Embryology

Question 1

What starts the beginning of the heart tube? What does it grow in conjunction with?

Answer 1

Blood islands induced by the underlying endoderm that secretes VEGF; grows in conjunction with the pericardial cavity

Question 2

How do the heart tubes receive bloodflow?

Answer 2

At each heart tube’s inferior end it connects to a vitelline vein (comes from the yolk sac) and blood exits superiorly through the dorsal aorta

Question 3

What is the bloodflow to the singular primitive heart tube?

Answer 3

The dorsal aortas fuse into the aortic sac (superiorly) which drains the heart (outflow) and the L and R vitelline vein fuse into the sinus venosus (inferiorly) which supplies the heart (inflow)

Question 4

How is the singular primitive heart tube formed?

Answer 4

Through lateral folding of the embryo

Question 5

What are the 2 layers of the heart tube?

Answer 5

1. endothelial lining; endocardium

2. cardiac myoblasts; myocardium

Question 6

List the 4 segments (between the aortic sac and sinus venosus) of the primitive heart tube from cranial –> caudal

Answer 6

1. truncus arteriosus
2. bulbus cordis
3. primitive ventricle
4. primitive atrium

Question 7

After looping, how does the atrium communicate with the ventricle?

Answer 7

The atrioventricular canal, the atrioventricular sulcus can be seen (surface groove encircling the heart separating the primitive atria from the primitive ventricles)

Question 8

What’s the transverse pericardial sinus?

Answer 8

formed by looping of the primitive heart tube, it’s the space behind the outflow and in front of the inflow (can stick a finger through)

Question 9

Where does the sinus venosus collect blood from?

Answer 9

The placenta, yolk sac and the body

Question 10

Explain the transition that the L and R sinus horns undergo

Answer 10

At first, the L and R sinus horns are equal in size

BUT as venous return shifts to the R, the L sinus horn recedes (and forms the coronary sinus) and the enlargening Right atria absorbs the R sinus horn

Question 11

Where do the right atria develop from? Where does it receive drainage from?

Answer 11

Most of the primitive atrium, (and absorbs the R horn of the sinus venosus) which forms its smooth wall portion. It receives drainage from the vena cava (superior and inferior) and the coronary sinus.

Question 12

Where does the left atrium develop from?

Answer 12

A small portion of the primitive atrium and absorbs proximal parts of pulmonary veins

Question 13

Explain why atria have differing textures

Answer 13

Rough: remainder from the primordial atrium
Smooth: where primordial veins have been sucked in

Question 14

Why is inflow dorsal to outflow? Which ventricle (R or L) is closest to the inflow/outflow tract?

Answer 14

Because during looping the atrium is dorsal to the bulbous cordis.

The primordium of the R ventricle is closest to outflow tract
The primordium of the L ventricle is closest to inflow tract

Question 15

What is the auricle?

Answer 15

The remainder of the rough portion (primitive part) of the L atrium

Question 16

What forms the smooth portion of the L atria?

Answer 16

All 4 branches of the pulmonary veins have been sucked into the L atrium

Question 17

What is the oblique pericardial sinus and when does it form?

Answer 17

As the L atrium expands and absorbs the pulmonary veins the oblique pericardial sinus is formed. It is the “cull-de-sac” formed by the palm of your hand when you hold a heart

Question 18

How do the aortic arches begin and which number is missing from the arched vessels?

Answer 18

The aortic arches sprout from the aortic sac and form a bilaterally symmetrical system of arched vessels, the aortic arch 5 has no derivatives in humans.

Question 19

What does the 4th, 6th and 3rd aortic arch form?

Answer 19

4th aortic arch:
R: proximal R subclavian artery
L: aortic arch

6th aortic arch: Recurrent laryngeal nerve
R: R pulmonary artery
L: L pulmonary artery and ductus arteriosus

3rd: internal carotid arteries

Question 20

What is the ductus arteriosus?

Answer 20

A blood vessel in the developing fetus connecting the trunk of the pulmonary artery to the proximal descending aorta to bypass the developing lungs

Question 21

What is the first step in septation between the atria and ventricles?

Answer 21

The endocardial cushions, they grow towards each other (anterior and posterior) to divide the heart into R and L channels (R and L atrioventricular canals)

Question 22

What does the division of the common atrium form?

Answer 22

2 septa with 3 holes

Question 23

What is ostium primum and septum primum?

Answer 23

Septum primum: is the first septal structure appearing between the R and L atria, grows from the roof of the common atrium towards the endocardial cushions

Ostium primum: gap underneath the growing septum primum

Question 24

What is ostium secondum? What closes shortly after this develops?

Answer 24

Hole in the septum primum, the ostium primum closes shortly thereafter

Question 25

What is the septum secondum? What forms as soon as it is complete, and what is this new structure’s purpose?

Answer 25

Septum secondum is the second septal structure that forms adjacently and to the R of the septum primum

The septum secondum does not fuse with the endocardial cushions like the septum primum does, and the gap it leaves is called the foramen ovale. The channel between the foramen ovale and ostium secondum creates the R-L shunt.

Question 26

What is the fossa ovalis?

Answer 26

It is the adult remnant of a thin fibrous sheath that covered the foramen ovale

Question 27

What are the 2 portions of the ventricular septum?

Answer 27

1. The muscular component: grows upwards from the apex and forms most of the septum. It leaves a hole known as the primary interventricular foramen
2. The membranous component is growing downwards from the fused endocardial cushions and ‘fills the gap’ by closing the primary interventricular foramen

Question 28

What is the membranous portion of the ventricular septum derived from and what does that structure do?

Answer 28

Derived from the spiral septum: this grows to separate the truncus arteriosus into the outflow vessels: aorta and pulmonary trunk (semilunar valves develop shortly after)

Question 29

What is especially important in the development of fetal lungs?

Answer 29

Important they do not receive a lot of blood so they’re not damaged

Question 30

Explain the fetal circulation

Answer 30

The fetus receives oxygenated blood from the placenta through the ductus venosus (bypasses the liver) into the inferior vena cava via the umbilical vein.

Blood reaches the R atria where it is shunted into the L atria via the foramen ovale.

The blood is then pumped around the body via the aorta. The ductus arteriosus allows the blood to pass from the pulmonary trunk to the aorta and bypass the baby’s fluid-filled non-functional lungs

Question 31

How does circulation change (and the function of the 3 shunts) as soon as the fetus is born?

Answer 31

When the fetus takes its first breath, this massively decreases pressure in the lungs - resulting in more blood entering the lungs and more blood returning to the L atrium. Once the L atrial pressure > R atrial pressure the foramen ovale is closed and the ductus arteriosus contracts. Once placental support has been removed the ductus venous also closes.

Question 32

What is the adult fate of the following…

a) foramen ovale
b) ductus arteriosus
c) ductus venosus
d) umbilical vein

Answer 32

a) fossa ovalis
b) ligamentum arteriosum
c) ligamentum venosum
d) ligamentum teres (hepatitis)

Question 33

What separates the primitive ventricle from the bulbus cordis?

Answer 33

The bulbloventricular sulcus

Question 34

What does the bulbus cordis form?

Answer 34

The R ventricle and aids the formation of the outflow tracts for both ventricles (connected to the truncus arteriosus)

Question 35

When is the epicardium formed?

Answer 35

Week 4, formed by the visceral pericardium

Question 36

What forms the SA and AV node?

Answer 36

In the wall of the sinus venosus (R atria) a group of myocardial cells organize and synchronize their electrical discharge

AV node: myocardial cells specialize in the atrioventricular septum

Question 37

Where and when does the development of the heart start?

Answer 37

In the ‘heart field’ areas of the mesoderm during week 3 (horseshoe-shaped)