Bio 580, Developmental Biology.  Wednesday, May 7, 1997
Limb Pattern Specification  

• Phenomenology is universal?
  Chicken limb, Newt Limb, Cockroach limb, Drosophila imaginal discs
• Limb Fields
  - It is possible to excise an area which then precludes (timely) regeneration
  - Induction of forelimb field by mesonephros (Insulin-like Growth Factor I, ILGF-I) (Fig. 19.4)
  
• Apical Ectodermal Ridge (AER)
  - induction by limb mesenchyme (Fig. 19.7)
  - chicken genes or chicken/quail hybrid experiments
  - mesenchymal mutations (Polydactylus)
  - ectodermal mutations (Udiplopodia, Limbless)
  - Fibroblast growth factor 2 (FGF-2) replaces AER (Fig. 19.13)
• Limb regeneration  
• Limb bud transplantation and augmentation.
  - Fig. 19.12 chick limb bud progress zone phenomenology
  - Cockroach augmentation and transplant polarity experiments
• Models of Pattern Formation
  
  • Mathematical models
    - Turing's (1952) reaction diffusion model
    - requires diffusion in a small space (e.g. embryos or blastemas)
    - can be modeled with defined chemicals and chemical reactions

• Morphogen Model:
  • A diffusible substance forms a gradient.
  
  - Zone of Polarizing Activity (ZPA)  (Fig. 19.19)
  - Mapped to be highly localized but moves distally as limb grows (Fig. 19.20)
  - ZPA material can be replace by a retinoic soaked bead (?).
  -
• Polar Coordinate Model [vs Cartesian Coordinate Model (D/V vs A/P)]:
  - the CC-Model in Drosophila uses the D/V and A/P gradients to place structures at particular levels along the animal anterior posterior axis
  • Cell surface properties form a gradient
    - (or very short distance diffusible substances)
  
  
  • Self rectification rules:
    • Shortest intercalation rule
    • Complete circle rule
      
  • Explains results of abnormal transplants (heterologous handed Fig. 19.27)
    - keloid scars (?)
  • Genetic control elements
    • Insect Proximal-Distal Limb Polarity
    • Hox-D gene series expression in chick limb bud (Fig. 19.30-19.32).
    •