Development of the heart Flashcards
What is the first day of contraction of the heart?
Day 22 of embryology
What are the 3 layers the heart is composed of? What do they do?
- 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
What is vasculogenesis?
- 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
What happens once the endocardial tubes form?
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).
What does the myocardium secrete?
The myocardium secretes a thick layer of extracellular matrix (cardiac jelly).
Why is cardiac jelly important?
Acellular jelly- important in cardiac looping and septation of the heart- called endocardial cushions.
How does the heart tube develop into the thorax?
Craniocaudal folding brings the developing heart tube into the thorax- sagittal section
What does the heart tube consist of?
- 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
What region has 3 veins in an embryo? What do they do?
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
What connects the 2 dorsal aortae in a 4 week embryo?
Cranial region connects to 2 dorsal aortae
What are the 5 dilations after further differentiation of the heart tube?
- Truncus arteriosus
- Conus arteriosus
- Ventricle
- Atrium
- Sinus venosus
What does the truncus arterioles and conus arterioles make up?
Bulbus cordis
What happens on day 23?
The Heart Tube starts to fold
What happens during cardiac looping?
- Bulbus cordis moves caudally, ventrally and to the right
- Primitive ventricle is displaced before moving back to midline
- Primitive atrium displaces cranially and dorsally
What happens to the sinus venosus?
- 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
What dos the right horn form?
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
What is the crista terminals?
Clear border between the trabeculated part of the right atrium and the sinus vernarum
How are the ventricles formed?
The majority of the ventricular wall is formed by the primitive ventricle with a small contribution from the conus arteriosus.
What forms the smooth walls of the LV and RV?
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
What forms the rough walls of the ventricles?
• The rest of the ventricular wall is trabeculated (rough) and formed from the primitive ventricle – trabeculae carneae
How is the atria further differentiated?
- 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
What is intussusecption?
- 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
What happens at the end of week 4?
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
How is the foramen primum formed?
As the septum primum extends, the diminishing connection between left and right sides of the primitive atrium is called the foramen primum.
How are the dorsal and ventral endocardial cushions formed?
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.
What do the dorsal and ventral endocardial cushions then do?
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.
By week 6 what happens?
The septum primum has fused with the atrioventricular septum, obliterating the ostium primum
How is the foramen secundum formed?
Apoptosis occurs in the upper part of the septum primum to form the foramen secundum
How is the septum secundum formed?
At the same time, a second crescent shaped projection forms from the dorsal wall of the atrium – septum secundum
What is the foramen ovale? How does it form?
- 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
What are the two origins of the intraventricular septum?
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)
How is the trunks arteriosus septated?
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.
What do the conotruncal swellings contain? How are they useful?
- 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
How does foetal circulation occur?
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)
What is ligament venosum?
Termination of the umbilical circulation causes the ductus venosus to close and degenerate – ligamentum venosum in adult.
How does circulation occur straight after birth?
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
