26. Which one of the following mRNAs is a BMP inhibitor and can rescue the dorsal structures of ventralized Xenopus embryo when injected into it?
    (1) beta-catenin (2) Noggin
    (3) Disheveled (4) Siamos

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Introduction

During early embryonic development in Xenopus (a model amphibian), establishing the dorsal-ventral axis is essential for proper body patterning. BMP (Bone Morphogenetic Protein) signaling promotes ventral development, and excessive BMP activity leads to ventralized embryos lacking dorsal structures. Noggin is a secreted protein that inhibits BMP signaling and is critical for dorsal development.

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Role of Noggin as a BMP Inhibitor

• Noggin binds directly to BMPs such as BMP2, BMP4, and BMP7, preventing them from activating their receptors.

• This inhibition reduces BMP signaling on the dorsal side, allowing dorsal cell fates and neural tissue to form.

• Noggin mRNA injections into ventralized Xenopus embryos can block BMP signaling regionally and restore dorsal structures such as the neural tube and notochord.

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Experimental Evidence

• Injection of Noggin mRNA leads to dorsalization; embryos develop normally structured dorsal tissues even if originally ventralized due to BMP overactivity.

• Noggin’s ability to induce a secondary axis when ectopically expressed demonstrates its power in shifting developmental fate through BMP antagonism.

• Other molecules like β-catenin, Disheveled, and Siamos have roles in developmental signaling but do not directly act as BMP inhibitors to rescue dorsal structures as Noggin does.

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Biological Significance

• Noggin is vital in regulating embryonic patterning by balancing BMP signals.

• Its dysfunction or absence disrupts axis formation, neural induction, and proper embryonic development.

• Understanding Noggin’s mechanisms informs developmental biology and regenerative medicine.

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Conclusion

Noggin is a crucial BMP inhibitor that can rescue dorsal structures in ventralized Xenopus embryos by blocking BMP signaling. Its injection is a classical experimental approach to demonstrate BMP inhibition’s importance in dorsal-ventral patterning.

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Final Answer:
(2) Noggin