Describe the formation of the primitive heart tube
The cardiogenic field, from which the heart, blood vessels and blood cells develop, is created during gastrulation and at first lies at the cranial end of the embryo before folding occurs.
During folding, LATERAL folding creates a heart tube and CEPHALOCAUDAL FOLDING brings the tube into the thoracic region.
As the development of the CVS gets underway, a pair of endocardial tubes develops within the cardiogenic field in the 3rd week of development.
The endocardial tubes are brought together during embryonic lateral folding and fuse in the mid-line to create the primitive heart tube.
What is the Cardiogenic field?
Progenitor heart cells lie in the epiblast, immediately adjacent to the cranial end of the primitive streak.
From there they migrate through the streak and into the splanchnic layer of lateral plate mesoderm where they form a horseshoe shaped cluster of cells called the Primary Heart Field (PHF).
The Secondary Heart Field appears slightly later than the PHF.
Once cells establish the PHF, they are induced by underlying endoderm to form cardiac myoblasts and blood islands that will form blood cells and vessels (vasculogenesis).
With time, the Islands unite and form horseshoe-shaped endothelial lined tube surrounded by myoblasts. This region is known as the cardiogenic region; the intraembryonic cavity over it later develops into the pericardial cavity.
Describe the primitive heart tube in situ
Suspending in the pericardial cavity by a membrane that subsequently degenerates (ruptures) so the primitive heart tube is completely free in the pericardial cavity and able to undergo morphological change.
As the embryo grows, the primitive heart tube grows within the confirms of the peritoneal cavity so looping occurs.
The primitive heart tube is linear at first (receiving blood - venous drainage/inflow at its caudal pole and pumping blood - outflow at the first aortic arch from its cranial pole)
Name the regions of the developing heart
The primitive atrium
The primitive ventricle
(+ the Sinus venosus which is the first chamber in the heart to receive blood from the veins and contracts to force the blood superiorly into the atrium)
Describe the looping of the primitive heart tube
Continued elongation results in bending/looping.
It begins ~day23 and is completed by ~day28.
It develops very quickly.
The heart tube cannot grow up or down but can spread out:
- The cephalic portion bends ventrally, caudally and to the right (forwards, downwards, right)
- The caudal protein bends dorsally, cranially and to the left (backwards, upwards, left)
What does looping result in?
Looping places both the inflow and outflow cranially with the inflow dorsal to (behind) the outflow.
Puts primordium of RV closest to outflow tract Puts primordium of LV closest to inflow tract
Puts atrium dorsal to bulbis Cordis (I.e. Inflow is dorsal to outflow) Also forms the transverse pericardial sinus
What is the clinical significance of the transverse pericardial sinus?
Passage between aorta and pulmonary artery (front) and superior vena caves behind can be used to pass ligature during cardiac surgery - this is the space behind the outflow and in front of the inflow, through which a finger can be inserted
What happens after looping?
The atrium communicates with ventricle via atrioventricular canal. This is a constriction between the two, making the first division between the atrium and ventricle
How does the evidence for the embryonic structures of the primitive heart tube persist in the adult?
With further growth and development:
The primitive atrium contributes a small component to each atrium
The bulbis Cordis gives rise to part of the RV
The LV is derived from the primitive ventricle
The truncus Arteriosus ultimately gives rise to the roots and proximal portions of the pulmonary trunk and aorta.
Describe what happens to the sinus venosus
The embryo collects blood from the placenta, yolk sac and the body.
All goes to the sinus venosus.
At first the right and left sinus horns are equal in size but as venous return shifts to the right hand side, the left sinus horn recedes.
The enlarging RA absorbs the right sinus horn.
Describe in brief the development of the Right Atrium
Develops from most of the primitive atrium and absorbs the right horn of the sinus venosus
It receives venous drainage from the body (venue cava) and the heart (coronary sinus)
Describe in brief the development of the Left Atrium
Develops from a small portion of the primitive atrium and absorbs proximal parts of pulmonary veins (wall of left atrium develops from pulmonary veins)
Receives oxygenated blood from the lungs
Explain about the Oblique Pericardial Sjnus
Blind ending passage posterior to the heart formed by the reflections of the visceral and parietal pericardium layers into the vessels traversing the space.
The oblique pericardial sinus is formed as the left atrium expands absorbing the pulmonary veins.
Explain the fetal circulatory shunts
Allow some organs and chambers to be by-passed. In the foetus, the lungs are non-functional and fluid-filled
Gas exchange occurs at the placenta - receives oxygenated blood via placenta and umbilical vein transports the blood to the heart, bypassing the lungs.
The deoxygenated blood returns to the placenta via umbilical arteries.
What is the Ductus Arteriosus?
Shunt (specialised blood vessel) that provides a connection between the pulmonary trunk of pulmonary artery and proximal descending aorta.
It allows the fluid-filled and non-functioning lungs to be by-passed.
Remains open in the foetal life.
A small volume of blood is permitted to go from right atrium to right ventricle to allow chambers to develop appropriately by allowing them to squeeze and contract. Too much blood however could damage the lungs irreparably.
What happens to the ductus Arteriosus at birth?
Upon the closure of the DA at birth, it becomes a fibrous cord - the ligamentum arteriosum.
It is a ligament attached to the superior surface of the left pulmonary artery and the proximal descending aorta.
It is closely related to the left recurrent laryngeal nerve, a branch of the left vagus nerve.
Describe in brief the aortic arches
Early arterial system begins as a bilaterally symmetrical system of arched vessels
They undergo extensive remodelling to create the major arteries leaving the heart.
Aortic arch 5 has no derivative in humans.
Describe the derivatives of the 4th Aortic Arch
Right side: 4th arch forms the proximal part of the right subclavian artery
Left side: 4th arch forms part of the arch of the aorta (between the left common carotid and the left subclavian artery)
Describe the derivatives of the 6th Aortic Arch
Right side: becomes the proximal segment of the right pulmonary artery
Left side: becomes the left pulmonary artery and ductus Arteriosus
Explain the recurrent laryngeal nerves
Each aortic arch has a corresponding nerve.
The nerve corresponding to the 6th arch is the recurrent laryngeal nerve (branch of the vagus nerve aka 10th cranial nerve)
The right recurrent laryngeal descends to T1-T2
The left recurrent laryngeal descends to T4-T5
The recurrent laryngeal nerve innervates the intrinsic muscles of the larynx
What factors influence the course of the recurrent laryngeal nerve on left and right sides?
1. Caudal shift of the developing heart from the neck region into the thoracic region and expansion of the developing neck region
2. The need for a fetal shunt between the pulmonary trunk and aorta. As the heart "descends" the nerve hooks around the 6th aortic arch and "turns back on itself"
The left recurrent laryngeal nerve becomes hooked around the shunt between the pulmonary trunk and aorta
What is needed to create the 4 chambers and to achieve selective outflow (to create a dual circulation "2 pumps in series")?
Septation of ventricular outflow tract- pulmonary trunk and aorta
(Note: these events are all happening at the same time)
Describe the first step of SEPTATION
Endocardial cushions (derived from neural crest cells that migrate to primitive heart tube) are developing in the atrioventricular region.
The cushions continue to grow (project) into the atrioventricular canal.
These grow towards each other (meet in the middle) to divide the developing heart (primitive atrium and primitive ventricle) into right and left channels.
What are endocardial cushions?
Bumps in the wall at the junction between the atrium and the ventricle.
Eventually the dorsal endocardial cushion and the ventral endocardial cushion meet at the midline, causing the division.
The endocardial cushions grow a platform for which the septa which are dividing the chambers can grow toward.
Describe Atrial Septation
A foetal circulation (shunt) is needed.
Involves the formation of two septa with three holes
Septum primum grows down from the roof of the primitive atrium towards the platform of the fused endocardial cushions.
The Ostium primum (first hole) is present before the septum primum fuses with the endocardial cushions.
Before the Ostium primum closes, the Ostium secundum appears in the septum primum (apoptosis) to maintain continuity (so foetal circulation can grow).
Finally a second crescent shaped septum, the septum secundum, grows down.
The hole in the septum secundum is the foramen ovale as the septum secundum is incomplete due to its shape.
What the complicated developmental process of atrial septation result in?
Allows the right atrium to left atrium shunt during foetal life - which allows oxygenated blood entering from the inferior vena cava into the right atrium into the left atrium and is then pumped around the body.
Risk of atrial septal defects.
What is the summary of the atria after atrial SEPTATION?
Both left and right atria have components derived from the primitive atrium (I.e. Auricles)
The right atrium (as it expands) absorbs the sinus venosus
The left atrium sprouts the pulmonary vein. The left atrium then grows (expands) to absorb the proximal part of the pulmonary vein and its first four branches.
The fossa ovalis (depression in the right atrium) is the adult remnant of the shunt (foramen ovale) used in utero to bypass the lungs.
What do you find when you open the left atrium?
4 openings of the pulmonary veins but the wall is very smooth- indicates vascular origin, reflects the sequence of development which absorbed the proximal part of the pulmonary vein
The auricle of the LA is the roughened wall of tissue which is indicative of primitive atrium derivation.
What do you find when you open up the right atrium?
You see a rough area (anterior wall) which indicates a derivative from the primitive atrium
Describe the process of ventricular septation?
The ventricular septum (between L and R ventricles) has 2 components: Muscular and Membranous
The Muscular portion forms most of the septum. The muscle of the primitive ventricle wall grows upwards towards the fused endocardial cushions (at the atrioventricular junctions) leaving a small hole (the primary interventricular foramen).
The primary interventricular foramen is closed by the membranous portion of the interventricular septum - formed by connective tissue that grows down to the endocardial cushions to "fill the gap" It is derived from the spiral septum that grows to separate the truncus Arteriosus into the outflow vessels.
Describe the Septation of the Outflow Tract
The contruncal septum divides up the truncus Arteriosus.
Additional endocardial cushions appear in the truncus Arteriosus region As they grow towards each other they twist around each other - they are not directly opposite each other - so they form a spiral septum, dividing an aortic side and a pulmonary trunk side.
Right ventricular outflow is lined up with the pulmonary trunk
Left ventricular outflow is lined with the aorta
What separates the aorta and the pulmonary arteries?
How do we make the transition from support from our mother to independent life at birth?
After birth, the change in circulation must happen immediately.
The baby takes its first breath, massively decreasing the pressure in the lungs. This results in more blood entering the lungs and as a result, more blood returns to the LA.
When pressure in the LA > RA, the foramen ovale is closed (septum primum is pushed against septum secundum)
The Ductus Arteriosus contracts because the tissue in the wall of the vessel is sensitive to increased O2 saturation. Physiological closure initially but over time becomes anatomical (ductus Arteriosus becomes fibrotic)
What happens to the Ductus Venosus at birth?
It closes because there is no longer blood coming through the umbilical cord - placental support has been removed so no blood is entering the umbilical vein of subsequently the ductus venosus. The septum primum is pushed against the septum secundum because the two holes don't align anymore.
Pressure in RA decreases because there is no longer oxygenated blood entering the IVC from the umbilical cord.
Ductus Venosus becomes Ligamentum Venosum
How is foetal circulation possible?
Oxygenated blood from the placenta passes into the foetus through the umbilical vein,
This oxygenated blood bypasses the liver to go straight into the inferior vena cava via the Ductus Venosus.
The blood reaches the right atrium where it is shunted from right --> left through the foramen ovale. From here it is pumped around the body via the aorta.
The ductus Arteriosus allows blood to pass from the pulmonary trunk to the aorta bypassing the lungs,
What are the fates of the foetal shunts?
Foramen Ovale: Fossa ovalis
Ductus Arteriosus: ligamentum arteriosum
Ductus Venosus: ligamentum venosum
Umbilical vein: ligamentum teres (hepatitis)
What are the fates of the sinus venosus, primitive atrium, primitive ventricle and bulboventricular sulcus?
Sinus venosus: RA except for left horn
Primitive atrium: Auricles of definitive atria
Primitive ventricle: LV
Bulboventricular Sulcus: Primary interventricular foramen
What is the fate of the Bulbus Cordis?
Proximal 1/3: RV (trabeculated)
Conus Cordis: outflow tract of L and RVs
Truncus Arteriosus: roots and proximal aorta and pulmonary trunk
What is achieved in the process of Septation?
The primitive heart tube becomes divided into chambers and the outflow tract is subdivided into pulmonary trunk and aorta.
Firstly the junction between the atrium and ventricle becomes constricted, creating a narrow channel called the atrioventricular canal.
This narrowing provides a framework by which the interatrial and interventricular septa are formed.
Why is atrial Septation complicated
The circulatory needs of the embryo/foetus are different to those of the adult. Thus a right to left shunt (foramen ovale) must be maintained during life in utero but this must be instantly sealable at birth.