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female canaries, Nottebom also noticed that it was
larger in males, which learn complex songs, than in
female birds, which do not sing. Moreover, if adult
females were given testosterone injections, the HVC
nucleus grew by 90% and the female birds acquired
male song (Nottebom, 1985).
To determine whether new neurons were added to
the nucleus in response to the testosterone, female
birds were injected with 3H-thymidine as well as
testosterone, and the animals were sacrificed for analy-
sis five months later. The researchers found that in
both the testosterone-treated and control birds there
were many thymidine-labeled cells, and many of these
had morphological characteristics of neurons. They
also analyzed birds immediately after the injections
and found that the new neurons were not produced in
the HVC itself, but rather were generated in the ven-
tricular zone of the telencephalon and migrated to the
nucleus, analogous to the way in which the nucleus is
initially generated during embryogenesis. Subsequent
studies have shown that the newly produced neurons
migrate along radially arranged glial processes from
the ventricular zone to the HVC (Garcia-Verdugo et al.,
1998). Thus, ventricular zone neurogenesis is a nor-
mally occurring phenomenon in adult canaries (Figure
3.31). The progeny of the cells produced in the SVZ
migrate to the HVC soon after their generation. There
they differentiate into neurons, about half of which dif-
ferentiate into local interneurons and half into projec-
tion neurons, which send axons out of the nucleus to
connect with other neurons in the brain and form part
of the functional circuit for song learning.
Thus, there appears to be a seasonally regulated
turnover of neurons in the HVC and in other song
control nuclei in the brain of the adult songbird. The
turnover of neurons may correlate with periods of
plasticity in song learning. Canaries modify their
songs each year; each spring breeding season, they
incorporate new syllables into the basic pattern, and
then in the late summer and fall they sing much less
frequently. Combining thymidine injections with
measures of cell death and overall neuronal number in
the HVC over a year, one can see two distinct periods
of cell death, and each one is followed by a burst in the
number of new neurons in the nucleus. Both of these
periods of high neuronal turnover correlate with peaks
in the production of new syllables added to the song.
The neurogenesis is balanced by cell death, and during
periods of new song learning the nucleus adds cells,
while during periods when no song is generated, the
song-related nuclei undergo regression. Is the rate of
neurogenesis in the ventricular zone controlled by the
seasonal changes in testosterone in the male birds?
When the number of labeled cells in the ventricular
HP
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Cb
LPO
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OL
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FM
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FIGURE 3.31 Neurogenesis in the brains of adult birds and
rodents. A. Sagittal section of an adult canary brain showing the
widespread pattern of mitotically active cells, many of which will
go on to differentiate as neurons. B. Dorsal and sagittal views of an
adult mouse brain showing thymidine labeling of proliferating sub-
ventricular zone cells and the rostral migratory stream (Modified
from Smart, 1961; see also Jacobson, 1991).
zone is compared in testosterone-treated and
untreated female birds, there are no differences—indi-
cating that the rate of neurogenesis does not change in
response to the hormone. However, it appears instead
that the survival of the neurons in the HVC is season-
ally regulated—neurons generated in the spring have
a much shorter average lifespan than those generated
in the fall. Thus, the seasonal changes in neuron
number in the songbird HVC are not dependent on
changes in the number of newly added cells, but rather
relate to seasonally and hormonally regulated differ-
ences in the survival of the newly produced neurons.
Neurogenesis also occurs in the mature mammalian
brain. Although for many years this view was
regarded as somewhat heretical, it has become well
accepted in recent years. The thymidine birthdating
studies of Altman and Bayer, described at the begin-
ning of this chapter, thoroughly documented the time
and place of origin of neurons and glia of many regions
of the rodent brain. It was found that many brain
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