What signaling molecule is secreted from mesodermal tissue underneath the spinal cord during induction?

The signaling molecule that is secreted from mesodermal tissue underneath the spinal cord during induction is Sonic Hedgehog (SHH). This protein plays a critical role in the development of the nervous system, particularly in the patterning and differentiation of neural progenitor cells in the developing spinal cord.

Sonic Hedgehog is essential for establishing the dorsal-ventral axis of the spinal cord, influencing the fate of neurons and the overall architecture of the neural tube. It stimulates the proliferation and differentiation of progenitor cells, leading to the formation of specific types of neurons, such as motor neurons, which are crucial for locomotion.

While other molecules listed, including Nerve Growth Factor (NGF), Brain-Derived Neurotrophic Factor (BDNF), and Platelet-Derived Growth Factor (PDGF), are important for various neural growth and support functions, they do not play the same pivotal role in the early patterning and induction of spinal cord development as Sonic Hedgehog does. Therefore, SHH is specifically recognized for its role in inductive signaling during embryonic development, making it the correct answer.