The patterning of the anterior-posterior (thumb-to-little finger) axis of the vertebrate limb and the dorsal-ventral axis of the nerve plate is critically regulated by the morphogen Sonic hedgehog (Shh). Shh acts as a signaling molecule that provides positional information, guiding tissue differentiation and axis specification during embryogenesis.

Role of Sonic Hedgehog in Limb Development and Neural Patterning

• Anterior-Posterior Limb Axis:

  • Shh is secreted from a specialized group of cells in the limb bud known as the zone of polarizing activity (ZPA), located at the posterior margin of the limb.

  • It forms a concentration gradient that specifies positional identity among developing limb cells, determining digit identity and patterning along the anterior-posterior axis.

  • Experimental grafting of ZPA cells to the anterior limb margin can cause mirror-image duplications of digits, demonstrating Shh’s role as a morphogen in positional specification.

• Dorsal-Ventral Nerve Plate Axis:

  • Shh signaling also contributes to the dorsal-ventral patterning of the neural tube, including the nerve plate, influencing the fate of ventral progenitor cells.

  • It interacts with other signaling pathways to establish motor neuron populations ventrally and sensory neurons dorsally through graded signaling mechanisms.

Why Other Options Are Incorrect

• Bicoid: This is a key anterior morphogen in Drosophila embryogenesis, not involved in vertebrate limb or nerve plate patterning.

• Pax3: Controls aspects of dorsal neural tube and muscle development but does not specify the anterior-posterior limb axis or dorsal-ventral nerve plate axis.

• Cactus: A Drosophila protein involved in immune response and dorsal-ventral embryo patterning in flies, not relevant to vertebrate limb or nerve plate development.

Conclusion

The anterior-posterior axis of the limb and dorsal-ventral axis of the nerve plate are predominantly specified by the concentration-dependent signaling of Sonic hedgehog (Shh). This morphogen provides essential positional information guiding proper limb formation and neural differentiation during vertebrate embryogenesis.

The correct answer is:
(2) Sonic hedgehog