MSK embryology

Question 1

When does the development of limbs begin?

Answer 1

Near end of 4th week

Question 2

What part of the germ layer does the development of limbs originate from?

Answer 2

Mesenchymal cells activated in somatic lateral mesoderm

Question 3

What genes regulate patterning of limb development?

Answer 3

Homeobox genes. Determine which skeletal element is formed

Question 4

What homeobox genes code for expression in upper limb?

Answer 4

HOXd 9-13. Eg remove HOXa 11 and HOXd 11 and radius and ulna don’t form

Question 5

What proliferates causing the limb buds to elongate?

Answer 5

Mesenchyme

Question 6

What do upper limb buds develop opposite?

Answer 6

Caudal cervical segments

Question 7

What do lower limb buds develop opposite?

Answer 7

Lumbar and sacral segments

Question 8

What happens at the apex of each limb bud?

Answer 8

Ectoderm thickens to form apical ectodermal ridge (AER)

Question 9

Within the AER, what does the mesenchyme consist of?

Answer 9

Undifferentiated, rapidly proliferating cells

Question 10

Proximal to this undifferentiated zone what occurs?

Answer 10

Mesenchymal cells differentiating into blood vessels and cartilage bone models

Question 11

When do the distal ends of limb buds flatten and condense to form digital rays?

Answer 11

End of 6th week (UL) and 7th week (LL)

Question 12

What causes the tissue breakdown in the interdigital regions?

Answer 12

Apoptosis mediated by bone morphogenetic proteins

Question 13

How do cartilaginous precursors of limb bones develop?

Answer 13

By chondrification within mesoderm in 6th week

Question 14

When do primary centres of ossification appear?

Answer 14

Weeks 8-12

Question 15

What is the only limb bone that has membranous ossification with the rest having endochondrial?

Answer 15

Clavicle

Question 16

How do the upper and lower limbs rotate?

Answer 16

• Upper limbs: 90 degrees laterally so extensors are on lateral and posterior surfaces (thumb is lateral)
• Lower limbs: 90 degrees medially so extensors are on anterior surface (big toe is medial)

Question 17

How do dermatomal patterns of limbs form?

Answer 17

Peripheral nerves grow from developing limb plexuses in 5th week

Question 18

What can cause birth defects to arise?

Answer 18

Hereditary or teratogens

Question 19

What is total/partial absence of limbs called?

Answer 19

• Total absence: amelia

- Partial absence: meromelia

Question 20

What is long bone absence known as?

Answer 20

Phocomelia

Question 21

What is small limbs known as?

Answer 21

Micromelia

Question 22

Is lobster claw foot autosomal dominant of recessive?

Answer 22

Autosomal dominant

Question 23

When is the critical period of limb development?

Answer 23

24-36 days after fertilisation

Question 24

What is polydactyly?

Answer 24

Presence of more than 5 digits on hands/feet. Either inherited or teratogen induced

Question 25

What are the two types of syndactyly (common defect in hand/foot)?

Answer 25

Cutaneous syndactyly - simple webbing between digits. Common

Osseous syndactyly - fusion of bones. When notches between digital rays fail to develop

Question 26

What defects can occur due to maternal ingestion of thalidomide?

Answer 26

• Meromelia
• Intestinal atresia
• Cardiac abnormalities

Acts by disrupting cell adhesion and angiogenesis

Question 27

What happens in club foot/talipes equinovarus (common birth defect)

Answer 27

Sole of foot turned medially and foot inverted

Question 28

Causes of club foot?

Answer 28

1) Multifactorial (genetic and env.)

2) Abnormal positioning or restricted movement of fetus lower limb (disputed)

Question 29

Discuss developmental dysplasia of hip?

Answer 29

Common after breech (buttock) deliveries, breech posture during late part of pregnancy (Causes abnormal development of acetabulum)

Joint laxity can be multifactorial and is associated with congenital hip dislocation