CNRS: UMR 6558, Département des Neurosciences, Université de Poitiers, Poitiers, France
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Abstract |
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Introduction |
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The notion that cortical cells remain uncommitted for a particular areal fate for a relatively long period in embryogenesis has received support from transplantations between different regions of the cerebral cortex (O'Leary and Stanfield, 1989). In the past decade, however, a number of studies have indicated that late embryonic cortical cells grafted heterotopically into various cortical sites of newborn hosts retain certain hodological characteristics corresponding to their embryonic locus of origin (Barbe and Levitt, 1992
, 1995
; Ebrahimi-Gaillard et al., 1994
; Garnier et al., 1995
; Ebrahimi-Gaillard and Roger, 1996
; Létang et al., 1997
; Frappé et al., 1999
). No comprehensive study has yet been undertaken on the development of the connectivity between cortical grafts and specific nuclei of the host thalamus.
The main goals of the present study were thus to provide quantitative data on the thalamic connectivity developed by grafts of embryonic parietal or occipital cortex placed into the parietal or occipital cortex of newborn rats and to examine the cytoarchitectonic organization developed by the grafts.
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Materials and Methods |
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The results of the present study are based on the use of 50 Wistar rats of both sexes supplied by R. Janvier (Le Genest-St Isles, France). All experiments were performed in accordance with NIH guidelines for animal care and use. Neocortical grafts were obtained from rat fetuses removed surgically at 16 days of embryonic (E) age (the day following insemination is designated E0) from dams anesthetized with chloral hydrate (300 mg/kg, i.p.). The fetuses were placed in sterile 0.6% glucose0.9% saline medium. The membranous skull was opened and the meninges were stripped from the cerebral cortex. Small fragments (11.5 mm2) of presumptive parietal and occipital cortex were then dissected out of the left and right hemispheres of each donor brain and transferred into the glucosesaline medium. The cortical fragments were labeled following a 1 h incubation in glucosesaline medium containing a 0.1% proteingold complex consisting of gold particles (10 nm) conjugated to wheatgerm agglutininhorseradish peroxidase (see Ebrahimi-Gaillard et al., 1994). Finally, the blocks were rinsed in glucosesaline medium and then transplanted. The donor dams were killed immediately afterwards by anesthetic overdose.
The fragments of embryonic cortex were engrafted into the left cerebral hemisphere of newborn (P0P1) rats anesthetized by ether. The recipient animals were divided into three groups (Table 1). They either received into the parietal cortex a block of embryonic parietal (parietal- to-parietal group; n = 19) or occipital (occipital-to-parietal group; n = 20) cortex or received into the occipital cortex a block of embryonic parietal cortex (parietal-to-occipital group; n = 11). The recipient animals sustained an aspirative unilateral lesion of part of the left presumptive parietal or occipital cortex immediately prior to grafting. The blocks of embryonic tissue were aspirated into a glass cannula and then slowly injected into the host lesion cavity. Care was taken to maintain the normal dorso-ventral orientation.
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Two to four months after grafting, the host animals were anesthetized with chloral hydrate (300 mg/kg, i.p.) and one of the following two sensitive neurotracers was injected into the left thalamic ventrobasal complex (VB) or dorsal lateral geniculate nucleus (DLG) (Table 1). However, tracers could not be injected into the DLG in cases with parietal-to-occipital transplants because of the massive atrophy that systematically affected this nucleus following the occipital lesion and transplantation. Biotinylated dextran-amine (BDA), a primarily antero- grade neurotracer, was selected to examine the distribution of the thalamic fibers within the transplants. The B subunit of the cholera toxin (CTB), a primarily retrograde neurotracer, was chosen to analyze the distribution of the transplant neurons projecting to the thalamus. The tracers were delivered iontophoretically through glass micropipettes (internal tip diameter: 810 µm). The animals injected with BDA (10%; Molecular Probes, Leiden, The Netherlands) were left for 5 days, then were deeply anesthetized and perfused transcardially with a prerinse of 200 ml of physiological saline followed by 750 ml of 4% paraformaldehyde in 0.1 M phosphate buffer (PB; pH 7.4). The animals injected with CTB (1%; List Biological Lab.) were left for 4 days and then were perfused with a fixative solution containing 0.01 M sodium m-periodate, 0.075 M DL-lysine and 3% paraformaldehyde in 0.01 M PB. The brains were im- mediately removed, flattened between two slides and postfixed overnight in the same fixative at 4°C. The brains were then stored in a mixture of 20% glycerol and 2% dimethyl sulfoxide in distilled water for 24 h and 40 µm sections were then cut horizontally on a freezing microtome. The sections were collected in four parallel series. One series (one out of six sections) was reacted for silver intensification of gold particles, to help subsequent identification of the transplant localization, and coverslipped unstained. A second series (one out of four sections) was processed for cytochrome oxidase (CO) activity according to the method of Wong-Riley (Wong-Riley, 1979
). The remaining two series were processed for the immunohistochemical detection of the tracers (Gaillard et al., 1997
). The sections treated for BDA were rinsed in Tris-buffered saline (TBS; 0.05 M, pH 7.6) containing 1% Triton X-100. The sections were incubated for 30 min with a solution of 200 µl H2O2 (30%) in 100 ml of 100% methanol to eliminate endogenous peroxidase activity and then with TBS containing 10% fetal bovine serum (Life Technologies) to block nonspecific binding sites. The sections were subsequently reacted for 2 h with avidin and biotinylated horseradish peroxidase (HRP; Vectastain, ABC Elite Kit), then rinsed in TBS and treated for 15 min with a solution containing 0.05% 3,3-diaminobenzidine (DAB; Sigma), 0.01% H2O2 and 1.5% nickel ammonium sulfate in 0.1 M acetate buffer (pH 6). One series of sections was counterstained with cresyl violet and the other was coverslipped unstained. The sections reacted for CTB were first rinsed and then incubated with rabbit normal serum as above. Afterwards, the sections were incubated firstly in a goat anti-choleragenoid (1/5000; List Biological Labs) for 24 h (4°C) and secondly with biotinylated rabbit anti-goat (1/200; Dako) for 2 h. The final part of the processing was identical to that of the BDA.
The distribution of the labeling through the transplant was plotted on magnified drawings of the sections with the aid of a camera lucida. The quantification of the labeling was performed on five equally spaced (80 µm) sections through the transplant. Though BDA and CTB are most frequently used as unidirectional tracers, both anterograde and retro- grade labeling images systematically occurred following use of either tracer. Nevertheless, anterograde quantification was only assessed in those cases that had received BDA whereas counts of labeled cells were only carried out in cases injected with CTB. Each labeled cell or fiber was counted within the grafts, using a high-magnification objective (x20), where labeling density was highest. For each transplant group, cell- or fiber-labeling indexes were obtained by calculating the mean (± SEM) total number of labeled cells or fibers in each group.
Statistical analyses were performed using the Statview program. Differences in cell and/or fiber labeling indexes were assessed by analysis of variance followed by the post-hoc Scheffe F-test. Null hypotheses were rejected at the alpha level of 0.05.
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Results |
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Parietally Placed Grafts
Parietal-to-parietal Grafts
. Following tracer injection into the VB, numerous anterogradely or retrogradely labeled elements were found within the transplants. However, the overall labeling density was consistently weaker than that seen in the adjacent parietal cortex of the host. Representative patterns of transplant labeling are illustrated in Figures 2 and 3. The distribution of the labeled cells or fibers did not conform to that of the host cortex. Indeed, the typical radial orientation of the labeled cells and the specific laminar distribution of the labeling were not maintained in the transplants. The labeling was distributed relatively dif- fusely throughout (Fig. 3B
) or in restricted parts (Fig. 3C
) of the grafts. In only a few cases were loose patches of labeled fibers found in the grafts or, occasionally, labeled neurons were found in continuity with those labeled in the host adjacent cortex. In parietal-to-parietal transplants, the mean (± SEM) fiber and cell labeling indexes were respectively 472.12 (± 47.48) and 331.6 (± 65.36) (Fig. 4A,B
).
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Occipital-to-parietal Grafts
. In this group, tracer injections into the VB returned only low labeling indexes within the grafts. The fiber and cell labeling indexes were, respectively, 74.16 (± 11.22) and 28.0 (± 12.78) (Fig. 4A,B). These values were significantly lower than those found in the parietal-to-parietal group (P < 0.01). Typical illustrations of the labeling are provided in Figures 5 and 6
. Although dense cell labeling was present within the host parietal cortex even in the immediate vicinity of the graft (Fig. 5A
), only few cells were labeled in the graft itself and most of these cells were located in the periphery of the graft. In the same way, though numerous thalamic axons were distributed within the host cortex adjacent to the graft, only few of them crossed the hostgraft border (Figs 5A and 6A,B
). The few axons which went across the border remained confined to the peri- phery of the graft (Fig. 6A,B
). Only exceptionally did we find labeled fibers in the core of the graft.
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Parietal-to-occipital Grafts
. In this group, tracers injections into the host VB returned significant labeling within the graft. The fiber and cell labeling indexes were respectively 214.33 (± 29.27) and 109.4 (± 22.06) (Fig. 4A,B). These fiber and cell labeling indexes were significantly lower than those obtained in parietal-to-parietal grafts following similar VB injections (P < 0.01 and P < 0.05, respectively). Although the labeling indexes were higher than those of the occipital-to-parietal grafts, the difference did not reach statistical significance. Figures 9 and 10
provide representative illustrations of the graft labeling within the host occipital cortex, which was, as anticipated, completely free of labeling. A sharp demarcation of the labeling was found at the grafthost interface, particularly evident in those cases injected with BDA (Fig. 9B
). Within the grafts, diffuse patterns of labeling were found.
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Parietal-to-parietal Grafts
In Nissl-stained sections, the normal barrel cytoarchitectonic organization was present within the host parietal cortex at the layer IV level (Fig. 11A). Inspection of the CO-stained sections provided more conclusive evidence of the presence of individual barrels within the host (Fig. 11B
). High CO activity was found inside the barrels whereas practically no staining of the septa occurred between barrels (Waters et al., 1995
). In the grafts, no evidence of barrel organization was found in Nissl-stained sections (Fig. 11A
). In some grafts, however, a few clusters of cells were identified but their disposition and size did not conform to that of typical SI barrels (Woolsey and Van der Loos, 1970
). Indeed, these cell clusters were systematically randomly distributed and the diameters of the clusters were extremely variable, often greatly exceeding the 400 µm admitted value for barrels in the posteromedial barrel subfield of the rat (Welker and Woolsey, 1974
).
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Occipital-to-parietal Grafts
Inspection of Nissl-stained sections throughout the graft failed to provide any evidence of barrel organization. In all cases considered the cell density was extremely variable (Fig. 12A). A few cell clusters of variable sizes were also identified in some of these grafts. As mentioned above for parietal-to-parietal grafts, these rare clusters were substantially different from typical barrels, in terms of both distribution and size. More- over, CO staining systematically returned uniform, low levels of enzymatic activity (Fig. 12B
). Despite thorough examination of CO-stained sections, no patterned organization could be found within the grafts even in those cases in which cell clusters were found in adjacent Nissl-stained sections.
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Discussion |
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In this study we have employed two different tracers, CTB and BDA, to examine the connectivity established between the transplant and host thalamus. CTB is mostly used as a retrograde tracer whereas BDA is mostly used as an anterograde one. Both tracers, however, possess bi-directional transport properties and they provided congruent results. In addition, (i) the accuracy of the injection sites was assessed following careful examination of the labeling distribution within the intact host cortex, (ii) the transplants were systematically labeled by a proteingold com- plex in order to precisely assess their limits and (iii) the results were derived from a quantitative examination of the patterns of labeling within the grafts.
Parietal-to-parietal Grafts
This study for the first time shows that transplants of E16 parietal cortex placed in the parietal cortex of newborn recipients develop a substantial set of reciprocal connections with the ventrobasal complex of the host thalamus. These findings are consistent with those of previous reports establishing the exist- ence of thalamic afferents or efferents of grafts of embryonic frontal or occipital cortex placed in a homotopic position in the cortex of newborns (Floeter and Jones, 1985; Chang et al., 1986
; Castro et al., 1989
; Sørensen et al., 1992
; Ebrahimi-Gaillard et al., 1994
; Létang et al., 1997
, 1998
; Frappé et al., 1999
). The organization of the thalamic connections of the graft differed from that of the normal cortex in two major aspects. The density of the connections between the host VB and the graft was weaker than that of the normal connections between the VB and intact parietal cortex, an observation which is in agreement with the findings of most studies dealing with the connectivity of neocortical transplants in newborn hosts (Roger and Ebrahimi- Gaillard, 1994). Most importantly, we found that the distribution of the thalamic axons within the grafts fails to display the specific arrangement seen within the normal parietal cortex (Lorente de No, 1922
; Jensen and Killackey, 1987
; Agmon et al., 1993
; Welker et al., 1996
). Following restricted BDA injection into the VB, labeled axons were restricted to a few adjacent barrels in the intact parietal cortex (Cases et al., 1996
; Welker et al., 1996). No evidence of radial or laminar organization was noted in the grafts and only occasionally did we find clusters of labeling. Instead, a mostly diffuse terminal distribution was found. The absence of any specific arrangement of thalamic afferents within the graft probably relies upon the conjunction of several factors. One of these concerns the mechanical perturbation of the embryonic tissue, inherent to the grafting procedure, that is likely to interfere with cell migration. At E16, indeed, those cells that are postmitotic within the somatosens- ory area are still in their migratory process (Bayer and Altman, 1991
) (these investigators consider the day of insemination as E1; the embryonic ages as used by these investigators have been adapted to match our definition). This is probably one of the reasons why most studies dealing with cortical transplantation fail to report preservation of stratification within the grafts (Roger and Ebrahimi-Gaillard, 1994
). Another factor involves the heterochronic status of the host cortex and grafted tissue. By the time of transplantation (P0P1), host thalamic axons have reached the deep part of the cortex, a region corresponding at this stage to layers VI and V (Erzurumlu and Jhaveri, 1990
; Catalano et al., 1991
; Agmon et al., 1993
), and some of them have even reached the location of their target neurons in layer IV (Jensen and Killackey, 1987
; Agmon et al., 1993
; Killackey et al., 1995
; Catalano et al., 1996
). At 16 days of embryonic age, most layer VI and V cells have been generated, but it is not until E18 that most layer IV neurons are generated (Bayer and Altman, 1991
). Further, in vitro studies have established that thalamic axons are not able to invade developing cortex significantly until late in embryogenesis (Molnar and Blakemore, 1991
; Götz et al., 1992
). Therefore, by the time of transplantation, the grafted cor- tex is not yet growth permissive and, several days later, thalamic axons might lose some of their capacity to interact with putative targets cells being generated within the graft. Ultimately, even though VB axons are able to invade the graft, the high degree of topographic organization and the clustered distribution that characterize the thalamic projection to the parietal cortex are not maintained. In our study, all cases incurred a deep aspirative lesion of the host parietal cortex prior to the transplantation, damaging part or all of the subplate. A number of studies have proposed one role of the subplate to be the guidance of thalamic axons towards their appropriate cortical target areas (Götz et al., 1992
; Allendoerfer and Shatz, 1994
). As mentioned above, by the time of birth some thalamocortical axons had already invaded the lower cortical layers VIV and lower parts of the cortical plate, and were thus axotomized. Other thalamocortical axons were still in the white matter (Agmon et al., 1993
) and were not affected by the lesion. It was not possible to determine in our experimental conditions whether intact or axotomized thalamic afferents behave differently when confronted with the graft cells. Importantly, however, our findings indicate that, despite partial or complete damage to the subplate, thalamocortical axons retain some capacity to invade developing cortex of an appropriate tangential origin.
Occipital-to-parietal Grafts
In contrast to what was found in grafts of parietal origin, grafts of occipital cortex transplanted into the parietal cortex dev- eloped only a minor set of reciprocal connections with the host VB. Instead, they developed a significantly dense system of connections with the DLG. In vitro studies have reported an absence of selectivity in the ingrowth of thalamic axons into explants of various cortical areas. Explants of E16 DLG co- cultured with slices of P6 occipital or frontal cortex did not show any hint of preference for occipital cortex invasion (Molnar and Blakemore, 1991). In addition, no difference in the degree of outgrowth from thalamic explants was found even when cultured with slices of E19 cortex, whatever the sites of origin of the thalamic or cortical explants (Molnar and Blakemore, 1991
, 1995
). However, Bolz and Götz showed that DLG outgrowth on surfaces treated with P7 cortical membranes was about twice as large when the thalamic explants were placed on membranes from occipital than from frontal cortex (Bolz and Götz, 1992
). In line with the findings of the latter in vitro study, our results indicate that a relatively high level of specificity is also present in vivo in the development of thalamocortical projections. However, Schlaggar and O'Leary found an organized ingrowth of VB axons into occipital-to-parietal grafts 812 days post- transplantation (Schlaggar and O'Leary, 1991
). The presence of thalamic axons was assessed by analyzing the distribution of acetylcholinesterase (AChE), an early marker for primary sensory afferents (Kristt, 1979
). However, acetylcholinesterase histochemistry also labels intensely thalamorecipient zones in the visual (Robertson et al. 1988
) and auditory (Robertson et al., 1991
) cortex, in addition to the SI. Therefore, AChE activity within the grafts cannot provide a decisive indication as to the nucleus of origin of the thalamic input. Moreover, marked differences between rodent species in enzyme expression in the SI and in individual nuclei of the ventrobasal complex call for caution in the use of AChE histochemistry as a marker for immature thalamocortical axons (Sendemir et al., 1996
). Our findings based on the use of two different, reliable neurotracers show that grafts of E16 occipital cortex do not seem to support or even allow the ingrowth and stabilization of developing VB axons. Evidence in favor of a non-permissivity of the heterotopic grafts for VB axon ingrowth can be found in the clear-cut border of the terminal field distribution at the hostgraft interface. It is worth noting that the inhibitory effect is not due to a develop- mental heterochrony between graft tissue and host brain since in the same experimental conditions VB axons were capable of innervating grafts of parietal embryonic origin. The non-permis- sive effect seems therefore to be tissue-specific.
In addition, occipital-to-parietal grafts are contacted by DLG axons which by the time of transplantation have already reached the deep part of the occipital cortical plate (Kageyama and Robertson, 1993; Woo and Finlay, 1996
). The existence of DLG projections to occipital-to-parietal grafts might simply result from the stabilization of an initial but transitory aberrant projection of this nucleus outside the occipital cortex. However, several studies based on retrograde tracer injections into various sites of the embryonic cortical subplate or early postnatal cortical plate have demonstrated the existence of a strict specificity of the thalamic axons as they grow toward their appropriate target area and in their subsequent invasion of the cortical plate. The distribution of the retrogradely labeled thalamic neurons is restricted to nuclei appropriate for the areal location of the tracer injection (O'Leary et al. 1995
). Consistent findings were reported by other investigators who showed that few errors occur in the initial projecting pattern (Miller et al. 1993
; Agmon et al., 1995
). Therefore, the probability that the DLG input provided to the heterotopic transplants relies entirely upon the stabilization of early aberrant projections beyond the occipital cortex is unlikely. This assumption is substantiated further by the findings of a recent study showing that E16 frontal cortex grafted into the occipital cortex of newborn rats has the capacity to receive significant input from ventral thalamic nuclei whose normal terminal fields are located even more rostrally, in the frontal cortex (Frappé et al., 1999
). Taken together, these results suggest that grafts of E16 cortical cells are capable of expressing features that can attract appropriate thalamocortical axons whose normal terminal fields are located at some distance from the grafts. The mechanisms upon which these effects would be based are largely unknown. Recently, however, a number of molecules have been identified as playing a key role in defining the trajectory of axon projection in the develop- ing brain. Among these, Sema III [one member of the chemorepulsent semaphorin family (Skaliora et al., 1998
)], receptors and ligands of the Eph family of tyrosine kinases (Gao et al., 1998
), membrane-associated (Mann et al., 1998
) or adhesion molecules such as N-cadherin (Huntley and Benson, 1999
) or transcription regulatory factors such as Pax-6 (Kawano et al., 1999
) were identified as involved in the guidance of thalamic axons towards their appropriate cortical territory.
In the same way, our results showed that in occipital-to- parietal grafts the number of neurons projecting to the DLG was much larger than that projecting to the VB. This finding is also suggestive of an early specification of the neocortical cells regarding the nature of their appropriate targets. In line with this observation, a number of studies using heterotopic trans- plantation paradigms or disrupting neocortical development with ionizing radiation reported an early commitment of graft cells to develop area-specific projections (Jensen and Killackey, 1984; Ebrahimi-Gaillard et al., 1994
; Garnier et al., 1995
; Ebrahimi-Gaillard and Roger, 1996
; Létang et al., 1997
). The trajectories of the axons of the graft neurons that reach the host DLG have not been examined in this study. It is therefore impossible to determine whether the graft axons adopt aberrant pathways or initially follow pre-existing corticofugal fibers of the host as a guide and then interact with appropriate cues within the internal capsule/diencephalon to reach the DLG.
Parietal-to-occipital Grafts
Equally, we found that grafts of E16 parietal cortex placed into the occipital cortex of P0P1 recipients developed a substantial set of reciprocal connections with the VB. This observation concurs with what we found in the other category of heterotopic grafts. That VB axons have the capacity to reach the occipitally placed grafts lends further support to the notion of a specific attraction exerted by developing neocortical cells towards appropriate thalamic axons. Also, graft neurons send axons to the thalamic nuclei appropriate for their embryonic origin. The parietal-to-occipital paradigm did not allow us to determine whether the grafts would develop connections with the DLG since in every case this nucleus exhibited massive atrophy. A number of studies have reported that neonatal damage to the occipital cortex induces severe atrophy of the DLG in rodents as well as in other species (Cunningham et al., 1979; Perry and Cowey, 1979
; Létang et al., 1998
; Frappé et al., 1999
). In this study, development of the DLG atrophy was most probably an unavoidable consequence of the aspirative lesion which was performed prior to grafting.
Cytoarchitectonic organization
One additional finding of this study was that grafts of E16 presumptive occipital cortex placed into the parietal cortex of a newborn rat failed to develop and maintain in adulthood the typical cytoarchitectonic organization of the parietal cortex. Layer IV of the primary area of the parietal cortex of rodents contains discrete cytoarchitectonic units called barrels, linked to peripheral receptors according to an isomorphic pattern and forming, collectively, the entire body representation (Woolsey and Van der Loos, 1970; Welker and Woolsey, 1974
; Welker, 1976
). In this study, the barrelfield organization was examined with different indicators. Although not providing in the rat as clear a definition of the barrel hollows as in mice (Woolsey and Van Der Loos, 1970
), Nissl staining yields obvious images of the rows of barrels (Welker and Woolsey, 1974
). In addition, CO histochemistry of sections through the parietal cortex was found to systematically return high enzymatic activity within the barrel hollows, leaving the sides and septa relatively free of activity (Wong-Riley and Welt, 1980
; Waters et al., 1995
; Rhoades et al., 1997
). With both indicators, barrels were clearly found within the parietal cortex adjacent to the grafts whereas within the grafts themselves there was no evidence of barrel existence, whatever the dorso-ventral plane of the section through the grafts.
Barrel formation is related to peripheral receptors through trigeminal and thalamic relay nuclei. Indeed, the topologic organization of the somatosensory pathway follows a peripheral- to-central progression during development (Belford and Killackey, 1979; Agmon et al., 1993
, 1995
), and lesions or genetic modifications of the receptors or interruption of the trigeminal input modify the pattern of organization of the barrels (Killackey and Belford, 1979
; Jeanmonod et al., 1981
; Welker and Van Der Loos, 1986
; Killackey et al., 1994
). Further, several studies have indicated that thalamocortical projections conform to the topography, size and shape of distinct barrels (Killackey, 1973
; Killackey and Leshin, 1975
; Keller et al., 1985
; Jensen and Killackey, 1987
; Land et al., 1995
; Cases et al., 1996
; Welker et al., 1996
). Land et al. found that following HRP injections into the CO-dark hollows of cortical barrels ~95% of retrogradely labeled neurons were located in the barreloid that was iso- morphic to the injected barrels and 5% were located in a single adjacent barreloid (Land et al., 1995
). In addition, Welker and colleagues reported that BDA injections into the VB reveal labeling confined to the hollows of distinct barrels in intact mice whereas in barrelless mice similar injections returned continuous labeling within the parietal cortex (Welker et al., 1996
). Diffuse and extensive terminal arbors of corticothalamic axons are coexistent in this mutant, with the absence of barrel parcellation within the SI. Also, it has been found in a transgenic mouse line (Tg8) deficient for the gene encoding monoamine oxidase A that the barrel organization is lacking. BDA injections into the VB of these Tg8 mice reveal that the normal clustering of axon terminals is replaced by an even distribution of the labeling within layer IV of the SI (Cases et al., 1996
). Taken together, these findings support the hypothesis that thalamo- cortical afferents from the VB provide immature SI with barrel-patterning information (Schlaggar and O'Leary, 1994
; O'Leary et al., 1994
).
In this study, diffuse VB axon arborizations systematically occurred in parietal-to-parietal transplants and only a few VB axons innervated occipital-to-parietal transplants, which resulted in similar defect of barrels in both category of grafts. Previous work in which the grafts were examined several days (P8P12) after the transplantation provided evidence that barrels form in E16E17 occipital cortex transplanted into the parietal cortex of newborn rats (Schlaggar and O'Leary, 1991). Our findings, however, indicate that barrel formation is not maintained in adulthood.
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Conclusion |
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We suggest that the pattern of connections developed by neocortical cells is also an early specified feature. At E16, heterotopic cortical grafts develop reciprocal connections with thalamic nuclei (this study) and develop efferent connections (Ebrahimi-Gaillard et al., 1994; Ebrahimi-Gaillard and Roger, 1996
) corresponding to their embryonic origin. Grafts of earlier embryonic ages (E12), however, develop connections that are appropriate to the new cell location (Barbe and Levitt, 1992
, 1995
). This suggests that the specification of cerebral cortical cells as to some of their hodological characteristics occurs between E12 and E16. Experiments are still in progress to get more precise information on the embryonic age of specification.
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Notes |
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References |
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