Musculoskeletal

Question 1

Musculoskeletal system development

Answer 1

• Begins near end of 4th week

- Mesenchymal cells of somatic lateral mesoderm activated

Question 2

Genes that control musculoskeletal system development

Answer 2

• Homeobox (HOX) genes
• Regulate proximo-distal patterning by regionally expressing combinations of SHH, FGF’s and WNT-7a
• Determine which skeletal element is formed
• HOXd 9-13: upper limb

Question 3

Limb bud development

Answer 3

Proliferation of mesenchyme

• Day 24: upper limb bud opposite caudal cervical segments
• Day 25/26: lower limb bud opposite lumbar and sacral segments

Question 4

Apex of bud

Answer 4

• Ectoderm thickens to form apical ectodermal ridge (AER)

Question 5

Mesenchyme surrounding AER

Answer 5

• Adjacent: undifferentiated rapidly proliferating cells

- Proximal: differentiate to blood vessels, cartilage and bone models

Question 6

Limb plexus development

Answer 6

• 5th week

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

Question 7

Brachial plexus nerves

Answer 7

C5-T1 (C3-T2)

Question 8

Lumbosacral plexus nerves

Answer 8

? L2-S3

Question 9

Cartilagenous precursors of limb bones

Answer 9

• 6th week

- Bone develops by chondrification within the mesoderm

Question 10

Hand and foot development

Answer 10

• Distal limb buds flatten
• 6th week: mesenchymal tissue forms digital rays in hand plates
• 7th week: mesenchymal tissue forms digital rays in foot plates
• 8th week: apoptosis of interdigital (webbed) regions mediated by bone morphogenetic proteins

Question 11

Primary centres of ossification

Answer 11

• 8-12 weeks
• Form in centre in response to growth factors and expand in both directions
• Infiltrated by developing blood vessels which deliver calcium
• Endochondral ossification for all limb bones (except clavicle)

Question 12

Secondary centres of ossification

Answer 12

• Will eventually fuse with primary centre

Question 13

Limb rotation

Answer 13

Upper limbs rotate 90 degrees laterally

• Extensors are lateral/posterior
• Thumb is lateral

Lower limbs rotate 90 degrees medially

• Extensors are anterior
• Big toe is medial

Question 14

Amelia

Answer 14

Total absence of limb

Question 15

Meromelia

Answer 15

Partial absence of limb

Question 16

Phocomelia

Answer 16

Some long bone absence

Question 17

Micromelia

Answer 17

All parts present but small

Question 18

Lobster claw foot

Answer 18

Autosomal dominant trait

Question 19

Polydactyly

Answer 19

• Inherited or teratogen induced
• Presence of >5 digits on hand/foot
• Extra digit incompletely formed and lacks muscular development

Question 20

Syndactyly prevalence

Answer 20

Common birth defect

• 1 in 1000
• More frequent in the foot

Question 21

Cutaneous syndactyly

Answer 21

• Simple webbing between digits

- Plastic surgery

Question 22

Osseous syndactyly

Answer 22

• Fusion of bones

- Notches between digital rays fail to develop

Question 23

Congenital/idiopathic clubfoot (Talipes Equinovarus)

Answer 23

Common birth defect: 1 in 1000 births

• Multifactorial (genetic, environmental factors)
• Sole of foot is turned medially, and foot is inverted

Question 24

Abnormal development of acetabulum

Answer 24

• 15% with congenital dislocations
• Common after breach delivery
• ? Breach position

Question 25

Generalised joint laxity

Answer 25

• Multifactorial
• Maldevelopment of ligaments
• Associated with congenital dislocation of hip

Question 26

Maternal thalidomide

Answer 26

• Sleeping tablet, anti-nauseant (1957-62). Now: cancer, leprosy, AIDS
• Absence/deformity of long bones, intestinal atresia and cardiac abnormalities
• Acts by disrupting cell adhesion and angiogenesis
• ‘Sensitive’ period 4-8 weeks