Flashcards in Development of the Heart and Great Vessels Deck (28):
Describe how lateral and cephalocaudal folding moves the cardiogenic area of the embryo.
Before folding the cardigenic area is in the worng place (too cranial), after folding the cardiogenic area is now in the right place (thoracic cavity), the lateral folding creates a heart tube and the cephalocaudal folding puts this tube into the correct region (ish).
Describe the cardiogenic field.
The cardiogenic field consists of specialised mesoderm and also a cavity for it to grow into. Blood islands are the beginning of blood vessels and on either side we get the formation of larger heart tubes, folding will push these into the correct place and fuse to produce the primitive heart tube.
How is the heart tube tethered?
The heart tube is initially suspended by a tiny membrane but over time this disappears so the heart is only tethered cranially and caudally.
Why does looping take place?
Looping occurs to form a second set of chambers. Bringing the heart closer to becoming a dual pump.
Describe the process of looping
The cephalic portion elongates ventrally, caudally and to the right whilst the caudal portion grows dorsally, cranially and to the left (embryo’s left). However as the pericardial sac is in a fixed space this causes some loopping to take place.
What drives looping and does it achieve by the end.
The looping is driven by the position of tethering and the limited space in the pericardial sac. Looping pushes both outflow and inflow to the cranial end and the outflow anterior to the inflow creating the sinuses.
How does looping cause communication between the ventricles and aorta?
During in looping the two are pushed together and fuse via the atrioventricular canal.
From top to bottom describe the 6 segments to the primitive heart tube
Aortic roots (All)
Truncus Arteriosus (The)
Bulbus Cordis (Best)
Sinus Venosis (Small)
How does the sinus venosus become asymmetrical?
Originally the embryo and heart is very symmetrical. The sinus venosus forms into both the vena cava along with the coronary sinus. As a result, the whole venous drainage of the embryo needs to shift to the right-hand side. This takes place as the sinus venosus is engulfed by the right atrium.
Describe the formation of the left atria and pulmonary veins
The lungs and left atria/right ventricle of the heart develops last as they are not needed yet in the embryo. The tiny part of the primitive atrium that forms the left atria sproute some new vessels which will form into the pulmonary veins and the left atrium engulfs this structure until there are now 4 opening into the left atrium from the pulmonary veins. This development of the pulmonary veins and left atrium is what produces the oblique sinus.
Describe the problems fetal circulation must overcome.
Bood is oxygenated in the placenta via the umbilical chord. On the way to the left side of the heart the blood must by-pass the lungs (as they are further behind in the developmental process and so are fragile) and liver (as the liver is very metabolically active). However the blood returning from the placenta is on the wrong side of the heart.
How does the fetal circulation overcome the issue of the developing lungs and arriving at the wrong side of the heart?
A very small amount of blood does go through the lungs just to give them the nutrients they need to survive but the majority is shunted from the pulmonary trunk into the aorta via the Ductus Arteriosus. The Foramen Ovale is the hole that by-passes the blood from the right to the left atrium.
Describe the descendants of the original symmetrical aortic arches.
The early arterial system begins as a bilaterally symmetrical system of arched vessels. Right hand 4th Arch forms the proximal part of the right subclavian Artery whilst the left hand of the 4th arch forms the arch of the aorta. Right hand 6th arch (also known as the pulmonary arch) forms the right pulmonary artery and the left hand 6th arch forms the left pulmonary artery and the ductus arteriosus.
What structure does each of the arches have associated with it?
Each of these arches has a corresponding nerve.
What nerve is associated with the 6th arch?
The nerve corresponding to the 6th arch is called the recurrent laryngeal nerve (recurrent meaning turns back on itself) that innervates the muscles of the larynx and allows us to speak.
Why are the recurrent laryngeal nerves special and what is the clinical significance?
They flow down through the thorax and then curls back up and this happens because as these arches develop and the heart moves more caudally the recurrent nerves (which branches off from the vagus nerve) get hooked on the 6th arch remnant – the right subclavian on the right hand side and the ductus arteriosus on the left hand side. As a result, the left hand recurrent laryngeal nerve hangs much lower and right next to the heart. This is important as a hoarse voice is often an indication of pathology of the heart area.
What is the endocardial cushion?
In the atrioventricular canal an endocardial cushions appear (i.e. tissue grows in the wall of the endocardium) and divides the heart into left and right channels by fusing in the middle.
Describe how atrial septation occurs.
Atrial Septation - The septum primum grows down towards the fused endocardial cushions with a hole called the ostium primum. Before this hole closes another hole called the ostium secundum appears within the septum primum. Finally, a second crescent shaped septum grows called the septum secundum with a hole called the foramen ovale in.
What are the auricles
Both of the atria have components derived from the primitive atrium which is what forms the auricles.
Describe ventricular septation
Ventricular Septation - the muscular component grows up towards the endocardial cushion but doesn’t quite make it all the way to the cushion leaving a small gap called the primary interventricular foramen the gap is filled from a growth of tissue from the endocardial cushion this is the membranous part.
Describe septation of the outflow tract
Endocardial cushions appear in the truncus arteriosus and as they grow towards each other they twist forming a spiral called the spiral septum. This is how the pulmonary trunk and aorta wrap around each other.
What is the purpose of the ductus arteriosus?
The small amount of blood sent to the lungs to allow resistance for the RV to contract against is returned to the aorta by the Ductus arteriosus.
How is foramen ovale reconciled after the first breath?
When the first breath is taken the lungs send oxygenated blood to the left atrium there by increasing left atrial pressure dramatically. As a result, the foramen ovale closes forming the thumb imprint called the fossa ovalis when the two septum are pushed together by the opposing pressures.
How is ductus arteriosis reconciled after the first breath?
The increase in oxygenated blood causes the ductus arteriosus to contract and close eventually forming a fibrous string called the ligamentum arteriosum (foetal blood has a lower oxygen saturation).
Describe the purpose of the ductus venosus
Ductus venosus allows blood to by-pass the liver as it returns to the heart from the umbilical cord.
How is the cirucaltion surrounding the liver and umbilical cord reconciled
The ductus venosus will close with the removal of the placental support and forms into the ligamentum venosus whilst the umbilical vein that supplied this shunt forms into the ligamentum teres (hepatis).
What structures form the rough edges in the ventricles?
The inside of the ventricles are covered in trabeculae carneae which are rounded edges of muscles that project.