Definition: Brain Development

The study of neural development draws on both neuroscience and developmental biology to describe the cellular and molecular mechanisms by which complex nervous systems emerge during embryonic development and throughout life.

Some landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory.

Typically, these neurodevelopmental processes can be broadly divided into two classes: activity-independent mechanisms and activity-dependent mechanisms. Activity-independent mechanisms are generally believed to occur as hardwired processes determined by genetic programs played out within individual neurons. These include differentiation, migration and axon guidance to their initial target areas. These processes are thought of as being independent of neural activity and sensory experience. Once axons reach their target areas, activity-dependent mechanisms come into play. Neural activity and sensory experience will mediate formation of new synapses, as well as synaptic plasticity, which will be responsible for refinement of the nascent neural circuits.

Developmental neuroscience uses a variety of animal models including the fruit fly Drosophila melanogaster , the zebrafish Danio rerio, Xenopus laevis tadpoles and the worm Caenorhabditis elegans, among others.

Neurulation

Neurulation is the formation of the neural tube from the ectoderm of the embryo. It follows gastrulation in all vertebrates. During gastrulation cells migrate to the interior of embryo, forming three germ layers— the endoderm (the deepest layer), mesoderm and ectoderm (the surface layer)—from which all tissues and organs will arise. In a simplified way, it can be said that the ectoderm gives rise to skin and nervous system, the endoderm to the guts and the mesoderm to the rest of the organs.

After gastrulation the notochord—a flexible, rod-shaped body that runs along the back of the embryo—has been formed from the mesoderm. The notochord sends signals to the overlying ectoderm, inducing it to become neuroectoderm. This results in a strip of neuronal stem cells that runs along the back of the fetus. This strip is called the neural plate, and is the origin of the entire nervous system.

The neural plate folds outwards during the third week of gestation to form the neural groove. Beginning in the future neck region, the neural folds of this groove close to create the neural tube. The anterior (front) part of the neural tube is called the basal plate; the posterior (rear) part is called the alar plate. The hollow interior is called the neural canal. By the end of the fourth week of gestation, the open ends of the neural tube (the neuropores) close off.

Formation of brain parts

Late in the fourth week, the superior part of the neural tube flexes at the level of the future midbrain—the mesencephalon. Above the mesencephalon is the prosencephalon (future forebrain) and beneath it is the rhombencephalon (future hindbrain).

The optical vesicle (which will eventually become the optic nerve, retina and iris) forms at the basal plate of the prosencephalon. The alar plate of the prosencephalon expands to form the cerebral hemispheres (the telencephalon) whilst its basal plate becomes the diencephalon. Finally, the optic vesicle grows to form an optic outgrowth.

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