(423) The three stages of neuronal development are proliferation , migration and differentiation( synaptogenesis). In the proliferation stage stem cells multiply mitotically to provide enough cells for development of the nervous system, In the migration phase neuroblasts lose their mitotic ability and migrate to their destinations.. In the third phase, each migrating neuron, guided by many chemical signals in its environment, finds its target cell and synapses with. If correct targets are not contacted neuronal development is abnormal and dysfunctional.
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The nervous system originates embryonically from the neural tube, and the neural crests. The neuroepithelial cells of the walls of the neural tube start their development/differentiation towards the status of neurons during the second month of pregnancy. The three common divisions of the neuronal development process are proliferation, migration and synaptogenesis. But programmed cell death , apoptosis, also plays a significant role in neuronal development; it has been estimated that almost two thirds of all the neurons formed during embryogenesis die before birth. The growth of an axon towards its target is guided by maany signals. In this process each axon produces a fan-like growth cone which generates many process and moves in an amoeboid fashion ; the movement is aided by actin protein filaments, and guidance is provided by chemical signals . Some of the chemical signals, for example, cell surface adhesion protein, and nerve cell adhesion molecule (N-CAM). provide anchorage, and signal permission to move, respectively. Neurotrophins also give important instructions to the growth cone: ephrins signal "go away"; semaphorins signal , " stop"; nerin is attractive and invites the growth cone to come in its direction--follow along its concentration gradient. The differentiating neuron also needs the nutritional support of nerve growth factor(NGF) at every stage. . At the target, cell adhesion molecules connect axon and target ( presynaptic cell couples with post synaptic cell) together, and vesicles with synaptic secretions are formed . The target cells are not passive in the process of synaptogenesis. During this process thrombospondin released by astrocytes induces dendrites to reach out and grasp migrating axons to form synapses. Axons that do not contact their targets, or do not synapse correctly, do not survive. This a natural part of the process of neural development.