- Email: [email protected]
Efferents of frontal or occipital cortex grafted into adult rat's motor cortex
Neuroscience Letters 180 (1994) 265-268
ELSEVIER
NEORDSCIIHC[ LHTERS
Efferents of frontal or occipital cortex grafted into adult rat's motor cortex Jos6 Guitet, Cyril Garnier, Afsaneh Ebrahimi-Gaillard, Michel Roger* Laboratoire de Neurophysiologie, CNRS, URA 1869, Universit~ de Poitiers, Facultb des Sciences, 40 Av. du Recteur Pineau, 86022 Poitiers Cedex, France
Received 31 May 1994; Revised version received 16 August 1994; Accepted 16 August 1994
Abstract
Phaseolus vulgaris leucoagglutinin (PHA-L) was used to examine the efferent connectivity of embryonic (E 16) frontal (homotopic) or occipital (heterotopic) neocortical transplants placed into - or in the vicinity o f - lesion cavities made in the frontal cortex of adult recipients. Homotopic transplants projected towards the host sensorimotor cortex and, in most cases, into the lateral caudate-putamen (CPu). Heterotopic transplants projected into the anterior cingulate cortex and, in most cases, distributed terminals into the medial CPu. It is suggested that embryonic neocortical tissue placed into a damaged cortical site of an adult recipient develops a pattern of efferents corresponding to its cortical origin. Key words: Neurotransplant; Homotopic embryonic neocortex; Heterotopic embryonic neocortex; Motor cortex; Rat
Several studies have examined the efferent connectivity of embryonic neocortical tissue placed into the neocortex of adult recipients by injecting retrograde neurotracers into various centers of the host CNS. Following injection of retrograde tracers into the ipsilateral (relative to the graft) cortex [8] or thalamus [4,5], labeled cells were identified in transplants of fetal frontal [5,8] or gustatory [4] neocortex placed homotopically in the damaged neocortex of an adult host. However, following injection of Fluoro-Gold into the host ipsilateral thalamus or contralateral cortex, no labeled cells were identified in suspension grafts placed into the excitotoxically lesioned neocortex [7]. Recent studies in which a sensitive anterograde neurotracer (Phaseolus vulgaris leucoagglutinin: PHA-L) was injected into the transplant gave discordant results. Some studies [11,14] reported the existence of labeled fibers and terminals within the host CNS, whereas Schulz et al. [13] found no evidence of host axonal labeling, except in the area immediately adjacent to the transplant. Recently, we have established that embryonic frontal cortex placed in homotopic position in newborn host
*Corresponding author. Fax: (33) (49) 45 40 14. E-mail: [email protected]. 0304-3940/94/$7.00 © 1994 Elsevier Science Ireland Ltd. All rights reserved S S D I 0304-3940(94)00702-0
cortex develops a set of projections with a topographic organization corresponding to that of normal sensorimotor cortex. In contrast, the pattern of projections from embryonic occipital cortex heterotopically placed into the host frontal cortex corresponds to that of normal occipital cortex [3]. The present study was undertaken to examine the capacity of transplants of embryonic neocortical tissue placed into the frontal cortex of an adult recipient to develop efferents into the host CNS. In addition, this study sought to determine whether the embryonic origin of the transplanted tissue - homotopic or heterotopic -exerts an influence on the development of the transplant efferents in adult recipients. Unilateral neocortical lesions were produced under chloral hydrate (30 mg/100 g) anesthesia in 30 twomonth-old rats by aspirating part of the neocortex lying rostral to the coronal suture. The transplantations were performed 3 days after the lesions. Neocortical grafts were obtained from Wistar rat fetuses (El6) according to a procedure described previously [3]. In our experimental conditions, the level of integration of the grafts with the host tissue being generally higher (e.g. lesion cavities were systematically filled up) with cell suspension than with tissue block, frontal cortical tissue was transplanted in most cases as a cell suspension but, on
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Fig. 1. Camera-lucida drawings illustrating the axonal labeling in 2 representaUve cases with homotopic (A C: graft's extent: +4.7 to + 1.2) or heterotopic (D F; graft's extent: +4.2 to + 1.7) transplants. The dark areas design the PHA-L injection sites and the arrows indicate the localization of the neostriatal labeling. Scale bar = 2 ram. C C , corpus callosum; DR dorsal pedoncular cortex; Cgl 3, cingulate cortex, areas I 3: CPu, caudate putamen: Frl 3, frontal cortex, areas 1 3: 11, infi:alimbic cortex: LV, lateral ventricle; Parl, parietal cortex, area l: T. transplant.
occasions, tissue blocks were also grafted. Occipital cortical tissue was only transplanted as a cell suspension. Cell suspensions were prepared according to the procedure described by Bj6rklund et al. [1]. The cortical fragments were labeled with gold particles (10 nm) conjugated to W G A - H R P (Sigma) [3]. The cell suspensions (2 yl) were injected into the host cortex just rostral to the lesion cavity whereas the tissue blocks were simply layered into the cavity. Two to 5 months after grafting, the hosts were anesthetized and PHA-L was iontophoretically injected [3] into the transplants. Following a survival time of 9 days, the brains were fixed in situ under deep anesthesia and then sectioned on a freezing microtome. The sections (40 y m thick) were treated in 3 parallel series. One series was treated for silver intensification of the gold particles [10] to help subsequent identification of the transplant localization. The two remaining series were reacted for PHA-L immunohistochemistry [3]. One series was counterstained with Cresyl violet. The other series was reacted with a nickel enhanced- diaminobenzidine solution [3]. Within a given structure, the labeling density was estimated by counting the number of positive fibers found in a given representative section (weak: 1 5 fibers; moderate: 6 25 fibers; dense: 26 and more fibers). Nine cases with homotopic transplants and 4 cases
with heterotopic transplants had a PHA-L injection site unequivocally restricted to the transplant (no tracer spread outside the graft) and were considered in this study. In the homotopic group, the distribution of efferents from tissue block grafts (n = 2) being very similar to that of cell suspension grafts (n = 7), the results of both types of grafts were pooled. The transplants, either homo- or heterotopic, were located in the rostral part of the frontal cortex (extreme caudal and rostral limits of the graft's extensions: +0.5 to +5.2 mm, relative to bregma). The transplants filled the major part of the rostral division of Fr2 (nomenclature of Zilles [16]) and, in several cases, encroached upon the medial part of Frl. The distribution of the efferents from one representative case with homotopic transplant is given in Fig. 1A C. in all cases, a dense array of labeled fibers and terminals was found in the parts of Fr2 adjacent to the transplant. Anterograde labeling was also found systematically in Frl (Fig. 2B,C) with moderate (5 cases) to weak density. Moderate to weak labeling was also found in Fr3 (5 cases), FL (2 cases), Parl (3 cases) and Par2 (1 case). Labeled fibers were found in the corpus callosum, ipsilaterally (8 cases) and contralaterally (2 cases). An occasionnal fiber was also labeled contralaterally in Frl (2 cases: Fig. 2E), Fr2 and FL (one case). In addition,
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I
Fig. 2. Photographic illustration of host axonal labeling following PHA-L injection into homotopic (A-E,G) or heterotopic (F) transplants. A: labeled fibers leave the transplant (upper left corner) and course ventrally towards the corpus callosum. The arrow indicates host-transplant limit. B,C: labeled fibers and terminals in the ventral part of Frl. B: labeling within short distance (400,um) from the transplant. The arrow designates a bundle of thin fibers coursing mediolaterally. C: labeled fibers bearing boutons and varicosities (2 mm from the transplant) with a branching point (arrowhead). D: axonal labeling within the ipsilateral claustrum. E: contralaterally, branching fibers are present at the white/gray matter interface and some labeled fibers (arrowheads) are found deeply in layer VI of Frl. F: heterotopic transplant: Axonal labeling in the medial sector of the CPu. The arrow designates a bouquet close to the lateral ventricle and the arrowheads indicate fibers extending more deeply into the neostriatum. G: homotopic transplants; labeled fibers and terminals in the lateral part of the CPu, close to the external capsule (ec). Scale (A) = 150pm; (B,C) = 40pm; (D) = 60 /lm; (E,F,G) = 75/,tm. (A C,E,F): Cresyl violet counterstaining; (D,G): non-counterstained material.
w e a k a n t e r o g r a d e labeling was n o t e d in the r o s t r o l a t e r a l division o f C g l (3 cases). In f o u r cases, m o d e r a t e (Fig. 2D) to w e a k labeling was f o u n d in the claustrum. Finally, in 6 o u t o f the 9 cases, m o d e r a t e (5 cases) to weak labeling was identified in the c a u d a t e - p u t a m e n (CPu). T h e labeling was restricted to the external p a r t o f the d o r s o l a t e r a l o r v e n t r o l a t e r a l sectors (Figs. 1B,C; 2G). In 2 cases, the striatal labeling e x t e n d e d c a u d a l l y up to - 2 . 5 ram, relative to b r e g m a . In one case, 1-2 fibers with t e r m i n a l s were f o u n d c o n t r a l a t e r a l l y in the d o r s o l a t e r a l sector. In no case were efferents t r a c e d at a distance g r e a t e r t h a n 7-8 m m f r o m the transplants. T h e d i s t r i b u t i o n o f the efferents f r o m h e t e r o t o p i c t r a n s p l a n t s was m a r k e d l y different. The p a t t e r n o f efferents o f one r e p r e s e n t a t i v e case is illustrated in Fig. 1 D - F .
Practically no t e r m i n a l s were f o u n d in the s e n s o r i m o t o r cortex except in Fr2, the subfield into which the transp l a n t s were placed. In contrast, m o d e r a t e to w e a k labeling was s y s t e m a t i c a l l y identified in C g l a n d , occasionally, in Cg3 (3 cases) a n d i n f r a l i m b i c (1 case) cortex (Fig. I D,E). In 3 cases, labeled fibers were f o u n d in the c o r p u s c a l l o s u m ipsilaterally a n d / o r c o n t r a l a t e r a l l y to the transplant. In a d d i t i o n , m o d e r a t e to w e a k labeling was f o u n d in the m e d i a l sector o f the CPu, ipsilaterally in 3 cases (Fig. 1F) a n d b i l a t e r a l l y in one case. L a b e l e d fibers a n d t e r m i n a l s were f o u n d d o r s a l l y a n d / o r v e n t r a l l y in the n e o s t r i a t a l sector a d j a c e n t to the lateral ventricle (Fig. 2F). In one o f these 3 cases, a n t e r o g r a d e labeling was also f o u n d in the d o r s a l p a r t o f the C P u (Fig. 1F). A great v a r i a b i l i t y was f o u n d a m o n g cases in the den-
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sity and expansion of the transplant efferents. This variability was probably due to the diversity in the expansion of the injection site of the neurotracer and in the degree of integration of the graft with the host cortex. Our findings indicate that fetal frontal cortex transplanted into the previously lesioned sensorimotor cortex of an adult host has the capacity to develop efferent projections over some distance into host cortical and subcortical centers. Our findings confirm and extend previous results with the PHA-L method indicating that the grafted cells develop efferent projections into host cortical areas adjacent to the transplant [11,13]. In the present study, however, homotopic transplants, though being systematically located close to Cgl, distributed only few terminals and in only few cases into this area. In contrast, homotopic transplants systematically projected towards motor cortical area Frl and, in some cases, towards additional sensorimotor or somatosensory subfields which, in some instances (e.g. contralateral sensorimotor cortex), were relatively distant from the transplant. In addition, this study indicates that homotopic transplants are susceptible to project to the claustrum and confirms the findings of previous studies which suggested that some transplant cells distribute fibers and terminals into the CPu [6,11,14]. The topographic organization of the neostriatal projection arising from homotopic transplants corresponds to that arising from intact sensorimotor cortex [2,9]. In marked contrast, transplants of embryonic occipital cortex did not project to any of the sensorimotor subfields other than Fr2, the area into which the grafts were placed. Instead, heterotopic transplants systematically projected into Cgl which receives its main neocortical afferents from the occipital cortex [15]. In addition, most of the heterotopic transplants distributed terminals into the dorso- and/or ventromedial part of the CPu, a region which receives afferents from the occipital cortex [9]. It seems, therefore, that embryonic neocortical tissue placed into a damaged neocortical site of an adult host develops a pattern of efferents corresponding to its cortical origin rather than to the cortical site into which it is placed. These results are in agreement with what has been reported in newborn recipients [3]. Transplants of fetal neocortex into an adult CNS have the capacity to develop efferents which seem to grow over significant distance within the host corpus callosum but, in most cases, fail to penetrate deeply into the gray matter. The density of the efferent projections is, however, weaker than that seen in newborn hosts. As compared to newborn CNS, adult CNS is likely to contain low levels of growth factors and, therefore, the trophic
promotion towards the axons of the transplanted neurons is probably weaker in adult than in newborn recipients [12]. [1] Bj6rklund, A., Stenevi, U., Schmidt, R.H., Dunnett, S.B. and Gage, F.H., Intracerebral grafting of neuronal cell suspensions, Acta Physiol. Scand., 522 (1983) 1 7. [2] Ebrahimi, A., Pochet, R. and Roger, M., Topographical organization of the projections from physiologically identified areas of the motor cortex to the striatum in the rat, Neurosci. Res., 14 (1992) 39-60. [3] Ebrahimi-Gaillard, A., Guitet, J., Garnier, C. and Roger, M., Topographic distribution of efferent fibers originating from homotopic or heterotopic transplants: heterotopically transplanted neurons retain some of the developmental characteristics corresponding to their site of origin, Dev. Brain Res., 77 (1994) 271 -283. [4] Fernandez-Ruiz, J., Escobar, M.L., Pina, A.L,. Diaz-Cintra, S., Cintra-McGlone, F.L. and Bermudez-Rattoni, F., Time-dependant recovery of taste aversion learning by fetal brain transplants in gustatory neocortex-lesioned rats, Behav. Neural. Biol., 55 (1991) 179-193. [5] Gonzalez, M.F., Sharp, F. and Loken, J.E., Fetal frontal cortex transplanted to motor/sensory cortex of adult rats: reciprocal connections with host thalamus demonstrated with WGA-HRE Exp. Neurol., 99 (1988) 154-165. [6] Isacson, O., Wictorin, K., Fischer, W., Sofroniew, M.V. and Bj6rklund, A., Fetal cortical cell suspension grafts to the excitotoxically lesioned neocortex: anatomical and neurochemical studies of trophic interactions, Prog. Brain Res., 78 (1988) 13 26. [7] Isacson, O. and Sofroniew, M.V., Neuronal loss or replacement in the injured adult cerebral neocortex induces extensive remodelling of intrinsic and afferent neural systems, Exp. Neurol., 117 (1992) 151-175. [8] Labbe, R., Firt, A., Mufson, E.J. and Stein, D.G., Fetal brain transplants: reduction of deficits in rats with frontal cortex lesions, Science, 221 (1983) 470-472. [9] McGeorge, A.J. and Faull, R.L.M., The organization of the projections from the cerebral cortex to the striatum in the rat, Neuroscience, 29 (1989) 503-537. [10] Menetrey, D., Retrograde tracing of neural pathway with a protein gold complex. I. Light microscopic detection after silver intensification, Histochemistry, 83 (1985) 391-395. [11] Plumet, J., Ebrahimi, A., Guitet, J. and Roger, M., Partial recovery of skilled forelimb reaching after transplantation of fetal cortical tissue in adult rats with motor cortex lesion anatomical and functionnal aspects, Restor. Neurol. Neurosci., 6 (1993)9-27. [12] Roger, M. and Ebrahimi-Gaillard, A., Anatomical and functional characteristics of fetal neocortex transplanted into the neocortex of newborn or adult rats, Rev. Neurosci., 5 (1994) 11 26. [13] Schulz, M.K., Hogan, T.P. and Castro, A.J., Connectivity of fetal neocortical block transplants in the excitotoxically ablated cortex of adult rats, Exp. Brain Res., 96 (1993) 480~86. [14] Senatorov, V.V., Nyakas, C. and Fulop, Z., Visualization of outgrowing axons of grafted neurons by anterograde labelling with Phaseolus vulgaris leucoagglutinin in the motor cortex of the rat, Restor. Neurol. Neurosci., 5 (1993) 337 345. [15] Vogt, B.A. and Miller, M.W., Cortical connections between rat cingular cortex and visual, motor, and postsubicular cortices, J. Comp. Neurol., 216 (1983) 192-210. [16] Zilles, K., The Cortex of the Rat: A Stereotaxic Atlas, Springen Berlin, 1985.
ELSEVIER
NEORDSCIIHC[ LHTERS
Efferents of frontal or occipital cortex grafted into adult rat's motor cortex Jos6 Guitet, Cyril Garnier, Afsaneh Ebrahimi-Gaillard, Michel Roger* Laboratoire de Neurophysiologie, CNRS, URA 1869, Universit~ de Poitiers, Facultb des Sciences, 40 Av. du Recteur Pineau, 86022 Poitiers Cedex, France
Received 31 May 1994; Revised version received 16 August 1994; Accepted 16 August 1994
Abstract
Phaseolus vulgaris leucoagglutinin (PHA-L) was used to examine the efferent connectivity of embryonic (E 16) frontal (homotopic) or occipital (heterotopic) neocortical transplants placed into - or in the vicinity o f - lesion cavities made in the frontal cortex of adult recipients. Homotopic transplants projected towards the host sensorimotor cortex and, in most cases, into the lateral caudate-putamen (CPu). Heterotopic transplants projected into the anterior cingulate cortex and, in most cases, distributed terminals into the medial CPu. It is suggested that embryonic neocortical tissue placed into a damaged cortical site of an adult recipient develops a pattern of efferents corresponding to its cortical origin. Key words: Neurotransplant; Homotopic embryonic neocortex; Heterotopic embryonic neocortex; Motor cortex; Rat
Several studies have examined the efferent connectivity of embryonic neocortical tissue placed into the neocortex of adult recipients by injecting retrograde neurotracers into various centers of the host CNS. Following injection of retrograde tracers into the ipsilateral (relative to the graft) cortex [8] or thalamus [4,5], labeled cells were identified in transplants of fetal frontal [5,8] or gustatory [4] neocortex placed homotopically in the damaged neocortex of an adult host. However, following injection of Fluoro-Gold into the host ipsilateral thalamus or contralateral cortex, no labeled cells were identified in suspension grafts placed into the excitotoxically lesioned neocortex [7]. Recent studies in which a sensitive anterograde neurotracer (Phaseolus vulgaris leucoagglutinin: PHA-L) was injected into the transplant gave discordant results. Some studies [11,14] reported the existence of labeled fibers and terminals within the host CNS, whereas Schulz et al. [13] found no evidence of host axonal labeling, except in the area immediately adjacent to the transplant. Recently, we have established that embryonic frontal cortex placed in homotopic position in newborn host
*Corresponding author. Fax: (33) (49) 45 40 14. E-mail: [email protected]. 0304-3940/94/$7.00 © 1994 Elsevier Science Ireland Ltd. All rights reserved S S D I 0304-3940(94)00702-0
cortex develops a set of projections with a topographic organization corresponding to that of normal sensorimotor cortex. In contrast, the pattern of projections from embryonic occipital cortex heterotopically placed into the host frontal cortex corresponds to that of normal occipital cortex [3]. The present study was undertaken to examine the capacity of transplants of embryonic neocortical tissue placed into the frontal cortex of an adult recipient to develop efferents into the host CNS. In addition, this study sought to determine whether the embryonic origin of the transplanted tissue - homotopic or heterotopic -exerts an influence on the development of the transplant efferents in adult recipients. Unilateral neocortical lesions were produced under chloral hydrate (30 mg/100 g) anesthesia in 30 twomonth-old rats by aspirating part of the neocortex lying rostral to the coronal suture. The transplantations were performed 3 days after the lesions. Neocortical grafts were obtained from Wistar rat fetuses (El6) according to a procedure described previously [3]. In our experimental conditions, the level of integration of the grafts with the host tissue being generally higher (e.g. lesion cavities were systematically filled up) with cell suspension than with tissue block, frontal cortical tissue was transplanted in most cases as a cell suspension but, on
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Fig. 1. Camera-lucida drawings illustrating the axonal labeling in 2 representaUve cases with homotopic (A C: graft's extent: +4.7 to + 1.2) or heterotopic (D F; graft's extent: +4.2 to + 1.7) transplants. The dark areas design the PHA-L injection sites and the arrows indicate the localization of the neostriatal labeling. Scale bar = 2 ram. C C , corpus callosum; DR dorsal pedoncular cortex; Cgl 3, cingulate cortex, areas I 3: CPu, caudate putamen: Frl 3, frontal cortex, areas 1 3: 11, infi:alimbic cortex: LV, lateral ventricle; Parl, parietal cortex, area l: T. transplant.
occasions, tissue blocks were also grafted. Occipital cortical tissue was only transplanted as a cell suspension. Cell suspensions were prepared according to the procedure described by Bj6rklund et al. [1]. The cortical fragments were labeled with gold particles (10 nm) conjugated to W G A - H R P (Sigma) [3]. The cell suspensions (2 yl) were injected into the host cortex just rostral to the lesion cavity whereas the tissue blocks were simply layered into the cavity. Two to 5 months after grafting, the hosts were anesthetized and PHA-L was iontophoretically injected [3] into the transplants. Following a survival time of 9 days, the brains were fixed in situ under deep anesthesia and then sectioned on a freezing microtome. The sections (40 y m thick) were treated in 3 parallel series. One series was treated for silver intensification of the gold particles [10] to help subsequent identification of the transplant localization. The two remaining series were reacted for PHA-L immunohistochemistry [3]. One series was counterstained with Cresyl violet. The other series was reacted with a nickel enhanced- diaminobenzidine solution [3]. Within a given structure, the labeling density was estimated by counting the number of positive fibers found in a given representative section (weak: 1 5 fibers; moderate: 6 25 fibers; dense: 26 and more fibers). Nine cases with homotopic transplants and 4 cases
with heterotopic transplants had a PHA-L injection site unequivocally restricted to the transplant (no tracer spread outside the graft) and were considered in this study. In the homotopic group, the distribution of efferents from tissue block grafts (n = 2) being very similar to that of cell suspension grafts (n = 7), the results of both types of grafts were pooled. The transplants, either homo- or heterotopic, were located in the rostral part of the frontal cortex (extreme caudal and rostral limits of the graft's extensions: +0.5 to +5.2 mm, relative to bregma). The transplants filled the major part of the rostral division of Fr2 (nomenclature of Zilles [16]) and, in several cases, encroached upon the medial part of Frl. The distribution of the efferents from one representative case with homotopic transplant is given in Fig. 1A C. in all cases, a dense array of labeled fibers and terminals was found in the parts of Fr2 adjacent to the transplant. Anterograde labeling was also found systematically in Frl (Fig. 2B,C) with moderate (5 cases) to weak density. Moderate to weak labeling was also found in Fr3 (5 cases), FL (2 cases), Parl (3 cases) and Par2 (1 case). Labeled fibers were found in the corpus callosum, ipsilaterally (8 cases) and contralaterally (2 cases). An occasionnal fiber was also labeled contralaterally in Frl (2 cases: Fig. 2E), Fr2 and FL (one case). In addition,
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I
Fig. 2. Photographic illustration of host axonal labeling following PHA-L injection into homotopic (A-E,G) or heterotopic (F) transplants. A: labeled fibers leave the transplant (upper left corner) and course ventrally towards the corpus callosum. The arrow indicates host-transplant limit. B,C: labeled fibers and terminals in the ventral part of Frl. B: labeling within short distance (400,um) from the transplant. The arrow designates a bundle of thin fibers coursing mediolaterally. C: labeled fibers bearing boutons and varicosities (2 mm from the transplant) with a branching point (arrowhead). D: axonal labeling within the ipsilateral claustrum. E: contralaterally, branching fibers are present at the white/gray matter interface and some labeled fibers (arrowheads) are found deeply in layer VI of Frl. F: heterotopic transplant: Axonal labeling in the medial sector of the CPu. The arrow designates a bouquet close to the lateral ventricle and the arrowheads indicate fibers extending more deeply into the neostriatum. G: homotopic transplants; labeled fibers and terminals in the lateral part of the CPu, close to the external capsule (ec). Scale (A) = 150pm; (B,C) = 40pm; (D) = 60 /lm; (E,F,G) = 75/,tm. (A C,E,F): Cresyl violet counterstaining; (D,G): non-counterstained material.
w e a k a n t e r o g r a d e labeling was n o t e d in the r o s t r o l a t e r a l division o f C g l (3 cases). In f o u r cases, m o d e r a t e (Fig. 2D) to w e a k labeling was f o u n d in the claustrum. Finally, in 6 o u t o f the 9 cases, m o d e r a t e (5 cases) to weak labeling was identified in the c a u d a t e - p u t a m e n (CPu). T h e labeling was restricted to the external p a r t o f the d o r s o l a t e r a l o r v e n t r o l a t e r a l sectors (Figs. 1B,C; 2G). In 2 cases, the striatal labeling e x t e n d e d c a u d a l l y up to - 2 . 5 ram, relative to b r e g m a . In one case, 1-2 fibers with t e r m i n a l s were f o u n d c o n t r a l a t e r a l l y in the d o r s o l a t e r a l sector. In no case were efferents t r a c e d at a distance g r e a t e r t h a n 7-8 m m f r o m the transplants. T h e d i s t r i b u t i o n o f the efferents f r o m h e t e r o t o p i c t r a n s p l a n t s was m a r k e d l y different. The p a t t e r n o f efferents o f one r e p r e s e n t a t i v e case is illustrated in Fig. 1 D - F .
Practically no t e r m i n a l s were f o u n d in the s e n s o r i m o t o r cortex except in Fr2, the subfield into which the transp l a n t s were placed. In contrast, m o d e r a t e to w e a k labeling was s y s t e m a t i c a l l y identified in C g l a n d , occasionally, in Cg3 (3 cases) a n d i n f r a l i m b i c (1 case) cortex (Fig. I D,E). In 3 cases, labeled fibers were f o u n d in the c o r p u s c a l l o s u m ipsilaterally a n d / o r c o n t r a l a t e r a l l y to the transplant. In a d d i t i o n , m o d e r a t e to w e a k labeling was f o u n d in the m e d i a l sector o f the CPu, ipsilaterally in 3 cases (Fig. 1F) a n d b i l a t e r a l l y in one case. L a b e l e d fibers a n d t e r m i n a l s were f o u n d d o r s a l l y a n d / o r v e n t r a l l y in the n e o s t r i a t a l sector a d j a c e n t to the lateral ventricle (Fig. 2F). In one o f these 3 cases, a n t e r o g r a d e labeling was also f o u n d in the d o r s a l p a r t o f the C P u (Fig. 1F). A great v a r i a b i l i t y was f o u n d a m o n g cases in the den-
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sity and expansion of the transplant efferents. This variability was probably due to the diversity in the expansion of the injection site of the neurotracer and in the degree of integration of the graft with the host cortex. Our findings indicate that fetal frontal cortex transplanted into the previously lesioned sensorimotor cortex of an adult host has the capacity to develop efferent projections over some distance into host cortical and subcortical centers. Our findings confirm and extend previous results with the PHA-L method indicating that the grafted cells develop efferent projections into host cortical areas adjacent to the transplant [11,13]. In the present study, however, homotopic transplants, though being systematically located close to Cgl, distributed only few terminals and in only few cases into this area. In contrast, homotopic transplants systematically projected towards motor cortical area Frl and, in some cases, towards additional sensorimotor or somatosensory subfields which, in some instances (e.g. contralateral sensorimotor cortex), were relatively distant from the transplant. In addition, this study indicates that homotopic transplants are susceptible to project to the claustrum and confirms the findings of previous studies which suggested that some transplant cells distribute fibers and terminals into the CPu [6,11,14]. The topographic organization of the neostriatal projection arising from homotopic transplants corresponds to that arising from intact sensorimotor cortex [2,9]. In marked contrast, transplants of embryonic occipital cortex did not project to any of the sensorimotor subfields other than Fr2, the area into which the grafts were placed. Instead, heterotopic transplants systematically projected into Cgl which receives its main neocortical afferents from the occipital cortex [15]. In addition, most of the heterotopic transplants distributed terminals into the dorso- and/or ventromedial part of the CPu, a region which receives afferents from the occipital cortex [9]. It seems, therefore, that embryonic neocortical tissue placed into a damaged neocortical site of an adult host develops a pattern of efferents corresponding to its cortical origin rather than to the cortical site into which it is placed. These results are in agreement with what has been reported in newborn recipients [3]. Transplants of fetal neocortex into an adult CNS have the capacity to develop efferents which seem to grow over significant distance within the host corpus callosum but, in most cases, fail to penetrate deeply into the gray matter. The density of the efferent projections is, however, weaker than that seen in newborn hosts. As compared to newborn CNS, adult CNS is likely to contain low levels of growth factors and, therefore, the trophic
promotion towards the axons of the transplanted neurons is probably weaker in adult than in newborn recipients [12]. [1] Bj6rklund, A., Stenevi, U., Schmidt, R.H., Dunnett, S.B. and Gage, F.H., Intracerebral grafting of neuronal cell suspensions, Acta Physiol. Scand., 522 (1983) 1 7. [2] Ebrahimi, A., Pochet, R. and Roger, M., Topographical organization of the projections from physiologically identified areas of the motor cortex to the striatum in the rat, Neurosci. Res., 14 (1992) 39-60. [3] Ebrahimi-Gaillard, A., Guitet, J., Garnier, C. and Roger, M., Topographic distribution of efferent fibers originating from homotopic or heterotopic transplants: heterotopically transplanted neurons retain some of the developmental characteristics corresponding to their site of origin, Dev. Brain Res., 77 (1994) 271 -283. [4] Fernandez-Ruiz, J., Escobar, M.L., Pina, A.L,. Diaz-Cintra, S., Cintra-McGlone, F.L. and Bermudez-Rattoni, F., Time-dependant recovery of taste aversion learning by fetal brain transplants in gustatory neocortex-lesioned rats, Behav. Neural. Biol., 55 (1991) 179-193. [5] Gonzalez, M.F., Sharp, F. and Loken, J.E., Fetal frontal cortex transplanted to motor/sensory cortex of adult rats: reciprocal connections with host thalamus demonstrated with WGA-HRE Exp. Neurol., 99 (1988) 154-165. [6] Isacson, O., Wictorin, K., Fischer, W., Sofroniew, M.V. and Bj6rklund, A., Fetal cortical cell suspension grafts to the excitotoxically lesioned neocortex: anatomical and neurochemical studies of trophic interactions, Prog. Brain Res., 78 (1988) 13 26. [7] Isacson, O. and Sofroniew, M.V., Neuronal loss or replacement in the injured adult cerebral neocortex induces extensive remodelling of intrinsic and afferent neural systems, Exp. Neurol., 117 (1992) 151-175. [8] Labbe, R., Firt, A., Mufson, E.J. and Stein, D.G., Fetal brain transplants: reduction of deficits in rats with frontal cortex lesions, Science, 221 (1983) 470-472. [9] McGeorge, A.J. and Faull, R.L.M., The organization of the projections from the cerebral cortex to the striatum in the rat, Neuroscience, 29 (1989) 503-537. [10] Menetrey, D., Retrograde tracing of neural pathway with a protein gold complex. I. Light microscopic detection after silver intensification, Histochemistry, 83 (1985) 391-395. [11] Plumet, J., Ebrahimi, A., Guitet, J. and Roger, M., Partial recovery of skilled forelimb reaching after transplantation of fetal cortical tissue in adult rats with motor cortex lesion anatomical and functionnal aspects, Restor. Neurol. Neurosci., 6 (1993)9-27. [12] Roger, M. and Ebrahimi-Gaillard, A., Anatomical and functional characteristics of fetal neocortex transplanted into the neocortex of newborn or adult rats, Rev. Neurosci., 5 (1994) 11 26. [13] Schulz, M.K., Hogan, T.P. and Castro, A.J., Connectivity of fetal neocortical block transplants in the excitotoxically ablated cortex of adult rats, Exp. Brain Res., 96 (1993) 480~86. [14] Senatorov, V.V., Nyakas, C. and Fulop, Z., Visualization of outgrowing axons of grafted neurons by anterograde labelling with Phaseolus vulgaris leucoagglutinin in the motor cortex of the rat, Restor. Neurol. Neurosci., 5 (1993) 337 345. [15] Vogt, B.A. and Miller, M.W., Cortical connections between rat cingular cortex and visual, motor, and postsubicular cortices, J. Comp. Neurol., 216 (1983) 192-210. [16] Zilles, K., The Cortex of the Rat: A Stereotaxic Atlas, Springen Berlin, 1985.