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development, while the smaller neurons (or microneu-
rons) are generated later in development, through the
postnatal period and even into adulthood. These later
generated microneurons are then integrated into the
framework provided by the macroneurons. Altman
pictured this second stage of neurogenesis as a way for
environmental influences to regulate the neurogenesis
and produce a brain ideally suited to its environment.
Although it has been difficult to prove that persistent
neurogenesis in a particular region of the brain is nec-
essary for behavioral plasticity in that brain region,
recent studies are consistent with Altman's hypothesis.
In one of the last reviews of his work, Altman (Altman
and Das, 1965) summed up his hypothesis: “We postu-
late that this hierarchic construction process endows
the brain with stability and rigidity as well as plasticity
and flexibility.” It is possible that if you are going to
remember anything you have just read in this Chapter
you have to make new neurons in your hippocampus!
tube and brain vesicles. The progenitor cells from the
early embryonic nervous system undergo many sym-
metric cell divisions to make more progenitor cells,
while the progenitor cells in the late embryo are more
likely to undergo an asymmetric division to generate
neurons and glia. The production of both neurons and
glia from the progenitor cells is under tight molecular
control, and this allows the proper numbers of both
neurons and glia to be produced for the proper func-
tioning of the brain. Interactions between the neurons
and the progenitor cells regulate their proliferation in
both positive and inhibitory ways. Overall, a remark-
able coordination takes place to regulate proliferation
in the nervous system during development, and muta-
tions in specific genetic pathways involved in neuro-
genesis can lead to childhood tumors and gliomas in
adults. Once the developmental period of neurogene-
sis is complete, most areas of the brain do not gener-
ate new neurons, even after damage. This has led to
the concept that you are born with all the neurons that
you are going to ever have. However, in recent years,
it has become clear that certain regions of the brain, the
hippocampus and the olfactory bulb, continue to add
new neurons throughout life. This continual addition
of neurons in these regions may allow for greater plas-
ticity in these specific brain circuits.
The enormous numbers of neurons and glia in the
brain are generated by progenitor cells of the neural
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