L3 Embryology Development of the Heart

Question 1

What day does contraction of the heart begin?

Answer 1

Day 22

Question 2

What is the structure of the heart wall? What is each layer derived from?

Answer 2

The heart is made up of 3 layers: a endocardium (derived from the heart tube), myocardium (derived from visceral mesoderm) and epicardium.

The heart begins as a tube developed from embryonic mesoderm that differentiate into:

Mesothelium - which goes goes onto form the pericardium
Endothelium - which becomes the endocardium, blood vessels and the lymphatics
Myocardium - which goes onto form the heart muscle

Question 3

How is the heart tube formed?

Answer 3

Above the angiogenic cell clusters are primitive cariogenic cells. These cells are stimulated to differentiate by the endoderm. The endoderm forms the inner epithelial lining of the respiratory system. The epithelium stimulates the mesoderm to differentiate into endothelial cells which eventually form endocardial tubes with overlying myocardial cells.

This endocardial tube and the myocardial cells comes from the mesoderm. It is formed from the lateral plate mesoderm. The angiogenic cell clusters are the cells that form the two endocardial heart tubes. The tow tubes fuse to form one tube. Around the endocardial tube there is an outer myocardium from the myocardial cells which were next to the endocardial tube. The myocardium creates a cardiac jelly and secretes it into the extracellular matrix.

Question 4

How does folding of the embryo create the heart tube?

Answer 4

The folding of the embryo causes the heart tube to move ventral to the gut tube and move into there thoracic cavity. The two heart tubes fuse to form one heart tube.

Question 5

When is the heart tube developed?

Answer 5

The heart tube Starts to develop at day 18-19 and is functional at day 22.

Question 6

What is the function of cardiac jelly?

Answer 6

The cardiac jelly is an acellular jelly that is important in the septation of the heart and cardiac looping. It forms endocardial cushions. The cardiac jelly is also believed to have a role in the differential growth fo the heart tube leading to 5 dilations of the heart.

Question 7

What are the layers of the heart tube?

Answer 7

The heart tube is made of an inner endocardium surrounded by cardiac jelly. This is surrounded by the myocardium and the pericardium.

Question 8

How does the vascular system form in week 4?

Answer 8

Caudal region there is 3 paired veins which drain into the tubular heart in week 4 via the right and left horn of the sinus venosus. The cranial region connects 2 dorsal aortae.

Question 9

How does the heart tube further differentiate?

Answer 9

Moving from the cranial end to the caudal end there is a: bulbous cordis (Truncus arteriosus and Conus arteriosus), primitive ventricle, primitive atria and sinus venosus.

The Bulbous cores is made up of the Truncus ateriosus and the Conus arteriosus, these form the outflow tract for blood. the sinus venosus is made up of a left and right horn.

Question 10

When does the heart tube start to fold?

Answer 10

Day 23

Question 11

When does the sinus venous degenerate?

Answer 11

Week 5

Question 12

What does the right and left horn of the sinus venous go onto form?

Answer 12

Right horn goes onto form part of the right atria.

The left horn goes onto form the oblique vein of the left atrium and coronary sinus.

Question 13

What is the origin of the right atria?

Answer 13

The right horn forms the smooth part of the right atria - Sinus vernarum.

The rough part of the atria was derived from the primitive atria.

The border between the trabeculated part of the right atria and the sinus vernarum is the crest terminals.

Question 14

What is the origin of the ventricles?

Answer 14

the majority fo the ventricles is formed from the primitive ventricle with a small contribution form the conus arteriosus. The conus arteriosus forms the smooth walls of the left and right ventricle that lead into the aorta (aortic vestibule) and pulmonary trunk (conus arteriosus). the rest of the ventricular wall is roughened and is formed from the primitive ventricle.

Question 15

How does the atria further differentiate in week 5?

Answer 15

The left atrial wall is smooth incorporation to the right due to intussusception. From the left atrium we sprout an out growth of pulmonary veins. As the atria continue to develop, it takes up the vessel wall and incorporates it into the lining of the atrium. Therefore some of the wall is derived from the pulmonary veins. The pulmonary veins bifurcates into primary veins until there is eventually four pulmonary veins.
The roughened part of the left atria is derived from the primitive atria.

Question 16

When does the septum premium develop?

Answer 16

Week 4

Question 17

What is the foramen primium?

Answer 17

In the developing heart, the atria are initially open to each other, with the opening known as the foramen primum. The foramen lies beneath the edge of septum primum and the endocardial cushions.

Question 18

What are dorsal and ventral endocardial cushions? What are their function?

Answer 18

The endothelium lining the boundary between the atrium and ventricle expand to form a left, right, dorsal and ventral endocardial cushions. The cushions are outgrowths from the endothelium.

Question 19

What is the septum secundum?

Answer 19

The septum secundum is a fold of the dorsal atrial myocardium, begins forming from the cranial aspect of the atria, and forms the roof of the foramen ovale. This develops on the right side of the septum premium. The atrial septum membrane is made up of 2 layers.

Question 20

What is the foramen secundum?

Answer 20

By week 6, the septum primum has fused with the atrioventricular septum, obliterating the ostium primum.
Apoptosis occurs in the upper part of the septum primum to form the foramen secundum

Question 21

How does blood bypass the lungs in the embryo?

Answer 21

• Through the foramen ovale, straight into the left atrium
  Some blood does pass to the right ventricle and is ejected into the pulmonary trunk. In the pulmonary trunk only a small amount is sent to the lung. There is a bypass mechanism called the ductus arteriosus in which we can bypass the lungs.

Question 22

How does separation of the ventricles occur?

Answer 22

There is two origins of septation of the primitive ventricle: a muscular part, derived from the floor of the ventricles separating into a right and left ventricle, and a membranous part derived developing septum between the vessels - The outflow tract. The intraventricular septum has 2 origins: a muscular and membranous part.

At the end of week 4, the muscular wall projects from the floor of the primitive ventricle towards the endocardial cushions leaving an interventricular foramen. A membranous part that projects inferiorly from the endocardial cushion to close the interventricular foramen.

Question 23

How is the outflow tract separated?

Answer 23

The truncus arteriosus is divided into 2 channels by endocardial swellings – conotruncal ridges (swellings). The ridges spiral and fuse together to form a conotruncal septum. This separates the outflow of the left and right ventricles with the inter ventricular septum. The conotruncal swellings don’t fuse in a straight line but spiral. The conotruncal septum forms the membranous part of the intraventricular septum. It separates the aorta from the left ventricle and the pulmonary trunk form the right ventricle. Now deoxygenated and oxygenated blood is separated. Neural crest cells migrate and differentiate to form some of the conotruncal septum.

Question 24

What is the fossa ovalis?

Answer 24

An indentation of the foramen ovale found in an adult heart.

Question 25

What is the ligamnetum venosum and ligamentum arteriosum?

Answer 25

The ligamentum venosum : is the fibrous remnant of the ductus venosus of the foetal circulation. the ductus venosus is a shunt between the inferior vena cava and the umbilical vein.

Ligamentum arteriosus: is a small ligament that is the remnant of the ductus arteriosus formed within three weeks after birth. The duct acted as a shunt between the aorta and the pulmonary artery.

Question 26

What is the foreman ovale?

Answer 26

An opening in the septum secdundum.