If the ZPA (zone of polarizing activity) is transplanted from the posterior margin of one limb bud to the anterior margin of another, what develops?
AThe host limb develops normally — the ectopic ZPA has no effect
BA mirror-image duplication of digits forms, with the ectopic ZPA inducing posterior-type digits (like digit 4 and 5) at the anterior margin, producing a limb with digits arranged 5-4-3-2-3-4-5
CThe entire limb bud degenerates due to conflicting signals
DA second complete limb grows from the transplant site
This classic experiment by Saunders and Gasseling (1968) demonstrated that the ZPA is the source of anterior-posterior patterning information. The ZPA produces Sonic Hedgehog (Shh), which forms a posterior-to-anterior gradient that specifies digit identity. Transplanting the ZPA to the anterior creates a second, opposing Shh gradient, producing mirror-duplicated digits. Cells near either ZPA receive high Shh and become posterior digits (4, 5); cells between the two sources receive intermediate levels and form digit 3. This experiment proved that a single morphogen gradient can specify the identity of five different digits.
Question 2 True / False
The AER (apical ectodermal ridge) is required only for the initial stages of limb outgrowth; once the limb bud is established, removing the AER has no effect.
TTrue
FFalse
Answer: False
Removing the AER at progressively later stages produces progressively more complete but still truncated limbs. Early removal produces a limb with only a humerus; later removal produces humerus + radius/ulna; even later removal produces those plus some carpals but no digits. This demonstrates that the AER is continuously required for distal outgrowth — FGF signals from the AER maintain the underlying mesenchyme in a proliferative, undifferentiated state. As the limb grows, proximal structures differentiate first (they are farthest from the AER's FGF signal), while distal structures form last. Removing the AER at any point truncates all structures that have not yet been specified.
Question 3 Short Answer
How do the AER, ZPA, and dorsal ectoderm maintain each other through reciprocal signaling?
Think about your answer, then reveal below.
Model answer: The three signaling centers form a mutually reinforcing loop: FGF from the AER maintains Shh expression in the ZPA; Shh from the ZPA maintains FGF expression in the posterior AER (via Gremlin-mediated BMP inhibition); and Wnt7a from the dorsal ectoderm maintains Shh expression in the ZPA. Disrupting any one center collapses the others. For example, removing the AER eliminates FGF, which leads to loss of Shh in the ZPA, which leads to loss of posterior AER maintenance — the entire patterning system disintegrates. This reciprocal dependence ensures that limb growth and patterning are tightly coordinated and that all three axes are specified simultaneously.
This tripartite signaling loop is a beautiful example of developmental robustness through interdependence. The mutual maintenance ensures that the patterning system operates as a coherent unit — you cannot pattern one axis without the others, preventing malformed partial limbs.