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Life support (Year 1) > CVS: Development of the Heart and its Conduction System > Flashcards

CVS: Development of the Heart and its Conduction System Flashcards

1
Q

Describe early heart development

A
  • Tube formation - muscle wrapped around endothelium
  • Tube attached to arteries at one end, veins at the other
  • Tube elongates by addition at each end of cells from the SHF
  • Tube bends, loops + coils
  • Neural crest cells migrate into outflow region to make outflow cushions
  • Epicardium cells come from proepicardial organ
  • Tube divided by partitioning into chamber
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2
Q

When does the heart begin to develop?

A

Weeks 3-10 post conception

First recognisable as a crescent-shaped tube of myocardium, lumen of tube lined by endocardium

Heart is first organ to develop and function

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3
Q

What is the cardiac crescent derived from?

A

Mesodermal cells

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4
Q

Describe the expansion of the primary heart tube

A

Medial part of crescent expands - later becomes left ventricle

Endocardial tube attaches to developing aortic arches cranially (outflow) and systemic veins caudally (inflow)

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5
Q

Describe the elongation of the heart tube

A

Cells from 2nd heart field added at both outflow and inflow

Ends of tube fixed, so heart must bend as it elongates

Direction of looping is genetically controlled (leftward)

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6
Q

What are the primitive chambers formed by?

A

Expansion (ballooning) of tube

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7
Q

How is the heart tube divided?

A

Dividing heart tube = Septation

Divided into:

  • Atrioventricular septation
    • Divides atria from ventricles
    • Divided by AV cushions
  • Atrial septation
    • Divides LA + RA
    • Divided by interatrial septum
  • Ventricular septation
    • Divided LV + RV
    • Divided by interventricular septum
  • Outflow tract septation
    • Divides pulmonary artery from aorta
    • Divided by OFT cushions
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8
Q

What 2 types of septation divide the heart tube?

A

Cushions:

  • AV and OFT cushions
  • Valve formation, stenosis and atresia defects if goes wrong
  • Form when cardiac jelly (ECM) secreted by myocardium endocardial cells undergo epithelial-mesenchymal transition and populate the jelly. Neural crest cells contribute significantly to OFT cushions

Muscular walls:

  • 1° and 2° interatrial septa, interventricular septum
  • Defects give rise to holes
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9
Q

What are the FHF and SHF?

A

FHF - First Heart Field

SHF - Second Heart Field

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10
Q

What does the FHF form in heart embryology?

A

Forms primary heart tube, mostly left ventricle, parts of RA and LA

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11
Q

What does the SHF form in heart embryology?

A

Forms cells of outflow tract, RV + portions of RA and LA

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12
Q

What is the process called by which the heart tube is divided into 4 chambers?

A

Elongation

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13
Q

Explain the process by which the 4 chambers form

A
  • Tubes (made from mesodermal cells) from either side midline fused
  • Medial side expands (forming LV primarily)
  • Arterial + venous sides need to move so they are on same side, done via twisting and looping
  • Endocardial tubes attach to developing aortic arch (cranially, i.e. outflow) and systemic veins (caudally i.e. inflow)
  • Cardiac crest and initial heart tube formed
  • Cells from SHF add more tissue at outflow and inflow ends
  • Outflow and inflow ends (anterior + posterior) are fixed so as tissue added, heart tube starts to bend as it elongates
  • Bending = looping
    • Direction of looping genetically controlled (humans → LHS)
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14
Q

What is dextrocardia?

A

Right hand side looping

This isn’t problematic as long as everything else is correctly reversed

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15
Q

Explain the later stages and actual forming of the chambers of the heart

A
  • Tube elongates + loops
  • Ballooning out of certain areas, mapping out chambers, specifically ventricles
  • Interventricular groove on external surface that begins to separate the LV from RV on outside
  • Venous region wrapped behind heart so LA and RA have been position on cranial region of heart
  • Process of ballooning and folding continues until appendages of heart tube fuse to side of heart in adult position
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16
Q

What is the characterical difference between atria and ventricles in development?

A

Coronal section:

  • On RHS - ballooning chamber
  • Atria - smooth
  • Ventricles - Honeycomb, trubecular layers
17
Q

Discuss what aortic arch arteries are

A
  • Form sequentially with looping + ballooning of chamber
  • Initially → Aortic arch I, until aortic arch VI
  • As each forms the earlier forms will degenerate so that adult structures formed from 2, 4, 6
    • When 4th forms, 1st degenerates
    • When 5th forms, 2nd degenerates
  • Remodelled at later stages, giving rise to structures in aortic arch
  • Symmetrical
  • In embryo - called pharyngeal arteries (transient)
  • Form in sequence from anterior → posterior
18
Q

Outline the remodelling of the aortic arch vessels and how it links to outflow tract separation

A

Makes the great vessels

  • Cells responsible for separation of outflow tract are neural crest cells (from neural tube)
  • Cells migrate from dorsal aspect of neural tube down pharyngeal arteries towards outflow tract
  • Neural crest cells migrate to outflow tract and populate it + divide into aorta and pulmonary artery
    • Causes change in BF in the R+L side of common trunk
    • This BF is what causes degeneration of aortic arch arteries
19
Q

Describe CHD effects

A

Acyanotic - Left to Right shunt

  • VSD
  • ASD
  • AVSD
  • Persistent Ductus Arteriosus

Cyanotic - Right to Left shunt

  • Transposition of great arteries
  • ToF
  • Truncus arteriosus
  • TAPVC (Total anomalous pulmonary venous connection)

Holes in septums are usually acyanotic (pressure is higher on left than right)

20
Q

What are the 2 different types of septation that divide the heart tube?

A

Muscular walls:

  • Primary and Secondary interatrial septa, interventricular septum
  • Defects give rise to holes

Cushions:

  • AV and OFT cushions - also give rise to valves
  • Valve formation, stenosis and atresia defects if this goes wrong
21
Q

Explain how the AV septum is formed

A
  • AV cushion growing from dorsal wall towards midline of heart and also growing from medial wall towards midline
  • Ventral + dorsal cushion meet in midline and fuse
  • On either side of AV cushion are narrow channels which go on to form tricuspid + mitral valve, allowing flow of blood b/w LA + LV and RA + RV

H hole - 2 vertical lines which are the narrow channels

22
Q

In what condition is an AV septum defect more common?

A

AVSD - Common AV junction, common in Down’s syndrome

23
Q

What is the first step in interatrial septum formation?

A
  • LA with the orifice of the pulmonary vein (broken arrow)
  • RA with orifice of the systemic venous sinus (star)
  • Primary interatrial septum (arrow) is growing to dive L-R
    • First to form to separate atria
    • Grows downwards
    • Will grow down, meet AV cushion + fuse
24
Q

Explain how interatrial septum is formed

A
  • Primary interatrial septum grows down + fuses with AV cushion
  • Trailing edge of septum that starts breaking down - allows blood to flow b/w LA and RA
    • This hole is the foramen ovale and it exists to allow blood to flow around the lungs.
  • Secondary interatrial septum is forming and that grows down, adjacent to primary
  • As it grows it covers the foramen ovale
    • Trailing edge of primary can move and allows shunting of blood
25
Q

Outline statistical evidence that shows the severity of congenital heart disease and which types are more common and which lead to more infant death

A

CHD - Most common causes of infant mortality due to birth defects (30-50%)

26
Q

Describe the origins of the cardiac conduction system

A
  • Conduction tissue is specialised 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 fibrous sheath
27
Q

What is the hole between the atria called?

A

Ostium secundum or foramen ovale

28
Q

Explain how the atrial septum is formed

A
  • Primary interatrial septum grows down, fuses with AV cushions
  • Trailing edge primary septum break down, allows blood to continue flowing from RA → LA
  • Septum secundum grows down to form flap valve
  • LA pressure increases after birth, closes valve, eventually seals
29
Q

Describe the different ASDs

A

Atrial septal defects:

  • Persistent foramen ovale
    • Flap valve doesn’t completely seal
    • Common (10% population) seen as probe patency
    • Possible risk factor for stroke and in divers
  • Ostium primum defect - Located near lower portion of atrial septum, may be associated with defects in mitral and tricuspid
  • Ostium secundum defect - Located in centre of atrial septum
30
Q

Explain how the interventricular septum is formed

A

IVS grows from wall, towards AV cushions, initially as thickened trabeculum

31
Q

Describe ventricular septal defects

A
  • 75% perimembranous
  • 25% muscular
  • Small
    • Small L→ R shunt
  • Medium
    • Moderate L→ R shunt
  • Large
    • Large L→ R sunt
    • Surgically repaired in childhood
32
Q

Explain how the outflow tract is separated

A
  • Single tube separated into aorta and pulmonary artery
  • Must be attached to L + R ventricles respectively
  • 2 cushions spiral through truncus arteriousus
  • Complex remodelling at each end
33
Q

Explain how the outflow tract is formed

A
  • Spiralling cushions separate aorta + pulmonary artery
  • Neural crest cells contribute to AP septum
  • Complex remodelling of aortic arches
  • Proximal cushions fuse with the IVS and AVS
34
Q

What are the effects of OFT defects?

A

Several types of OFT defect:

  • Different types that result in a common truncus
  • OFT defects associated with neural crest defects
  • Unequal division of OFT can lead to aortic or pulmonary stenosis
35
Q

Describe transposition of the great arteries (TGA)

A
  • Aorta connected to RV
  • Pulmonary trunk connected to LV
  • Arterial trunks usually side-by-side
  • Probably caused by abnormal OFT cushions
  • Lethal without VSD, ASD or PDA
  • Cyanotic
  • Can be surgically correct
36
Q

Describe Tetralogy of Fallot

A

4 characteristic features:

  • VSD (large)
  • Pulmonary stenosis
  • RV hypertrophy
  • Overriding aorta

Right → Left shunt

Cyanotic

Complex phenotype may result from abnormal looping, leading to malalignment of segments

Life support (Year 1) (24 decks)

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