Which substance guides neuronal growth during embryonic development and helps sustain neurons in adults?

Nerve Growth Factor (NGF) plays a crucial role in neuronal growth and survival during both embryonic development and throughout adulthood. It is a type of neurotrophin, a family of proteins that are vital for the growth, maintenance, and survival of neurons.

During embryonic development, NGF promotes the growth and differentiation of neurons from precursor cells, ensuring that the nervous system develops correctly. It influences the formation and maintenance of synapses, allowing neurons to connect and communicate effectively. In adult organisms, NGF continues to support neuronal health by facilitating the survival of certain types of neurons and playing a critical role in neuronal repair processes following injury.

Neurotransmitters such as dopamine and serotonin are important for communication between neurons and have various roles in mood regulation and many other functions. However, they do not directly guide neuronal growth or sustain neurons in the way NGF does. While they contribute to neurotransmission and modulating neural activity, their primary functions do not include the promotion of neuron growth and survival in the developmental context. Thus, NGF is the most appropriate answer to the question regarding substances involved in neuronal growth and sustenance.