Embryology Course - Session 6

8/4/2019 Embryology Course - Session 6

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Origin of the Heart Tube

The cardiac progenitor cells originate lateral to the primitive streak and invaginate through it to

migrate cranially to the cardiogenic field where they lie within splanchnic mesoderm, rostral to the

buccopharyngeal membrane, as two separate groups, one on each side; each group is stimulated by

the endoderm of the pharynx to form cardiac myoblasts; Blood islands also appear in the splanchnic mesoderm and form a horseshoe-shaped endothelial

tube for each group of cardiac myoblasts; hence the endocardium is derived from the blood islands,

while the myocardium is derived from the cardiac progenitor cells

With lateral folding of the embryo, the two groups come together and begin fusion craniocaudally; a

complete heart tube thus is formed


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Fusion of the Heart Tubes and Pericardial Coeloms


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The Dorsal Mesocardium

The heart tube bulges more into the pericardial cavity; initially it remains attached to the dorsal wall

of the cavity by the dorsal mesocardium (connection between the splanchnic and somatic layers of

the pericardial cavity); later the dorsal mesocardium breaks down, connecting both sides of the

cavity together, and forming the transverse pericardial sinus in the adult; the heart is thensuspended by blood vessels at its cranial and caudal ends


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Fusion of Heart Tubes

With lateral folding of the embryo, the two groups come together and begin fusion craniocaudally; a

complete heart tube thus is formed

Blood islands appear in the splanchnic mesoderm and form a horseshoe-shaped endothelial tube

for each group of cardiac myoblasts; hence the endocardium is derived from the blood islands, whilethe myocardium is derived from the cardiac progenitor cells


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Looping of the Heart

The heart tube continues to elongate and bend at the beginning of the 4th week; the cephalic

portion of the tube bends ventrocaudally and to the right, while the caudal portion bends

dorsocranially and to the left; this process is called dextral looping, which is complete by the 28th

day; opposite bending leads to dextrocardia Local expansions in the tube appear dividing it, from caudal to cranial, into: sinus venosus, common

atrium, AV junction, left ventricle, primary interventricular foramen, and bulbus cordis

Bulbus cordis is divided into three parts, from proximal (i.e. caudal) to distal: trabeculated part of

right ventricle, conus cordis (future outflow tracts, or smooth parts of both ventricles), and truncus

arteriosus (roots and proximal ends of aorta and pulmonary artery)


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Looping of the Heart

Local expansions in the tube appear dividing it, from caudal to cranial, into: sinus venosus, common

atrium, AV junction, left ventricle, primary interventricular foramen, and bulbus cordis


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Ventricles and Atria

When looping is complete, trabeculae begin to appear on both sides of the primary interventricular

foramen, forming the trabeculated part of both ventricles

Formation of two transverse dilations of the common atrium on either side of the bulbus cordis,

causes shifting of the conotruncus from the right side to a more median position


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The Sinus Venosus

Sinus venosus has two sinus horns, each receiving blood from three veins: umbilical, common

cardinal, and vitelline (or omphaloenteric); later the left common cardinal and left vitelline veins

are obliterated, causing shunting of blood to the right horn (left-to-right shunts) which results in its

enlargement; only remnant of left horn is the coronary sinus and oblique vein of left atrium Connection of the sinus with the common atrium is initially wide, but later shifts to the right and the

right horn is incorporated into the right atrium as the sinuatrial orifice, guarded by right and left

venous valves; the valves fuse dorsocranially forming the septum spurium


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The Sinus Venosus

The right horn is incorporated into the right atrium as the sinuatrial orifice, guarded by right and

left venous valves; the valves fuse dorsocranially forming the septum spurium; then the left valve

and the septum spurium fuse with the septum secundum on their left side; the superior portion of

the right venous valve disappears completely, while the inferior portion develops into the valves ofthe inferior vena cava and coronary sinus

In the adult, the crista terminalis on the inside (sulcus terminalis on the outside) is the dividing line

between the original trabeculated part of the right atrium and the smooth part (sinus venarum)

derived from the sinuatrial orifice


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Conducting System of the Heart

Initially the pacemaker is located in the caudal part of the left heart tube; later it lies within the

sinus venosus; later when the sinus is incorporated into the right atrium, the sinuatrial node is

established; the AV node and bundle arise from the left wall of the sinus venosus and the AV canal

and eventually come to lie in the interatrial septum


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Formation of Septa

The ways by which a septum may be formed are three:

Two masses of tissue growing towards each other until they fuse

One mass of tissue growing towards another mass until it fuses with it

Presence of a narrow strip of tissue which fails to grow in an expanding wall, resulting in expansion of the walls on its sides


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Formation of AV Septum

Initially blood passes through AV canal only to the left ventricle in a direct manner because it is

separated from the bulbus cordis by a mass of tissue, the bulbocono ventricular flange; later this

flange is obliterated and blood can then pass directly to both ventricles

Two mesenchymal cushions, the superior and inferior AV endocardial cushions appear in the wall ofthe canal and grow towards each other until they fuse and divide the canal into right and left parts


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Initially blood passes through AV canal only to the left ventricle in a direct manner because it is

separated from the bulbus cordis by a mass of tissue, the bulbocono ventricular flange; later this

flange terminates at the superior border of the canal and blood can pass directly to both ventricles

Two mesenchymal cusions, the superior and inferior AV endocardial cushions appear in the wall ofthe septum and grow towards each other until they fuse and divide the septum into right and left


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Formation of AV Valves

After fusion of the AV septa, each orifice is surrounded by local proliferations of mesenchymal

tissue which are hollowed and thinned by blood on the ventricular side, forming valves which lose

their muscular tissue and are filled with dense connective tissue covered by endocardium

These valves (tricuspid on the right and biscuspid on the left) are connected by chordae tendinae tothe papillary muscles in the wall of the ventricles


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Formation of the Atrial Septum

First, a sickle-shaped crest, the septum primum, grows from the roof of the atrium; its limbs grow

down to the AV cushions; the opening between the lower rim of septum and cushions is the ostium

primum; as this opening is filled by tissue from the cushions, other foramina appear in the upper

part of the septum and fuse to form another opening, the ostium secundum


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First, a sickle-shaped crest, the septum primum, grows from the roof of the atrium; its limbs grow

down to the AV cushions; the opening between the lower rim of septum and cushion is the ostium

primum; as this opening is filled by tissue from the cushions, other foramina appear in the upper

part of the septum and fuse to form another opening, the ostium secundum


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Then another septum, the septum secundum, appears from the roof of the atrium, to the right of

the septum primum; it also has limbs that extend down to the AV septum, but it never forms a

complete septum, leaving the foramen ovale; when the septum secundum fuses with the septum

spurium and left venous valve, it overlies the ostium secundum; when the upper part of the septumprimum disappears, it forms the valve of the foramen ovale


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Then another septum, the septum secundum, appears from the roof of the atrium, to the right of

the septum primum; it also has limbs that extend down to the AV septum, but it never forms a

complete septum, leaving the foramen ovale; when the septum secundum fuses with the septum

spurium and left venous valve, it overlies the ostium secundum; when the upper part of the septumprimum disappears, it forms the valve of the foramen ovale


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Fate of Foramen Ovale

After birth, pressure on the left atrium increases because of respiration, and the valve of the

foramen ovale is pressed against the septum secundum until they fuse, forming the fossa ovalis; in

about 20% of individuals, fusion never occurs and a narrow oblique cleft remains, which is called

probe patent foramen ovale, but does not allow shunting of blood between the atria


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Further Differentiation of Atria

Initially, a single pulmonary vein grows from the wall of the left atrium, to the left of the septum

primum, and connects with veins of the developing lungs; similar to incorporation of right sinus horn

into right atrium, this pulmonary vein is later incorporated into the wall of the left atrium along

with three other pulmonary veins formed later In the adult heart, the original portions of the atria are respresented by the trabeculated

appendages, while the incorporated portions represent the smooth parts


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Formation of the Conotruncal Septum

A pair of opposing ridges appear in the truncus arteriosus; these are the right superior and left

inferior truncus swellings; both of them grow distally (i.e. upwards) in opposite directions, thus

twisting around each other and forming the aorticopulmonary septum

Meanwhile, two conus swellings also appear; these are the right dorsal and left ventral conusswellings; they grow towards each other and fuse to form the conus septum which divides the conus

into an anterolateral portion (outflow tract for the right ventricle) and a posteromedial portion (for

the left ventricle); the conus swellings also grow distally (i.e. upwards) toward the truncus swellings


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Formation of the Conotruncal Septum

Neural crest cells from the hindbrain region also contribute to the formation of this septum; since

they also contribute to craniofacial skeleton, it is not uncommon to see abnormalities in the face and

the heart in the same patient


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Formation of the Semilunar Valves

When partitioning of the truncus is near completion, a pair of small tubercles becomes visible on

each of the two truncus swellings; one of each pair is assigned to each of the aorta and the

pulmonary trunk; then a third tubercle appears in each channel opposite the fused main truncus

swellings; thus three cusps are formed which are hollowed on their upper surface to form thesemilunar valves of the aorta and the pulmonary trunk


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Formation of the Ventricular Septum

Continuous growth of the two ventricles causes their medial walls to fuse and form a muscular

interventricular septum that does not completely separate them; the interventricular foramen

above the septum is bounded above by the conus septum and behind by the AV septum

Later, cells proliferating from the inferior AV endocardial cushion close the foramen and form themembranous part of the interventricular septum thus completely separating the two ventricles


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Layers of the Heart

The endocardium, inner layer of the heart, is derived from the blood islands forming in the

splanchinic mesoderm

The myocardium, middle muscular layer, forms from the cardiac progenitor cells originating lateral

to the primitive streak and invaginating towards the cardiogenic field The epicardium, outer layer, is derived from two sources:

Proepicardial cells derived from the septum transversum near the sinus venosus

Mesothelial cells originating in the outflow tract region


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Formation of the Arterial System

In addition to the cardiogenic field, other blood islands appear bilaterally parallel to the midline of

the embryo; these form the dorsal aortae; later in development, the truncus arteriosus is divided by

the aorticopulmonary septum into the ventral aorta and the pulmonary trunk

The most distal portion of the truncus arteriosus, called the aortic sac, gives rise to two horns: theright one forms the brachiocephalic artery, while the left one forms the aortic arch

Each pharyngeal arch receives its own artery from the ventral arteries arising from the aortic sac and

terminating dorsally at the dorsal aorta; six arches form sequentially; the fifth arch either never

forms or forms and then disappears


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Fate of the Arch Arteries

The first arch disappears and only a small part of it persists as the maxillary

artery

The second arch disappears and only contributes to the hyoid and stapedial

arteries The third arch forms the common carotid and part of the internal carotid

arteries (the rest of the internal carotid is derived from the dorsal aortae;

the external carotid is formed by sprouting from the third arch)


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The forth arch on the right side forms the proximal portion of the

subclavian artery while on the left side it forms the part ofthe aortic arch

between the common carotid and the left subclavian

The rest of the right subclavian artery and entire left subclavian artery arederived from the 7th intersegmental artery, which is a branch of the dorsal

aortae

The proximal portion of the aortic arch is derived from the left horn of the

aortic sac, the right horn of which forms the brachiocephalic artery


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The fifth arch either never forms, or forms incompletely and then

disappears

The sixth arch on both sides gives an important pulmonary branch that

grows towards the developing lungs; however, on the right side the distalportion of the arch disappears and loses connection with its dorsal aorta,

while on the left side it persists as the ductus arteriosus (ligamentum

arteriosum in the adult) which connects it to the future descending

thoracic aorta


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Other Changes in the Arterial System

The portion of dorsal aorta between entry of 3rd and 4th arches, called the

carotid duct, is obliterated

The portion of the right dorsal aorta disappears that lies between the right

7th intersegmental artery and its union with the left dorsal aorta As a result of the descent of the heart, the carotid and brachiocephalic

arteries elongate considerably and the left subclavian shifts its origin close

to the left common carotid


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Other Changes in the Arterial System

Because of the difference in the fate of the 6th

arches on the right and left, the recurrent

laryngeal nerve on the left hooks around the

ductus arteriosus (ligamentum arteriosum in theadult) while on the right it hooks around the

right subclavian artery


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Vitelline and Umbilical Arteries

The paired vitelline arteries, which arise from the ventral aspect of the dorsal aorta, later form three

vessels which supply the gut: the celiac, superior mesenteric, and inferior mesenteric, supplying the

foregut, midgut, and hindgut, respectively

The paired umbilical arteries arise from the dorsal aorta; later the proximal portion of each oneacquires a secondary connection with the common iliac artery, branch of dorsal aorta, and loses its

original connection; in the adult the proximal portions contribute to the internal iliac and form the

superior vesical arteries while the distal portions form the medial umbilical ligaments

h


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The Coronary Arteries

The coronary arteries are derived from two sources:

Angioblasts formed at other places and distributed over the heart by the migration of the proepicardial cells

The epicardium of the heart itself, by undergoing an epithelial-to-mesenchymal transition, induced by underlying myocardium,

and then contributing to both the endothelial lining and the muscular coat of the coronary arteries

Neural crest cells also contribute to the muscular wall of these arteries

Their connection with aorta occurs by the arteries invading the wall ofthe aorta

i f h V S


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Formation of the Venous System

Initially, three pairs of veins can are recognized in the embryo and drain into the sinus venosus: the

vitelline veins, the umbilical veins, and the cardinal veins (each receiving anterior and posterior

cardinal branches)

Th Vi lli V i


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The Vitelline Veins

The vitelline veins, on their way to the sinus venosus, form a plexus around the duodenum and

pierce the septum transversum; later, when the liver cords penetrate the septum transverum, they

interrupt the course of the veins and form the hepatic sinusoids that connect with the vitelline veins

With the left-to-right shunting of blood, the right vitelline vein, now called the right hepatocardiacchannel, enlarges and forms the hepatocardiac portion of the inferior vena cava; the left vitelline

disappears completely; the superior mesenteric vein also derives from the right vitelline vein

Th Vit lli V i


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The Vitelline Veins

The vitelline veins, on their way to the sinus venosus, form a plexus around the duodenum and

pierce the septum transversum; later, when the liver cords penetrate the septum transverum, they

interrupt the course of the veins and form the hepatic sinusoids that connect with the vitelline veins

With the left-to-right shunting of blood, the right vitelline vein, now called the right hepatocardiacchannel, enlarges and forms the hepatocardiac portion of the inferior vena cava; the left vitelline

disappears completely; the superior mesenteric vein also derives from the right vitelline vein

Th U bili l V i


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The Umbilical Veins

Both veins initially pass on either side of the liver to the sinus venosus; later they acquire connection

with the liver; gradually the proximal parts of both veins and the remainder of the right vein are

completely obliterated, leaving only the distal part of the left umbilical vein which becomes the

definitive umbilical vein that carries nutrition from the placenta to the fetus With further development, a direct connection forms between the left umbilical vein and the

hepatocardiac portion of inferior cava through the ductus venosus, which allows blood to bypass

the sinusoidal plexus and reach the heart faster; after birth, both veins are obliterated (umbilical vein

becomes ligamentum teres hepatic and ductus venosus becomes ligamentum venosum)

Th U bili l V i


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The Umbilical Veins

Both veins initially pass on either side of the liver to the sinus venosus; later they acquire connection

with the liver; gradually the proximal parts of both veins and the remainder of the right vein are

completely obliterated, leaving only the distal part of the left umbilical vein which becomes the

definitive umbilical vein that carries nutrition from the placenta to the fetus With further development, a direct connection forms between the left umbilical vein and the

hepatocardiac portion of inferior cava through the ductus venosus, which allows blood to bypass

the sinusoidal plexus and reach the heart faster; after birth, both veins are obliterated (umbilical vein

becomes ligamentum teres hepatic and ductus venosus becomes ligamentum venosum)

Th C di l V i


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The Cardinal Veins

The anterior and posterior cardinal veins on each side drain into the common cardinal veins; later,

other vein pairs develop: subcardinal veins (draining the kidneys mainly), sacrocardinal veins

(draining the lower limbs), and supracardinal veins (draining the posterior body wall)

Later, anastomosis occurs between the veins such that most of the blood from the left is shunted tothe right; anastomosis of the anterior cardinal veins forms the left brachiocephalic vein; only a small

portion of the left posterior cardinal vein persists as the left superior intercostal vein

The Brachiocephalic Vein


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The Brachiocephalic Vein

Anastomosis of the anterior cardinal veins forms the left brachiocephalic vein; only a small portion

of the left posterior cardinal vein persists as the left superior intercostal vein which drains into the

left brachiocephalic vein and drains the 2nd and 3rd intercostal spaces

The Cardinal Veins


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The Cardinal Veins

The superior vena cava is formed from the right common cardinal vein and the proximal portion of

the right anterior cardinal vein

Most of the posterior cardinal veins are obliterated; the right supracardinal vein forms the azygos

vein which drains the 4th

to 11th

intercostal spaces and enters the superior vena cava, while the leftsupracardinal forms the hemiazygos vein which drains the 4th to 7th intercostal spaces and enters

the azygos vein

The Cardinal Veins


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The Cardinal Veins

The anastomosis of the subcardinal veins forms the left renal vein; then only the distal portion of

the left subcardinal vein persists as the left gonadal vein; the right subcardinal enlarges and forms

the subcardinal portion of the inferior vena cava

This is the reason why in the adult the left gonadal vein enter the renal vein while the right gonadalvein enters the inferior vena cava directly

The Cardinal Veins


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The Cardinal Veins

The anastomosis of the sacrocardinal veins forms the left common iliac vein; the right sacrocardinal

becomes the sacrocardinal portion of the inferior vena cava; thus the inferior vena cava is complete,

formed from three portions: hepatocardiac, subcardinal, and sacrocardinal

The Lymphatic System


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The Lymphatic System

The lymphatic system may either develop from mesenchyme in situ or as outgrowth from the veins;

six main lymph sacs form: twojugular, two iliac, one retroperitoneal, and one cisterna chyli

Of the many lymphatic channels that form, two main thoracic ducts form; later, anastomosis

between these two ducts results in the formation of the definitive thoracic duct from the distalportion of the right duct, the anastomosis, and the proximal portion of the left duct, and the

formation of the right lymphatic duct from the proximal end of the right duct

Fetal Circulation


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Fetal Circulation

Read the explanation in the book. Give attention to the sites where oxygenated and deoxygenated

blood mix.

Circulatory Changes at Birth Neonatal Circulation


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Circulatory Changes at Birth Neonatal Circulation

Changes that occur at birth or soon thereafter are: closure of umbilical arteries and vein, closure of

ductus venosus and arteriosus, and closure of oval foramen; details in the book are required


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Embryology Course - Session 6

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