Key points

■ During early embryonic development, the neuroectoderm forms the neural tube and neural crest; these structures become the central nervous system (CNS) and cells of the peripheral nervous system (PNS), respectively.

■ The neural tube forms the spinal cord and brain, and the neural canal forms the inner ventricular system. The brain divides into prosencephalon, mesencephalon and rhombencephalon.

■ Dorsal outgrowths of the neural tube give rise to the cerebrum and cerebellum. The degree of cerebellar development at birth determines the mobility of a neonatal animal.

■ Germinal cells surrounding the neural canal give rise to neurons and glial cells; these cells migrate to form grey and white matter. The germinal layer becomes the ependyma in the postnatal animal.

■ Neuronal differentiation is determined by extracellular morphogens. Sonic Hedgehog protein triggers motor neuron development in the ventral spinal cord, while Bone Morphogenetic Protein induces sensory neuron differentiation in the dorsal spinal cord. Different homeobox proteins induce the longitudinal differentiation of the spinal cord.

■ Neuronal precursors migrate in response to molecular signals. Axonal growth is directed by chemoattractants or chemorepellants in the extracellular environment.