​The following statements pertain to limb development in chick. Each statement describes an experiment and the expected outcome.

A. Targeted loss of retinoic acid synthesis in the forelimb causes a reduction of Tbx4 expression and the failure to form forelimbs.

B. When Fgf10-secreting beads are placed at a somite level that induces a limb bud expressing Tbx5 in the anterior and Tbx4 in the posterior part, a chimeric limb can be formed.

C. Constitutive activation of FGF receptors results in the loss of the forelimb field, demonstrating that expression of Fgf8 functions to inhibit forelimb development.

Which one of the following option(s) is/are correct?

Correct option is D

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Explanation:

Statement A- Retinoic Acid (RA) and Limb Initiation

• RA is required for forelimb formation by inducing Tbx5 expression.
• Tbx4 specifies hindlimb identity, not forelimbs → Statement A is incorrect because it links RA to Tbx4 instead of Tbx5.

Statement B- Fgf10 and Limb Bud Formation

• Fgf10 is essential for initiating limb formation by activating limb bud outgrowth.
• When Fgf10 beads are placed at an intermediate somite level, Tbx5 (forelimb) and Tbx4 (hindlimb) are co-expressed, leading to chimeric limb formation → Statement B is correct.

Statement C- Fgf8 and Limb Field Inhibition

• Fgf8 inhibits RA signaling, which is necessary for Tbx5 expression and forelimb initiation.
• Overactivation of FGF receptors represses RA, leading to forelimb loss → Information Booster:

Information Booster:

·       Retinoic Acid (RA) is essential for forelimb formation by inducing Tbx5 expression.

·       Fgf10 is a key inducer of limb bud formation.

·       Fgf8 inhibits forelimb development by repressing RA signaling.