Lecture 19 - Cardiogenesis Flashcards
(26 cards)
What is cardiogenesis?
Development of heart
- heart is a mesodermal-derived organ - it is also a highly patterned organ
Explain how the human heart develops
Tube formation occurs after 3 weeks. Linear heart tube is contracting. Tube undergoes LOOPING MORPHOGENESIS (complex change in shape). This occurs over the next 3 weeks & sets the blue print of which the structures of the heart will be formed. Once looping occurs, regions of looping heart tube start ballooning out, which will form the chambers. Atria then ventricles. Elaboration of chambers occurs, which then leads to the heart having a more functional use.
It is a complicated process, that is made from a simple linear structure in the embryo.
Formation of valves also occurs. Ventricles have a muscular region (thicker than atria), due to trabecular layer - important for pumping blood after high pressure.
What models are used to study heart development?
Hard to study human heart, as many women don’t know they are pregnant at 3 weeks.
Human - mouse - chick (all have 4 chambered-heart).
Why is a Xenopus a good model to study heart development?
Interesting for exploration of septation
Why is a Zebrafish a good model to study heart development?
Don’t need 4 chambers, as it only has a single circulatory system (from heart to gill epithelium to rest of body).
Why is a Drosophila a good model to study heart development?
Tubular heart which doesn’t undergo looping morphogenesis
What are key common morphological steps in heart development?
- Cardiac precursor cells found as BILATERAL POPULATIONS of cells around the midline.
- Migrate to the midline & fuse to form the HEART TUBE.
- Heart tube undergoes an asymmetric bending morphogenesis termed HEART LOOPING, that leads to the correct positioning & elaboration of the 4 chambers.
- The heart undergoes MATURATION, resulting in formation of structures required for proper function - valves, septa, trabeculae.
What is the process of Zebrafish heart development?
Precursors which start of as a disks which is seen on the midline (21 hrs post fertilisation). The precursors migrate & form linear tube. Linear tube positioned asymmetrically - positioned to the left of embryo, under left eye. Still looping morphogenesis occurs (48 hours after fertilization), creating S-loop helps distinguish the different regions of the heart that forms atrium & ventricle, & restriction in tissue in the part of the tube that lies between where the valve develops. Bi-lateral precursors are found. Migration then occurs & fusion to midline, forming heart tube - asymmetric bending morphogenesis (looping), before maturation.
Describe the origins of cardiac cells - 2 heart fields
The heart is constructed from 2 populations of cardiac cells which are specified early during development, & exhibit spatiotemporal differentiation into the heart.
FIRST HEART FIELD (FHF) - left ventricle & right atria
SECOND HEART FIELD (SHF) - right ventricles, left & right atria, outflow tract.
Linear heart tube & looped heart are constructed from 2 different precursor cells in the embryo
Describe specification of cardiac cells
Cardiac cells are specified from mesodermal tissue along the primitive streak (mouse) or embryonic margin (fish) - CONTROLLED BY COMBINATORIAL MORPHOGEN SIGNALLING.
Once specified they migrate anteriorly to form the primitive heart tube (FHF) & SHF in the adjacent mesoderm.
All cardiac cells are exposed to Wnt signalling, then they are exposed either to Wnt, B-catenin & FGF or BMT, which then causes segregation of cardiac precursors, into either First Heart Field progenitors or Second Heart Field progenitors
Describe regional patterning, growth & functional regionalization
Chamber vs non-chamber (contractile vs non-contractile myocardium)
Atrial vs ventricular contractility (directional conduction)
Inflow (pacemaker/SA node), atrioventricular canal (valve/AV node), outflow (valves).
Contraction in different ways
How does heart patterning begins at tube stage?
Setting up patterning in different regions of the heart tube is a complicated interplay of activating & inhibitory interactions.
2 main pathways that regulate formation of chamber & non-chamber
Notch-Tbx20 pathway promotes chamber identity in cardiomyocytes
BMP-tbx2 pathway promotes atrioventricular canal identity
How does Tbx2 specify the atrioventricular canal?
Tbx2 represses the chamber program & promotes the AV canal/valve program
Tbx2 mutants = abnormal valve morphology. NPPA gene is a hormone expressed only in chamber myocardium. NPPA gene (which should be restricted to chamber myocardium) can expand throughout the region where usually is, in Tbx2 mutants.
In a normal situation, Tbx2 repressed expression of NPPA in the atrioventricular canal region.
Tbx2 mutation - linked to atrial septal defects.
How does Tbx20 specify the chambers?
Tbx20 mutants = loss of expression of NPPA - loss of chamber identity. Expression of Tbx20 is expanded outside of atrioventricular canal region & into the region where chambers usually from in heart.
Normally, Tbx20 normally represses atrioventricular canal program, & instead promotes the chamber program.
Regionalized patterning due to Tbx genes - either repress/establish chamber of canal program.
Where are Secondary Heart Field cells found?
SHF found on the periphery of linear heart tube. Overtime, some of SHF cells are found in the heart tube, due to looping morphogenesis.
What is expressed in SHF progenitor cells?
Islet1
How does the heart grow?
Heart grows as it undergoes looping morphology. At early stages, there isn’t a lot of proliferation (typical way of organ growth). Nevertheless, growth still has to occurs.
Transcription Factor Islet1 is expressed in tissue of the body wall next to the heart (not in the heart)
Where is Islet 1 (Isl1) expressed?
LacZ lineage tracing was used to understand where the Isl1 (Islet1) expressing cells ended up.
LacZ lineage tracing involves taking involves a Cre Driver & cross it with tracer mouse. The tracer mouse is one that is expressing the LacZ operon globally throughout the embryo. It is under the control of lock sites. Crossing of these 2 mice leads to Cre recombining at lock sites & expression of LacZ (galactosidase). Any cell expressing Isl1 will now express LacZ. Embryos are taken & LacZ staining occurs. This allows identification of cells that are (or had expressed) Isl1.
Isl1 is expressed in the SHF field, but the cells end up in the heart.
LacZ lineage tracing shows that Isl1 positive cells are now sitting in the heart.
This tells us Isl1 is expressed in the wall of the embryo early on, and then in later stages of heart development (after looping morphogenesis) are found in the heart. This suggests that there is a movement/deployment of cells of the body wall of the embryo into the heart tube during looping morphogenesis. This important for growth.
What does Isl1 do?
It is important for the movement of SHF cells, which is important for growth. However, a lack of Isl1 also leads to cells in the body wall not migrating into the heart & preventing growth. A lack of isl1 also leads to lack of chamber formation & other functions involved in the process of looping morphogenesis.
How is left-right asymmetry important for heart looping morphogenesis?
Heart is highly asymmetric, which is important for sending blood around the dual circulatory system, with each side having different requirements.
Explain how individuals with heterotaxia (L/R asymmetry abnormalities) often have congenital heart defects
Situs invertus - usually asymptomatic
Situs ambiguous/heterotaxia - associated with significant pathology. Abnormal cardiac development & function is common in heterotaxia patients.
Organs patterned different across L/R axis.
Specific congenital heart defects associated with Sinus heterotaxia.
What is the role of Nodal in creating Organ asymmetry?
- Nodal is asymmetrically expressed in the left lateral plate mesoderm of embryos prior to organ formation.
- loss of Nodal function results in disrupted organ asymmetry
How do we generate & maintain asymmetric expression of Nodal and what does it do?
Highly conserved pathway - spaw is a Nodal homolog
In ball-shaped structure, there is cilia which undergo rotation & generate fluid-flow which induces calcium signalling to become elevated on the left side of the embryo, at the posterior. This elevation of calcium signalling results in an elevation in Nodal expression, on the left size, which propagates anteriorly through the left plate mesoderm & induces asymmetric gene expression in organ.
How does Cilia influence Nodal expression?
Cilia are motile & undergo rotation which induces fluid flow. This results in asymmetric elevation of Nodal signalling on the left side of the lateral plate mesoderm. Nodal signalling is initiated in the posterior region & propagates anteriorly, via the left lateral plate (LPM) mesoderm to more anterior regions of the LPM. Once it reaches the anterior, it signals to the regions where organs will be formed, and instructs which side will be left. Nodal induces expression of a few genes - including cyclops & lefty2.
asymmetric movement of cells to form the heart tube, but already been patterned into L/R side. This L/R patterning is what instructs the cells to undergo specific migration that allows it to make the heart tube, that is positioned asymmetrically under the left eye.
Nodal is important for sending signals to pattern disc, which also give instruction of how that heart tube should be assembled in the correct way.
