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The brain is a system which allows for complex information processing and control. It is comprised of a network of billions of neurons that communicate with one another to sense the body and the surrounding world and to orchestrate the appropriate responses. The brain coordinates our basic involuntary and essential functions, such as breathing and heartbeat. It is responsible for homeostasis, and monitors our skeletal muscles as well as the function of our internal organs. The brain is also responsible for our communication and learning abilities, our thoughts, consciousness and feelings. As befitting such a central organ, the brain is protected by multiple layers of neural crest origin, including the skull bones, associated connective tissues, and meningeal membranes. The brain and its structures form in a complex multistep process, which is further fine-tuned in the postnatal period. The key steps of embryonic brain development are described below.
The brain and spinal cord develop from the ectoderm. Following formation of the neural ectoderm, the neural preplate is formed and splits to form the neural plate. Closure of the neural plate forms the neural tube in a process called neurulation (see description in "Neural Tube" overview). The central hollow space of the neural tube later forms the fluid-filled brain ventricles. Neuroepithelium is generated in the neural tube walls and gives rise to immature nerve precursors called neuroblasts, the majority of which migrate and grow leading axonal appendages, and then aggregate in specific, genetically determined locations that will become the brainstem and spinal cord.The neuroectoderm then subdivides into ventricular and subventricular zones, which produce separate waves of migrating neuroblasts.
Following expansion of the anterior part of the neural tube and of the neural crest region, the tube twists and indentations appear and form the primary brain vesicles - the prosencephalon (forebrain), mesencephalon (midbrain) and the rhombencephalon (hindbrain) . Following subsequent divisions to the secondary brain vesicles, the prosencephalon gives rise to the telencephalon and diencephalon, the mesencephalon gives rise to sub-compartments, and the rhombencephalon gives rise to the metencephalon and myelencephalon.
The developing neurons differentiate and form all brain areas, including the medulla, pons, substantia nigra, hypothalamus, thalamus, and later the limbic system, striatum, and the six layers of the neocortex. Final maturation of neurons including differentiation, axonal and dendritic growth and pathfinding, establishment of synapses, and myelination drive the functional development of the medulla, first followed by the pons, then the mesencephalon, and last, by the prosencephalon, which does not begin to functionally mature until near term.