Development of the heart

Question 1

What is the first day of contraction of the heart?

Answer 1

Day 22 of embryology

Question 2

What are the 3 layers the heart is composed of? What do they do?

Answer 2

• Epicardium – This is the visceral layer of the pericardium and is also derived from the visceral mesoderm
• Myocardium – derived from the visceral mesoderm overlying the heart tube
• Endocardium – derived from the heart tube

Question 3

What is vasculogenesis?

Answer 3

• The endoderm induces some cells of the overlying visceral/splanchnic mesoderm to differentiate into angioblasts
• Angioblasts differentiate into endothelial cells and form tubes (vasculogenesis) – endocardial tubes

Question 4

What happens once the endocardial tubes form?

Answer 4

The endocardial tubes fuse during lateral folding to form the primitive heart tube.

The visceral mesoderm surrounding the primitive heart tube differentiates to form the myocardium (heart muscle).

Question 5

What does the myocardium secrete?

Answer 5

The myocardium secretes a thick layer of extracellular matrix (cardiac jelly).

Question 6

Why is cardiac jelly important?

Answer 6

Acellular jelly- important in cardiac looping and septation of the heart- called endocardial cushions.

Question 7

How does the heart tube develop into the thorax?

Answer 7

Craniocaudal folding brings the developing heart tube into the thorax- sagittal section

Question 8

What does the heart tube consist of?

Answer 8

• Endocardium: forming the internal endothelial lining of the heart
• Myocardium: muscular wall
• Epicardium : covering the outside of the heart tube (Note this outer layer is responsible for formation of the coronary arteries)
• Note also presence of cardiac Jelly: gelatinous connective tissue separating the myocardium and heart tube endocardium

Question 9

What region has 3 veins in an embryo? What do they do?

Answer 9

Caudal region 3 paired veins drain into the tubular heart of a 4 week embryo via the right and left horn of the sinus venosus: heart is already beating at this point

Question 10

What connects the 2 dorsal aortae in a 4 week embryo?

Answer 10

Cranial region connects to 2 dorsal aortae

Question 11

What are the 5 dilations after further differentiation of the heart tube?

Answer 11

• Truncus arteriosus
• Conus arteriosus
• Ventricle
• Atrium
• Sinus venosus

Question 12

What does the truncus arterioles and conus arterioles make up?

Answer 12

Bulbus cordis

Question 13

What happens on day 23?

Answer 13

The Heart Tube starts to fold

Question 14

What happens during cardiac looping?

Answer 14

• Bulbus cordis moves caudally, ventrally and to the right
• Primitive ventricle is displaced before moving back to midline
• Primitive atrium displaces cranially and dorsally

Question 15

What happens to the sinus venosus?

Answer 15

• The sinus venosus largely degenerates by week 5
• It remains as part of the wall of the right atrium (right horn) and contributes to the venous drainage (left horn) of the heart
• The left horn forms the oblique vein of the left atrium and coronary sinus

Question 16

What dos the right horn form?

Answer 16

The right horn forms the smooth-walled part of the right atrium – sinus venarum

This can be differentiated easily from the majority of the right atrial wall which was derived from the primitive atrium and appears rough – trabeculated

Question 17

What is the crista terminals?

Answer 17

Clear border between the trabeculated part of the right atrium and the sinus vernarum

Question 18

How are the ventricles formed?

Answer 18

The majority of the ventricular wall is formed by the primitive ventricle with a small contribution from the conus arteriosus.

Question 19

What forms the smooth walls of the LV and RV?

Answer 19

The conus arteriosus forms the smooth walls of the left and right ventricles that lead into the aorta (aortic vestibule) and pulmonary trunk (conus arteriosus) respectively

Question 20

What forms the rough walls of the ventricles?

Answer 20

• The rest of the ventricular wall is trabeculated (rough) and formed from the primitive ventricle – trabeculae carneae

Question 21

How is the atria further differentiated?

Answer 21

• While the right atrium enlarges by incorporating the right sinus horn, the left atrium also undergoes remodelling
• An outgrowth of the left atrial wall forms a single pulmonary vein

Question 22

What is intussusecption?

Answer 22

• The pulmonary vein branches into left and right veins which then bifurcate to form 4 pulmonary veins
• In week 5, the 4 pulmonary veins are incorporated into the wall of the left atrium -intussusception

Question 23

What happens at the end of week 4?

Answer 23

Septation of the Primitive Atrium

• The majority of the atrial wall is derived from the primitive atrium.
• The primitive atrium now needs to be divided into left and right atria
• This begins at the end of week 4 with a crescent-shaped outgrowth from the dorsal wall – septum primum

Question 24

How is the foramen primum formed?

Answer 24

As the septum primum extends, the diminishing connection between left and right sides of the primitive atrium is called the foramen primum.

Question 25

How are the dorsal and ventral endocardial cushions formed?

Answer 25

As the septum primum extends into the atrium, the endothelium lining the boundary between the atrium and ventricle expands to form dorsal and ventral endocardial cushions.

Question 26

What do the dorsal and ventral endocardial cushions then do?

Answer 26

These will fuse in the midline to form the atrioventricular septum with 2 channels communicating between the future left atrium and ventricle and the right atrium and ventricle.

Question 27

By week 6 what happens?

Answer 27

The septum primum has fused with the atrioventricular septum, obliterating the ostium primum

Question 28

How is the foramen secundum formed?

Answer 28

Apoptosis occurs in the upper part of the septum primum to form the foramen secundum

Question 29

How is the septum secundum formed?

Answer 29

At the same time, a second crescent shaped projection forms from the dorsal wall of the atrium – septum secundum

Question 30

What is the foramen ovale? How does it form?

Answer 30

• The septum secundum does not form a complete partition
• The opening left in the septum secundum is the foramen ovale

• This allows blood to flow from the right atrium to the left atrium, bypassing the lungs

Question 31

What are the two origins of the intraventricular septum?

Answer 31

1) End of week 4, a muscular part that projects from the floor of the primitive ventricle towards the endocardial cushions leaving an interventricular foramen
2) A membranous part that projects inferiorly from the endocardial cushion to close the interventricular foramen (week 7)

Question 32

How is the trunks arteriosus septated?

Answer 32

The truncus arteriosus is divided into 2 channels by endocardial swellings – conotruncal ridges (swellings).

These swellings fuse to form a septum (conotruncal septum) that separates the outflow of left and right ventricles and fuses with interventricular septum

The contruncal swellings don’t fuse in a straight line but spiral round each other. This is required for the right ventricle to connect with the pulmonary trunk and the left ventricle to connect with the aorta.

Question 33

What do the conotruncal swellings contain? How are they useful?

Answer 33

• As neural folds elevate and fuse, cells at lateral edge separate from the neural tube – neural crest cells
• Migrate laterally and ventrally, dispersing widely and differentiating into a variety of structures throughout the body
• Sometimes referred to as the 4th germ layer

Question 34

How does foetal circulation occur?

Answer 34

Mother’s circulation is oxygenating and detoxifying blood therefore the foetal circulation shunts blood away from the lungs and liver
• Oxygenated blood enters through the umbilical vein
• Some blood enters the liver and the rest enters the ductus venosus to bypass the liver and enter the inferior vena cava
• The IVC enters the right atrium and most of the blood passes through the foramen ovale to the left atrium, left ventricle and aorta
• Blood entering the right atrium from the superior vena cava is poorly oxygenated as is blood returning from the lungs
• This passes through the ductus arteriosus and reduces the oxygenation of the blood in the aorta (purple)

Question 35

What is ligament venosum?

Answer 35

Termination of the umbilical circulation causes the ductus venosus to close and degenerate – ligamentum venosum in adult.

Question 36

How does circulation occur straight after birth?

Answer 36

The first breath causes the pulmonary arteries to dilate and changes pressures in the atria. Blood returning from the lungs increases pressure in left atrium and the flimsy septum primum is pushed against the sturdy septum secundum closing the foramen ovale – fossa ovalis in adult
The first breath also causes changes in oxygen saturation in the ductus venosus that cause it to constrict – ligamentum arteriosum in adult