TBL12 - Development of the Heart and Great Vessels

Question 1

In the presomite embryo, where do angiogenic cell clusters (hemangioblasts) arise from? What are derivatives of hemangioblasts? What is vasculogenesis? What is angiogenesis?

Answer 1

1) In the presomite embryo, angiogenic cell clusters (aka hemangioblasts) arise from mesenchymal cells of the visceral (splanchnic) mesoderm
2) Endothelial cells and blood cells (hematopoietic cells) are derivatives of the hemangioblasts
3) Coalescence of endothelial cells to form endothelium-lined vessels is called vasculogenesis
4) The sprouting of new vessels from existing vessels is designated angiogenesis

Question 2

What heart related cell other than angiogenic cell clusters arise from mesenchymal cells of visceral mesoderm? What shape do these cells organize into to form the primary heart field? What produces the endocardial tube and what shape is it?

Answer 2

1) Myocardial cells (aka myoblasts) also arise from mesenchymal cells of the visceral (splanchnic) mesoderm
2) Original islands of the myoblasts organize into a horseshoe-shaped primary heart field
3) Among the myoblasts, vasculogenesis creates the crescent-shaped endocardial tube

Question 3

During lateral folding of the embryo to create the pericardial cavity, how does the endocardial tube fuse/close? How is the tube suspended from the posterior wall of the pericardial cavity?

Answer 3

1) During lateral folding of the embryo to create the pericardial cavity, the horseshoe-shaped endocardial tube fuses into a single heart tube with an inner endothelial lining and outer myocardial layer
2) The tube is suspended from the posterior wall of the pericardial cavity by the dorsal mesocardium, a fold of visceral mesoderm

Question 4

Where is the caudal end of the heart tube embedded? What cells form the visceral and parietal layers of the serous pericardium and where are they derived from? What forms the fibrous pericardium?

Answer 4

1) The caudal end of the heart tube is embedded in the septum transversum
2) Mesothelial cells and fibroblasts derived from mesenchymal cells in the septum transversum migrate onto the heart tube to form the visceral and parietal layers of the serous pericardium
3) The fibrous pericardium is formed by the pleuropericardial membranes

Question 5

Which way does the heart tube bend in response to elongation of the cranial end of the tube? Between the cranial and caudal ends of the tube, which is the outflow portion and which is the inflow portion? What eventually forms the transverse pericardial sinus and what is its function?

Answer 5

1) Elongation of the cranial end of the tube causes the bulging tube to bend caudally
2) The cranial end of the tube is the outflow portion and the caudal end is the inflow portion of the tube
3) Eventual degradation of the dorsal mesocardium forms the transverse pericardial sinus that interconnects both sides of the pericardial cavity

Question 6

During elongation of the heart tube, what do dilatations along its craniocaudal axis form?

Answer 6

During elongation of the heart tube, dilatations along its craniocaudal axis form the bulbus cordis, primitive ventricle, primitive atrium, and sinus venosus, which consists of left and right sinus horns

Question 7

Which way does the elongated cranial end of the heart tube bend? What is the function of this bending?

Answer 7

1) The elongated cranial end of the tube bends caudally
2) The caudal bending places the primitive atria, ventricles, bulbus cordis, and sinus venosus in their proper spatial relationships

Question 8

What maintains the connection between the primitive left and right ventricles, as well as the primitive left and right atria? What connects the atria to the ventricles?

Answer 8

1) The primitive left and right ventricles remain connected by the interventricular foramen and the primitive left and right atria also remain connected
2) The single atrioventricular canal connects the atria and ventricles

Question 9

What initiates septum formation in the atrioventricular canal? What separates the single atrioventricular canal into right and left canals?

Answer 9

1) Development of mesenchyme-derived superior (posterior), inferior (anterior), right and left endocardial cushions initiates septum formation in the atrioventricular canal
2) Fusion of the anterior and posterior endocardial cushions separates the single canal into right and left canals between the right and left atria and ventricles

Question 10

How are cusps of the tricuspid and mitral valves formed?

Answer 10

Cusps of the tricuspid and mitral valves are formed by thinning of the fused anterior and posterior cushions and the non-fused right and left endocardial cushions

Question 11

What is tricuspid atresia and which cardiac shunts are always associated with it?

Answer 11

1) Tricuspid atresia, which involves obliteration of the right atrioventricular orifice, is characterized by the absence or fusion of the tricuspid valves
2) Tricuspid atresia is always associated with patency of the oval foramen, ventricular septal defect (VSD), underdevelopment of the right ventricle, and hypertrophy of the left ventricle

Question 12

What does blood flow do to the ventricular walls and surfaces of valvular cusps? What retains attachment of the cusps to the ventricular walls? What develops at these attachment sites?

Answer 12

1) After blood flow hollows out the ventricular walls and ventricular surfaces of the valvular cusps, thin muscular cords retain attachment of the cusps to the ventricular walls
2) Cardiac myocyte-derived papillary muscles develop at the attachment sites

Question 13

What forms the chordae tendineae?

Answer 13

After muscle tissue within the cords degenerates, the remaining dense connective tissue forms the chordae tendineae

Question 14

What forms the trabeculated parts of the left and right ventricles? What do the midportion and distal portion of the bulbus cordis form?

Answer 14

1) The proximal portion of the bulbus cordis forms the trabeculated part (i.e., ridged cardiac muscle) of the right ventricle and the primitive ventricle forms the trabeculated part of the left ventricle
2) The midportion (aka conus cordis) of the bulbus cordis forms the smooth outflow tracts of both ventricles and the distal portion of the bulbus cordis forms the truncus arteriosus

Question 15

What does the truncus arteriosus develop into? Where do left and right pairs of conal and truncal swellings (cushions) develop? What forms the cushions?

Answer 15

1) The truncus arteriosus forms the aortic sac and pulmonary trunk
2) Left and right pairs of conal and truncal swellings (cushions) develop on the walls of the conus cordis and proximal truncus arteriosus
3) Neural crest cells migrate into the outflow tract to form the cushions

Question 16

How is the spiral aorticopulmonary septum formed? What does the septum separate?

Answer 16

1) While growing toward the aortic sac, ipsilateral conal and truncal cushions join to form left and right conotruncal ridges that twist around each other and fuse to form the spiral aorticopulmonary septum
2) The septum separates the left ventricular outflow tract and aortic sac from the right ventricular outflow tract and pulmonary trunk

Question 17

What creates the cusps of the semilunar valves?

Answer 17

Three valvular leaflets form on the pulmonary side of the septum and three similar leaflets form on its aortic side, thereby creating the cusps of the aortic and pulmonary (aka semilunar) valves

Question 18

What causes Tetralogy of Fallot and what are the four characteristic cardiovascular alterations?

Answer 18

1) Tetralogy of Fallot, the most frequently occurring abnormality of the conotruncal region, is due to an unequal division of the conus resulting from anterior displacement of the conotruncal septum
2) Displacement of the septum produces four cardiovascular alterations:
a) a narrow right ventricular outflow region, pulmonary infundibular stenosis
b) a large defect of the interventricular septum
c) an overriding aorta that arises directly above the septal defect
d) hypertrophy of the right ventricular wall because of higher pressure on the right side

Question 19

Which septal malformation causes transposition of the great vessels and why it is the leading cause of death in neonates with cyanotic heart disease?

Answer 19

1) Transposition of the great vessels occurs when the conotruncal septum fails to follow its normal spiral course and runs straight down
2) As a consequence, the aorta originates from the right ventricle, and the pulmonary artery originates from the left ventricle
3) Minimal oxygenated blood is getting to the body

Question 20

What are the main tributaries of the left and right sinus horns?

Answer 20

The umbilical, vitelline, and common cardinal veins are the main tributaries of the left and right sinus horns

Question 21

What does the umbilical vein transport and where is it transported to and from? What do the vitelline veins transport and where is it transported to and from?

Answer 21

1) The umbilical vein transports oxygenated blood from the placenta to the sinus horns
2) The vitelline veins transport venous blood from the temporary yolk sac to the sinus horns

Question 22

Initially, what amount of blood do the sinus horns deliver to the sinus venosus individually? Once a left-to-right venous shunt develops between the sinus horns, what happens?

Answer 22

1) The sinus horns initially deliver equal returns to the sinus venosus
2) After development of a left-to-right venous shunt, venous return shifts toward the right sinus horn

Question 23

After formation of the left-to-right venous shunt, what does the diminished left sinus horn form? What does the right sinus horn form?

Answer 23

1) After development of the venous shunt, the diminished left sinus horn forms the coronary sinus
2) The expanded right sinus horn forms the smooth portion of the right atrial wall

Question 24

What does one of the pulmonary veins in the left atrium form? How do the other three pulmonary veins form?

Answer 24

1) An outgrowth from the posterior wall of the left atrium forms one of the four pulmonary veins
2) The other three veins grow from the developing lungs into the left atrium to complete the formation of its smooth wall

Question 25

What initiates formation of the interatrial septum? How does it do so?

Answer 25

The narrow septum primum initiates formation of the interatrial septum by growing toward the endocardial cushions in the atrioventricular canal but atrial continuity is maintained by the ostium primum

Question 26

What creates the ostium secundum? How is the foramen ovale created?

Answer 26

1) The septum primum ultimately fuses with the endocardial cushions, but apoptosis of fibroblasts and endothelial cells in the upper portion of the septum primum creates the ostium secundum
2) Subsequent formation of the septum secundum fails to completely close the ostium secundum thereby creating the foramen ovale

Question 27

What becomes the valve of the foramen ovale? How is the fossa ovale formed?

Answer 27

1) After formation of the foramen ovale, the inferior part of the septum primum becomes the valve of the foramen ovale
2) After birth, increased left atrial pressure presses the valve against the septum secundum to transform the foramen ovale into the fossa ovale

Question 28

What are the two principal causes of the ostium secundum defect and why does shunting from the left atrium into the right atrium occur?

Answer 28

1) One of the most significant defects is the ostium secundum defect, characterized by a large opening between the left and right atria. It may be caused by excessive cell death and reportion of the septum primum, or by inadequate development of the septum secundum
2) Blood can now flow from left to right

Question 29

What forms the muscular portion of the interventricular septum? What maintains communication between the two ventricles?

Answer 29

1) Merger of the medial walls of the expanding ventricles forms the muscular portion of the interventricular septum
2) The open space between the free rim of the muscular septum and fused endocardial cushions maintains communication between the two ventricles

Question 30

What completes the separation of the ventricles?

Answer 30

Growth of the anterior endocardial cushion along the top of the muscular septum enables the cushion to fuse with the conal septum thereby forming the membranous portion of the interventricular septum and completing the separation of the ventricles

Question 31

Why do about 80% of VSD resolve during childhood? How do the other 20% of VSD affect blood flow into the aorta and pulmonary trunk?

Answer 31

1) Myoblasts continue to produce myocardial cells in the muscular portion of the interventricular septum, allowing for a VSD to resolve
2) Membranous IV septum VSD causes the other 20% and creates a left to right shunt of blood

Question 32

Using internet sources, what is VSD with Eisenmenger syndrome?

Answer 32

1) Eisenmenger syndrome refers to any untreated congenital cardiac defect with intracardiac communication that leads to pulmonary hypertension, reversal of flow, and cyanosis
2) The previous left-to-right shunt is converted into a right-to-left shunt secondary to elevated pulmonary artery pressures and associated pulmonary vascular disease
3) Lesions in Eisenmenger syndrome, such as large septal defects, are characterized by high pulmonary pressure and/or a high pulmonary flow state
4) Development of the syndrome represents a point at which pulmonary hypertension is irreversible and is an indication that the cardiac lesion is likely inoperable