What is the purpose of the first two weeks of embryonic development?
To create the tissues of the future embryo and future placenta, achieving the right cells of the right type in the right place
What does the third week of embryonic development achieve?
Creates the three germ layers, the ectoderm, mesoderm and endoderm
Where does the cardiovascular system develop from?
The mesoderm
What happens in the fourth week of embryological development?
It creates a recognisable body form Mesoderm begins to organise
Where does the cardiogenic area initially lie on the embryonic disc?
In the wrong place- anterior to the buccopharyngeal membrane
What is the cardiogenic field of the embryonic disc?
A collection of specialised mesodermal cells that’s creating vascular like tissue and all its constituents
What forms about the cardiogenic field?
The pericardial cavity
What happens within the pericardial cavity?
Specialisation of the mesoderm, and differentiation of blood cells, tiny vessels and ultimately, the primitive heart
What is found within the jelly of the pericardial sac?
Blood islands
What happens as blood islands continue to develop?
You get tiny blood vessels beginning to appear
What creates a heart tube?
Lateral folding
What brings the heart into the thoracic region?
Cephalocaudal folding
Where is the heart after cephalocaudal folding?
Still more cranial than is ultimately the case
What is the overall result of folding?
It puts the tube into the first big shared cavity, the intraembryonic coelom
What ultimately happens to the intraembryonic coelom?
This cavity ultimately becomes divided by the diaphragm, into the thorax and the abdominal cavity
How many heart tubes develop?
Two, one on either side of the midline
What happens when lateral folding pushes the two tubes together?
They fuse together to give the primitive heart tube
What has happened once cephalocaudal folding has put the heart tubes in the right place?
The primitive tube is sitting within a space that has opened up to accommodate it, the pericardial cavity
How is the heart tube suspended in the pericardial cavity?
A very thin membrane
What happens to the thin membrane suspending the heart tube?
It degenerates, leaving the heart tube tethered cranially and caudally, but free to move around
What does the primitive heart tube consist of?
Aortic roots Truncus arteriosus Bulbus cordis Ventricle Atrium Sinus venosus
What happens as the heart tube moves up from the atrium to the ventricle to bulbus cordis?
It becomes tapered, becoming narrower as we proceed out of the heart tube and into outflow
What is looping fundamental for?
The normal proceeding of cardiac development
What does continued elongation of the heart tube result in?
Bending
Why does the heart tube continue to elongate?
To meet the advancing needs of the foetus as it grows
Why does the heart tube bend as it elongates?
Because it is growing into a space that is fixed by the pericardial sac, and so as the tube gets longer, it needs to fold up
When does the elongation of the heart tube process begin?
Day 23
When is the elongation of the heart tube process complete?
Day 28
What does the early occurrence of the heart tube elongation illustrate?
That the embryo needs a cardiovascular system to support itself, as it’s outgrown the capacity to support its need via simple diffusion alone
In what direction does the cephalic portion of the heart tube grow?
Ventrally, caudally, and to the right
In what direction does the caudal portion of the heart tube grow?
Dorsally, cranially and to the left
How do certain zones of the heart tube differ from others?
They grow at different rates
Give an example of where a zone of the heart tube grows at a different rate to the other?
Theres lots of growth of the ventricles, which causes the specific way the heart tube folds
What drives looping?
The limitation of space caused by tethering
What happens once looping is completed?
It pushes the outflow anterior to the inflow, producing the transverse pericardial sinus, and so the arteries end up in front of the veins
What does looping essentially do?
Puts everything in the right place to ensure that positioning can go ahead
Where does looping put the primordiums?
Puts the primordium of the right ventricle closest to the outflow tract, and the primordium of the left ventricle closes to the inflow tract
Where does looping put the atrium?
Dorsal to the bulbus cordis
What is the positioning of the individual domains/areas on the primitive heart tube critical to?
Establishing normal partitioning
What happens after looping?
The atrium communicates with teh ventricles via the atrioventricular canal
What does the atrioventricular canal result from?
The pinching of the primitive heart tube at the junction between the primitive atrium and the primitive ventricle
Are the chambers at the point after looping?
No, it is all continuous
What is initially true of the left and right sinus horns?
They are of equal size
What is true of the venous system in the embryo?
It is very symmetrical, with a highly symmetrical arrangement of venous drainage of placenta, yolk sac and body
What quickly happens to the venous system?
The symmetrical arrangement is lost, and venous return shifts to the right hand side, and so with right sinus horn becomes dominant, with the left sinus horn receding
What happens to the right sinus horn?
It is absorbed by the enlarging right atrium
What does the primitive atrium become?
Essentially, the right atrium
What is the right atrium also developed from?
The sinus venosus, as the right horn gets engulfed
What does the right atrium receive?
Venous drainage from the body (venae cavae) and the heart (coronary sinus)
What is the coronary sinus?
The remnant of the left sinus horn
What does the left atrium develop from?
A small portion of the primitive atrium. It also absorbs the proximal parts of the pulmonary veins.
What does the left atrium receive?
Oxygenated blood from the lungs
What does the embryo not commit a lot of energy to?
Structures that it doesn’t need until the end of development- the limb’s and lungs
When do limbs and lungs develop?
Towards the end of foetal life
What happens from the tiny zone of the primitive atrium that ultimately gives the left atrium?
A new vessel sprouts, the primitive pulmonary veins
What happens to the left atrium as development of the heart and lungs continue?
It engulfs the proximal parts of the primordial pulmonary veins, and then proceeds further out, until it ultimately engulfs all the way up, leaving the entrance of the four pulmonary veins
Why is there an oblique sinus within the pericardial sac?
Due to development/remodelling of the venous inflow to the developing heart tube
How is the oblique sinus formed?
As the left atrium expands, absorbing the pulmonary veins
What needs to be established in the foetus?
A foetal circulation that is dependant on oxygen rich blood coming from a place that doesn’t exist after birth- the placenta
How is foetal circulation from the placenta achieved?
Through the umbilical circulation
What happens to the lungs in the foetus?
They are non functional until after birth
What is the result of foetal lungs being non-functional until after birth?
The foetus must receive oxygenated blood from the mother via the placenta
What happens at the placenta?
Gas exchange
How is blood passed from placenta to body?
By the umbilical vein
How does the circulation bypass the lungs and liver?
By action of shunts
Why must the umbilical vein bypass the liver?
Because the first thing the oxygenated blood from the umbilical vein encounters, but the foetal liver is very metabolically active, and so would use up all the oxygen and there would be none left for the rest of the body
What do we need to ensure happens of the shunts?
They close immediately after birth
What route does oxygenated blood from the mother take?
It enters the placenta, bypasses the liver, and enters the right atrium through the inferior vena cava
Does oxygenated blood enter the right ventricle in the foetal circulation?
A small amount does
Why does a small amount of blood enter the right ventricle in the foetal circulation?
They pass to the pulmonary trunk to give the lungs the nutrition they need to develop
How does the blood thats gone to the pulmonary trunk bypass the lungs?
By the ductus arteriosus
What does the ductus arteriosus connect?
The pulmonary trunk to the aorta
What happens to the majority of blood in the pulmonary circulation?
It bypasses the right ventricle and lungs, and goes directly to the left atrium, by action of the foramen ovale shunt, and then to the left atrium, to the aorta, which passes oxygenated blood to the body
What happens to deoxygenated blood?
It returns to the placenta and goes back to the mother
Why does blood in the foetal circulation bypass the lungs?
Partially because there is no point, as the blood is already oxygenated, and partially because we need to protect the developing lungs, as they are behind in their developmental programme, and so would not be able to cope if we were to send all of the cardiac output to the lungs
What does the outflow of the primitive heart tube feed into?
A structure known as the aortic sac
What does the aortic sac feed into?
An early arterial system
What does the early arterial system begin as?
A bilaterally symmetrical system of arched vessels
What happens to the early arterial system?
It undergoes extensive remodelling to create the major arteries of the heart
What does the early arterial system consist of?
5 arches, number 1-4, and 6
What happens during remodelling of the early arterial system?
There is a loss of all or parts of some of the arches
What happens to the 4th arch of the early arterial system?
It is remodelled so that the right part becomes the subclavian artery, and the left becomes the arch of the aorta
What is the 6th arch of the early arterial system known as?
The pulmonary arch
What happens to the pulmonary arch?
The right part becomes the right pulmonary artery, and the left part becomes the left pulmonary artery and ductus arteriosus
What does each aortic arch have?
A corresponding nerve
What is the nerve corresponding to the 6th aortic arch?
The recurrent laryngeal nerve (br vagus, CN X)
Where does the right branch of the recurrent laryngeal nerve descend?
To T1-T2
Where does the left branch of the recurrent laryngeal nerve descend?
To T4-T5
What do the branches of the recurrent laryngeal nerve innervate?
The larynx
Why are the branches of the recurrent laryngeal nerve anatomically unusual?
They descend down the thorax, then come back on themselves to innervate the muscles
What influences the course of the recurrent laryngeal nerves?
The caudal shift of the developing heart The expansion of the developing neck region The need for a foetal shunt between the pulmonary trunk and aorta
What links the primitive atrium and ventricle?
Atrioventricular canal
What is created to divide the two primitive atria?
An interatrial septum
What is created to divide the two primitive ventricles?
The interventricular septum
What is the result of the septation of the outflow tract?
Blood is diverted from two separate ventricles into separate circulations through the pulmonary trunk and the aorta
What does septation, and the creation of the interventricular and interatrial septums create?
4 chambers, and selective outflow
Where do the beginnings of septation occur?
The atrioventricular canal
What is the purpose of the atrioventricular canal?
After looping, the atrium communicates with the ventricle via this canal
What are endocardial cushions?
Tissue growing in the walls of endocardium
Where do endocardial cushions start developing?
In the atrioventricular region
What do the endocardial cushions do?
Protrude out into the canal, pushing into the initially patent channel. They grow towards each other and fuse in the muddle
What does the fusion of endocardial cushions create?
A platform dividing the developing heart into right and left channels, and so the beginning of a structure towards which walls can grow
Why is atrial septation complex?
Because need to separate and create two discreet atria, but also allow for a foetal shunt
What does division of the common atrium involve?
Formation of two septa, with three holes to allow the opportunity for blood to shunt in foetal life
What is the septum primum?
A wedge of tissue that grows down towards the fused endocardial cushions from the roof of the atrium
What is the ostium primum?
The hole present before the septum primum fuses with the endocardial cushions
What happens before the ostium primum closes?
A second hole, the ostium secondum, appears in the septum primum
What is the septum secundum?
A second, cresent shaped septum that grows
What is present in the septum secundum?
A small hole, the foramen ovale
What is true of the foramen ovale and ostium secundum?
They are staggered
Why are the foramen ovale and ostium secundum staggered?
So there is a right to left shunt, as the blood entering the right atrium via the vena cava causes a high pressure, pushing the septums apart so blood can flow
What do both the left and right atria have?
The auricles
What are the auricles?
Components derived from the primitive atrium
What does the right atrium absorb?
The sinus venosus
What does the left atrium absorb?
The pulmonary vein it sprouted, and its 4 branches
What is the fossa ovalis?
The adult remnant of the shunt used in utero to bypass the lungs
What are the components of the ventricular septum?
Muscular and membranous
What does the muscular portion of the ventricular septum form?
Most of the septum
How does the muscular portion of the ventricular septum grow?
Upwards towards the fused endocardial cushions
What does the growth of the muscular portion towards the endocardial cushions leave?
A small gap called the primary interventricular foramen
What is the membranous portion of the interventricular septum formed by?
Connective tissue derived from endocardial cushions
What does the membraneous portion of the interventricular septum do?
Fills the primary interventricular foramen
Where do endocardial cushions appear?
In the truncus arteriosus
What happens as endocardial cushions grow?
They twist around each other, forming a spiral septum
What is formation of the twisted endocardial cushions dependant on?
Contribution from the neural crest
Where does the neural crest migrate from?
Outside of the cardiogenic field
What is present before endocardial cushions appear?
A right and left channels
What happens to the left and right channels when endocardial cushions appear?
It creates a spiral divisions of the outflow tract
What does the spiral division of the outflow tract create?
The ability to separate outflow of the left ventricle from the right ventricle, and divert it to the appropriate site
What is the spiral arrangement of the outflow tract the reason for?
The aorta and pulmonary trunk seem to wrap around each other
What happens after birth?
Respiration begins, opening the pulmonary circulation
What is the result of the opening of the pulmonary circulation?
Left atrial pressure increases, and so forces the foramen ovale closed
What happens to the ductus arteriosus after birth?
It contracts in the presence of the higher oxygen saturation
What happens to the contraction of the ductus arteriosus over time?
It becomes an anatomical contraction, making this vessel fibrotic
What happens when the placental support is removed?
The ductus venous closes as there is no blood flowing through it
Why does the foramen ovale close at birth
Because the LA pressure is greater than the RA pressure, so the septum primum is pushed against the septum secundum, and so blood cannot pass through
What does the foramen ovale become?
The fossa ovalis
What does the ductus arteriosus become?
The ligamentum arteriosum
What does the ductus venosus become?
The ligamentum venosum
What does the umbilical vein become?
The ligamentum teres
