Embryology

Question 1

On what wall of the embryo do limb buds initially develop?

Answer 1

Ventrolateral

Question 2

Approximately what week do limb buds begin to appear?

Answer 2

4 weeks

Question 3

Where does the tissue for limb bud development originate from?

Answer 3

Lateral mesoderm, derived from the somatic layer of the lateral plate mesoderm.

Question 4

What does the remnant of the notochord become in an adult?

What condition is associated with it?

Answer 4

Nucleus pulposus

Slipped disc - herniated

Question 5

What is the differing function of the lateral plate mesoderm and somites in limb development?

Answer 5

Lateral plate mesoderm - limb skeleton

Somites - muscles

Question 6

Describe the three degress of asymmetry in limbs

Answer 6

Proximodistal axis - shoulder to fingers

Dorsovental axis - no hairy palms

Anteroposterior axis - left/right hands are mirror images

Question 7

What is the core of the limb bud made from?

Answer 7

Mesenchyme

Question 8

What is the function of the apical ectodermal ridge?

Answer 8

Orchestrates limb development from proximal to distal. Stops differentiation of the mesenchyme directly below it, final stage is the appearance of paddles before it regresses.

Question 9

What is the function of the zone of polarising activity?

Answer 9

Allows the generation of asymmetry in the limbs. (anteroposterior axis determination_
Is a signalling centre located at the posterior base of the limb bud.
Controls patterning and maintains the AER.

Question 10

Describe how digital rays form.

Answer 10

Mesenchyme condensation within the plates which become cartilagenous models of the digital bones.

Question 11

What portion of the spine does the upper and lower limb bud appear opposite?

Answer 11

Upper - caudal cervical spine segments

Lower - Lumbar and sacral spinal segments

Question 12

Describe how development of the brachial plexus determines innervation of specific muscles.

Answer 12

Muscles are compartmentalised and nerves grow into common muscle masses.

Anterior divisions regroup to form medial and lateral cords so flexors are derived from these.

Posterior divisions regroup to form the posterior cord so extensors are supplied by this.

Question 13

Describe the difference between a dermatome and myotome.

Answer 13

Dermatome - strip of skin supplied by a single spinal nerve

Myotome - muscle or group of muscles supplied by a single spinal nerve

Question 14

What does the cardiovascular system derive from?

Answer 14

Mesodery

Question 15

Describe the folding that leads to the final position of the heart.

Answer 15

Lateral folding creates the heart tube. A pair of primitive vessels forms and predominate either side. Early vessels pushed together to make one large tube at the same time as the gut.
Cephalocaudal folding brings it to the thoracic region.

Question 16

What are blood islands?

Answer 16

A composite of primitive blood cells and capillaries. Found either side of the midline.

Question 17

From cephalic to caudal, name the regions of the primitive heart tube.

Answer 17

Aortic roots
Truncus arteriosus
Bulbus cordis
Ventricle
Atrium
Sinus venosus

Question 18

What role does the pericardium play in heart development?

Answer 18

Forces the tube to bend as the size of the pericardium doesn’t increase in proportion to the heart. Causes the cephalic end to move ventrally and caudally to the right. Caudal end moves dorsally and cranially to the left.

Question 19

Describe development of the sinus venosus

Answer 19

Right and left sinus horns are equal in size. Venous return shifts to the right and the left sinus horn recedes. The right sinus horn is absorbed by enlargement of the right atrium.

Question 20

What does the right atrium develop from?

Answer 20

Most of the primitive atrium

Sinus venosus

Question 21

What does the left atrium develop from?

Answer 21

Small proportion of the primitive atrium

Absorbs the proximal pulmonary veins

Question 22

What causes the oblique pericardial sinus to form?

Answer 22

The left atrium expands, absorbing the pulmonary veins

Question 23

Explain why the right atrium has trabeculae and the left atrium is smooth.

Answer 23

The left atrium absorbs the primordial pulmonary veins.

Question 24

What allows foetal circulation to bypass the lungs?

Answer 24

Ductus arteriosus

Question 25

Describe the fate of the 4th and 6th arch in development of the great vessels.

Answer 25

4th - right to proximal part of right subclavian artery. Left to arch of aorta.

6th - right to right pulmonary artery. Left to left pulmonary artery and ductus arteriosus

Question 26

Describe the recurrent laryngeal nerves

Answer 26

A branch of the vagus.
Innervates the intrinsic laryngeal muscle, affecting speech.

Right branch drops to T1/T2
Left drops to T4/T5 then turns back up.

Question 27

Explain why the recurrent laryngeal nerves drop to the thoracic vertebrae then turn back up.

Answer 27

As the heart descends the nerve hooks around the 6th aortic arch and turns back on itself.

Question 28

Describe the atrioventricular canal

Answer 28

Point of constriction so the atria can communicate with the ventricle after looping

Question 29

State the structures formed during septation.

Answer 29

Interatrial septum
Interventricular septum
Septation of the ventricular outflow tract to form the pulmonary trunk and aorta.

Question 30

Describe the first step of septation.

Answer 30

Endocardial cushions develop in the atrioventricular region. Divides the developing heart into right and left channels. Grow into the lumen then connect in the centre.

Question 31

Describe the steps of atrial septation.

Answer 31

Septum primum grows down towards the fused endocardial cushions.
The ostium primum is present before the septum primum fuses with the endocardial cushions.
Before the ostium primum closes, the ostium secondum appears in the septum primum.
A second crescent-shaped septum (septum secondum) grows. The hole in this is the foramen ovale.

Question 32

What component of the left and right atrium is derived from the primitive atrium?

Answer 32

Auricles

Question 33

What part of the primitive heart tube is absorbed by the right atrium?

Answer 33

Sinus venosus

Question 34

What is the adult remnant of the foramen ovale?

Answer 34

Fossa ovalis

Question 35

What is the shunt in the heart allowing it to bypass the lungs?

Answer 35

Foramen ovale

Question 36

Describe the septation of the outflow tract.

Answer 36

Endocardial cushions form in the truncus arteriosus.
Grow towards and twist around each other, forming a spiral septum.
Creates the root of the aorta and pulmonary trunk.

Question 37

What structure allows the umbilical vein to bypass the liver?

Answer 37

Ductus venosus

Question 38

Describe the change in circulation at birth.

Answer 38

Respiration beginning causes a change in pressure so it’s greater in the left atrium than the right.
The foramen ovale shuts because the septum primum is pushed against the septum secondum, sealing the holes which fuse over time.
The smooth muscle in the ductus arteriosus closes and becomes fibrous over time.
Ductus venosus closes as the placental support is removed.

Question 39

State the fates of the foetal shunts

Answer 39

Foramen ovale - fossa ovalis
Ductus arteriosus - ligamentum arteriosum
Ductus venosus - ligamentum venosum
Umbilical vein - ligamentum teres (hepatis)

Question 40

State the fates of the regions of the primitive heart tube

Answer 40

Sinus venosus - right atrium
Atrium - auricles of atria
Ventricle - left ventricle
Bulboventricular sulcus - primary interventricular foramen
Bulmus cordis
(proximal third) - right ventricle
(conus cordis) - outflow tract of L/R ventricles
(truncus arteriosus) - roots, proximal aorta and pulmonary trunk