Development of the Heart and Blood Vessels

Question 1

What are the 5 stages of heart development and what are the possible defects?

Answer 1

1. Formation of the four chambered heart tube
   
   • Situs inversus.
2. Cardiac looping
   
   • Dextrocardia.
3. Division of the atrioventricular canal into left and right channels
   
   • Vetricular septal defect.
4. Formation of the atrial septa
   
   • Atrial septal defect – foramen ovale.
5. Formation of the conotruncal cushions and division of the outflow tract
   
   • Tetralogy of Fallot.

Question 2

What are the 4 stages of embryo development?

Answer 2

1. Morula
2. Early blastocyst
3. Late blastocyst
4. Gastrula

Question 3

List what is made from the three primary germ layers.

Answer 3

1. The Ectoderm becomes the skin (epidermis), brain, spinal cord (CNS), sensory organs, etc.
2. The Mesoderm becomes the heart, skeletal muscle, kidneys, urogenital organs, connective tissue, etc.
3. The Endoderm becomes the lining of the gastrointestinal tract and lungs, pancreas, liver, etc.

• In a simplified way, it can be said that the Ectoderm gives rise to skin and the nervous system, the Endoderm to the intestinal organs, and the Mesoderm to the rest of the organs.

Question 4

Describe the structure of the embryo at Day 18.

Answer 4

• In the embryonic disk cell if you look down on top of it, its already starting to change in shape and not be symmetrical.
• At the head end, we have a primary heart field with blood islands in it.
• At the tail end, we have the primitive node (a group of cells on the anterior portion of the primitive streak).
• This node is where gastrulation first begins.
• The heart develops from the primitive streak via the blood islands in the splanchnic mesoderm.
• Note that organ placement is determined by the asymmetry along the primitive streak.
• If you cut across the dotted lines you get a section that contains a notochord in the middle.
• The notochord is a cartilaginous skeletal rod that supports the body in all embryonic and some adult chordate animals (It acts as support before we get a vertebrae).
• It is also the source of signalling molecules.
• As signalling molecules radiate out of there it gives the embryo some concept of having a dorsal and ventral surface and a left and a right.

Chordate animals - They are animals with backbones. Humans are not chordate as we do not have a tail.

Question 5

What is gastrulation?

Answer 5

• Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a multilayered structure known as the gastrula.

Question 6

Describe the formation of the heart tube across Days 20, 21 and 22.

Answer 6

1. AT 20 DAYS:
   
   • The blood islands slowly fuse together into 2 tubes called cardiogenic cords.
2. AT 21 DAYS:
   
   • The tubes fuse down the middle to make one heart tube.
3. AT 22 DAYS:
   
   • Further development gives rise to a tube with asymmetric openings, forming an arterial and venous end.
   • There is the beginning of asymmetry in the bulge to the left (actual right).
   • The reason of the asymmetry is because the notochord that runs down the centre sends different signals to the left and the right.
   • If this goes wrong we can develop a condition called situs inversus.
   • At the superior end, we have the truncus arteriosus (divides into the aorta and the pulmonary trunk aka pulmonary vein), and at the inferior end, we have the sinus venosus.
   • The sinus venousus will attach to rudimentary (immature) circulation.

Question 7

What is situs inversus?

Answer 7

• It is a condition in which all the heart and other organs of the body are transposed through the sagittal plane (vertically) to lie on the opposite side from normal.
• It happens because the notochord has sent the left message to the right and the right message to the left.
• If it effect everything it is called situs inversus totalis.

Question 8

Describe the cardiac looping across Days 22, 23, 24 and 35.

Answer 8

1. AT 22 DAYS:
   
   • The cells at each end of the heart proliferate, making the tube longer.
   • The entire structure is in a confined space, so it begins to fold upon itself.
2. AT 23/24 DAYS:
   
   • The primitive atrium loops up above and behind the primitive ventricle.
   • The looping process brings the primitive areas of the heart into the proper spatial relationship for development.
   • A mistake in the looping can casue dextrocardia.
3. AT 24 DAYS:
   
   • The primitive atrium, when viewed from the side, is growing out of the back and becoming two atria.
   • The future right ventricle grows downwards at the front.
4. AT 35 DAYS:
   
   • The heart is beating by this stage (so blood is present inside the heart) and the foetal heart can be seen on the ultrasound.
   • However, the four interior chambers of the heart are not yet divided.

Question 9

What is dextrocardia?

Answer 9

• Dextrocardia is a rare heart condition in which your heart points toward the right side of your chest instead of the left side.
• Dextrocardia is congenital, which means people are born with this abnormality.

Question 10

Describe the division of the atrioventricular canal (at 30 days).

Answer 10

• The septum primum grows along the midsagittal plane, separating the atria (into left and right), except for a temporary space called the foramen primum.
• The left side of the atrium grows pulmonary veins, sending growing veins to the developing lungs.
• The posterior and anterior endocardial cushions fuse, diving the atria from the ventricles.
• A ventricular septum will eventually grow upwards to form a division between the two ventricles.
• It will attach to the divison between the atria and the ventricles.
• A defect in the division process can cause ventricular septal defect.

Question 11

Describe a ventricular septal defect, and the effects it has.

Answer 11

• One cause of a ventricular septum defect is the failure of the endocardial cushions to provide an anchor point for the developing ventricular septum.
• The extent of the opening may vary from pin size to complete absence of the ventricular septum.
• We can see this abnormality using an echocardiogram.
• This means that during ventricular contraction, some of the blood from the left ventricle will leak into the right ventricle, passing through the lungs and re-entering the left ventricle via the pulmonary veins and left atrium.
• We can hear this turbulence when listening to the heart.
• This has 2 main effects:
  
  • The systemic circulation doesn’t receive all the blood being pumped by the left ventricle.
  • The leakage of blood into the right ventricle elevates right ventricular pressure and volume, causing pulmonary hypertension. It can also cause right ventricular wall hyperplasia (thickening of ventricular wall), increasing the hearts O2 demand.

Question 12

Describe the formation of the atrial septa (across 40 and 43 days).

Answer 12

1. AT 40 DAYS:
   
   • The anterior and posterior endocardial cushions have fused, leaving the left and right atrioventricular canals.
   • Initially, the temporary hole in the septum primum called the foramen primum permits the right to left shunt of foetal blood, as there is no pulmonary circulation.
   • The foramen primum closes before a second hole called the foramen secundum opens.
   • A second, more muscular and robust septum called the septum secundum grows down from the roof of the atria, just lateral to the septum primum.
2. AT 43 DAYS:
   
   • The foramen secundum is partially obscured by the septum secundum, but it is not completely obliterated as it becomes the foramen ovale, providing an alternative left to right shunt.
   • The foramen ovale allows the unborn baby that does not use its lungs to get blood rich in oxygen. Instead, this blood comes from the mother’s placenta and is delivered through the umbilical cord. The foramen ovale makes it possible for the blood to go from the veins to the right side of the fetus’ heart, and then directly to the left side of the heart.
   • The ventricular septum grows up to fuse with the now fused endocardial cushions.

Shunt meaning - an alternative path for the passage of the blood or other body fluid.

Question 13

What happens to the foramen ovale when we are born?

Answer 13

• At birth, the lungs become functional.
• This means that the pulmonary vascular pressure decreases and the left atrial pressure exceeds that of the right, forcing the septum primum against the septum secondum, functionally closing the foramen ovale.
• In time, the septa eventually fuse, leaving behind the fossa ovalis.

Question 14

What is atrial septal defect?

Answer 14

• An atrial septal defect is a birth defect of the heart in which there is a hole in the wall (septum) that divides the upper chambers (atria) of the heart.
• A hole can vary in size and may close on its own or may require surgery.
• An atrial septal defect is one type of congenital heart defect.
• As a baby’s heart develops during pregnancy, there are normally several openings in the wall dividing the upper chambers of the heart (atria).
• These usually close during pregnancy or shortly after birth.
• If one of these openings does not close, a hole is left, and it is called an atrial septal defect.

Congenital meaning - present at birth.

Question 15

Describe the division of the outflow tract.

Answer 15

• At this stage, there is no separation between the ventricles, so all the blood from the heart is leaving through one vessel.
• Eventually, they are going to be separated into the left and right sides (the pulmonary artery and the aorta).
• The separation of the ventricles results from the union of the:
  
  1. Conotruncal septum.
  2. Endocardial cushions.
  3. Ventricular septum.
• The conotruncal septum grows as a spiral down the conus arteriosus.
• There is a division at the top of the tube which spirally grows down to create two intertwining tubes.
• The conotruncal septum meets with the endocardial cushions and the ventricular septum.
• The three of them make the final full separation of the left and right sides.

Question 16

Describe the Tetralogy of Fallot.

Answer 16

• If the conotruncal septum is misaligned, you either get a differently sized aorta and pulmonary artery, or it won’t meet with the endocardial cushion correctly, giving rise to a septal defect.
• The ultimate complication is the Tetralogy of Fallot.
  
  • A. PULMONARY STENOSIS:
    
    • When the spiral valve has grown off centre, so we end up with a large aorta and small pulmonary artery, restricting blood to the lungs.
  • B. OVERRIDING AORTA:
    
    • When the aortic opening is positioned over a ventricular septal defect, it allows the blood from both ventricles to enter the aorta, which means that some deoxygenated blood is pumped around the body.
  • C. VENTRICULAR SEPTAL DEFECT:
    
    • During systole, blood from the left ventricle leaks into the right ventricle. This increases pressure in the pulmonary circulation and also causes a volume overload in the left ventricle.
  • D. RIGHT VENTRICULAR HYPERTROPHY:
    
    • The right ventricular wall increases in size to deal with the obstruction in the pulmonary artery. This increases the heart’s oxygen demand.

Question 17

What is the ductus arteriosus?

Answer 17

• It’s a blood vessel connecting the main pulmonary artery to the proximal descending aorta.
• It allows most of the blood from the right ventricle to bypass the foetal fluid-filled non-functioning lungs.
• Upon closure at birth, it becomes the ligamentum arteriosum.