelline veins
Umbilical veins
Cardinal veins

Aortic Arches
Aortic arches arises from the aortic sac.
These arches are embedded in mesenchyme of the pharyngeal arches and terminal part in the right & left dorsal aorta.
The sac contributes a branch to each new arch as it form, giving rise to a total of five pairs of arteries.
During further development, this arterial pattern becomes modified, and some vessels regress comletely.

Aortic pulmonary septumdivides the truncus arteriosus ,causing the division of the outflow channel of the heart into ventral Aorta & pulmonary arteries.
As a result, the aortic sac forms the right horn, which gives rise to brachiocephalic arteries, and the left horn, which also gives rise to aortic arch.

Development of aortic arch
1 st aortic arch: disappears although small portion persist to form Maxillary arteries .
2 nd aortic arch: disappears , but the remaining parts are: Hyoid & Stapedial arteries .
With further development the aortic arch system loses its original symmetrical and the following changes occur :
3 rd aortic arch: forms common carotid artery and the first part of the internal carotid artery .
4 th aortic arch : on the left it forms part of the arch of the aorta. On the right it forms the most proximal segment of the right subclavian artery.rtical

5 th aortic arch: either never forms or forms incompletely and then regress.
6 th aortic arch: known as pulmonary artery. The distal portion of its right side loses the connection with the dorsal aorta and disappears. But the distal part of its left persists during intrauterine life as ductus arteriosus
There are other changes occur include:
The dorsal aorta btw 3th and4th arches , carotid duct , obliterate.
The right dorsal aorta disappears btw origin of the 7 th intersegmental artery & the junction with the left dorsal aorta.
The heart will pushed into thoracic cavity which resulted

From the cephalic folding and elongation of the neck.
As a result, the caudal shift of the heart and disappearance of the various portions of the aortic arches, the course of the recurrent laryngeal nerves differentiate on both side.

Vitelline artery
A number of paired vessels.
They fuse and form arteries in dorsal mesentry of the gut.
In the adult they represented by:
celiac: which supply foregut .
Superior mesenteric artery : supply the midgut.
Inferior mesenteric artery: supply the hindgut.

Umbilical arteries
> Umbilical arteries – paired branches of dorsal aorta – to placenta (allantois) in embryonic stalk or later in umbilical cord
> It persist as internal iliac and superior vesical arteries .

There are three major pairs of veins:
Vitelline vein.
Umbilical vein.
Cardinal vein.

Vitelline veins
Vitelline veins form apluxes around the duodenum and pass through the septum tranversum.
The growing liver interrupt the course of the vein and form hepatic sinusoids .
Right hepatocardiac channel is formed ,and as result, blood from left side of the liver is rechanneled to ward the right.
Network around the duodenum develops into single vessel Portal vein.
Distal portion of L.vetlline disappears.
The superior mesentric vein derived from R. vetilline.

Umbilical vein
They pass through each side of the liver but some connect to the hepatic sinusoid.
disappearance of the Proximal of both umbilical veins and right umbilical vein .
Duct veinouses will form B/w left umbilical vein nad right hepatocardinal channel.
After birth left umbilical vein obliterate and form legementum teres hepates.
Ductuses veinouse will form lemgentum venesoum.

Cardinal vein
Cardinal vein is the main venouses drainage system of the embryo.
this system consist of :
Anterior cardinal vein.
Posterior cardinal vein.

During the fifth to the seventh week ,number of veins will form:
subcardinal vein : which mainly drains the kidneys.
sacrocardinal vein: drain the lower extremities.
supracardinal veins: drain the body wall.

Vena cava system:
Posterior vena cava.
anterior vena cave.

Formation of the vena cava system is characterized by anastomoses .
Anastomoses B/w anterior cardinal will develpe into left brachiocephalic veins.
Anstomoses B/W subcardinal veins will form left renal vein
The anstomoses B/W sacrocardinal veins formthe left common iliac vein.

The fetal circulation Circulatory changes at birth Lymphatic system

Fetal circulation
The circulation of blood in the embryo is a good example of (economy) in which well oxygenated blood from the placenta is (shunted) to important structures such as the (brain) and the (heart) while relatively desaturated blood is left for less important structures.

PLACENTA umbilical vein ductus venosus I.V.C
Rt. ATRIUM Lt. atrium Lt. ventricle AORTA.

The umbilical veins transport blood rich in oxygen and nutrients from the placenta to the fetal body.
Most of the blood enters a shunting vessels called the ductus venosus , that bypasses (short-circuiting) the liver.
A smaller amount enters liver sinusoids and mixes with blood from the portal circulation.

Then the blood flows directly into the vena cava by way of ductus venosus.
There, the oxygenated blood from the placenta is mixed with deoxygenated blood from the lower parts of the fetal body. This blood continuous through the vena cava to the right atrium.

As the blood relative high in oxygen enters the right atrium of the fetal heart, a large proportion of it is shunted directly into the left through an opening in the atrial septum called oval foramen .

The more highly oxygenated blood that the left atrium through the foramen ovale is mixed with a small amount of the deoxygenated blood returning from the pulmonary veins. This mixture moves into the left ventricles and pumped into the aorta.

Circulatory changes at birth
At birth the lung expands.
Expansion of the lungs creates negative pressure inside the thoracic cavity which sucks blood and, therefore, the blood flow in the pulmonary arteries increases.
Blood now reaches the Lt. atrium from the lungs in the large quantities and as a result the pressure in the Lt. atrium “rises”

Ligature of the umbilical cord at birth, will stop the flow of blood through the umbilical vein and the ductus venosus to the I.V.C, as a result the pressure inside the Rt.atrium drops.

There will be a number of closures within the vascular system at the time of birth:
Closure of the umbilical arteries, accomplished by contraction of smooth musculature in their walls. Distal parts of the umbilical cord forms the medial umbilical ligaments , and the proximal part remain open as the superior vesicle arteries .
Closure of umbilical vein and ductus venosus, the U.V forms the ligamentum teres hepatis . The ductus venosus forms the ligamentum venosum .

Closure of ductus arteriosus, it forms the ligamentum arteriosum.
Closure of oval foramen is caused by an increased pressure in the left atrium, combined with a decrease in pressure on the right side.

The lymphatic system
The lymphatic system develops later than the cardiovascular system appearing at the fifth wk of gestation originating as five sacs:two jucular, two iliac,one retroperitonealand one cisterna chyli.
Numerous channels form to connect the sacs and provide drainage from other structures.

Cont’
Ultimately the thoracic duct forms from anastomosis of the right and left thoracic duct, the distal part of the right duct , and the cranial part of the left thoracic duct. The right lymphatic duct develops from the cranial part of the right thoracic duct.

CVS Embryology (Amoud University)

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