Hints from developing neurons

The embryonic development of the nervous system is one of the most interesting processes of ontogenesis, and its study has also been one of the richest sources of information on the strategies that embryos adopt to solve their problems (Bonner, 1988 Edelman, 1988). 

The first phase of neural development begins when a strip of ectoderm is induced to become nervous tissue by the underlying mesoderm, and comes to an end when neuroblasts complete their last mitosis, an event which marks the birth of a neuron. The extraordinary thing is that all that a neuron will do in its whole life is determined by its birth, and more precisely by the time and by the place of its birth. These two parameters leave an indelible mark in the cell, and somehow therefore they must physically be recorded in a true organic cell memory. 

The second phase is a period during which neurons migrate to the places of their final destination, places that they know because somehow they are written in the memory of their time of birth and of their birthplace. 

If neurons were to depend on precise geographical information about their paths and their targets, they would need far bigger genomes than they could possibly carry. It is clear therefore that they rely on a different type of information, but it is also clear that the final result is the same, since all organs end up with nerve connections in precise geographical areas. With a small amount of a special information, neurons manage to obtain the same result that could 

The organs that are to be innervated send off particular types of molecules, known as nerve growth factors, whose effect is truly dramatic the neurons which get them survive, while the others die. More precisely, neurons are programmed to commit suicide - i.e. to activate the genes for programmed cell death, or apoptosis - at the end of a predetermined period of time, and nerve growth factors are the only molecules that can de-activate their self-destruction mechanism. The result is that neurons which reach the right places survive, and the others disappear. 

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