Answer
The cells that constitute the nervous system originate in the neural tube and neural crest. The neuroepithelial cells of the walls of the neural tube begin to differentiate in the second month of pregnancy. In three or four linked processes the neuroepithelial cells form neuroblasts which proliferate, migrate to targets and form synapse with them..
The growth of the neurons is supported by nerve growth factor through all stages, and the presence of cholesterol is necessary for proper synapse formation Astrocytes provide essential support for developing neurons.
In their migration to targeted destinations, each neuron generates a growth cone with filopodia. These amoeboid structure enable the neuron to flow or roll towards its target, as it is guided by chemical cues in its environment. Some of these cues are adhesion proteins, and nerve cell adhesion molecule(N-CAM) which serve to anchor the growth cone. Several other chemicals issue chemical guidance instructions to the growth cones . Some of these chemicals are netrin-- which attracts the growth cone; semaphorin, which emits "Stop" instructions and ephrin, which essentially directs the growth cone to change direction. If all this guidance works as intended, the presynaptic cell will find a target and consummate the synaptic process. The postsynaptic dendrites play an active role by grasping and pulling in the presynaptic axon. If the developing neuron does not achieve proper synaptogenesis, it will die. Developing neurons that do not make proper connection die by apoptosis.
Work Step by Step
Cells of the nervous system develop from the neural tube, and the neural crests. Th neuroepithelial cells of the walls of the neural tube develop into neuroblasts which proliferate mitotically and provide all the cells that will form the nervous system.
Developing neurons migrate towards predestined cellular targets, where they differentiate, synapse, and become adult neurons. The migration is effected by an amoeboid, seeking movement of a growth cone which is generated by each developing neuron. If a neuron is able to follow chemical clues successfully, and accomplish synaptogenesis, it will become a functioning adult neuron. However, if a neuron fails to contact its correct target cell, and synapse with it correctly, the potential presynaptic neuron will die. Over sixty percent of all neurons produced during embryogenesis die before birth of the a fetus.
