MSK Embryology

Question 1

When do the upper and lower limb buds first appear?

Answer 1

• Upper: day 24

- Lower: 1-2 days later

Question 2

Where are Hox genes found and what do they do?

Answer 2

• Regionally expressed in limbs
• Determine which skeletal element formed
• HOXd 9-13 give gene expression in the upper limb

Question 3

Describe the formation of the limb buds

Answer 3

• Elongation by the proliferation of mesenchyme
• Upper limb buds develop opposite the caudal cervical segments
• Lowe limb buds form opposite the lumbar and sacral segments
• At the apex of each bud the ectoderm thickenss to form an apical ectodermal ridge

Question 4

Describe the development of hand and foot plates

Answer 4

• Distal ends of the limb buds flatten into paddle like hand plates and flipper like foot plates
• 6th week: mesenchymal tissue in hand plates has condensed to form digital rays
• 7th week: digital rays are formed in the foot plates

Question 5

Which proteins are responsible for the tissue breakdown in the interdigital regions?

Answer 5

Bone morphogenetic proteins

Question 6

Describe the final stages of limb development

Answer 6

• Cartilaginous precursors of the limb bones develop by chondrication within the mesoderm
• Primary centres of ossification appear in weeks 8-12
• Endochondrial ossification for all limb bones (except the clavicle)

Question 7

Describe the limb rotation

Answer 7

• Upper limbs rotate 90 degrees laterally (extensors are on the lateral and posterior surfaces + thumb is lateral)
• Lower limbs rotate 90 degrees medially so that the extensors are on the anterior surface

Question 8

Describe how the dermatomal patterns of the limbs form

Answer 8

Peripheral nerves grow from the developing limb plexus into the mesenchyme of the limbs

Question 9

List the categories of birth defects in the limbs

Answer 9

• Total absence (amelia)
• Partial absence (meromelia)
• Long bone absence (phocomelia)
• All parts present but small (micromelia)

Question 10

What is polydactyly and how is it caused?

Answer 10

• Presence of more than five digits on the hands or feet

- Inheritated or teratogen induced

Question 11

What are the two types of syndactyly?

Answer 11

• Cutaneous: simple webbing between the digits

- Osseus: fusion of bones caused when notches between the digital rays fail to develop

Question 12

What effect does thalidomide have on limbs?

Answer 12

• Absence or deformity of long bones

- Disrupts cell adhesion and angiogenesis

Question 13

What are the causes of congenital club foot?

Answer 13

• Genetic and environmental factors

- Abnormal positioning or restricted movement of the fetus’s lower limb in utero?

Question 14

What is developmental dysplasia of the hip and what are the causes?

Answer 14

• Abnormal development of acetabulum
• Breech deliveries
• Multifactorial and associated with congenital dislocation of the hip