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.