Embryology

Question 1

When do limb buds first appear in embryology

Answer 1

Limb buds appear towards the end of the 4th week as embryo prioritise development of cardiovascular system and nervous system

Question 2

Explain structure of limb buds

Answer 2

• Lower limb development lags ~2 days behind upper limb
• Core of proliferating mesenchymal cells with an ectoderm covering
• Limb bud has a mesenchyme core and flexible mesoderm
  
  • Elongation of limb bud is through proliferation of mesenchyme core
    
    • Thickened ectoderm at apex of limb bud

Question 3

Describe the 3 spatial axes in limb development

Answer 3

• Shoulder to fingertips (proximal - distal axis)
  
  • Controlled by apical ectodermal ridge (AER)
• Palmar surface of dorsal surface (dorsal - ventral axis)
  
  • Controlled by ectoderm
• Side to side (anterior - posterior axis)
  
  • Controlled by zone of polarising activity (ZPA)

Question 4

Explain the importance of apical ectodermal ridge (AER)

Answer 4

• Critical for limb bud outgrowth
• Orchestrates limb development proximal to distal
  
  • AER exerts an inductive influence on the immediately underlying mesenchyme - remains undifferentiated
  • Proximal mesenchyme - begins to differentiate into constituent tissues
  • AER induces development of the digits within the hand/foot plates
• Final stage is appearance of paddles
• AER then regresses

Question 5

Explain the importance of the zone of polarising activity

Answer 5

• Generation of asymmetry in the limbs (anterior - posterior axis determination)
• A signalling centre located at the posterior base of the limb bud
• Controls both patterning and maintains the AER
• Experiment - implantation of second zone of polarising activity creates a mirror image of the limbs

Question 6

How are hand and foot digits formed

Answer 6

• Digital rays - mesenchyme condensations within plates
  
  • Cartilaginous models of the digital bones
• Apoptosis of the tissue between the digits
  
  • AER breaks up and is maintained only over the tips of the digital rays
    
    • Interdigital spaces are progressively sculpted by programmed cell death

Question 7

How are muscles implemented into limb buds

Answer 7

• Myogenic precursors migrate into limbs from somites (body segments)
  
  • Combine into 2 common muscle masses around the newly formed skeletal elements
    
    • Ventral = flexor, dorsal = extensor
    • Individual muscles then split from common masses

Question 8

What is syndactylyl

Answer 8

Fusion of digits, may involve just connective tissue or bones may be fused

Question 9

What is polydactylyl

Answer 9

Extra digits, genetic recessive trait

Question 10

What is amelia

Answer 10

Complete absence of a limb

Question 11

What is meromelia

Answer 11

• Partial absence of one or more limb structures

- Phocomelia - hands or feet attached directly to the trunk

Question 12

What are underlying problems of morphogenesis

Answer 12

• Malformation - intrinsic error in coordination of morphogenesis
• Deformation - constriction bands
• Disruption - external agent e.g. Thalidomide which disrupts AER to stop elongation of limb bud
  
  • Infections