Oligodendrocyte Type-2 astrocyte (postnatal day 1) (postnatal days 8-10)

Figure 8 Postulated glial cell lineages in rat optic nerve.

invertebrate nervous systems. The first systematic report in this context was that of Hamburger and Levi-Montalcini, who observed massive cell losses by death in dorsal root ganglia of the growing chick at the upper cervical and thoracic levels. Subsequent studies showed similar neuronal degeneration in other nerve structures to the extent of 25-75% occurring during a well-defined period of days or weeks. For example, 30% of the cells of mouse cerebral cortex or lateral geniculate nucleus are lost between Postnatal Days 5 and 30, and chick trigeminal mesencephalic nucleus loses as much as 75% of the cells between Embryonic Days 11 and 13.

There are many reasons for the natural death of the neurons in the developing nervous system, but the best understood and most documented is the lack of maintenance from the target tissue. The target cells are known to secrete one or more trophic factors, the so-called neurotrophic factors, of which the nerve growth factor is the best characterized. These trophic factors are essential for the survival of the neurons and are made available to the neurons by retrogradic transport.

Many observations have clearly indicated the phenomenon of "competition" among neurons to gain access to the target tissue and obtain the necessary trophic factors. More active neurons have been shown to win and survive, whereas less active ones make fewer synapses, receive less trophic factors, and eventually degenerate. This is in agreement with the suggestion made more than 100 years ago by Roux that cells, including neurons, follow the Darwinian "struggle for existence—survival of the fittest" theory.

During early periods of development, many diffuse, inaccurate, and aberrant synaptic interconnections are formed in the nervous tissue that have to be removed for a precise and topographically ordered synaptic network characteristic of the nerve tissues to be established. Death of the neurons with such aberrant connections would be a way to achieve this. However, cell death is not the only means by which neuronal connections are eliminated. In many cases, this is achieved by pruning the axons of aberrant neurons without the death of the cells.

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