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Semester 2 - CVS > 6. Embryology > Flashcards

6. Embryology Flashcards

1
Q

Which embryonic tissue gives rise to the cardiovascular system?

A

Mesoderm

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2
Q

In which week of development does the embryo fold?

A

4th weeks

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3
Q

Does the folding of the embryo in the 4th week change the position of the cardio genic field?

A

Yes

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4
Q

What happens in the first two weeks of embryo development?

A

The first two weeks created tissues of the future embryo and future placenta

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5
Q

What happens in the third week of embryo development?

A

The third week created the three germ layers
• ectoderm, mesoderm and endoderm
• the primordia of all tissues

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6
Q

What happens in the fourth week of embryo development?

A

The fourth week created a recognisable body form and the mesoderm began to organise

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7
Q

What effect does folding of the embryo have on the early development of the heart?

A
  • Lateral folding - Creates a heart tube

* Cephalocaudal folding - Brings the tube into the thoracic region

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8
Q

What is the cardiogenic field and where is it located?

A

The field from which the heart, blood vessels and blood cells will develop from, this is created during gastrulation.
It’ll lie at the cranial end of the embryo before folding occurs.

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9
Q

What is the cardiogenic field composed of before folding?

A

Percardial cavity and blood islands (beginning of differentiated cells)

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10
Q

How is the primitive heart tube formed?

A
  • In the 3rd week of embryonic development a pair of endocardial tubes develop in the cardiogenic field called blood islands.
  • These blood islands on either side of the mid line are brought together during Lateral folding and fuse in the mid-line to create the primitive heart tube.
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11
Q

Describe the structure of the primitive heart tube

A 
When first made the primitive heart tube is linear at first, receiving blood (inflow) at its caudal pole and pumping blood (outflow) from its cranial pole.
Blood flow from the sinus venosus to the aortic roots.
From top to bottom:
 Aortic roots
 Truncus arteriosus
 Bulbus cordis
 Ventricle
 Atrium
 Sinus venosus
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12
Q

What 2 key things must happen for the primitive heart tube to resemble the mature heart

A

• 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

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13
Q

In which cavity can the primitive tube be found in?

A

The primitive heart tube is suspended in the pericardial cavity.
Suspended at the top and bottom end

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14
Q

Describe the looping of the primitive heart tube

A
  • The tube will elongate and get longer however as it’s fixed in this pericardial cavity where there’s minimal space it can’t go anywhere top to bottom.
  • Runs out of room
  • This causes it to twist and fold up -regular and predictable
  • This places the inflow and outflow in the correct orientation with respect to each other
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15
Q

What is the transverse pericardial sinus and how is it formed ?

A
  • The transverse pericardial sinus is the space posterior to the aorta and pulmonary artery and anterior to the superior vena cava.
  • It’s formed as a result of the looping of the primitive heart tube. - arteries in front of the veins
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16
Q

Explain how the sinus venosus develops in the embryo

A
  • Originally both sides of the sinus venosus are equal in size however this changes as the venous system undergoes remodelling.
  • At the fourth week, the sinus venosus is responsible for the inflow of blood to the primitive heart, and empties into the primitive atrium. It receives venous blood from the right and left sinus horns.
  • Over time, the venous return shifts to the right side of the heart, causing the left sinus horn to recede and form the coronary sinus (responsible for the drainage of venous blood from the heart itself). The enlarged right sinus horn is absorbed by the growing right atrium and eventually forms part of the inferior vena cava in the adult.
  • In the left atrium, a similar process occurs with the pulmonary veins. The four pulmonary veins are incorporated into the left atrium, forming the smooth inflow portion of the left atrium and the oblique pericardial sinus.
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17
Q

Explain how both the right atrium form in the embryo

A

RA develops from
• most of the primitive atrium
• contribution from sinus venosus
• receives venous drainage from the body (venae cava) and the heart (coronary sinus)

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18
Q

Explain how both the left atrium form in the embryo

A

LA develops from
• a small portion of the primitive atrium
• absorbs proximal parts of primordial pulmonary vein tissue which becomes part of the walls of the atrial chamber
• as it enlarges, it effectively consumes branches of left and right pulmonary veins
• receives oxygenated blood from the lungs

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19
Q

How is the oblique sinus formed?

A

The oblique pericardial sinus is formed as the left atrium expands absorbing the pulmonary veins

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20
Q

Compare cardiovascular circulation in the fetus after birth and before birth

A

In the mature circulation:
• Deoxygenated blood is collected from the body
• This deoxygenated blood is pumped to the lungs for reoxygenation and removal of CO2
• The reoxygenated blood is returned from the lungs to the heart
• It is then pumped around the body

However in the fetus before birth…
• The lungs don’t work
• Oxygenation and removal of CO2 will occur at the placenta
• This means that shunts are required to maintain fetal life
• As well as this these shunts must be reversible at birth

Give the fetal circulatory shunts

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21
Q

What happens in fetal circulation before shunts are made?

A

Blood arrives at the wrong side of the heart

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22
Q

What are the 3 fetal circulatory shunts?

A
  • Ductus Venosus
  • Foramen ovale
  • Ductu Arteriosus
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23
Q

Describe the foetal circulatory system

A

Oxygenated blood from the mother via the placenta
By passes the liver via ductus venosus to the Inferior Vena Cava and into the right artium then via the foramen ovale (by passing the lungs) to the Left atrium, ventricle, then aorta and to the body then to the placenta

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24
Q

In fetal circulation why is a small amount of blood allowed to drain from the right atrium to the right ventricle and what happens to it rather than to the left atrium?

A

Small drainage into the right ventricle and pulmonary trunk to allow normal development of the ventricle muscle.
This blood then by passes the weak developing lungs via the ductus arteriosus into the aorta

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25
Q

What is the foramen ovale?

A

An opening between the right and left atria in the embryo and fetus.

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26
Q

What is the function of the ductus venosus?

A

The ductus venosus shunts a portion of umbilical vein blood flow directly to the inferior vena cava.
It allows oxygenated blood from the placenta to bypass the liver.

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27
Q

What is the function of the foramen ovale?

A

The foramen ovale allows blood to enter the left atrium from the right atrium. It acts as a small passageway between the septum secundum and the ostium secundum.
It allows oxygenated blood to by pass the right ventricle and the lungs.

28
Q

What is the function of the ductus arteriosus

A

The ductus arteriosus, is a blood vessel in the developing fetus connecting the trunk of the pulmonary artery to the proximal descending aorta.
It allows most of the blood from the right ventricle to bypass the fetus’s fluid-filled non-functioning lungs.

29
Q

What role does the placenta have in the fetal circulation

A

The placenta is the site at which the fetus can receive oxygenated blood from the mother as well as the site at which deoxygenated blood that’s been circulated around the fetus can be returned back to the mother.

30
Q

How is the fetal circulatory system shunt deactivated at birth?

A
  • respiration begins
  • LA pressure increases foramen ovale closes
  • DA contracts
  • placental support removed
  • DV closes
31
Q

What are the aortic arches?

A
  • The aortic arches are a series of six paired embryological vascular structures which give rise to the great arteries of the neck and head.
  • Early arterial system begins as a bilaterally symmetrical system of arched vessels
  • Undergo extensive remodelling to create the major arteries leaving the heart
32
Q

Why do we not have a fifth early embryonic arched vessel

A

Present only in other species so skip 5

1, 2, 3, 4, 6 are present.

33
Q

What are the derivatives of the 4th aortic arches?

A
  • R = proximal part of R subclavian A

* L = arch of aorta

34
Q

What are the derivatives of the 6th aortic arches?

A

6th arch = “pulmonary arch”
• R = R pulmonary artery
• L = L pulmonary artery & Ductus Arteriosus

35
Q

Explain the association between the aortic arches and cranial nerves

A
  • When the aortic arches are remodelled their development occurs close to nerves such as the vagus nerve which branch into the laryngeal nerves.
  • The laryngeal nerve is used innervate the larynx.
  • There are 2 of these nerves on either side of the arches.
  • 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
36
Q

what is Patent ductus arteriosus (PDA)?

A

Patent ductus arteriosus (PDA) is a medical condition in which the ductus arteriosus fails to close after birth.
This allows a portion of oxygenated blood from the left heart to flow back to the lungs by flowing from the aorta, which has a higher pressure, to the pulmonary artery.

The ductus arteriosus is a fetal blood vessel that normally closes soon after birth. In a PDA, the vessel does not close, but remains patent (open), resulting in an abnormal transmission of blood from the aorta to the pulmonary artery.

37
Q

What causes the ductus arteriosus to close naturally?

A

There’s a specific type of smooth muscle in the wall of the ductus arteriosus.
This muscle will be exposed to a high concentration of oxygen after birth causing the muscle to undergo contraction.
This causes physiological closure and after a while this will become an anatomical closure.

38
Q

What is the atrioventricular canal?

A

This is a small opening between the primitive atrium and the primitive ventricle

39
Q

What have we got after looping of the heart tube?

A
  • atrioventricular canal
  • one enlarged atrium and one very enlarged ventricle
  • but don’t yet have the two pumps in series
  • the primitive chambers must be divided still
40
Q

What is septation and gives its function within the development of the cardiovascular system

A

Septation refers to the development of a septum.

The septum’s formed result in the creation of the 4 chambers of the heart and the achievement of selective flow.

41
Q

What needs to be separated after looping? What does this achieve

A
  • Interatrial septum
  • Interventicular septum
  • Septation of ventricular outflow tract
  • pulmonary trunk
  • aorta

to create the 4 chambers and to achieve selective outflow

42
Q

What are endocardial cushions and what do they do

A
  • They form in the atrioventricular canal and are migrations of cells
  • They grow and divide the developing heart into right and left chambers
  • You have one set of cushions on the ventral wall and another set on the dorsal wall.
43
Q

How does atrial separation occur?

A
  • In this process you get the division of the primitive atrium, this involves the formation of 2 septa with 3 holes.
  • In the developing heart, the atria are initially open to each other, with the opening known as the ostium primum.
  • It progressively decreases in size as the septum grows downwards, and eventually disappears with the formation of the atrial septum.
  • The first septum is the septum primum and it grows down towards the fused endocardial cushions. It’s used to separate the single primitive atrium into the right and left atrium.
  • Before the 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.
  • The structures formed allow the movement of blood from the right side of the atria to the left.
44
Q

What is the fossa ovalis?

A

The fossa ovalis is the remnant of the foramen ovale of the fetal heart used to by-pass the lungs.

45
Q

What are the relative pressures in the atria in utero?

A

Right atrial pressure is greater than left atrial pressure as oxygenated blood from the placenta arrives to the right atria via the umbilical vein and drains into the inferior vena cava

46
Q

What happens to the left atrial pressure with the first breath at birth?

A

The pressure in the left atria will increase because the pulmonary circulation starts.

47
Q

What happens to the foramen ovale after birth?

A

The foramen ovale normally closes at birth.
At birth, when the lungs become functional, the pulmonary vascular pressure decreases and the left atrial pressure exceeds that of the right.
This forces the septum primum against the septum secundum, functionally closing the foramen ovale as the Holes are not aligned.
In time the septa eventually fuse, leaving a remnant of the foramen ovale, the fossa ovalis.

48
Q

What happens to ductus arteriosus after birth?

A

The function of the ductus arteriosus is to by pass the lungs however at birth the lungs will fill with air so this structure is no longer needed.
It will usually close after birth to form the ligament ligamentum arteriosum.

49
Q

What is an atrial septal defect?

A

• An atrial septal defect is a flaw in the septum that divides the two atria of the heart. It causes blood in the RA and LA to mix.
• It can be due to a defect in the septum primum or in the septum secundum.
• For example the septum primum is resorted or too short.
Or the septum secundum is too small.

50
Q

What is hypoplastic left heart syndrome?

A

This is a condition where the left side of the heart doesn’t develop fully.
Whilst the exact cause isn’t known there are speculative theories.

51
Q

What are the possible causes of hypoplastic left heart syndrome?

A
  1. It could be as a result of a defect in the development of mitral and aortic valves.
    The development of valves is dependent on the migration of tissue in the developing heart. If they don’t form normally it can cause problems with flow around the heart. This can result in atresia and limited flow.
  2. Another cause could be that the ostium secundum is too small, this then prevents sufficient flow to the left side of the heart.
    This means that the left side of the heart doesn’t have enough work to do during development so it won’t be as well developed as normal.
52
Q

Explain what happens during ventricular septation

A
  • This starts with a singular ventricular chamber.
  • A ventricular chamber will then form, its made up of 2 components; the muscular (main) and the membranous components.
  • The muscular portion forms most of the septum and grows upwards towards the fused endocardial cushions • it leaves a small gap known as the interventricular foramen.
  • This gap allows communication between the left and right ventricles.
  • The membranous portion of the interventricular septum is formed by connective tissue derived from endocardial cushions to “fill the gap” and close the interventricular septum.
53
Q

What is a ventricular septal defect?

A

VSD is an abnormal opening between the right and left ventricles.
It’s usually due to an issue with the membranous portion of the interventricular septum.

54
Q

What is the effect of ventricular septal defect?

A
  • The opening between the ventricles allows for left to right shunting (pushing) of the blood due to higher pressure in the left side which results in excessive fatigue during exertion.
  • Excess blood in the right side increases the pulmonary blood flow and leads to pulmonary hypertension, which in later states causes thickening of the pulmonary arteries and increased resistance.
  • Thickening of these arteries leads to increased pulmonary resistance in circulation which may ultimately result in right to left shunting of blood. (Eisenmenger’s syndrome).
55
Q

Why must outflow be divided?

A

To get the routing right - routing oxygenated and deoxygenated blood appropriately

56
Q

Explain the process that results in the septation of the outflow tract

A
  • You have endocardial cushions made up of cells that migrate from outside of the cardiogenic region into the primitive tube.
  • These cushions appear in the truncus arteriosus, they appear in a staggered almost perpendicular arrangement as they’re not directly opposite one another.
  • As they grow towards each other they twist around each other to form a spiral septum.
  • This allows us to connect the left atrium to the aorta and the right ventricle to the pulmonary trunk.
  • This spiralling process ensures we get correct connections to the right chambers.
57
Q

What are congenital birth defects?

A

Congenital effects can either be structural abnormalities (so something’s wrong with the formation of a structure) or there’s a complete absence of a structure.

It can result from interference or the interruption of normal developmental processes.

58
Q

What are causes for congenital birth defects?

A
  • genetic (e.g mutations in genes)
  • exposure to chemicals / drugs / infectious agents (e.g rubella, warfarin and ACE inhibitors)
  • unexplained
59
Q

What are congenital heart defects?

A

Congenital heart defects occur when there’s:

  1. A structural defect (e.g of the chambers or of the vasculature)
  2. An obstruction
  3. Abnormal communication between pulmonary and systemic circulations
60
Q

why are congenital heart defects the most common form of birth defects?

A

This form of birth defect are more common due to the additional complexity because of the differing circulatory needs of the fetus as compared to the newborn (mature).

61
Q

What is the Transposition of the Great arteries?

A
  • Aorta arises from right ventricle
  • Pulmonary trunk arises from left ventricle
  • Can result in Cyanosis - Depending on what other if any defects are present
  • It’s thought to be because of a defect in the development of the aortic and pulmonary vessels during the septation of the outflow tract.
62
Q

What is cyanosis?

A

A bluish discoloration of the skin resulting from poor circulation or inadequate oxygenation of the blood

63
Q

What is Tetralogy of Fallot?

A

It’s a combination of 4 distinct defects.
1. Large ventricular septal defect
You have a large hole in the septal that separates the left and right ventricles.
2. Overriding aorta
The aorta sits across both left and right ventricles.
3. Right ventricular outflow tract obstruction
4. Right ventricular hypertrophy

  • Conotruncal septum formation defective
  • Importance of neural crest cells
64
Q

Importance of neural crest cells in the formation of the heart

A

These cells are important for creating endocardial cushions in the heart and without them everything is vulnerable to defect.

65
Q

Give the step by step process of blood flow through the developed fetal heart

A
  1. Blood drains into the right atrium
  2. Blood will then flow through the foramen ovale into the left atrium out into the left ventricle into the aorta.
  3. Any blood that goes into the the pulmonary circulation is redirected into the systemic circulation by the ductus arteriosus

Semester 2 - CVS (20 decks)

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