Embryology of Heart (Mon 16th)

Question 1

What are the cardiac progenitor cells?

Answer 1

In mesoderm have two type of cells: primary and secondary heart field. PHF forms heart tube (the heart starts as a tube) and SHF cells migrate in and add to heart. In ectoderm, cardiac neural crest cells develop. CNCC migrate to septa and blood vessels and help form them. Genetic defects in cardiac progenitor cells can lead to congenital heart defects (CHD)

Question 2

Describe all the anatomy of the right atrium

Answer 2

1. On the outside RA there is pectinate muscles (ridges on the outside of the RA) to increase contraction power.
2. Then there is Crista Terminalis, the origin of pectinate muscles
3. Then sinus venarum, the smooth inside of the RA
4. In the RA you can see the fossa ovalis (the hole between the atria that only fetus’ have) closed.
5. You can see the tricuspid valve too.

Question 3

Describe all the anatomy of the right ventricle

Answer 3

1. The cusps of the tricuspid valve can be seen.
   
   • Anterior cusp is the one closest to the front.
   • Septal cusp is joined to the septum
   • Posterior cusp is at the back.
2. The strings are known as chordae tendinae and prevent inversion.
3. They are joined to papillary muscles at the middle of the RV, which tighten the strings.
4. The moderator band AKA Septomarginal Trabecula is at the bottom of the RV, and contains the RAV bundle.
5. Trabeculae carneae prevent suction.

Question 4

Describe the anatomy of the left atrium?

Answer 4

• Pectinate muscle to increase contaction power
• Fossa Ovalis: where the right atria where once connected by a hole
• Bicuspid valve
• Right and Left Pulmonary Vein bringing oxygenated blood from the lungs.

Question 5

Describe the semi-lunar valves.

Answer 5

1. The semi-lunar valve in the pulmonary artery is known as the pulmonary valve. It has an anterior, right and left cusp.
2. The semi-lunar valve in the aorta is known as the aortic valve. It has a posterior, right and left cusp, but we call them sinus instead.

Question 6

Describe the conducting system of the heart.

Answer 6

• The heart cells are a connected, branching network. So the stimulation of one part of the heart, allows an impuse throughout of the heart.
• The Sinoatrial Node (loacted in the RA, near entrance of the superior vena cava) sends out an impulse at a rate o 70-80 BPM. Since the SAN initiates the contraction of the heart it is also known as the heart’s pacemaker.
• The impuse from the atria can’t pass directly to the ventricles due to the connective fibre seperating the atria from the ventricles.
• The nodal fibres of the SA node carry the impulse to the Atrioventricular Node. There is a small delay between the impulse being passed to the AV node, to allow the atria to finish contracting.
• The impulse then travels through the atrioventricular bundle in the septum. The atrioventricular bundle branches into the right and left bundle branch to pass the impulse to the bottom of each ventricle.
• This allows the ventricles to contract from the bottom first, squeezing blood up and out into the arteries.

Question 7

Describe the heart’s fibrous skeleton.

Answer 7

• The atria and ventricles are electrically isolated by the fibrous skeleton of the heart.
• All four valves have a fibrous ring.
• So the only way an impulse can pass to the ventricles is using the AV Node.

Question 8

Describe the embryology of the heart

Answer 8

1. Egg is fertilised and moves down the fallopian/uterine tubes toward the uterus.
2. The blastula (hollow ball of cells) implants into the uterine wall.
3. Single layered blastula becomes multilayered (glastrulation):
   
   • Ectoderm: the outer layer, this will develop into the epidermis/skin and the CNS.
   • Mesoderm: Split into
     
     • Paraxial, which is closest to the primitive streak: muscle cells
     • Medial (gonads and kidneys)
     • Lateral plates (further split into Somatic and Splanchic)
   • Endoderm: gives rise to pancreas/liver/ inner linings.
4. The splanchnic mesoderm has specialised cells:
   
   • Primary Heart Field Cells: form the heart tube (which becomes the heart)
   • SHF Cells: add to heart
5. The splanchic mesoderm forms a crescent shape so is called the Cardiac Crescent.
6. Neural Crest Cells from the ectoderm migrate to the splanchic mesoderm and form septa/ vessels.
7. At 3 weeks the embryo can’t receive enough nutrients by diffusion alone, and needs a specialised circulatory system.
8. The heart tube differentiates at each end and branches blood vessels at 20 days.
   
   • Blood enters at the bottom (sinous venoses/ in-flow tract) which will become the S/I vena cava
   • Blood leaves from the top of the heart tube, called the aortic sac/ out-flow tract. This will become the pulmonary trunk and aorta.
9. Then the heart tube differntiates further and has bulges in it at 21 days. This is the common (will split into 2) atria and ventricles.
10. The septa need to form:
    
    1. Atrio-ventricular Septum: forms from Subendocardial Cushion Cells fusing at 28 days. However there is still blood flow from the right and left of the atrioventricular septum, from the RA to RV, and LA to LV, untill the atrio-ventricular valves form properly.
    2. Atrial Septum: initially two half septums form that still allow blood flow between the atria- the foreman ovale. This should close to become the fossa ovalis at birth. If that doesn’t happen (due to a genetic defect in cardiac prgenitor cells) you get a patent foreman ovale and atrial septic defect.
    3. Ventricular Septum forms at 35+ days: First the muscular part of the septum forms at the bottom of the ventricles. Then Neural Crescent Cells will form the rest of the septum. This also forms the aortico-pulmonary septa seperating the pulmonary tract and aorta. If have genetic defect in cardiac progenitor cells:
    • You may get Aortico-Pulmonary Septation Defect: The A/P septum shifts to the right, so the aorta is wider (called an over-riding aorta, where the aorta covers both ventricles) and the pulmonary artery to be thinner, called PA stenosis. If the A/P septum shifts to the right, it doesn’t attatch properly to the muscular part of the ventricular septum, so the blood can flow between theventricles (VSD).

Question 9

What are fetal cardiac shunts, and give examples.

Answer 9

• Fetal Cardiac Shunts are mechanisms in a fetus for blood to bypass the lungs, as fetus lungs don’t function.
• Forman Ovale:
  
  • between right and left atria
  • blood bypasses pulmonary vessels
  • closes at birth – fossa ovalis
  • congenital HD: patent foramen ovale (Arterial Septic Defect)
• Ductus Arteriosus: In fetus’ there is a small vessel connecting the descending aorta to the pulmonary artery. This allows oxygenated and deoxygenated blood to mix, so bypass the lungs. At birth the ductus arteriosus should shrivel up and close. If it doesnt its called a patent ductus arteriosus.

Question 10

Congenital heart defects

Answer 10

• Commonly due to genetic defects in migration, adhesion or proliferation of progenitor cells
• Atrial Septal Defect/ Patent foreman ovale
• Patent Ductus Arteriosus: pulmonary artery (deoxygenated) and descending aorta are connected by a small vessel in fetus calleed ductus arteriosus.
• Ventral Septal Defects: blood flows between ventricles.
• Transposition of the great arteries (Aorta and PT swapped)
• Tetralogy of fallot due to a Cardiac Neural Crest Cell defect has four features:
  
  1. Pulmonary stenosis
  2. Over-riding aorta
  3. Ventricular septal defect
  4. Right ventricular hypertrophy

Question 11

What does the right side of the heart do?

Answer 11

1. The RA receives blood from the vena cava
2. The blood is pushed through the tricuspid valve into the RV
3. The blood is pushed out of the RV, through the seminar valve, to the PA to go to the lungs.

Question 12

What does the left side of the heart do?

Answer 12

1. LA receives oxygenated blood from PV
2. Blood pushed into LV through the bicuspid valve.
3. Blood sent to the body, using aorta and semi-lunar valve.

Question 13

Describe the anatomy of the left ventricle

Answer 13

• Has the biscupid valve AKA as the mitral valve (BTW the tricuspid valve doesn’t have any other weird name like the bicuspid valve does).
• The bicuspid/ mitral valve has two cusps: the anterior and posterior.
• Chordae Tendinae: to prevent the valve from inverting the wrong way up the atrium, rather than snapping shut.
• Pappilary muscles: to tighten the chordae tendinae.
• Trabeculae Carneae to prevent suction.

Question 14

How do the atrioventricular valves function?

Answer 14

žOpen to allow blood to flow from atria to ventricles.

žRaised ventricular blood pressure closes leaflets when ventricle contracts (systole). This prevents backflow into atria.

žPapillary muscles contract and chordae tendinae hold leaflets closed and prevent inversion.

This means that when the papillary muscles are relaxed, the choradae tendinae is not tight, and the valve is open.

If the heart is enlaged, the valve leaflets may not touch, when the papillary muscles contract, so some blood may flow back to the atria. The valve is not sealed.