Development of Heart Flashcards
How is the heart slightly twisted in situ?
Would predominantly see right side as left side predominantly towards posterior
When does formation of heart begin?
Week 3
When is first contraction of heart?
Day 22
What are the 3 layers of the heart?
Epicardium Myocardium Endocardium
What is the epicardium? What is it derived from?
Protective outer layer of heat BUT visceral (inner) layer of pericardium (heart cavity) Derived from visceral mesoderm
What is the myocardium? What is it derived from?
Middle –> goes on to form heart muscles Derived from the visceral mesoderm overlying the heart tube
What is the endocardium? What is it derived from?
Inner lining of heart Derived from the heart tube
What is vasculogenesis?
Creating blood vessels from scratch
What is angioblast?
The embryonic tissue from which blood vessels arise
How are angioblasts formed?
The endoderm induces some cells of the overlying mesoderm to differentiate into angioblasts
What do angioblasts then differentiate into?
Endothelial cells which form endocardial tubes with overlying myocardial cells
What does the lateral plate mesoderm get split into?
- Parietal layer
- Inner splanchnic (visceral) layer
What does the splanchnic (visceral) layer go on to form?
The endocardial tubes and myocardial cells
How is the primitive heart tube formed?
Endocardial tubes fuse during lateral folding to form the primitive heart tube
Why does lateral folding occur?
Due to the weight of the amniotic sac
Where is the endocardial (primitive heart) tube in relation to the gut tube?
Ventral to the gut tube
What does the visceral mesoderm surrounding the primitive heart tube differentiate to form?
The myocardium (heart muscle)
What does the myocardium secrete?
A thick layer of extracellular matrix (cardiac jelly)
What brings the developing heart tube into the thorax?
Craniocaudal folding (head and tail end come into closer proximity)
Describe the 3 layers of the resultant heart tube
- Endocardium
- Forming the internal endothelial lining of heart
- Endocardial tube surrounded by cardiac jelly - Myocardium
- Forms muscular wall (myocytes) - Epicardium
- Covering the outside of the heart tube
What is outer layer of heart tube (epicardium) responsible for?
Formation of the coronary arteries
What is purpose of cardiac jelly?
Is a gelatinous connective tissue separating the myocardium and heart tube endocardium
At the 4th week, what is responsible for the inflow of blood to the primitive heart?
The sinus venosus. Right and left sinus horns empty into primitive atrium.
What is the sinus venosus?
At caudal end of the developing heart tube. Largely degerates by week 5. Splits into left and right horn.
What does the cranial region connect to in the 4th week?
2 dorsal aortae
With further differential growth of the heart tube, what 5 dilations become apparent?
- Truncus arteriosus
- Conus arteriosus
- Ventricle
4 . Atrium
- Sinus venosus
These develop into the adult structures of the heart
In foetuses, the ventricle and atrium are the wrong way around. How do they change position?
Cardiac looping –> bending of the heart tube positions the heart in the left thoracic region and creates the ‘typical’ heart shpe with the atria posterior to the ventricles
How does the bulbus cordis move during cardiac looping?
Move caudually, ventrally and right
How does the primitive ventricle move during cardiac looping?
Displaced before moving back to midline
How does the primitive atrium move during cardiac looping?
Displaces cranially and caudally
When does sinus venosus mostly degenerate by?
Largely degenerates by week 5
What does remnants of left horn of the sinus venosus remain as?
Forms the oblique vein of the left atrium and coronary sinus (contributes to venous drainage)
What do vast majority of veins drain into?
The coronary sinus (except the superior and inferior vena cava that go straight to right atrium)
What does coronary sinus empty into?
Right atrium
What does remnants of right horn of sinus venosus remain as?
Remains as part of wall of right atrium –> forms smooth walled part of right atrium (sinus venarum)
What is the smoothed wall part of the right atrium called?
Sinus venarum
What does the majority of the right atrial wall (not smooth) appear as? Where is it derived from?
Appears rough (trabeculated). Dervied from the primitive atrium
How can the smooth (sinus venarum) and rough (trabeculated) parts of right atrial wall be differentiated?
Clear border –> crista terminalis
What is the majority of the ventricular wall (roughened) formed by?
The primitive ventricle with a small contribution from the conus arteriosus
What forms the smooth walls of the left and right vnetricles?
The conus arteriosus
What is the smooth part in the aorta called?
The aortic vestibule
What is the smooth part (of ventricle wall) in the pulmonary trunk called?
Conus arteriosus
What is the rest of the rough (trabeculated) ventricular wall called?
Trabeculae carneae (muscular ridges of ventricle)
What emerges from these trabeculae carneae?
Papillary muscles
During further differentiation of the atria, what happens to the right atrium? What does it incorporate?
Incorporates the right sinus horn (into the wall)
What happens to the left atrium during further differentiation?
- An outgrwoth of the left atrial wall forms a single pulmonary vein
- Pulmonary vein branches into left and right veins which bifurcate to form 4 pulmonary veins
- In week 5, the 4 pulmonary veins are incoportated into the wall of the left atrium
What is the process of incorporating the 4 pulmonary veins into the wall of the left atrium called?
Intussesception
What is the majority of the atrial wall derived from?
The primitive atrium
When does the primitive atrium start to be divided into left and right atria?
At end of week 4
How does primitive atrium start to be divided into left and right?
Crescent-shaped outgrowth from the dorsal wall called the septum primum
As the septum primum extends, it dimishes the connection between the left and right sides of the primitive atrium. What is this called?
Foramen primum
As the septum primum extends into the atrium, the endothelium lining the boundary between the atrium and the ventricle expands. What does this form?
Dorsal and ventral endocardial cushions
What are the endocardial cushions outgrowths from?
The endocardium
What happens to these endocardial cushions? What is then formed?
They fuse in the midline to form the atrioventricular septum with 2 channels communicating between the future left atrium and ventricle and right atrium and ventricle (how blood passes from atrium to ventricle in adults)
What separates the left and rigt atrium?
Septum primum
What separates the atrium and ventricle?
Endocardial cushions
What is the atrioventricular septum?
Separates atria and ventricle
The septum primum eventually fuses with the atrioventricular septum. What does this cause?
Obliteratures the foramen primum and separates the atria and ventricle
What is the foramen primum?
Connection between the left and right sides of primitive atrium (ostium primum)
What is the foramen/ostium secundum? How is it formed?
A foramen (hole) in the upper part of the septum primum. Formed by apoptosis
What is the septum secundum? How does it form?
A 2nd cresent shaped projection forms from the dorsal wall of the atrium. Develops on right side of septum primum.
What is the opening in the septum secundum called?
The foramen ovale
What is the opening in the septum primum called?
Foramen secundum
What is purpose of these openings in the septum primum and septum secundum?
Allows blood to flow from the right atrium to the left atrium and bypass the lungs (foetus does not need to breathe)
What is the truncus arteriosus?
Dilation at the cranial end
How are vessels leaving the heart (aorta and pulmonary trunk) separated?
Septation of the truncus arteriosus into the aorta and pulmonary trunk
How is the truncus arteriosus divided?
Divided into 2 channels by endocardial swellings (conotruncal ridges) that grow out from the outflow tract, spiral and fuse together
What do these endocardial swellings fuse to form?
The conotruncal septum
What does the conotruncal septum separate?
Separates the outflow of blood of the left and right ventricles
What does the conotruncal septum fuse with?
The interventricular septum, forming the membranous part of the interventricular septum
How do the conotruncal ridges (swellings) fuse? Why is this required?
Not in a straight line but spiral around each other. Required for right ventricle to connect with the pulmonary trunk and left ventricle to connect with the aorta
Separates oxygenated and deoxygenated blood
What do the conotruncal swellings contain?
Neural crest cells
What do neural crest cells do?
As neural folds elevate and fuse, the cells at the lateral edge separate from the neural tube (neural crest cells). These migrate laterally and ventrally, dispersing widely and differentiating into a variety of structures throughout the body
Sometimes referred to as 4th germ layer
In foetal circulation, why is blood shunted away from the liver and lungs?
The mother’s circulation is oxygenating and detoxifying blood
How does oxygenated blood pass to the foetus?
Through the umbilical cord
Where does the oxygenated blood go after reaching the foetus?
Some enters the liver, the rest enters the ductus venosus (bypassing the liver) and entering the inferior vena cava and then the right atrium
What is the ductus venosus?
Allows venous blood to bypass the liver and enter the inferior vena cava and then the right atrium
When oxygenated blood reaches the right atrium in foetuses, what then happens?
Most of the blood passes through the foramen ovale to the left atrium, left ventricle and aorta (bypasses lungs)
What is the ductus arteriosus?
A patent vessel in fetal circulation between the pulmonary trunk and aorta. A shunt in fetal circulatory system so blood can bypass the lungs. Closes shortly after birth to form the ligamentum arteriosum
After birth, why do these shunts have to be closed off?
As blood needs to be sent to/from the lungs
How is the ductus arteriosus closed off?
Increase in pressure in atria (due to blood now passing to lungs) causes septum primum to flap against septum secundum. This closes off the shunt between the atria and the ductus arteriosus to form the ligamentum arteriosum
What causes the ductus venosus to close and degenerate? What does it become?
Termination of the umbilical circulation.
Ligamentum venosum in adults
How is the foramen ovale closed?
During first breath:
- Pulmonary arteries dilate
- Changes pressure in atria
- Blood returning from lungs increases pressure in left atrium and flimsy septum primum is pushed against the sturdy septum secundum
- This closes the forament ovale to become the fossa ovalis in adults
What is the ligamentum arteriosum?
A small ligament that is the remnant of the ductus arteriosus formed within three weeks after birth
The heart begins to develop near the head of the embryo. What is this area called?
Cardiogenic area
Following signals from the underlying endodermm what does the cardiogenic area begin to form?
2 strands called the cardiogenic cords
What happens as the cardiogenic cords develop?
A lumen rapidly develops within them and they become endocardial tubes
What do the 2 endocardial tubes fuse together to form?
A single primitive heart tube
When are cardiogenic cords called endocardial tubes?
Once lumen forms within
What is the ductus venosus?
Shunts a portion of umbilical vein blood flow directly to the inferior vena cava. Thus, it allows oxygenated blood from the placenta to bypass the liver.
What is a serous membrane made of?
The thin membrane is made up of mesothelium tissue which originates from the mesoderm
What is a serous membrane?
a thin membrane that lines the internal body cavities and organs such as the heart, lungs, and abdominal cavity
What is the purpose of a serous membrane?
The serous membrane allows for frictionless movement in a number of vital organs.
What is the epicardium?
a serous membrane that forms the innermost layer of the pericardium
What is the function of the epicardium?
- A thin layer of elastic connective tissue and fat that serves as an additional layer of protection from trauma or friction for the heart under the pericardium
- Contains the coronary blood vessels, which oxygenate the tissues of the heart with a blood supply from the coronary arteries
What is the difference between endothelial and epithelial cells?
epithelial cells line both internal surfaces and external surfaces of the body whereas endothelial cells line the internal surfaces of the components of the circulatory system
What is the conotruncal septum?
Separates the truncus cordis into pulmonary trunk and aorta (formed as 2 endocardial swellings spiral and fuse together)
What is the conotruncal septum formed from?
From neural crest (conotruncal swellings contain neural crest cells)
