The Rockefeller University, Field Research Center, Millbrook, New York 12545, USA
Neurons are constantly added to the telencephalon of songbirds.
In the high vocal center (HVC), where this has been studied,
new neurons replace older ones that died. Peaks in replacement
are seasonal and affect some neuronal classes but not others.
Peaks in replacement coincide with peaks in information acquisition.
The new neurons are produced by division of cells in the wall
of the lateral ventricle. Where studied closely, the neuronal
stem cells proved to be radial glia. Life expectancy of the
new neurons ranges from weeks to months. New neuron survival
is regulated by vacancies, hormones, and activity. The immediate
agent of new neuron survival is, in some cases, brain-derived
neurotrophic factor (BDNF). The effect of BDNF is maximal 14-20
days after the cells are born, when they are establishing their
connections. These observations are now being extended to other
vertebrates and may apply, to varying degrees, to all of them.
The function of neuronal replacement in healthy adult brain
remains unclear. If synaptic number and efficacy sufficed as
mechanisms for long-term memory storage and could be adjusted
again and again to incorporate new memories, then neuronal replacement
would seem unnecessary. Since it occurs, it seems reasonable
to suppose that replacement serves to maintain learning potential
in a way that could not be done just by synaptic change. Long-term
memories may be encoded by long-term changes in gene expression
akin to a last step in cell differentiation. If so, neuronal
replacement may be the adult brain's way of striking a balance
between limited memory space and the need to acquire new memories.
The testing of this hypothesis remains in the future. This chapter
tells how neuronal replacement was discovered in the adult songbird
brain.
http://www.rockefeller.edu/labheads/nottebohm/nottebohm-lab.html 
