Development of the heart Flashcards
describe the formation of the body plan in the embryo
- one cell divides and gives rise to a group of identical cells
- these cells begin to differ from one another and form blastocysts
- blastocysts form 2 cells layers after implantation
- 2 cell layers form 3 primary germ layers during the process of gastrulation
- human embryo is arranged as a flat disk but has recognisable anterior-posterior , left-right and dorsal-ventral axes
describe the formation of blastocysts
- single fertilized egg (zygote) is encased in a transparent protein coat called zona pellucida. This stops the cell from getting any bigger as it continuously divides.
- cells near outside of embryo undergo a process called compaction where tight bonds with neighbouring cells are formed.
- this allows fluid filled space to develop on inner part of embryo called the blastocoel. now embryo is recognised as a blastocyst.
- outside epithelial cells are called the trophectoderm which give rise to extra embryonic material.
- inner cell mass is what the embryo will develop from.
- after implantation, inner cell mass cells give 2 different types of cells - epiblast(above) and hypoblast (below) which form the germ disc
describe the formation of the primitive streak and gastrulation
- the 2 layered embryo now becomes a 3 layered embryo during gastrulation
- the primitive streak begins to form at the posterior end of the germ disc and progresses to the anterior end
- The epiblast cells move inwards from the outside of the embryo and become concentrated in the middle.
- the cells then detach and become migratory these are known as mesenchymal cells
- the mesenchymal cells then invade the space between the epiblast and the hypoblast.
- these cells differentiate.
what are the 3 primary germ layers and where are they positioned?
- ectoderm - outside
- mesoderm - middle
- endoderm - inside
describe the formation of the cardiac crescent
- As gastrulation progresses, signals
from different regions of the
embryonic disc combine to induce
cardiogenic mesoderm (the cardiac
crescent) at the anterior end of the
embryo - The first heart field forms the primary
heart tube - The second heart field adds cells to
the inflow and outflow regions of this
tube - There is major additional
contribution to the heart from neural
crest cells (ectoderm-derived)
describe embryonic folding
- heart tissue that is anterior to nervous tissue at the forebrain is brought more posterior so it begins to lie towards its adult position in the thoracic region.
what type of blood do the left and the right side of the heart carry?
right side - deoxygenated
left side - oxygenated
describe the formation of the primary heart tube
- cardiac crescent forms a tube made from myocardium on the inside and the lumen is lined by endocardium
- middle part of crescent expands to become the Left Ventricle
- endocardial tube attaches to developing aortic arches cranially (outflow) and systemic veins caudally (inflow)
describe the elongation of the heart tube
- Cells from the second heart field are added at both the outflow and inflow
- The ends of the tube are fixed, so the heart must bend as it elongates
- The direction of looping is genetically controlled (leftward)
- coils and foils so that heart chambers start to lie in adult position.
- dextrocardia - looping of heart is flipped.
how are chambers of the heart formed ?
Primitive chambers formed by expansion (ballooning) of the tube
how are ventricles categorised ?
by the presence of trabeculae (honeycomb-like structure)
what are the 4 different types of septation of the heart tube ?
Atrioventricular septation
* Divides atria from ventricles
* Divided by AV cushions
Atrial septation
* Divides L from R atrium
* Divided by interatrial septum
Ventricular septation
* Divides L from R ventricle
* Divided by interventricular septum
Outflow tract septation
* Divides pulmonary artery from aorta
* Divided by OFT cushions
what are the two ways in which septation can occur ?
Cushions
* AV and OFT cushions
* separation in the horizontal plane
* Valve formation, stenosis, and atresia defects if this goes wrong
* formed from endocardial cells
muscular walls
* muscles growing up or down
* separation in the vertical plane
* defects give rise to holes
what is an atrioventricular septal defect?
- when the 2 cushions dont grow and meet properly
- common in patients with down syndrome.
describe atrial septation - the primary septum
Atrial septation of the primary septum (septum primum) is the process that divides the primitive atrium into the right and left atria during heart development.
Steps of Atrial Septation (Primary Septum Formation):
* Septum Primum Grows – A thin tissue flap called the septum primum grows downward from the roof of the common atrium.
* Foramen Primum Forms – A small gap, the foramen primum, remains open at the bottom, allowing blood to flow between atria.
* Foramen Primum Closes – As the septum primum reaches the endocardial cushions, the foramen primum disappears.
* Foramen Secundum Forms – Before complete closure, small holes appear in the septum primum, forming the foramen secundum, ensuring continued blood flow.
describe atrial septation - foramen ovale and septum secundum
- 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
- Left atrial pressure increases after birth, closes valve, eventually seals
describe fetal vs postnatal circulation
- in the embryo our blood supply comes from our mother into the right side of the heart
- right-sided pressure is high as blood circulation is coming from the mother
- this pushes primary septum to open the hole between L and R atria
- after birth we start breathing this increases resistance of L side of heart the pushes flap against R side against secondary septum that’s formed
- flap is pushed and kept against secondary septum and will eventually fuse and close
- oval window between R and L atria is foramen ovale.
what are atrial septal defects ?
Persistent Foramen Ovale
* primary septum isnt properly connected to secondary septum
* Flap valve doesn’t seal completely
* Common (10% of population), seen as probe patency
* Possible risk factor for stroke and in divers
* Ostium primum defect - primary septum hasn’t grown down far enough to meet AV cushions
* Ostium secundum defect- secondary septum hasn’t grown down far enough to meet trailing edge of primary septum
what is an acyanotic defect ?
when pressure on the left is higher than on the right
describe interventricular septum formation
- interventricular septum grows between 2 ventricles that will meet and fuse with the atrioventricular septum and separate the left and the right ventricles
what are ventricular septal defects?
- holes form at various points around the heart.
- will allow ox blood to move from L to R but not the other way around
describe the septation of the outflow tract
- A single tube (labelled bulbus and conus ) is separated into the aorta and pulmonary artery
- These must be attached to the left and right ventricles, respectively
- Two cushions spiral through the truncus arteriousus
- Complex remodeling at each end
describe the formation of the outflow tract
- Spiralling cushions separate the aorta and pulmonary artery
- Neural crest cells contribute to the AP septum
- Complex remodelling of aortic arches
- Proximal cushions fuse with the IVS and AVS
what is the ductus arteriosus ?
- Defect is acyanotic
- Allows fetal circulation to bypass lungs
- Normally closes after birth and becomes the ligamentum arteriosum
what are OFT defects ?
- Several types of OFT defect
- Different types that result in a common truncus
- OFT defects are associated with neural crest defects and some genetic conditions (eg 22q11 Deletion Syndrome)
- Unequal division of the OFT can lead to aortic or pulmonary stenosis
what is transposition of the great arteries ?
- Aorta connected to right ventricle
- Pulmonary trunk connected to left ventricle
- Arterial trunks usually side-by-side
- Probably caused by abnormal outflow tract cushions
- Cyanotic
- fixed via surgery
what is tetraology of fallot
Four characteristic features
* VSD (large)
* Pulmonary stenosis
* Right ventricular hypertrophy
* Overriding aorta
- Cyanotic
- Complex phenotype may result from abnormal looping, leading to malalignment of segments
describe the cardiac conduction system
- Conduction tissue is specialized myocardium, not nerves (they come from neural crest cells)
- Variations in conduction properties caused by differences in ion channel and connexin (gap junctions) expression
- Conduction system differentiates by progressive, localised recruitment from heart tube myocardium
Electrical insulation layers: - Fibro-fatty layer at AV junction (ex cushions)
- Ventricular bundle branches wrapped in a fibrous sheath
describe atrioventricular septum formation
the atrioventricular (AV) septum forms during heart development to separate the atria (upper chambers) from the ventricles (lower chambers).
1.Endocardial Cushions Appear – Around the 4th week of embryonic development, small tissue swellings called endocardial cushions develop in the heart’s central region.
2. Cushions Grow and Fuse – These cushions grow toward each other and eventually merge, dividing the common AV canal into the right and left AV openings.
- Septum Formation Completes – The fused cushions contribute to the formation of the AV septum, helping separate the atria from the ventricles and ensuring proper blood flow direction.
