Analysis of Statements

A. Positive feedback loop between AER-derived FGF and Shh in the ZPA

• Correct. FGFs (particularly FGF4 and FGF8) secreted by the Apical Ectodermal Ridge (AER) maintain Shh expression in the Zone of Polarizing Activity (ZPA) in a positive feedback loop, stabilizing limb patterning along the anterior-posterior axis.

B. Inhibitory loop where Shh inhibits Gremlin, which normally blocks BMPs so BMP-mediated inactivation of AER FGFs is prevented

• Correct. Shh indirectly supports maintenance of FGFs by suppressing Gremlin, an antagonist of BMPs. This negative feedback loop regulates BMP activity to prevent premature cessation of FGF signaling, ensuring limb outgrowth.

C. Wnt7a expression in the ventral ectoderm forms the dorsal-ventral axis

• Incorrect. Wnt7a is expressed in the dorsal ectoderm, not ventral, and is critical for establishing dorsal limb cell fates.

D. BMPs mediate both apoptosis and mesenchymal differentiation into cartilage

• Correct. BMP signaling plays a dual role: it promotes apoptosis to sculpt the forming limb (e.g., interdigital cell death) and induces differentiation of mesenchymal cells into chondrocytes for cartilage formation.

Correct Combination of Statements

The true statements, based on developmental biology evidence, are A, B, and D, while C is false because of the incorrect localization of Wnt7a.

Thus, the correct choice is:
(2) A and D

Summary

• FGF and Shh positively regulate each other’s expression, coordinating anterior-posterior limb patterning.

• Shh also modulates Gremlin and BMP signaling, forming an inhibitory loop critical for balanced regulation of limb outgrowth.

• Wnt7a is essential for dorsal ectoderm patterning, not ventral.

• BMPs direct both programmed cell death and cartilage differentiation, shaping the limb structurally.

Final answer:
(2) A and D