Heart Muscle
Differentiation
Heart Muscle Differentiation
THE HEART
• Structure and function
• Anatomy of the heart
HEART DEVELOPMENT
• Overview of Heart Formation
• Genetic factors regulating cardiac
muscle differentiation
WHAT HAPPENS WHEN IT ALL GOES WRONG?
• Congenital Heart Disease
WHAT ARE THE REQUIREMENTS
OF A PUMP???
1. Receiving Chambers
Left and Right
Atria
2. Delivery Chambers
Left and Right
Ventricles
WHAT ARE THE REQUIREMENTS
OF A PUMP???
3. Valves to direct the flow of blood
pulmonary
Valve
tricuspid
Valve
Aortic
Valve
Mitral/
Bicuspid Valve
WHAT ARE THE REQUIREMENTS OF A PUMP?
4. Strongly contractile wall to provide the force
required to propel blood
Intercalated
Disc
Nucleus
CARDIAC MUSCLE
CELLS
•Cardiomyocytes
• contractile element of cardiac muscle
• elongated cells
• centrally located nucleus
• branched cells
• separated from each other by the
presence of intercalated discs
• filled with rod-like bundles of
myofibrils (contractile proteins s.a.
myosin and Actin)
Intercalated
Disc
Nucleus
THE 6 REQUIREMENTS OF A PUMP
1. Receiving chambers
2. Delivery chambers
3. Valves to direct the flow of blood
4. Strong contractile wall to propel blood
5. Vessels to deliver blood
6. Conduction system to regulate the pump
5. Vessels to Deliver Blood
6. Conduction System
AORTA
SUPERIOR
VENA CAVA
ASCENDING AORTA
PULMONARY ARTERY
LEFT
ATRIUM
SA NODE
RIGHT
ATRIUM
LEFT ATRIUM
AV NODE
MITRAL VALVE
RIGHT
ATRIUM
AORTIC
VALVE
PULMONARY VALVE
RIGHT VENTRICLE
TRICUSPID
VALVE
INFERIOR
VENA CAVA
LEFT VENTRICLE
LEFT ANTERIOR DESCENDING
CORONARY ARTERY
INTERVENTRICULAR
SEPTUM
CONDUCTION SYSTEM
HEART FORMATION
• Gastrulation - formation of 3
germ layers
• ENDODERM
• ECTODERM
• MESODERM
• Heart is derived from the
mesoderm
• First indication of human heart
development is around day 16-19
• How do we progress from a single layer of
mesoderm to the complex 3 dimensional
structure of the heart???
differentiation
• Genes
Morphogenesis
• Process of Heart
Formation
Cardiogenic Mesoderm
Cardiac
Cresent
Cardiac
Progenitor
cells)
HEART FORMATION
1. Formation of endocardial
tubes derived from
mesodermal cells
2. Formation of the Primitive
heart tube
Fusing
Heart
tubes
Day 18
Endothelial
(endocardial) Tubes
Day 22
Primative/linear
Heart Tube
HEART FORMATION
3. Primitive heart tube develops into 5 distinct regions
Truncus
Arteriosus
Bulbus
Cordis
Fusing
Heart
tubes
Ventricle
Day 22
Atrium
Sinus Venosus
HEART FORMATION
4. Primitive heart tube Twists
Truncus Arteriosus
Bulbus Cordis
Day 23
Ventricle
Atrium
Sinus
Venosus
Looping brings the distinct regions of the heart into the
basic pattern that prefigures the adult structure
HEART FORMATION
The bulbus cordis and truncus arteriosus have divided into two
vessels forming the aorta and pulmonary trunk
Superior
vena cava
Aorta
Pulmonary
Trunk
Atrium
Inferior
Vena cava
Ventricle
Week 7 - the interatrial and interventricular septa have formed
partitioning the atria and ventricles into L and R compartments
Mouse Heart Formation
Cardiac crescent
stage
(E7.75)
Intra-embryonic coelom
Heart progenitors
Linear heart tube
Myocardium
(E8.0)
Endocardium
Conotruncal cushions
Endocardial cushions
Looping heart
(E10.5)
Remodelling heart
(E12.5)
RV
LV
RA
RV
Inter-ventricular septum
Trabeculae
LA
LV
Atrial septum
Endocardial cushions
Inter-ventricular septum
Trabeculae
Early identification of Cardiac Progenitors
in Mouse Embryo Day 7/7.5
Neural Plate
Ectoderm
Cardiac
Cresent
Cardiac
Progenitor
cells)
Head
Mesoderm
Cardiac
Mesoderm
E7.75
Whole mouse embryo
Transverse section
Heart Development in the Mouse
J.M. Icardo, 1997
AT WHAT STAGE DOES THE
HEART START PUMPING?
• Human primitive heart begins contracting at day 22
• Mouse heart starts to contract around day 8
• Why does the heart start to contract so early???
GENETIC CONTROL OF HEART MUSCLE
CELL DIFFERENTIATION
• Genetic dissection of heart (dorsal vessel) formation in
Drosophila has led to the identification of a number of
genes implicated in heart determination
Dhand
dp
p
ba
tinman
gp
i
pe
tw
is t
2
f
e
Dm
What does tinman do?
• homeobox gene
• identified in 1989 by
Kim and Nirenberg
• localised to the
dorsal mesoderm
• later stages to
heart precursors
and mesoderm
• Does tinman play a
role in heart
formation?
Does tinman play a role in
heart development?
• Disco - identify cardial cells of the
dorsal vessel
• visceral mesoderm expression in
wt embryos
• tinman k/o - no heart or visceral
mesoderm formation
• tinman expression is crucial for
heart formation in drosophila
TRANSCRIPTIONAL CASCADE FOR
CARDIOGENESIS IN DROSOPHILA
TWIST
Ventral
D-mef2 mesoderm
tinman
Dpp
Wg
tinman
Cells committed
to a cardiogenic fate
D-mef2
Activation of
transcription
Contractile protein genes
Dorsal
mesoderm
Heart
Precursors
• twist is essential for mesoderm
formation
• 6 different genes related to tinman
were isolated from divergent
species
Nkx2-3
Nkx2-5 (1993 Kamuro and Izumo,
Nkx2-6
Nkx2-7
Nkx2-8
Nkx2-9
Harvey 1996)
• evolutionary conservation
• Does Nkx2-5 have a similar
function to tinman?
Localisation of Nkx2.5 positive cells in the
developing mouse embryo using Lac Z Expression
Cardiac
Cresent
Cardiac
Progenitor
cells)
E7.75
• Nkx2-5 expressed in the
mesoderm
• later stages it is only
expressed in the heart
• panel E shows no expression
of Nkx2-5 in the lungs
• k/o Nkx2.5 there is no effect
on early heart formation
•
looping/twisting
morphogenesis is
affected
Muscle
Peri-cardial
VM
Expression of Cardiac NK2-class Homeobox genes
tinman Drosophila
Mouse
Nkx2-5
Chick
Zebrafish
Frog Nkx2-5
GENETIC CONTROL OF HEART MUSCLE
CELL DIFFERENTIATION
• Nkx2 class hoemobox genes are expressed during
gastrulation in the lateral plate mesoderm
(mouse, frog, avian and fish embryos)
• critical determinants of cardiac development
• Studies in Drosophila have shown tinman expression
to be essential for heart development
• Absence of Nkx2-5 in the mouse doesn’t prevent the
formation of the heart tube but blocks looping
and septum formation
CONSERVATION OF GENETIC PATHWAYS
IN HEART DEVELOPMENT
CONSERVATION OF THE GENETIC PATHWAYS
IN HEART DEVELOPMENT
Congenital Heart Disease
• Defects present from birth
• affect <1 % of all children
• Morphogenesis of the heart has many stages
Pulsating
tube
Twisting/rotating
heart
Septa Formation
4 Chamber
Organ
• Ample opportunity for something to go wrong
• Not surprising that abnormalities occur, perhaps
more surprising that the occurrence of abnormality
is so infrequent.
Congenital Heart Disease
• What kind of abnormalities occur????
• Incomplete septa formation (holes between chambers)
• Incorrect connection between chambers and vessels
• valves that don’t function properly
• Atrial septal defect
• ventricular septal defect
• Tetralogy of Fallot
Atrial Septal Defect
• incomplete closure between 2 upper chambers
• Blood returning from lungs flows through the hole
• More blood flows through R side of heart
• pulmonary hypertension
• in most cases surgery can rectify this problem
Ventricular Septal Defect
• incomplete closure between 2 ventricles
• Initially blood flows from R to L
• heart dilates, pressure increases resulting in
pulmonary hypertension due to increased workload
• in most cases spontaneous closure occurs
•If needed surgery can rectify this problem
Tetralogy of Fallot
1. Ventricular septal defect
2. Pulmonary stenosis (narrowing of pulmonary artery)
3. Hypertrophy (thickening of the RV wall)
4. Overriding aorta
Decreased blood flow to the lungs and mixing of blood
Nkx2-5 and Human Cardiac defects
• Recent investigations have mapped a mutation
causing atrial septal defects to the NKX2-5 gene
(Schott et al 1998)
• Decreased capacity of this transcription factor to
bind DNA
• implicates Nkx2-5 in atrial septation and
conduction system development
More recent reviews
Buckingham et al 2005 Building The Mammalian Heart From
Two Sources Of Myocardial Cells Nature reviews –genetics
6:826-835
Srivastava 2006 Making or Breaking the Heart: From
Lineage Determination to Morphogenesis Cell 126:1037-1048
NKX2-5 in ventricular muscle – Srivastava Nature 2004