Embryology Flashcards
What effect does folding of the embryo have on the early development of the heart
• Lateral folding
• Creates a heart tube
• Cephalocaudal folding
• Brings the tube into the thoracic region
Primitive heart tube - beginning of functioning cvs
Describe the formation of the primitive heart tube
Once embryos folds in the plane, the tubes fuse to form heart tube
Endocardial tubes fuse to form a single tube
Embryo cannot survive unless it has own circulatory system. Which is hwy it develops early on
What must happen to the primitive heart tube?
• The primitive heart tube must be divided
• to create the 4 chambers
• The inflow vessels and the outflow vessels must be remodelled
• creating the familiar configuration of vessels returning blood from the systemic circulation and the great vessels taking blood away from the heart
Tube- blood squeezed alone - suffiecint for early embryo
But needs to be divided into 4 chambers
What are the regions of the primitive heart tub e
Blood enters the sinus venosus Atrium Ventricle Bulbus cordis Truncus arteriosus Aortic roots (blood leaves)
Blood is squeezed up and ou
Which regions of the primitive heart tube are located in the pericardial sac
Ventricle, bulbus cordis, truncus arteriosus
What is looping?
- Tube elongates - expansion of zones within it
- Runs out of room
- Twists and folds up - ventricle moves forwards and downwards, atria pushed backwards and upwards
- regular and predictable
- Places the inflow and outflow in the correct orientation with respect to each other
How does the sinus venosus develop?
Sinus venosis is venous inlet - blood coming up into tube
Initially 2 horns - initially same size - receiving veins from placenta, yolk sac & embroyinic body
Over time venous return shifts to RHS, L horn recedes, right horn absorbed by enlarging oprimitive atrium
One of the contributors to wall of left atrium is wall of vessels
How do the atria develop?
- RA develops from
- most of the primitive atrium
- sinus venosus
- receives venous drainage from the body
- LA develops from
- a small portion of the primitive atrium
- absorbs proximal parts of primordial pulmonary veins to leave 4 entrances
- receives oxygenated blood from the lungs (venae cava) and the heart (coronary sinus)
How does the oblique pericardial sinus form?
See slide for positioning
• Oblique pericardial sinus formed as left atrium expands absorbing the pulmonary veins
LA expands and stretches out pericardial sac around pulmonary veins
Why does the fetal heart need shunts?
Lungs don’t work - sending blood to pulmonary circulation could damage lungs
• Oxygenation and removal of CO2 occur at the placenta - circulation takes blood from uteroplacenta circulation to heart
• So shunts are required to maintain fetal life
• AND – these shunts must be reversible at birth
Oxygenated blood comes from umbilicus
Highest pO2 in umbilical vein - comes in at right side of heart
Decsribe fetal circulation
Oxygenated blood coming in at interface between maternal circulation and fetal circulation - from placenta to umbilicus
Drains into IVC (draining into aorta would be a short circuit)
Highest pO2 blood in RA - not where we want it to be
Need it in left - need to bypass right ventricle and lungs to get oxygenated blood into left side (hole between RA and LA)
Small amount of blood enters the RV from RA - its muscle so needs blood to develop normally
RV empties into pulmonary trunk - needs to bypass lungs to get to aorta
High pO2 blood in the LA goes to LV then round to body - back to mother in uteroplacenta circulation to lose the CO2
See slide for diagram
What are the fetal circulatory shunts?
Ductus venosis - short vessel between placenta and IVC
Foramen ovale - hole in heart between atria
Ductus arteriosus - vessel that connected pulmonary trunk to aorta - ensures that we protect lungs - functional only in fetal life
How are the shunts closed at birth?
Need to close the shunts at birth
When baby takes first breath - pulmonary circulation starts
LA pressure increases - rise over RA pressure - closes FO
DA contracts - wall has special tissue sensitive to rises in pO2
Initial breath - increase in pO2 - causes vessel to contract and obliterate the connection between pulmonary trunk and aorta
When placental support removed, the DV closes because no longer a big volume of blood coming through umbilical vein
Closing of shunts - resolve circulation
Which major arteries are formed early on?
- arteries that supply head & upper limb
- route blood to lungs for oxygenation
- ascending, arch and descending thoracic aorta
Hold egos the early arterial system begin?
- Early arterial system begins as a bilaterally symmetrical system of arched vessels
- Undergo extensive remodelling to create the major arteries leaving the heart
See slide for diagram
What are 2 important arches and what are their derivatives?
- 4th arch
- R = proximal part of R subclavian A
- L = arch of aorta
- 6th arch = “pulmonary arch” - gives vessels to supply pulmonary circulation
- R = R pulmonary artery
- L = L pulmonary artery & Ductus Arteriosus
How are the aortic arches remodelled?
- The aortic arches are remodelled to create the mature disposition
- As the heart “descends” the nerve hooks around the 6th aortic arch and “turns back on itself”
- The left recurrent laryngeal nerve becomes hooked around the shunt between the PT & aorta
Remodelling of 4th and 6th arteries - variant remodelling on either side
Purple vessel = ductus arteriosis
Yellow - nerves - recurrrent laryngeal nereve - descends down into thorax and turns back on itself - this never gets tangles in system of reorganisation of aortic vessel - SEE SLIDE
What is patent ductus arteriosus?
Patent ductus arteriosis
DA should normally contract with first breath - in this condition it doesnt
Persistent route of communication between aorta and pulmonary artery
Initial spasm of artery - originally prostaglandin based - becomes anatomical
Blood shunt from higher to lower pressure- left to right
What happens after looping of the heart tube?
• Atrioventricular canal links atrium & ventricle
• Don’t yet have the “two pumps
in series” configuration
What is septation?
Interatrial septum - Divide primitive atrium into left and right chambers
Left atrium gets bulk of chamber size from incorporation of pulmonary vermin
Interventricular septum - Divide primitive ventricle into left and right
Create 4 chambers to achieve selective outflow
What is the first step in septation?
Endocardial cushions - develop in AV region - narrowing of heart tube at this point - new tissues calls endocardial cushions forming - divides developing heart into left and right channels
Narrowing of junction between primitive atrium and primitive ventricle = atrioventricular canal
Division of cells - bumps on dorsal and ventral walls - endocardial cushions - dependent on a particular subgroup of cells migrating in - neural crest cells
Bumps for and meet in the middle - above it primitive atrium, below it primitive ventricle
Describe atrial septation
- Septum primim grows down from room of primitive atrium onto fused cushions
- As first septum is forming, as it grows will maintain an opening for blood to travel freely between
- Ostium primum is the hole present before the septum primum fuses with the endocardial cushions
- Before ostium primum closes, a second hole, the ostium secundum appears in the septum primum
- Finally a second crescent shaped septum, the septum secundum grows; the hole in the septum secundum is the foramen ovale
During embryonic/fetal life, when RA pressure higher than left, umbilical vein to IVC - relative pressure difference will push 2 leaves apart -
Holes staggered so blood flows
Exploiting the real active pressures in the 2 chambers
Give a summary of the development of the atria
- Both left & right atria have components derived from the primitive atrium (i.e. auricles)
- The right atrium absorbs the sinus venosus
- The left atrium sprouts the pulmonary vein then grows to absorb it and its first 4 branches
- Interatrial septum forms to divide the chamber into left and right chambers
- The fossa ovalis is the adult remnant of the shunt used in utero to by- pass the lungs
What are ostium secundum defects?
- Ostium secundum defect
- septum primum
- resorbed
- too short
- septum secundum too small
What is hypoplastic left heart syndrome?
• Exact cause not known • Some embryological speculation...... • defect in development of mitral and aortic valves - Resulting in atresia - Limited flow • ostium secundum too small • therefore right to left flow inadequate in utero • use it or loose it rule applies • left heart is underdeveloped
What are the 2 components of the ventricular septum?
- Starting with a single ventricular chamber
- Ventricular septum forms, which has 2 components - muscular and membranous
- Muscular portion forms most of the septum and grows upwards towards the fused endocardial cushions
How does the interventricular septum form?
Muscular portion grows upwards towards the endocardial cushions leaving a small gap, the 1° interventricular foramen
Membranous portion of the interventricular septum formed by
connective tissue derived from endocardial cushions to “fill the gap”
What is ventricular septal defect?
• (most commonly) Membranous portion of interventricular septum involved
Failure of formation of membrane in intraventricylar septum leads to ventricular septal defect - membrane didnt completely obliterate gap
How is the outflow tract separated?
- Endocardial cushions also appear in the truncus arteriosus - they are staggered and grown into lumen of tube
- As they grow towards each other they twist around each other
- Form a spiral septum so blood from the right side is channeled to pulmonary trunk and blood from left side channeled into aorta
What are congenital birth defects?
Congenital birth defects • Can be • structural abnormalities • complete absence of a structure • Result from interference with / interruption of normal developmental processes
- Causes can be
- genetic
- exposure to chemicals / drugs / infectious agents
- unexplained
What are congenital heart defects?
• The developing heart is subject to the same vulnerabilities as all other systems
• Occur when there is
• a structural defect - Failure of structure or valve to develop normally
- of the chambers
- of the vasculature
• There is an obstruction
• There is communication between pulmonary and systemic circulations
•because…….
• Additional complexity due to the differing circulatory needs of the fetus as compared to the newborn (mature)
What is transposition of the great arteries?
• Aorta arises from right ventricle
• Pulmonary trunk arises from left ventricle
• What will happen?
• Cyanosis
• Likely to relate to the development of the
• Depending on what other if any defects are present
aortic and pulmonary values which need to be carefully positioned to ensure normal “plumbing”
What is tetralogy of Fallot?
4 features
• Large ventricular septal defect
• Overriding aorta - over both ventricles
• Right ventricular outflow tract obstruction
• Right ventricular hypertrophy
Underlying problem
• Conotruncal septum formation defective
• Importance of neural crest cells
