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5.4 Limb Development — Test 1
Q1. The apical ectodermal ridge (AER) of the limb bud is responsible for:✓ Proximal-distal outgrowth of the limb
Q2. Removal of the AER from a limb bud causes:✓ Cessation of distal limb outgrowth
Q3. After AER removal, limb outgrowth can be rescued by implanting beads soaked in:✓ FGF (e.g. FGF8, FGF4 or FGF2)
Q4. The zone of polarizing activity (ZPA) controls which limb axis?✓ The anterior-posterior axis
Q5. The ZPA exerts its effect by secreting:✓ Sonic hedgehog (Shh)
Q6. Grafting an extra ZPA to the anterior margin of a limb bud results in:✓ Mirror-image duplication of the digits
Q7. The dorsal-ventral axis of the limb is set in part by the expression of ____ in the dorsal limb ectoderm:✓ Wnt7a
Q8. The proximal-to-distal order in which the tetrapod limb forms is:✓ Stylopod, then zeugopod, then autopod
Q9. The distal-most part of the tetrapod limb (the hand or foot) is the:✓ Autopod
Q10. In mice, the gene Tbx5 is transcribed in the limb fields of the:✓ Forelimbs
Q11. In mice, the genes Islet1, Tbx4 and Pitx1 are expressed in the presumptive:✓ Hindlimbs
Q12. The limb bud of a tetrapod is specified by:✓ Both Hox genes and retinoic acid
5.4 Limb Development — Test 2
Q13. Sonic hedgehog specifies digit identity mainly according to:✓ The duration of Shh expression and, to a lesser extent, its concentration
Q14. The programmed cell death that separates the digits is controlled by which pathway?✓ BMP
Q15. Placing chick leg mesenchyme beneath the wing AER results in:✓ Distal hindlimb (toe) structures forming at the wing tip
Q16. These grafting experiments show that the AER's role is to:✓ Sustain outgrowth, while the mesenchyme specifies limb type
Q17. The proliferative mesenchyme just beneath the AER that fuels limb outgrowth is the:✓ Progress zone
Q18. In the limb-bud gene network, FGF from the AER and Shh from the ZPA form a:✓ Positive feedback loop that maintains both centres
Q19. Gremlin in the limb bud acts by:✓ Blocking BMP, which would otherwise shut off the AER FGFs
Q20. A human homozygous for a HOXD13 mutation would show:✓ Hand and foot abnormalities with fused digits (synpolydactyly)
Q21. In mice homozygous for a mutant Sonic hedgehog (shh) allele, a likely outcome is:✓ A reduced facial midline with a single central eye (cyclopia)
Q22. Human polysyndactyly (joined, extra digits) results from a homozygous mutation in:✓ One of the HoxD genes
Q23. Nail-Patella syndrome (and double-dorsal limbs in mice) arises from loss of dorsal-patterning genes such as:✓ Lmx1b (and Wnt7a)
Q24. The three signalling centres of the developing limb — AER, ZPA and dorsal ectoderm — each control one limb axis. The AER controls the:✓ Proximal-distal axis
5.4 Limb Development — Test 3
Q25. FGF10 from the lateral plate mesoderm is important early in limb development because it:✓ Induces the overlying ectoderm to form the AER
Q26. A graft of the ZPA to the anterior margin of a wing bud produces a mirror-image set of digits because:✓ The new Shh source patterns the anterior as if it were posterior
Q27. Replacing the ZPA graft with a bead soaked in Sonic hedgehog protein gives the same mirror-image digits, showing that:✓ Shh is the active signal of the ZPA
Q28. Digit identity in the limb depends on a gradient of Shh such that the digit nearest the ZPA (most posterior) is:✓ The most posterior digit (e.g. the little finger side)
Q29. The limb bud progress zone model proposes that a cell's proximal-distal identity depends on:✓ How long it spends in the progress zone before leaving
Q30. Hoxa and Hoxd genes are expressed in nested patterns that help specify the:✓ Stylopod, zeugopod and autopod identities along the limb
Q31. Interdigital webbing is removed in most birds and mammals by BMP-driven apoptosis. In ducks, the webbing is retained because:✓ BMP-induced cell death is locally inhibited (e.g. by Gremlin)
Q32. The dorsal-ventral patterning gene Lmx1b acts in the dorsal mesenchyme downstream of:✓ Wnt7a (from the dorsal ectoderm)
Q33. The positive feedback loop that keeps the limb growing links the AER and ZPA via:✓ FGF → Shh → Gremlin (which blocks BMP) → maintains AER FGF
Q34. When the AER is removed progressively earlier in development, the limb that forms is:✓ Truncated at progressively more proximal levels
Q35. Overall, the vertebrate limb is a premier model in developmental biology because it shows how:✓ Three signalling centres coordinate to pattern a three-dimensional structure