Heart development Flashcards
Why are many different model organisms used when looking at the heart?
- Different model organisms have different advantages
- Have different structures to their heart - show different developmental and evolution of the heart
What is the structure of a 4-chambered heart?
2 atrium
2 ventricles
Separated by valves and septum
2 separate circulatory systems (pulmonary and body circuits)
Oxygenated and deoxygenated blood kept separate in the heart
Which organisms have a 4-chambered heart?
Human
Mouse
Chick
What is the structure of a 3-chambered heart?
2 atrium
1 ventricle
2 separate circulatory systems
Oxygenated and deoxygenated blood kept separate in the heart
What organism has a 3-chambered heart?
Xenopus
What is the structure of a 2-chambered heart?
1 atrium
1 ventricle
1 circulatory system
Blood from ventricle –> to gills (where becomes oxygenated) –> rest of body
What organism has a 2-chambered heart?
Zebrafish
What is the structure of a tubular heart?
Single tube
Lined with valves - promote blood flow in ONE direction
What organism has a tubular heart?
Drosophila
Where does the heart derive from in the human?
From the cardiogenic mesoderm
What are the basics of how the heart develops in humans?
1) Cardiogenic mesoderm forms cardiac crescent
2) Cells within the cardiac crescent migrate towards the MIDLINE and COALESCE to form a SINGLE TUBE
3) Tube then undergoes highly ASYMMETRIC CARDIAC LOOPING to form the heart
When is the cardiac crescent of the heart formed in humans?
2 weeks pf
What does the cardiac crescent consist of at 2 weeks pf?
Mesodermal tissue and 2 endocardial tubes either side of the crescent
When is the single heart tube in humans formed?
3 weeks pf
How long does heart looping in the human take?
1 week
At what point is the heart well-looped?
4 weeks pf
Why is heart looping required?
- To ensure the future chambers of the heart are well aligned
- So, when the heat undergoes MATURATION - everything is in the right place
What happens in the human during maturation of the heart?
- Formation of the septa and valves
- Connection of the inflow/outflow tracts
When is the heart well-formed?
7 weeks pf
How is the development of the heart in the mouse different/same to in humans?
Quicker process but highly CONSERVED:
- Cardiac crescent formation at E7.5
- Heart looping at E8.5 - E9.5
- Well looped heart and emergence of chambers at E10.5
- Mature, functional heart at E13
What is the process of heart formation in the zebrafish?
1) Migration of 2 groups of precursors from the left and right sides of the heart tube to form the CARDIAC DISC
2) Cardiac disc undergoes EXTENSION to form the HEART TUBE
3) Heart tube undergoes looping
Why is studying the formation of the zebrafish heart advantageous?
Everything happens very quickly - advantageous if looking at the very EARLY stages of heart development
When does the cardiac disc in the zebrafish from?
20 hours pf
In the zebrafish, where does the cardiac disc undergo extension to form the heart tube?
24 hours pf
In the zebrafish, where does the heart tube undergo looping?
36 hpf
In the zebrafish, when has the heart finished looping?
48hpf
At 48hpf, what 2 things can be seen in the zebrafish heart?
- Restriction of the atrioventricular canal
- Initiation of the formation of the valves
What happens once the heart has formed?
Undergoes maturation
Where do cardiac cells originate from?
2 separate origins (2 heart fields)
From the mesodermal tissues
What are the heart fields?
2 separate sets of cardiac precursors that are specified very early on in development:
- First heart field
- Second heart field
How are the 2 heart fields different from each other?
- Very distinct spatial arrangement during morphological heart development
- Contribute to very specific structures of the heart
What structures do the FHF cells contribute to in the developed heart?
- LEFT ventricle
- Left and right atria
What structures do the SHF cells contribute to in the developed heart?
- RIGHT ventricle
- Left and right atria
- Outflow tract
Where are heart field cells specified?
Along the primitive streak (mouse)
OR
Along the embryonic margin (fish)
What is the specification of the heart field cells controlled by?
Combinatorial MORPHOGEN signalling
Describe the specification of FHF cells
1) Cardiac mesoderm subjected to NON-CANONICAL WNT signalling
2) BMP signalling
Describe the specification of SHF cells
1) Cardiac mesoderm subjected to NON-CANONICAL WNT signalling
2) Beta-catenin and FGF signalling
Once the cardiac cells are specified, what must the cells do?
Express specific transcription factors that demark them as:
- Cardiac cells
AND
- Either FHF or SHF SPECIFIC cardiac precursors
What transcription factor is common to both FHF and SHF cells?
What does this show?
NKX2.5
Shows that NKX2.5 is important to show have CARDIAC MYOCYTES
What transcription factor is only expressed in FHF cells?
Tbx5
What transcription factor is only expressed in SHF cells?
Islet1
What are the heart field specific transcription factors required for?
Cardiac differentiation and specific aspects of cardiac function
Once the heart field cells are specified, what happens?
They migrate ANTERIORLY to form the primitive heart tube
What HF cells initially make up the heart tube?
Where are the the other HF cells?
FHF cells make up the heart tube
SHF cells are in the adjacent mesoderm
What is important about the heart tube, even when it is linear?
- Patterning of the heart tube
- There is distinction between the different regions of the heart - it is coordinated
What is different kinds of patterning in the heart tube important for?
Examples?
Subsequent development and function of the heart between the different parts
Examples:
- Chamber vs non chamber
- Contractile vs non-contractile
- Atrial vs ventricular contractility
- Inflow vs atrioventricular canal vs outflow
What are the non-contractile parts of the heart?
Valves
Septum
Atrioventricular canal
Connective tissue
Why is there a need for different pattering in the atria and ventricles?
What do they express differently?
- Must contract differently and in a DIRECTED way for proper flow of blood through the heart
- Ventricles undergo trabeculation - must have genes expressed for this
Express different MYOSINS
What signalling needs to occur in the heart tube?
1) Notch - tbx20 pathway
2) Bmp2
What is the Notch - tbx20 pathway important in?
What occurs in this signalling pathway?
What does it result in?
Specifying CHAMBER myocardium in the heart
Pathway:
1) Notch activates tbx20
2) Tbx20 inhibits BMP2 and tbx2
Results in:
Specification of chamber myocardium in the heart
What is CHAMBER myocardium characterised by?
Expression of Nppa and Nppb
What does BMP2 do?
What does this result in?
Activated tbx2
Results in the specification of the atrioventricular canal
What is tbx2 a marker of?
Atrioventricular myocardium
What does tbx2 KO in mice cause?
- Abnormal valve morphology and Nppa expression
- Widened atrioventricular canal
- Expansion of Nppa into what would have been the atrioventricular canal
What occurs in tbx20 KO in mice?
- Loss of chamber identity
- Expression of tbx2 (valve markers) throughout the heart
When heart looping occurs, which way does the heart loop?
Rightward looping
What model organism is useful in studying heart looping?
The zebrafish
What processes occur during heart looping?
1) Changes in cell shape
2) Asymmetric cell movements at the POLES of the heart
3) Regional changes in ECM composition
4) Lateralised cell signalling from the embryo
What happens the cell shape/size when the heart loops?
INCREASE in shape/size
Describe the regional DIFFERENCES in cell shape change during heart looping
Cells on INNER curvature of ventricle:
- Stay CUBOIDAL
Cells on OUTER curvature of ventricle:
- Grow MORE
- Elongate MORE
- Change orientation
How does the heart tube grow during heart looping?
Why does this occur?
SHF cells migrate to both poles of the heart and add into the heart tube
Helps the heart tube to undergo morphogenesis
During heart looping, what occurs at the poles of the heart?
What does this help to occur?
- Rotation of some cells
- Occur in OPPOSITE directions at either end of the heart tube
Helps the heart tube to undergo buckling/bending
Describe the regional changes in ECM composition when the heart undergoes looping
What does this allow?
Regionalised EXPANSION of the ECM at the ATRIOVENTRICULAR CANAL (where the valves form)
Allows:
- The valves to emerge from the heart tube
- Changes to the stiffness of the tube
Why is lateralised signalling from the embryo needed during heart looping?
To tell the heart tube which direction to bend in - this is very important
How can cardiac cells be visualised?
- Using the promoter of myl17 to drive GFP expression
- So that all cardiac cells are expressing GFP - able to visualise the linear heart time
What transcription factor is a marker of the specified SHF cells?
What does in situ hybridisation show of this transcription factor?
Islet1
ISH shows this transcription factor is expressed in the MESODERM adjacent to the heart early in development
What does lineage tracing of Islet1 expressing SHF cells show?
What reporter is used in lineage tracing?
That the cells that WERE expressing Islet1 end up inside the heart itself
LacZ reporter
What happens to Islet1 as the SHF cells enter the heart?
Islet 1 is switched off
What does ISH of myl17 in Islet mutant mice show?
Heart is REDUCED in size and is INCOMPLETELY looped
What is myl17 a marker of?
Cardiac cells
What is heterotaxia?
L/R asymmetry abnormalities
What do people with heterotaxia often have, in relation to the heart?
What does this show?
Congenital heart defects - abnormal cardiac development
Shows that it is important that heart looping occurs int he correct direction
What is situs invertus?
Complete reversal of the organ organisation
What is situs ambiguious/heterotaxia?
NOT a full reversal of organ organisation
What is organ asymmetry controlled by?
The ASYMMETRIC expression of NODAL
How is nodal expressed in the embryo?
What causes this?
In the LEFT LATERAL PLATE MESODERM prior to organ formation
Due to elevated calcium expression on the left of the node/Kupffer’s vesicle
What does loss of nodal function result in?
Disrupted organ asymmetry
What is the nodal homologue in zebrafish?
South paw (spaw)
What forms at the posterior of the embryo during somatogenesis?
What is this called in mice? Zebrafish?
Cup-shaped organ
Called:
The node (mouse)
Kupffer’s vesicle (zebrafish)
What is the structure of the node/Kupffer’s vesicle?
Cup-shaped organ
Lined with motile cilia
What substance is elevates in the left of the embryo?
How?
Calcium
Cilia of the Kupffer’s vesicle/node beat in a CLOCKWISE direction
Causing a directional fluid flow
What does the elevated calcium on the LEFT of the node/Kupffer’s vesicle result in?
1) Asymmetric expression of NODAL in the LEFT lateral plate mesoderm
2) This nodal propagates itself from the POSTERIOR part of the embryo (where the node is) to more ANTERIORLY in the LPM
What happens once nodal reaches the more anterior part of the embryo?
What does this help?
Starts to turn on ASYMMETRIC gene expression in the regions where the organs are going to form
- Helps the heart tube to form in the CORRECT POSITION
- Goes onto dictate the DIRECTION of organ looping
What is expressed where the gut is going to form?
Asymmetric expression of pitx2
What is expressed where the heart is going to form?
Cyclops and lefty2
Where are cyclops and lefty2 expressed in the heart?
In the LEFT part of the cardiac disc
What is expressed where the brain is going to form?
Lefty1
What happens if there is a loss of nodal FLOW?
What causes this loss of nodal flow?
RANDOMISATION of lateralised gene expression
Loss of nodal flow due to the KV/Cilary genes not working
What happens if there is loss of nodal (but the node/KV is still working)?
ABSENCE of lateralised gene expression
When does the heart beat during development?
Even BEFORE the heart tube forms
What are the 2 sources of mechanical force in the heart?
1) Cardiac contractility
2) Flow of blood (caused by cardiac contractility)
What is blood flow important for?
How does this occur?
Spatiotemporal gene expression of valves
- Induction and maintenance of FLOW-RESPONSIVE genes
What is an example of a flow responsive gene?
What does it require to be turned on?
klf2
Requires blood flow through the heart to be turned on
Describe the blood flow in the heart over devleopment
What does this cause?
- As heart undergoes morphogenesis - different patterns of blood flow through the heart
- Causes differences in the STRESS in different parts of the heart
What happens to the blood flow where there is narrowing of the heart tube?
What is this sensed by?
What does this cause?
There is retrograde (backwards) flow
Sensed by the cells in the adjoing regions
Causing these cells to turn on flow-responsive genes that are required for VALVE formation
How does the heart form different internal structures?
Sense SPECIFIC flow patterns in DIFFERENT regions of the heart
How are the differential patterns of blood flow through the heart as it develops
Architecture of the tissue changes
Due to the tissues undergoing MORPHOGENESIS
(When the heart loops - the sides of the vessels come closer together)
