Embryology

Question 1

Describe the early stages of the develoment of the MSK system.

Answer 1

○ Begins near the end of the fourth week
○ Activation of mesenchymal cells (undifferentiated cells) in the somatic lateral mesoderm
○ Homeobox (Hox) genes regulate patterning
○ Day 24: upper limb bud appears
○ 1 to 2 days later: lower limb bud appears

Question 2

What are HOX genes?

Answer 2

○ Regionally expressed in limbs
○ Determine which skeletal element formed
○ HOX gene expression results from a combination of the expression of
- SHH (sonic the hedgehog)
- FGF’s (fibroblast growth factor)
- WNT-7a

Question 3

How do limbuds develop?

Answer 3

○ Limb buds elongate by the proliferation of mesenchyme
○ Upper limb buds develop opposite the caudal cervical segments
○ Lower limb buds from opposite the lumbar and sacral segments
○ At the apex of each bud the ectoderm thickens to form an apical ectodermal ridge (AER)

Question 4

What is an apical ectoderm ridge?

Answer 4

○ AER is restricted to end of the limb bud
○ Multi-layered epithelial structure
○ Exerts an indictive influence on the limb mesenchyme
○ The mesenchyme adjacent to the AER consists of undifferentiated, rapidly proliferating cells
○ Whereas mesenchymal cells proximal to it differentiate into blood vessels and cartilage bone models

Question 5

How do the hand and foot lates develop?

Answer 5

○ The distal ends of the limb buds flatten into paddle-like hand plates and flipper-like foot plates
○ By the end of the 6th week, mesenchymal tissue in hand plates has condensed to form digital rays
○ During the 7th week digital rays are formed in the foot plates
○ Programmed cell death (apoptosis) is responsible for the tissue breakdown in the interdigital regions- mediated by bone morphogenetic proteins

Question 6

What are the final stages of limb development?

Answer 6

○ Cartilaginous precursors of the limb bones develop by chondrification within the mesoderm in the 6th week
○ Primary centres of ossification appear in weeks 8 to 12 in response to growth factors
- By the time of birth all primary centres are there: forms the shaft of the bone
- Secondary centres develop into the epiphysis
○ Endochondral ossification for all limb bones except the clavicle (membranous)

Question 7

What happens in limb rotation?

Answer 7

○ Upper limbs and lower limbs rotate
○ Upper limbs rotate 90° laterally so that extensors are on the lateral and posterior surfaces (and the thumb is lateral)
○ Lower limb surfaces rotate 90° medially so that the extensors are on the anterior surface (and the big toe is medial)

Question 8

How do dermatomal patterns of the limbs develop?

Answer 8

During the 5th week, peripheral nerves grow from the developing limb plexus (brachial and lumbosacral) into the mesenchyme of the limbs

Question 9

What is the clinical term for total abcence?

Answer 9

amelia

Question 10

What is the clinical term for partial abcence?

Answer 10

meromelia

Question 11

What is the clinical term for some long boen abcence?

Answer 11

phocomelia

Question 12

What is the clinical term for all arts being there but small?

Answer 12

Micromelia

Question 13

When is criticcal limb devlopment

Answer 13

24-36 days after fertilisation

Question 14

What is polydactyly?

Answer 14

• Presence of more than 5 digits on the hands or feet
• Extra digit id incompletely formed and lacks muscular development
• May be inherited or teratogen induced

Question 15

What is syndactyly?

Answer 15

• Common birth defect of the hand or foot
• Cutaneous syndactyly (simple webbing between the digits) is a common limb defect
• More frequent in the foot than the hand
• Osseous syndactyly (fusion of the bones) occurs when notches between the digital rays fail to develop

Question 16

How does thalidomide affect foetuses?

Answer 16

• Absence or deformity of long bones (intestinal atresia and cardiac abnormalities)
• “sensitive” period 4-8 weeks
• Acts by disrupting cell adhesion and angiogenesis
  AIDS patients

Question 17

What is congenital clubfoot?

Answer 17

• Also known as Talipes Equeinovarus
• Relatively common birth defect
• Sole of the foot is turned medially and the foot is inverted

Question 18

What are the causes of congenital clubfoot?

Answer 18

□ Multifactorial (genetic and environmental factors)

□ Abnormal positioning or restricted movement of the foetus’s lower limb in utero (Evidence is inconclusive)

Question 19

What is abnormal develoment of the acetabulum?

Answer 19

□ 15% of infants with congenital dislocation of the hip
□ Common after breech deliveries: breech posture during late part of pregnancy (abnormal development of acetabulum and head of femur)

Question 20

How is generalised joint laxity of the hi caused?

Answer 20

□ Multifactorial

□ Associated with congenital dislocation of the hip