Heart development

Question 1

Summarise Heart development

What is the pro-epicardial organ (PEO)?

What are the cardiac neural crest cells (CNCCs) derived from and what do they form?

Answer 1

1. Here we can see the end region (anterior) and tail region (posterior) of the embryo. We can see the primitive streak where gastrulation is being initiated and cells of the epiblast and migrating in between the two bilateral layers to generate the endoderm and mesoderm. The mesodermal cells are of interest here, they will migrate and form the heart field in the shape of a crescent. We have the first and second heart field shown in different colours.
2. From the cardiac crescent the cells have grown to form a single heart tube, the arterial pole is at the anterior end of the embryo and the venus pole is at the posterior end of the embryo. At the top end we have the outflow tract, the cells of the heart field primarily form the primitive heart tube (which will go onto to form tissue of the left ventricle) and the second heart field will form regions of the outflow tract, the right ventricle and atrial tissue).
3. This is summarised in the last diagram
4. The dots shown in purple are the pro-epicardial organ (PEO) – these are initially clustered towards the venus pole of the heart. They will migrate out and will surround the whole of heart and will form epicardium (shown in purple). The epicardial cells are important because they will form the coronary vasculature
5. The cardiac neural crest cells (CNCCs) are derived from the neural tube and will give rise to tissues of the heart such as aortic arch and pulmonary artery and divide these two vessels from the OFT

Question 2

How common is coronary heart disease?

What percentage of birth defects do they make up?

Answer 2

8/1000 births

25% of all birth defects

Ventricular septal defects are the most common

Question 3

When does heart development begin and end?

Answer 3

• The heart is the first organ to develop and function which forms at week 3 up till week 10

Question 4

What is the cardiac crescent derived from?

Answer 4

• The cardiac crescent is derived from mesodermal cells of the embryo
• It is the first recognisable as a crescent-shaped tube of myocardium

Question 5

What happens to the myocardial heart tube?

Answer 5

• The myocardial heart tube will start to loop and fold and extend
• This will re-arrange the arterial and venus poles
• The venus pole will be brought up toward towards the arterial pole
• The chambers of the heart will expand
• The four chambers will begin to separate via the growth of various walls and structures that will divide the adult heart into four chambers

Question 6

What is tinman?

What is its homologue?

Answer 6

A transcription factor part of the nk2 family in flies

Its homologue in mammals is nkx2.5.

• NKX is controlled by Dp, FGF and Wg (its homologue is the BMP family)
• The tinman gene controls heart related transcription factors that turn on effector genes of the heart. It will turn on gata expression in the hand, tbx gene and MEF2 which turns on muscle genes in the heart.

In mammals NKX will feed into GATA genes, TBX genes involved and the hand genes

Question 7

What is the heart crescent composed off?

Answer 7

Mesodermal tissue

Question 8

What are the angioblastic cords?

Answer 8

They give rise to the heart tube

Question 9

Where does the heart tube move to?

Answer 9

• Remember the embryo will fold towards the midline along the AP axis forming the gut tube. This brings the heart in towards the adult position.
• Heart crescent and majority of cells added to it are from mesoderm
• Relative position of heart shifts from cranial to thoracic due to major embryonic morphogenesis, particularly of the neural tube and gut

Question 10

How is the heart forming mesoderm set aside during gastrulation?

Answer 10

o BMP molecules are expressed at margin of the embryonic disk and anterior endoderm
o Wnt inhibitors anterior to embryonic disk and Wnt ligands posterior to embryonic disk
o Overlap of Wnt inhibitors and BMPs induces cardiac mesoderm
o BMP and FGF signals drive NKX2.5 and GATA 4,5 and 6. These transcription factors will turn on Mef2 and will turn on cardiac specific muscles.

Question 11

What is the neural tube secreting and repressing?

What induces cardiac mesoderm?

Where is noggin and chordin being secreted from?

Answer 11

In a transverse section we can see the neural tube on the RHS which is secreting Wnt that will repress cardiac mesoderm so we get non-cardiac mesoderm adjacent to the neural tube. Signals from the anterior endoderm and lateral edges of the embryonic disk such as BMPs will induce the cardiac mesoderm.

As development proceeds Noggin and Chordin are being secreted from the Notochord that will form somatic mesoderm (non-cardiac mesoderm) adjacent to the neural tube. This will then secrete Wnt that supress cardiac mesoderm within the somatic mesoderm. Lateral to this we have BMP and FGF signals will drive NKX2.5 and GATA 4,5 and 6. These transcription factors will turn on Mef2 and will turn on cardiac specific muscles.

Question 12

Summarise the origin of heart cells

Answer 12

• Bilateral heart-forming regions of mesoderm migrate from primitive streak
• Signals from endoderm and midline required; BMPs, Wnts and their antagonists & FGFs
• GATA, Nkx and Mef-2 transcription factors, and co-factors (e.g. FOGs – Friend of GATA), are induced in HFR
• GATA 4 required for fusion of paired HFR

Question 13

What parts of the crescents fuse to form the left ventricle?

What does the endocardial tube attach to?

Answer 13

• Medial part of crescent expands
o This will become left ventricle
• Endocardial tube attaches to developing aortic arches cranially (outflow) and systemic veins caudally (inflow)

Question 14

What regions make up heart crescent?

Answer 14

• Two distinct regions form the heart crescent: the first and second heart fields.

Question 15

What will the first and second heart field give rise to?

Answer 15

• The first heart field will form the initial balloon structures of the heart tube, and will give rise to form the left ventricle and also the right and left atrium
• The second heart field will mainly contribute towards the cells of the outflow tract, the right ventricle and portions of the left and right atrium.
• The first heart field comes from the more anterior and ventral portion of the crescent

Question 16

Where does the second heart field lie to the first heart field?

What does the second heart field express?

Answer 16

• The second heart field lies dorsal to the primary heart field which is characterised by the expression of a TF called Islet-1 along with NKX2.5

Question 17

What does the second heart field divided into and what do these regions express?

Answer 17

• The second heart field can further be divided along the AP axis. The anterior portion will express FGF-10 and the posterior will express Tbx5 and the most posterior region is characterised by the expression Tbx18

Question 18

How does the second heart field interact with the neural crest cells?

Answer 18

The second heart field will interact with the neural crest cells that will migrate from the neural tube into the pharyngeal arches and will interact with the heart field. As they migrate through the outflow tract they will spiral to populate the cardiac cushion mesenchyme and separate the common outflow tract into the aorta and pulmonary artery.

Question 19

Summarise the second heart lineage

Answer 19

• The cardiac crescent (horseshoe) makes mostly left ventricle
• The second heart lineage lies dorsal and medial to crescent and adds to the heart at outflow and inflow poles
• Before addition to the heart, SHL cells express islet1 (a few islet1 cells can be found in adult heart - may be cardiac stem cells?)
• SHL has two components:
o “anterior” which expresses FGF10 and makes right ventricle and OFT
o “posterior”: an atrial, or inflow, component
• The proliferation of the SHF is controlled by Wnt and Tbx1, which control FGF expression
• NCC influence the SHF, inhibiting FGF signalling to allows outflow tract separation

Question 20

Where are cells of the second heart field added to?

Answer 20

• Cells from the second heart field are added at both the outflow and inflow

Question 21

The end of the heart tube are fixed so what happens to the heart?

What is the direction of the looping controlled by?

Answer 21

• The ends of the tube are fixed, so the heart must bend as it elongates
• The direction of looping is genetically controlled (leftward)

Question 22

What direction does a normal heart loop?

If the heart loops to the right what is it called?

Answer 22

Leftwards

If the heart loops to the right it is called dextrocardia

Question 23

How is LR asymmetry established in the heart?

Answer 23

• Left-right asymmetry is established by signals at the node, involving motile and non-motile cilia
• Differential activation of nodal pathway
• Activation of Pitx2 on left

Question 24

There are 5 morphological processes that are left-right asymmetric?

Answer 24

1. Looping
2. AV canal placement
3. Systemic venous remodelling
4. Pulmonary venous / atrial remodelling
5. Outflow septation

Question 25

What 3 cells types make up the heart tube?

Answer 25

• The heart will form from three cell types which consists of endocardial cells (on the inside of the tube), the myocardial cells (muscle cells) and epicardial cells (derived from the proepicardial organ).

Question 26

Describe chamber formation seen in day 21, 25, 28, 30 and 35

Answer 26

* These are diagrams of looping human hearts * Day 21 – we have a simple tube that has begun to elongate and loop. Aortic arch arteries are forming throughout * Day 25 – further elongation and looping, ballooning out of the ventriclar region in particular * Day 28 – formation of the intraventricular groove on the external surface of the heart that will begin to separate the left from the right ventricle. The venus region ha wrapped behind the heart, the atriums are now found at the cranial regions of the heart. * If we take a coronal section through the heart at day 28: we can see the ballooning chambers of the heart. The atrium (smooth surface) are distinct morophologically from the ventricles (have trabecular layers). * Day 30 – ballooning and folding has continued. * Day 35 – appendadges will fuse to the side of the heart

Question 27

What is Atrioventricular septation?

Answer 27

o Divides atria from ventricles o Divided by AV cushions (middle central cushion on the dorsal wall of the heart and will meet other cushion on opposite side of the heart)

Question 28

What is Atrial septation?

Answer 28

o Divides L from R atrium | o Divided by interatrial septum

Question 29

What is Ventricular septation?

Answer 29

o Divides L from R ventricle | o Divided by interventricular septum (will grow from the bottom of the heart to the AVC where it will fuse)

Question 30

What is outflow tract separation?

Answer 30

• Outflow tract septation o Divides pulmonary artery from aorta • Divided by OFT cushions derived from cardiac neural crest cells

Question 31

What are the two types of septation divide the heart?

Answer 31

Muscular walls • 1° and 2° interatrial septa, interventricular septum • Defects give rise to holes Cushion formation • AV and OFT cushions • Valve formation, stenosis and atresia defects if this goes wrong

Question 32

What is cardiac jelly? What is the OFT cardiac jelly populated with?

Answer 32

The cardiac jelly is extracellular matrix material that is secreted by cells of the myocardium. Growth factors secreted into the cardiac jelly will cause endocardial cells lining the heart tube to undergo a process called epithelial to mesenchymal transition (epithelial cells lose cell contact and can migrate away). They will migrate into the cardiac jelly and populate it – these cells will then give rise to the cushions and valves. In the OFT the cardiac jelly is populated by neural crest cells to form the OFT cushions.

Question 33

Describe the induction of endocardial cushion mesenchyme

Answer 33

* Cardiac jelly (acellular ECM) made by myocardium, initially throughout heart tube * Signals from restricted regions of myocardium (AVC & OFT) cause endocardium to delaminate into cushion in epithelial/mesenchymal transformation and therefore migrate into the cardiac jelly. This is driven by signals from the myocardium to the endocardium * Signals are in large complexes known as adherons that contain a variety of growth factors * TGF-1, -2, & -3 and receptors involved. They bind and endocardial cells undergo EMT transition, the cells will migrate and populate the cardiac jelly * This occurs in the atrioventricular canal and the ventricles, we have tbx20 and also suppressing cushion formation. Within the AV canal we have bmp4 signals driving the expression of tbx2

Question 34

What are valves of the heart formed by? What are the AV and semilunar valves held in place by?

Answer 34

remodelling of the cushions The AV valves will be held in place by tendons binding to the muscular wall The semilunar valves will be flat and unidirectional

Question 35

What forms the AV septum?

Answer 35

* The cells of the AV cushion are growing towards us from the dorsal wall towards the midline of the heart * From the ventral side there is another cushion that is growing in towards the midline where they will then fuse * There are also narrows channels on either side of the AV cushion which will allow communication between the left and right atriums and ventricles.

Question 36

What type of heart defect is common in people with downs syndrome?

Answer 36

AVSDs are common in Down’s Syndrome (~40%)

Question 37

Atrial septation – the primary interatrial septum

Answer 37

• Left atrium with the orifice of the pulmonary vein (broken arrow) • Right atrium with orifice of the systemic venous sinus (star) • Primary interatrial septum (arrow) is growing to divide L-R 1. The primary intratrial septum will form (a thin wall) where it will meet the AV cushion and fuse with it

Question 38

Describe how the foramen ovale closes

Answer 38

* Primary interatrial septum grows down to fuse with the AV cushions * The trailing edge of the primary septum breaks down to allow blood to continue to flow from the right to left atrium * Septum secundum grows down to form a flap valve that permits shunting of the blood from the right to the left atrium * Left atrial pressure increases after birth, closes valve, eventually seals

Question 39

Describe the fetal circulation system

Answer 39

* In the fetus there are no lungs so there is no oxygenation of blood. Oxygenated blood is supplied by the mother through the umbilical vein which will mix with the deoxygentaed blood of the fetus. This blood will then enter the right venticle. Blood will then flow from the right atrium into the left atrium. Blood can then pass through the left venticle into the systemic circulation. * Blood in the pulmonary artery can enter the aorta through ductus arteriosus which will degernate after birth to give rise to the ligamentum arteriosum * After birth the lungs will inflate, which will relieve pressure in the pulmonary trunk and so the pressure in the left hand side of the heart will become greater than the right. Increased pressure of the left atrium pushes the flat valve closed (primary septum is pushed against the secondary septum, they will fuse and close).

Question 40

What is a Persistent Foramen Ovale?

Answer 40

o Flap valve doesn’t seal completely

Question 41

What is an Ostium primum defect? What is an Ostium secundum defect?

Answer 41

Primary atrial septum has not grown down all the way to fuse with the AV cushion septum secundum has not grown down sufficiently to meet the primary interatrial septum

Question 42

Why are ASDs acyanotic?

Answer 42

* Pressure on the left is higher than on the right | * Defect is acyanotic (normal levels of oxyhaemoglobin in systemic circulation)

Question 43

What comes together to form the atrial septum?

Answer 43

• The superior and inferior atrioventricular cushions (sAVC and iAVC) are the two major cushions that develop in the central portion of the AVC • Two minor cushions, left and right lateral AV cushions (llAVC and rlAVC), form laterally at the AVC • The mesenchymal cap (MC) is a tissue that caps the leading edge of primary atrial septum (PAS) that grows from the atrial roof towards the AV canal • The dorsal mesenchymal protrusion (DMP) protrudes from the dorsal mesocardium into the atrial chamber o RA, right atrium; LA, left atrium; RV, right ventricle; LV, left ventricle; IVS, interventricular septum • (C) The composition of the atrial septum

Question 44

Where does the IVC grow?

Answer 44

• The IVS will growth upwards and will meet with and EC

Question 45

What types of Ventricular septal defects?

Answer 45

``` • 75% perimembranous • 25% muscular • Small o Small L>R shunt • Medium o Moderate L>R shunt • Large o Large L>R shunt o Surgically repaired in childhood ```

Question 46

Are ventricular septal defects acyanotic?

Answer 46

Yes