36. Following figure shows the early interactions between the Apical Ectodermal Ridge (AER) and the limb bud mesenchyme.
    The red lines with block head indicate repression while the black lines indicate activation.

    The following statements were made regarding the development of a tetrapod limb:
    A. When the limb bud grows Shh creates a new signalling centre that induces the posterior- anterior polarity.
    B. When the concentration of FGFs rises, it can inhibit Gremlin thus allowing BMPs to begin repressing the AER-FGFS.
    C. FGFs 4, 9 and 17 from the AER inhibit Shh to stabilize the ZPA.
    D. Repression of Gremlin synthesis helps maintain the AER.
    Which one of the following options represents the combination of all correct statements?
    (1) A and B (2) A and C
    (3) B and D (4) C and D

Role of Sonic Hedgehog (Shh) in Anterior-Posterior Axis Specification

• Shh, secreted from the zone of polarizing activity (ZPA) located at the posterior limb bud margin, is crucial for establishing the anterior-posterior polarity of the limb.

• Shh expression creates a gradient that instructs digit identity and positional information along this axis.

• The initiation of Shh expression is tightly regulated by signals from the AER, particularly the fibroblast growth factors (FGFs).

Fibroblast Growth Factors (FGFs) and Their Interaction with Shh

• FGFs such as FGF4, FGF9, and FGF17 are secreted by the AER and are essential for maintaining the proliferative state of the mesenchymal cells beneath the ridge.

• These FGFs activate Shh transcription in the ZPA, thereby stabilizing this signaling center and maintaining the limb patterning program.

• The positive feedback loop between FGFs and Shh sustains anterior-posterior polarity during limb outgrowth.

Regulatory Role of Gremlin and Bone Morphogenetic Proteins (BMPs)

• Gremlin acts as an antagonist of BMPs and is vital for controlling BMP activity within the limb bud.

• Shh signaling inhibits Gremlin expression, which allows BMP activity to be modulated.

• When Gremlin is inhibited, BMPs become active and repress FGF expression in the AER, regulating the timing and cessation of limb growth.

• This inhibitory loop between Gremlin, BMPs, and FGFs ensures balanced growth and proper morphogenesis.

Clarifying the Statements

• Statement A: Correct. Shh creates a new signaling center to induce posterior-anterior polarity during limb development.

• Statement B: Correct. Rising concentrations of FGFs inhibit Gremlin, allowing BMPs to repress AER FGFs, which modulates limb growth.

• Statement C: Incorrect. FGFs do not inhibit Shh; rather, they positively regulate Shh expression to reinforce the ZPA.

• Statement D: Incorrect. Repression of Gremlin would lead to increased BMP activity that suppresses FGF expression, which would eventually lead to AER regression, not maintenance.

Conclusion

Development of the tetrapod limb relies on a complex interplay where Shh forms the posterior signaling center specifying limb polarity, and FGFs from the AER maintain this signaling through a positive feedback loop. The Gremlin-BMP axis provides a balanced inhibitory mechanism controlling the duration of FGF activity and thus limb outgrowth. This coordinated regulatory network is essential for correct limb patterning and growth.

Final Answer:
The correct combination of all true statements is:
(1) A and B