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The Journal of Neuroscience, February 1, 2003, 23(3):937
Nonrenewal of Neurons in the Cerebral Neocortex of Adult Macaque
Monkeys
Daisuke
Koketsu1,
Akichika
Mikami2,
Yusei
Miyamoto1, and
Tatsuhiro
Hisatsune1
1 Department of Integrated Biosciences, University of
Tokyo, Kashiwa 277-8562, Japan, and 2 Primate Research
Institute, Kyoto University, Inuyama 484-8506, Japan
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ABSTRACT |
The concept that, after developmental periods, neocortical neurons
become numerically stable and are normally nonrenewable has been
challenged by a report of continuous neurogenesis in the association
areas of the cerebral cortex in the adult Macaque monkey. Therefore, we
have reexamined this issue in two different Macaque species using the
thymidine analog bromodeoxyuridine (BrdU) as an indicator of DNA
replication during cell division. We found several BrdU+/NeuN+
(neuronal nuclei) double-labeled cells, but cortical neurons,
distinguished readily by their size and cytological and
immunohistochemical properties, were not BrdU positive. We examined in
detail the frontal cortex, where it is claimed that the largest daily
addition of neurons has been made, but did not see migratory streams or
any sign of addition of new neurons. Thus, we concluded that, in the
normal condition, cortical neurons of adult primates, similar to other
mammalian species, are neither supplemented nor renewable.
Key words:
adult neurogenesis; cerebral cortex; primates; DNA
synthesis; glial markers; 3D immunohistochemistry
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INTRODUCTION |
Most neurons of the mammalian brain
are generated from neuroepithelial cells near cerebral ventricles
during well defined developmental stages and then act as substrates of
neural circuitry throughout life (Rakic, 1985 ). However, recent
findings of adult neurogenesis in selected structures of the mammalian
brain present the possibility that this concept does not apply for all
brain regions, in particular, the neural circuits of the olfactory bulb and dentate gyrus of the hippocampus, which may be changing their basic
circuitry with the arrival of newly born neurons (Rochefort et al.,
2002; van Praag et al., 2002). In addition, the application of new
staining methods in the adult Macaque monkeys uncovered some new
neurons in the olfactory bulb (Kornack and Rakic, 2001b) and dentate
gyrus of the hippocampus (Gould et al., 1999a; Kornack and Rakic,
1999).
In terms of the cerebral neocortex, studies in various mammalian
species that range from mice and rats to ferrets and cats have
indicated that neurogenesis in this structure stops after well defined
developmental periods (Hicks and D'Amato, 1968; Caviness and Sidman,
1973; Luskin and Shatz, 1985; Jackson et al., 1989; Takahashi et al.,
1996). Furthermore, no sign of neuronal renewal has been found in the
cortex of adult rodents (Magavi et al., 2000). An extensive analysis of
Macaque monkeys exposed to [3H]thymidine
indicated that the neocortical neurons in primates are generated
prenatally (Rakic, 1974) and not normally renewed during adulthood
(Rakic, 1985, 2002a). It was suggested that a stable population of
neurons in the cortex might be an important mechanism for continuity of
learning and preserving memory over a lifetime (Rakic, 1985). Contrary
to this theory, Gould and colleagues assert, on the basis of labeling
with the thymidine analog bromodeoxyuridine (BrdU), that streams of new
neurons are generated continuously and added to the association
neocortex of adult Macaque monkeys (Gould et al., 1999b), where many
form connections and survive for weeks before dying (Gould et al.,
2001). The daily addition of thousands of new neurons to the principal
sulcus alone (Gould et al., 1999b) in the absence of net growth
indicated high turnover. To verify this claim, Kornack and Rakic
(2001a) have performed experiments using the BrdU method in adult
Macaque monkeys. They found BrdU-labeled non-neuronal cells, but they
did not detect newly born neurons in the neocortex, nor did they find
the stream of migrating neurons from the proliferative subventricular
zone to the principal sulcus of the prefrontal cortex (Kornack and Rakic, 2001a). This discrepancy in basic findings required a
reexamination in other laboratories (Nowakowski and Hayes, 2000).
Therefore, we addressed the question of whether new neurons are
generated and continuously added in the neocortex of juvenile and young
adult Macaque monkeys using BrdU methods for labeling DNA synthesis in
the dividing brain cells (Nowakowski et al., 1989) that were used by
previous investigators. We examined in detail the frontal cortex, where
it has been previously claimed that the largest daily addition
of neurons is made (Gould et al., 1999b). We selected juvenile and
young adult monkeys because they presumably have more neuronal
plasticity and to avoid the possibility of BrdU labeling during
unscheduled DNA synthesis in the process of cell death (apoptosis) in
aged animals (Yang et al., 2001). We also avoided high doses of BrdU to
avoid its possible mutagenic and stimulation effect on DNA synthesis
(Nowakowski and Hayes, 2001; Rakic, 2002b). To identify the phenotypes
of BrdU-labeled cells in the neocortex, an immunohistochemical analysis
was performed with neuronal markers NeuN, a transcriptional factor that
is expressed in the nucleus and cytoplasm of neurons (Mullen et al.,
1992), and doublecortin (DCX) (Gleeson et al., 1999; Nacher et al.,
2001) and also with markers of other non-neuronal cell types. GFAP and S-100 (astroglial cell), O4 (oligodendrocyte), and Iba-1 (microglial cell) (Ito et al., 1998) were used.
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Materials and Methods |
Animals. Two young adult female Macaca
fascicularis (Cynomolgus monkey; 5 years of age) and two juvenile
female M. fuscata (Japanese monkey; 2 years of
age) were used. Monkeys were housed at the Primate Research Institute
of Kyoto University and were maintained in individual cages with up to
16 cages facing each other in a single room. Thus, the monkeys could
see and interact with other monkeys. In addition, Japanese monkeys were
provided with a piece of wood that was hung in the cage as a playing
tool and a food pickup toy that was placed in front of the cage every day for several hours. The size of the toy was 60 × 60 cm with 480 holes, half of which were covered with a piece of plate so that the
monkey needed to use its fingers to carefully pick up the food
(Saguinus, Monmouth, IL). The food used was raisins, and the monkeys
usually picked up most of the raisins within several hours. The monkeys
were cared for according to the Guide for the Care and Use of
Laboratory Animals from the National Institutes of Health and the
Guide for Care and Use of Laboratory Primates published by
the Primate Research Institute, Kyoto University.
Injections of BrdU. Monkeys were injected with BrdU (Wako,
Osaka, Japan) dissolved in 0.9% NaCl with 0.007 M NaOH. Two M. fascicularis were
injected intravenously, whereas the M. fuscata received
intraperitoneal injections. The first M. fascicularis (M. fascicularis I) received once-daily BrdU injections for
3 consecutive days and one more injection after another 7 d with a
dose of 100 mg/kg body weight per injection; it was perfused 10 d
after the initial BrdU injection, i.e., 1 day after the final BrdU
injection (Fig. 1). The second M. fascicularis (M. fascicularis II) received five
once-daily BrdU injections every other day with a dose of 75 mg/kg per
injection; then, it was perfused 22 d after the initial BrdU
injection, i.e., 14 d after the final BrdU injection. The first
M. fuscata (M. fuscata I) received once-daily
BrdU injections for 5 consecutive days with a dose of 75 mg/kg per
injection; it was perfused 29 d after the initial BrdU injection,
i.e., 25 d after the final BrdU injection. The second M. fuscata (M. fuscata II) received the same series of
BrdU injections as M. fuscata I, but it was perfused 30 d after the initial BrdU injection, i.e., 26 d after the final
BrdU injection. All animals were killed by intracardiac
perfusion with 4% paraformaldehyde. All removed brains were postfixed
with fresh 4% paraformaldehyde for 2 d at 4°C. This combination
of injections and perfusion schedule allowed the exposure of
newly generated cells as well as the ability to follow their migration
and differentiation to the cortex as done on the hippocampus (Markakis
and Gage, 1999).
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Figure 1.
BrdU injection schedule. The day of BrdU injection
is represented by the filled circle, and the day of
perfusion is represented by the arrowhead. M.
fascicularis I received once-daily BrdU injections for 3 consecutive days and one more injection after 7 d and then was
perfused 1 d after the final injection. M.
fascicularis II received five once-daily injections every other
day and then was perfused 14 d after the final injection.
M. fuscata I and M. fuscata II received
once-daily injections for 5 consecutive days. Then, M.
fuscata I and M. fuscata II were perfused 25 or
26 d after the final injection, respectively. The
number of days represents the period between the first
injection and perfusion.
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Preparation of tissue sections. Postfixed brains were
dissected into right and left hemispheres. Each hemisphere was cut into blocks of 5 mm. The brain blocks were put into O.C.T. Compound (Sakura,
Tokyo, Japan), and frozen at  80°C. The frozen blocks were sliced
into 40 µm coronal sections with a cryostat (MICROM, Walldorf,
Germany). The sliced sections were preserved in a cryoprotectant solution (30% ethylene glycol, 30% glycerol in 0.05 M phosphate buffer) at 20°C until they were processed.
Immunohistochemistry. Sections were washed twice with
Tris-buffered saline (TBS) for 10 min. Sections were then treated in 10 M citric acid buffer at 90°C for 5 min. The
heated sections were left at room temperature for 30 min and washed
with TBS for 10 min. The sections were incubated in 1 M HCl at 37°C for 30 min and neutralized by
rinsing with 0.1 M borate buffer and washed twice
with TBS. The sections were then blocked with a blocking solution (5%
normal donkey serum and 0.3% Triton X-100 in TBS) for 30 min at
room temperature with gentle shaking. The blocked sections were reacted
with primary antibodies at 4°C. The concentration of Triton X-100 in
the blocking solution dissolving primary antibodies with 1 or 3 d
reaction time was 0.3 or 0.1%, respectively. Monoclonal rat anti-BrdU
antibody (Harlan, Leicestershine, UK) with a 1:200 dilution and
monoclonal mouse anti-BrdU (Becton Dickinson, Franklin Lake, NJ) at
1:33 were used. Some antibodies to identify cell types were also used.
As antibodies to the neuronal marker, monoclonal mouse anti-NeuN
antibody (1:1000; Chemicon, Temecula, CA) and polyclonal goat anti-DCX
antibody (1:100; Santa Cruz Biotechnology, Santa Cruz, CA) were used.
The reaction time of NeuN was 1 d and that of DCX was 3 d. As
antibodies to the astroglial marker, polyclonal rabbit anti-GFAP
antibody (1:10,000; Dako, Carpinteria, CA) and polyclonal rabbit
anti-S-100 antibody (1:5000; Swant, Bellinzona, Switzerland) were
used. As the antibody to oligodendrocyte marker, monoclonal mouse IgM
anti-O4 antibody (1:10; Chemicon) was used. Then, as the antibody to
microglial marker, polyclonal rabbit IgG anti-Iba-1 antibody (1:250;
kind gift from S. Kohsaka, National Institute of Neuroscience, Tokyo,
Japan) was used. The reaction time of GFAP, O4, and Iba-1 was 1 d
and that of S-100 and O4 was 3 d. After immunoreaction with
these primary antibodies, sections were washed with TBS. The sections
were then reacted with fluorochromo-conjugated secondary antibodies. As
secondary antibodies, Alexa 488 (1:1000; Molecular Probes, Eugene, OR)
and rhodamine Red-X (1:200; Jackson ImmunoResearch, West Grove, PA)
were used. Similarly as with the primary antibody, the sections were
put into secondary antibodies dissolved in the blocking solution and
incubated for 2 hr at room temperature with gentle shaking. The
sections were then washed with TBS. The stained sections were mounted
on glass slides and incubated for 15 min in DAPI (Sigma, St. Louis, MO)
dissolved in 0.1% Triton X-100 in TBS and then coverslipped by using
Immu-Mount (Shandon, Pittsburgh, PA) with 2%
1,4-diazabiccydo-2,2,2-octane (Sigma).
Confocal imaging and data analysis. Stained sections
were observed using confocal laser scanning microscopy (TCS SP2;
Leica, Wetzlar, Germany). Obtained image data were processed using
image processing software (LCS; Leica) and reconstructed to
three-dimensional (3D) images. With this software, a cross section of
the scanning area can be confirmed, and the acquired image data can be
analyzed in greater detail.
Quantification of BrdU-labeled cells. Three coronal sections
of the anterior, middle, and posterior parts of the principal sulcus
and central sulcus were prepared. Then, BrdU-labeled cells were counted
in both banks of the principal sulcus and central sulcus by using a
fluorescent microscopy (Olympus BX50, Tokyo, Japan). Furthermore, in
the ventral part of area 14 (rectus gyrus) on the section of the
principal sulcus, BrdU-labeled cells were also counted.
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Results |
As reported by previous investigators, we observed a number of
BrdU-labeled cells in the cerebral cortex of each monkey (Table 1). The region of rectus gyrus (area 14)
in juvenile M. fuscata II had the highest frequency of BrdU+
cells (88.7 cells per cubic millimeter) among all the cortical
regions. The principal sulcus of the frontal association cortex had a
higher frequency of BrdU-labeled cells than the motor cortex and the
somatosensory cortex. BrdU-labeled cells were detected in the white
matter as well as the cortex. Among the four monkeys, M. fascicularis I had the highest frequency of BrdU-labeled cells in
the white matter. However, we did not detect any stream of BrdU-labeled
cells within the white matter, which might indicate cell migration from
the subventricular zone (SVZ) to the cerebral cortex. In the
SVZ, all four monkeys had BrdU-labeled cells, but the largest number of
BrdU-labeled cells appeared to be in the SVZ of M. fascicularis I, which had a single injection of BrdU 24 hr before
it was killed (data not shown). In the cerebral cortex, some of the
BrdU-labeled cells were clearly glial and endothelial cells. To
determine the phenotypes of these BrdU-labeled cells, we used glial
cell markers. As a result, few BrdU+ cells (<1%) were colabeled with
S-100, the astroglial marker, and then few BrdU+ cells (< 1%)
expressed Iba-1, the microglial marker (Fig.
2A-J).
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Figure 2.
3D images of BrdU+ cells double-labeled with glial
markers in the prefrontal cortex of Macaque monkeys.
A-E, A cell double-labeled with BrdU and S-100
(astroglial marker indicated by arrowhead).
A-C, Images projected from a stack of 42 optical slices
at a 0.5 µm interval. A, BrdU (red).
B, S-100 (green).
C, Overlay. D, The
y-z cross section of the red
line of C. E, The
x-z cross section of the blue
line of C. F-J, A cell double-labeled with BrdU
and Iba-1 (microglial marker indicated by arrowhead).
F-H, Images projected from a stack of 53 optical slices
at a 0.5 µm interval. F, BrdU (red).
G, Iba-1 (green).
H, Overlay. I, The
y-z cross section of the red
line of H. J, The
x-z cross section of the blue
line of H. Scale bars, 8 µm.
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To investigate the possibility that BrdU+ cells are colabeled with the
neuronal marker, sections were double-stained with BrdU and NeuN. Some
BrdU-labeled nuclei appeared to be colabeled with NeuN and, on initial
inspection of our material, could be indicative of newly generated
neurons (Fig. 3A). In such
cases, we performed a detailed confocal z-series analysis, which
revealed that most of these BrdU-labeled nuclei actually belonged to
cells that were closely apposed to neurons but were themselves
immunonegative for NeuN (Fig. 3B-I). This part of
our findings is very similar to the results reported by Kornack and
Rakic (2001a). Those flat BrdU+ cells, which are usually sticking to
the cell body of neurons, are anatomically known to be satellite glial
cells. In the prefrontal cortex of a young adult M. fascicularis II or juvenile M. fuscata II, 37.4 or
35.7%, respectively, of all BrdU+ cells were satellite glial cells,
which were not stained with glial markers such as GFAP, S-100, O4,
and Iba-1.
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Figure 3.
3D images of a BrdU-labeled satellite glial cell.
A, An image projected from a stack of 36 optical slices
at a 0.5 µm interval. A cell double-labeled with BrdU
(red) and NeuN (green) appears to
be found (arrowhead). Scale bar, 8 µm.
B, C, In fact, in the cross sections, the
cell is not double-labeled. A BrdU-labeled cell is closely apposed to
the soma of a NeuN-labeled neuron. B, The
y-z cross section of A.
C, The x-z cross section
of A. D-I, Six different optical slices
at a 1.5 µm interval of A also indicate that these are
two separate cells.
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To characterize and quantify the remaining population of BrdU+ cells,
we performed an extensive double-staining immunohistochemical analysis
of the prefrontal cortex as well as other areas of the cerebral
neocortex. We found only 18 BrdU+/NeuN+ cells in the >500 cortical
slices from four monkeys. Because each slice possesses between 300 and
1400 BrdU+ cells, this incidence of BrdU+/NeuN+ cells is between 18 in
150,000 and 18 in 700,000 (0.0026-0.012%). However, because each
animal received four or five BrdU injections, the number labeled per
day has to be considerably smaller. Nine of 18 BrdU+/NeuN+ cells were
localized in the principal sulcus, and five were observed in the
ventral part of area 14. However, the
localization of NeuN protein was limited to the cell nucleus (Figs. 4
A-E,
5A-J), which was not
the case with the surrounding neurons. Furthermore, the oval,
triangular, or spindle-shaped nuclei of these cells were significantly
smaller than of any cortical neurons in the same tissue. Because NeuN
may not be a definitive neuronal marker (Wolf et al., 1997; Teo et al.,
1999), we also performed anti-DCX staining, which labels perinuclear
cytoplasm of migrating young neurons (Gleeson et al., 1999; Nacher et
al., 2001). With this method, we observed two BrdU+ cells that were weakly costained with this marker in >100 slices examined (Fig. 4F-J). This is an incidence of <2 in 30,000 which is a lower rate of double-positive cells than we detected with
NeuN and BrdU. In addition, those BrdU+/DCX+ cells were substantially
less intensely stained with DCX antibody than the migrating cells
observed in the olfactory bulb of the very same animals (see below). At
any rate, DCX antibody was reported to stain nondividing cells in the
brain regions where there are no new neurons, such as the corpus
callosum (Nacher et al., 2001), and thus those faintly stained cells
cannot be taken as evidence for migrating neurons in the cortex.
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Figure 4.
3D images of BrdU+ cells in the prefrontal cortex
of a juvenile Macaque monkey. A-E, A cell
double-labeled with BrdU and NeuN (arrowhead). The
localization of NeuN protein in the double-labeled cell is limited to
the cell nucleus. A, BrdU (red).
B, NeuN (green). C,
Overlay. D, The y-z
section view of C. E, The
x-z section view of C.
F-J, A cell double-labeled with BrdU and DCX
(arrowhead). The DCX staining of the double-labeled cell
is weak. F, BrdU (red). G,
DCX (green). H, Overlay.
I, The y-z section view
of H. J, The
x-z section view of H.
Scale bars, 8 µm.
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Figure 5.
3D images of cells double-labeled with BrdU and
NeuN in the prefrontal cortex of a young adult Macaque monkey.
A-E, A cell double-labeled with BrdU and NeuN
(arrowhead) in the prefrontal cortex of M.
fascicularis I. The localization of NeuN protein in the
double-labeled cell is limited to the nucleus. A, BrdU
(red). B, NeuN
(green). C, Overlay.
D, The y-z section view
of C. E, The
x-z section view of C.
F-J, A cell double-labeled with BrdU and NeuN
(arrowhead) in the prefrontal cortex of M.
fascicularis II. As in M. fascicularis I, the
distribution of NeuN protein is localized in the nucleus.
F, BrdU (red). G, NeuN
(green). H, Overlay.
I, The y-z section view
of H. J, The
x-z section view of H.
Scale bars, 8 µm.
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To check the validity and reliability of our methods, we examined the
presence and nature of BrdU-labeled cells in the olfactory bulb where
new neurons have been reported to exist in monkeys (Kornack and Rakic,
2001b) by using juvenile M. fuscata. Indeed, in the
olfactory bulb we observed BrdU+/NeuN+ cells situated around the
glomerular layer of the olfactory bulb (Fig.
6A-E). Judging from
their size and shape, these cells seemed to be interneurons. We usually
observed several BrdU+/NeuN+ cells per sagittal section of the
olfactory bulb, clearly demonstrating the presence of a small but
consistent number of newborn neurons in the olfactory bulb in this
species. This incidence is therefore at least 100 times greater than
the incidence of BrdU+/NeuN+ neurons in the prefrontal cortex. This
finding stands in contrast to the lack of such cells in the cerebral
neocortex of the very same specimens stained with the same method.
Importantly, we observed a number of BrdU+/DCX+ double-stained cells
located just away from the subventricular zone of the olfactory bulb
(Fig. 6F-J). The DCX staining of these cells
was strong and located over the cytoplasm, equivalent to the staining
of the neighboring neurons. Together, our comparative analysis reveals
a small but steady cell turnover of interneurons in the olfactory bulb
and the absence of such turnover in the cortical areas examined in the
same specimens. This indicates that the methods that we have used are
suitably sensitive to detect neuron production when it is present.
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Figure 6.
3D images of BrdU+ cells in the olfactory bulb of
a juvenile Macaque monkey. A-E, Two cells
double-labeled with BrdU and NeuN (arrowheads) are
located around the glomerular layer. A, BrdU
(red). B, NeuN
(green). C, Overlay.
D, The y-z section view
of C. E, The
x-z section view of C.
F-J, A cell double-labeled with BrdU and DCX
(arrowhead) is located on the rostral migratory stream
as it enters the olfactory bulb. F, BrdU
(red). G, DCX
(green). H, Overlay.
I, The y-z section view
of H. J, The
x-z section view of H.
Scale bars, 8 µm.
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Discussion |
The present study was conducted in two different species of
Macaque monkeys that have been maintained in an environment enriched with toys and social interactions. We applied the most advanced methods
of labeling new cells and their phenotypes. Our comprehensive and
detailed analysis shows that although there is an easily detectable amount of cell proliferation that leads to BrdU-labeled cells in the
cerebral cortex, we could not unequivocally identify a single, newly
produced neuron. Thus, our results support the concept that neuronal
populations of the cerebral neocortex in healthy, sexually mature
primates become stable and are normally nonrenewable (Rakic, 1985;
Kornack and Rakic, 2001a). Most of the BrdU-labeled cells in the
neocortex were non-neuronal, and few (<2%) of them were stained with
glial markers such as S-100 and Iba-1. Satellite glial cells
(35-38%), which generally are closely adhered to the surface of
neocortical neurons, turned out not to be stained with any of the glial
markers that we used (GFAP, S-100, O4, and Iba-1). These satellite
glial cells have been observed previously in the substantia nigra of
adult rat (Lie et al., 2002). They also did not determine the phenotype
of these satellite glial cells. In the end, we could not determine the
phenotype of the rest (~60%) of the BrdU-labeled cells.
Of a total of several hundred thousand BrdU-labeled cells observed,
only 18 NeuN and 2 DCX double-labeled cells could be found. Importantly, the size, shape, and staining properties of these cells
differed from the neurons in the adjacent neocortex, and the double
labeling of these cells was qualitatively different from that which we
observed in the olfactory bulb. Furthermore, DCX protein can be
expressed in differentiating neurons and does not necessarily indicate
that the immunostained neuron is new or migrating (Nacher et al.,
2001). This casts doubt on the identity of any of these cells as newly
produced neurons. Thus, our extensive examination of the tissue from
juvenile and young adult Macaque monkeys did not reveal evidence of the
turnover of neurons in the prefrontal or any other area of the cerebral neocortex.
What cell type are the small BrdU+/NeuN+ cells that are occasionally
encountered within the primate cerebral cortex? One possibility is that
some of them are degenerating neurons that increase their DNA synthesis
in response to damage (Sanes and Okun, 1972; Klein et al., 2002) or as
a component of naturally occurring programmed cell death (Yang et al.,
2001). Second, the BrdU label might come from the blood-derived stem
cells fused with neurons (Steindler and Pincus, 2002; Terada et al.,
2002; Wurmser and Gage, 2002). Third, some cells could be intrinsic
multipotent progenitors that express this protein but are normally
prevented from differentiating into mature cortical neurons in the
cortical environment (Kukekov et al., 1999; Gage, 2000; Laywell
et al., 2000). Finally, there is the possibility that some of them are
glial progenitors that potentially give rise to NeuN+ tumors (Wolf et
al., 1997; Teo et al., 1999). Importantly, even if they are neurons,
the incidence would be very small (perhaps, one to two neurons per day
for the large portion of the prefrontal cortex that we sampled) and is consistent with the early report that the production of new neurons in
the primate brain and in particular the neocortex is extremely "limited" compared with the high rate of neuronal renewal observed in brains of fish, amphibians, and some birds (Rakic, 1985).
It should be emphasized that neocortical neurons in other mammalian
species, including rodents, are also generated during a restricted
developmental period (for review, see Rakic, 2002a). Except for the
report on granule cells in the hippocampus (Eriksson et al., 1998),
there are no signs of natural neuronal turnover in the human forebrain
(Seress et al., 2001). However, it was reported recently that in the
neocortex of adult rodents neurogenesis could be induced under a
specific neurodegenerative condition, in particular after a photolytic
deletion of projection neurons with the use of the retrograde transport
of a dye and laser illumination (Magavi et al., 2000). A small number
of potential neuronal precursors may exist in the primate cerebral
cortex, but they do not normally differentiate into cortical neurons
attributable to local conditions (Kirschenbaum et al., 1994;
Pincus et al., 1998; Gage, 2000; Laywell et al., 2000; Steindler and
Pincus, 2002). Such cells theoretically could be induced to
differentiate into neurons by way of support of the conducive
neurogenetic microenvironment (Leavitt et al., 1999; Laywell et al.,
2000; Song et al., 2002). Therefore, if a technique and growth
factor(s) capable of inducing appropriate neuronal differentiation are
developed, neuronal replacement therapies for neurodegenerative disease
may become possible through manipulation of endogenous neural
precursors even in areas such as the primate cerebral neocortex, where
neuronal replacement does not normally occur.
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FOOTNOTES |
Received Sept. 13, 2002; revised Nov. 4, 2002; accepted Nov. 20, 2002.
This study is supported by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science, Sports and Culture of Japan. We
thank Dr. Jose Antonio Campos-Ortega and Dr. Yukio Hirata for
informative discussion and technical advice. We also thank Dr. Shinichi
Kohsaka for kindly providing an anti-Iba-1 antibody.
Correspondence should be addressed to Tatsuhiro Hisatsune, Department
of Integrated Biosciences, Graduate School of Frontier Sciences, The
University of Tokyo, Bioscience Building 402, 5-1-5 Kashiwanoha,
Kashiwa, Chiba 277-8562, Japan. E-mail:
hisatsune{at}k.u-tokyo.ac.jp.
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ABSTRACT
INTRODUCTION
