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Biomedical subjects

G Raisman

Publications and source records attributed to G Raisman.

At least 37 records · Page 2Linked to original sources
PubMed1997

Embryonic tissue induces growth of adult axons from myelinated fiber tracts.

Suspensions of late embryonic hippocampal tissue were microinjected so as to be completely enclosed within the myelinated fiber bundles of the adult rat fimbria. Previous studies have shown that the axons from such transplanted neurons readily cross the graft/host interface and extend rapidly through the host fiber tract. The present study shows that the adult axons from the host fiber tract can also cross this interface in the opposite direction and enter the transplants. Biotin dextran tracing shows that the adult host fimbrial axons traverse the embryonic grafts and also form terminal arborizations within the transplants. Electron microscopy of orthograde electron-dense degeneration confirms that these host axons form synaptic terminals accounting for at least 6.6% of the synapses in the neuropil of the transplant. Thus, contact with embryonic nervous tissue can induce elongative growth by the adult fibers in a myelinated central tract.

Age Factors↗
PubMed1997

Integration of transplanted cultured Schwann cells into the long myelinated fiber tracts of the adult spinal cord.

A suspension of about 10,000 purified Schwann cells cultured from the neonatal rat sciatic nerve was transplanted into a discrete site in the upper cervical level of the corticospinal tract of one side in adult rats. From 4 days after transplantation immunostaining for p75 (low-affinity neurotrophin receptor) showed that the transplants consisted of a central mass of Schwann cells and cuffs of elongated Schwann cells along the perivascular space of curving blood vessels (most of which had been formed in response to the transplantation). Schwann cells leaving the central mass and perivascular cuffs migrated in strictly linear orientation along the rostrocaudal axis of the host corticospinal tract. According to the territory through which they migrated, the transplanted Schwann cells adopted two quite different forms: (1) The row Schwann cells, which migrated singly or in groups within the rows of host oligodendrocytic and astrocytic cell bodies, were non-process-bearing, rather cuboidal, brick-like cells (about 8 x 12 microm in size). (2) In contrast, the interfascicular Schwann cells, which migrated singly or intertwined in rope-like small groups interspersed among the axons of the host corticospinal tract, were larger, symmetrically bipolar cells, with processes about 100-120 microm long and 2 microm wide and bulging, ovoid nuclei, located in centrally placed cell bodies about 10 microm across. After about 6 weeks, the p75 immunoreactivity of the interfascicular Schwann cells had become down-regulated. However, from as early as 10 days after transplantation, immunostaining for the peripheral myelin protein, P0, semithin sections, and electron microscopy showed that these Schwann cells were not lost, but that they had myelinated the segments of the host corticospinal axons in the region of the transplant. In contrast, the row Schwann cells did not express P0 or form myelin. They retained their p75 immunoreactivity at long survivals (presumably because they were secluded from contacting the tract axons). The row Schwann cells also migrated farther than the interfascicular Schwann cells (possibly a function of their maintained p75 expression), becoming dispersed singly for at least 8 mm from the original transplant site. Our previous study of corticospinal tract lesions had shown the formation of a "closed" scar formed by hypertrophic astrocytic processes, which walled off a central astrocyte-free region and totally disrupted the normal longitudinal alignment of the tract astrocytic processes. In contrast, while the present Schwann cell transplants induced a comparable astrocytic hypertrophy over the same time course, the astrocytic processes remained able to penetrate the transplant site, which was not walled off, so that the longitudinal arrangement of the host corticospinal tract astrocytic skeleton was preserved intact across the region of the transplant. These observations show that Schwann cells can be intimately integrated into the cytoarchitecture of the myelinated adult host corticospinal tract. This integration is not a random dispersal in damaged areas: it involves direct interaction with the cell elements present in the host tract, it respects the complex and regular organization of the host tract glial cells, and it results in the formation of a precisely arranged mosaic of central and peripheral tissue.

Age Factors↗
PubMed1997

An urge to explain the incomprehensible: Geoffrey Harris and the discovery of the neural control of the pituitary gland.

Geoffrey Harris is responsible for our view that the brain controls the endocrine system by an exquisitely regulated pattern of synthesis and release of individual members of a family of peptide hormones. These hormones are carried through a portal vascular system that passes from the hypothalamus to the pituitary gland, where they selectively regulate the secretion of the six anterior pituitary hormones. This family of hypothalamic hormones is highly conserved in all vertebrates, including humans. They are essential for all aspects of reproduction--courtship, mating, pregnancy and young rearing--and they are responsible for the seasonal regulation of breeding. The hypothalamic control mechanism for reproduction is sexually dimorphic, with a basic female pattern that becomes masculinized under the influence of specific steroid hormones acting during development. Other members of the hypothalamic hormone family specifically regulate the secretion of pituitary growth hormone and the anterior pituitary hormones controlling the functions of the thyroid and adrenal glands. The secretion of the hypothalamic hormones is itself regulated by the feedback of the target gland hormones (such as estrogen and progesterone), which concurrently act on the brain to elicit appropriate behavior patterns. The hypothalamo-hypophysial axis plays a crucial role in the struggle for the survival of the species. By bringing the endocrine system under the control of the brain, it allows access to external environmental inputs, learned behavior patterns, and the whole of the central integrative machinery needed for the bodily functions to be sensitively and optimally adapted to the ever-changing challenges and opportunities in the outside world.

Animals↗
PubMed1997

Use of Schwann cells to induce repair of adult CNS tracts.

The ability of transplanted Schwann cells to modify the sprouts formed by cut central axons, and in particular to induce branching and extension of axon sprouts, is an encouraging sign for their possible future use in repair. The accessibility of the Schwann cells in the culture stage before transplantation offers a practical opportunity for genetic engineering (e.g. to introduce genes directing the expression of specific growth factors) which might be useful in designing a future method for the repair of human spinal injury. It must be borne in mind, however, that even the most successful cases of peripheral nerve grafts have shown only a limited proportion of axons growing back from the grafts into the environment of the CNS (Carter et al., 1989). When we constructed Schwann cells transplanted into the thalamus (Brook et al., 1994), we did not observe axons leaving the artificial tracts. In our experiments with Schwann cells transplanted into the spinal cord (Li & Raisman, 1994), the axons have only been studied within the graft, and we have as yet not been able to assess the extent to which they re-enter the CNS. For effective regeneration to occur, regenerating axons must not only be able to re-enter their original pathways and elongate along them, but also leave them in a correct manner--i.e. by making appropriate choices from a wide range of destinations. Therefore the effectiveness of a Schwann cell "bridging" repair must depend upon the self-organising capacity of the adult CNS (e.g. Florence et al., 1996).

Adult↗
PubMed1996

Conditionally immortalized neural progenitor cell lines integrate and differentiate after grafting to the adult rat striatum. A combined autoradiographic and electron microscopic study.

Neural progenitor cell lines, generated by conditional immortalization from the embryonic CNS, have previously been shown to survive and integrate after transplantation to the adult brain. The present study was designed to investigate the in vivo differentiation and morphological features of grafted neural progenitors using combined autoradiography and transmission electron microscopy of two temperature-sensitive neural progenitor cell lines, HiB5 and ST14A, labeled with 3H-thymidine prior to grafting. Two weeks after transplantation to the striatum the cells were found dispersed over an area extending about 1.5 mm from the injection site. Labeled cells located within the myelinated fiber bundles of the internal capsule were closely associated with myelinated axons and presented profiles similar to oligodendrocytes, while most of the grafted cells in the grey matter had morphological features of astroglia. Some labeled cells occurred also in close association with small blood vessels, morphologically resembling host pericytes. The results show that the immortalized neural progenitors can differentiate into mature glial cells, including astrocytes, oligodendrocytes and pericytes, after implantation into the adult striatum. The ability of the cells to become fully integrated with the resident glial population suggests that they will be highly useful as vehicles for intracerebral transgene expression in ex vivo gene transfer.

Animals↗
PubMed1996

Connectional specification of regenerating entorhinal projection neuron classes cannot be overridden by altered target availability in postnatal organotypic slice co-culture.

Layer II neurons of the entorhinal cortex project to the dentate gyrus and field CA3 and also send collaterals to the subiculum; layer III neurons project to the subiculum and field CA1, but not to the dentate gyrus; and layer IV neurons project to the perirhinal cortex. We have previously shown that these specific differences between the projections of the layer II and III neurons are maintained and can regenerate in organotypic slice culture. In the present experiments we have confronted Postnatal Day 7 (P7) rat entorhinal cortex with P7 tissue selected from restricted parts of the overall entorhinal projection field. (1) When entorhinal slices were co-cultured with target slices containing only dentate gyrus, extracellular uptake of biotin dextran from crystals placed on the dentate gyrus retrogradely labeled neurons in layer II, and those of layer III were not labeled. (2) When entorhinal slices were co-cultured with target slices that contained only the subiculum and the hippocampal field CA1 (but not dentate gyrus), neurons in both layers II and III of the entorhinal area were labeled. (3) When entorhinal slices were co-cultured with target slices containing the perirhinal area and no hippocampal or dentate tissue, the neurons of entorhinal layer IV were labeled, but (4) when co-cultured with control target slices taken from the rostral parietal neocortex, no entorhinal neurons were labeled. Thus the exclusive relationship of layer II entorhinal neurons to the dentate gyrus has already been established by 1 week of age and is maintained by the regenerating entorhinal axons. Layer III entorhinal neurons cannot be induced to project to the dentate gyrus even when deprived of their own target, and layer IV neurons are specified to project to the perirhinal area and will not project to any part of the hippocampal complex or to the rostral parietal cortex. Thus, deprivation of the normal target tissue and presentation of an incorrect target tissue (even when it is the correct target for one of the other classes of entorhinal neurons) are not sufficient to override the specificity of the entorhinal projection neurons.

Age Factors↗
PubMed1996

Regeneration of cut adult axons fails even in the presence of continuous aligned glial pathways.

The present study tests whether lesions small enough to allow the rapid reestablishment of a normally aligned tract glial framework would provide a permissive environment for the regeneration of cut adult CNS axons. We made penetrating microlesions which cut a narrow beam of axons in the adult rat cingulum, but caused minimal damage to the tract glial framework and no cavitation. The proximal tips of cut axons were identified by enhanced immunoreactivity for low affinity neurotrophin receptor, p75. From 1 day they became expanded into large growth-cone-like structures. At later times some axons turned back and extended in the reverse direction. Up to 14 days (after which time p75 could no longer be used as a marker), no axons advanced beyond the line of the lesion. From 1 to 2 days, OX42 immunostaining and electron microscopy showed that the lesion site was densely infiltrated by macrophages, which disappeared by 3 to 4 days. This was followed by a local hypertrophy of the OX42 immunoreactive resident tract microglial cells and an increase in both GFAP and vimentin immunoreactivity of the tract astrocytes. These responses were greatly reduced by 8 days, when the longitudinal alignment of glial processes across the lesion site was similar to that of an undamaged tract. The large growth-cone-like structures formed at the ends of the cut axons resemble those of developing axons exposed to chemorepulsive factors. This suggests that cellular elements in adult tract lesions may also exert chemorepulsive influences blocking regeneration of axons even in an apparently "open" tract framework.

Animals↗
PubMed1996

Expression of type 1 inositol 1,4,5-trisphosphate receptor during axogenesis and synaptic contact in the central and peripheral nervous system of developing rat.

Release of intracellular Ca2+ is triggered by the second messenger inositol 1,4,5-trisphosphate, which binds to the inositol 1,4,5-trisphosphate receptor and gates the opening of an intrinsic calcium channel in the endoplasmic reticulum. In order to understand the importance of this mechanism in development, we have examined the distribution of the type 1 inositol 1,4,5-trisphosphate receptor during development, in some areas of the rat brain and spinal cord and in peripheral neurons, using in situ hybridization and immunohistochemistry. In brain, we find that type 1 inositol 1,4,5-trisphosphate receptor is expressed in neurons from very early in development; low levels of expression are first detected after the neurons have migrated to their final positions, when they start to differentiate and begin axonal growth. Increasing levels of expression are observed later in development, during the time of synaptogenesis and dendritic contact. Glial cells do not express type 1 inositol 1,4,5-trisphosphate receptor, except for a transient period of expression, probably by oligodendrocytes, in developing fibre tracts during the onset of myelination. In contrast with the brain, both grey and white matter of the spinal cord express type 1 inositol 1,4,5-trisphosphate receptor throughout development, and it remains present in the adult spinal cord. We also show, for the first time, that type 1 inositol 1,4,5-trisphosphate receptor is expressed in the peripheral nervous system. Strong labelling was observed in the dorsal root ganglia and during development this expression seems to coincide with the onset of axogenesis. These results suggest that type 1 inositol 1,4,5-trisphosphate may be involved in the regulatory mechanism controlling Ca2+ levels in neurons during the periods of cell differentiation, axogenesis and synaptogenesis.

Animals↗
PubMed1995

Failure of axon regeneration in postnatal rat entorhinohippocampal slice coculture is due to maturation of the axon, not that of the pathway or target.

Horizontal slices which included the entorhinal area in continuity with the hippocampus were taken from the ventral levels of the cerebral hemispheres of rat pups from two age groups, from the 6th to the 8th postnatal days ('young') and the 12th to the 15th days ('old'). The slices were divided into an entorhinal part and a hippocampal part (which consisted of the hippocampus proper, dentate gyrus and subiculum) by a knife cut passing through the deep white matter of the entorhinal area. The slices were recombined in their normal orientation by matching the cut edges in the following age combinations: young/young, old/old, young/old and old/young. After 14 days in culture, crystals of biocytin were placed on the superficial layers of the entorhinal area. In the young/young combination the same placement of biocytin simultaneously labelled projections passing in both directions across the interface, i.e. (i) orthograde transport of biocytin taken up by entorhinal projection neurons resulted in labelling of axons passing from the entorhinal area across the interface between the cocultures to reach the correct terminal zone in the outer molecular layer of the dentate gyrus, and (ii) retrograde transport of biocytin taken up by axons and their terminals in the entorhinal area labelled the slender subicular and adjacent hippocampal field CA1 pyramidal cells whose axons project to the entorhinal area. In the old/old cocultures there were no projections in either direction. In the mixed age combinations, young entorhinal cortical tissue projected correctly across the interface to old dentate gyrus, but old entorhinal tissue did not project to young dentate gyrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMed1995

Sprouts from cut corticospinal axons persist in the presence of astrocytic scarring in long-term lesions of the adult rat spinal cord.

Small, circumscribed electrolytic lesions were made in the corticospinal tract at the upper cervical level of the adult rat spinal cord. At increasing survival times, immunohistochemistry of glial fibrillary acidic protein and electron microscopy showed that the predominantly longitudinal astrocytic processes underwent a progressive hypertrophy, which spread from the lesion, increasing in intensity from 1 week and reaching a maximum at between 9.5 and 13 weeks, by which time the lesion was completely surrounded by a dense astrocytic scar. A previous study with orthograde transport of axonal tracers showed that from 2 weeks after the lesion the main axonal stems of both cut and adjacent uncut corticospinal axons had large varicosities. The swollen ends of the cut axons, and also the adjacent uncut axons, emitted extensive arborizations of sprouts directed into the central, macrophage-filled area of the lesion. The present experiments indicated that the axon sprouts persisted apparently undiminished over the period (from 9.5 to 13 weeks) when the astrocytic scarring process was reaching its maximum. Surrounding the center of the lesion was an area in which the axons had become demyelinated. By 3 weeks a few axons were remyelinated with peripheral myelin formed by Schwann cells which had migrated into the lesions. By 4 months the scar region was densely colonized by Schwann cells, which now had remyelinated a wide swath of both cut and uncut axons. The cut axons were myelinated by Schwann cells as far as their large terminal expansions, which were sheathed, but not myelinated, by satellitic Schwann cells. Thus, at survivals long enough for the formation of a dense, astrocytic scar, cut corticospinal axons retain extensive terminal and collateral arborizations even in the macrophage-filled central lesion area and are myelinated or ensheathed by endogenous Schwann cells.

Animals↗
PubMed1994

Axons regenerate with correct specificity in horizontal slice culture of the postnatal rat entorhino-hippocampal system.

We have used slice culture of the entorhino-hippocampal system to investigate (1) whether nerve fibres which are cut postnatally are able to regenerate and (2) whether the regenerating fibres are able to establish correct selective target specificity in the formation of their terminal fields. Slices of tissue were taken in the horizontal plane through the caudo-ventral pole of the cerebral hemisphere of 9- to 10-day-old rats. Such slices maintain the entorhinal cortex in continuity with the hippocampus and intervening retrohippocampal areas. However, because of the dorsal inclination of the entorhino-hippocampal projection fibres in situ, the segments of the entorhinal cortex and hippocampus contained within each individual horizontal slice were disconnected from each other. During subsequent culture, the formation of fibre connections between the entorhinal area and the hippocampal complex was studied by the extracellular and intracellular anterograde transport of biocytin or biotin dextran, the retrograde transport of biotin dextran or carbocyanine dyes, and by electrical stimulation and recording. For the first 24 h after taking the slice, there were no entorhinal projections beyond the deep white matter, and no fibres reached the hippocampus or dentate gyrus. After 3 days in culture a small number of growing fibres had perforated the subiculum and entered the target areas. Between 6 and 14 days these projections increased and matured. As in the normal adult brain, entorhinal layer II stellate cells projected correctly to the dentate gyrus and hippocampal field CA3, whereas layer III pyramidal cells projected to hippocampal field CA1 and the subiculum. The new fibres grew along both alvear and perforant pathways. Anterograde and retrograde labelling showed that the reciprocal projections from the pyramidal cells of the subiculum and CA1 to the entorhinal area had also been severed at the time of taking the slices, and had similarly regenerated. Our results demonstrate that by taking tissue slices in appropriate planes it is possible to study the regeneration of axons in the tissue environment through which they normally run. This approach avoids the use of coculture and the concomitant difficulties associated with the need for fibres to cross a coculture interface. In horizontal slices of postnatal tissue, severed fibre projections between the entorhinal cortex and the hippocampal complex can regenerate in both directions and re-establish their correct laminar, pathway and target specificity.

Animals↗
PubMed1994

Multifocal pattern of postnatal development of the macroglial framework of the rat fimbria.

The development of the rat fimbria over the first postnatal month is associated with an approximate doubling of the tract diameter, a large increase in the number of glial cells, and the transformation of the prenatal radial glial skeleton into the adult interfascicular glial rows of solitary astrocytes and contiguous myelinating oligodendrocytes. The ventricular zone is reduced from a heterogeneous germinal layer of three or more cells thick at birth to the mature adult unicellular ependyma of homogeneous pale, mitotically inactive cells by the end of the second postnatal week. Mitoses are present throughout the body of the tract at all times, and persist, at reduced levels, in the adult. At birth the interior of the fimbria has only few scattered glial cell nuclei, largely solitary, or at most in longitudinal pairs. Over the first two postnatal weeks, the numbers and density of the interfascicular glia increase continuously. The scattered cells and cell clusters become progressively transformed into longer unicellular rows, which are aligned along the longitudinal axis of the tract, and which finally coalesce to form the continuous regular astrocyte/oligodendrocyte units that make up the interfascicular glial rows of the adult fimbrial glial skeleton. The increased cell packing density of the developing fimbrial glia is associated with a substantial decrease in nuclear and cytoplasmic size. From the end of the second postnatal week, the characteristic, large pale solitary astrocytes, and the smaller, more numerous, densely stained, closely packed oligodendrocytes are recognisable. Immunostaining for glial fibrillary acidic protein shows that immediately after birth the characteristic embryonic pattern of regular parallel radial glial processes starts to be modified by the progressive accumulation of longitudinal astrocytic processes, so the prenatal radial glial framework is rapidly transformed into the adult type of rectilinear array of radial and longitudinal processes. The development of the oligodendrocytes is shown clearly by immunostaining for myelin basic protein in enlarged, cytoplasm-rich, symmetrically placed cell pairs first seen at around P7. At P8-P10, there is a characteristic pattern of simultaneous multifocal maturation in which a single oligodendrocyte in each cluster develops a full complement of parallel, rather varicose myelinating processes. By P14 myelination is becoming confluent, oligodendrocytes are smaller, darker, with little cytoplasm, and individual myelinating processes cannot be discerned. Even at the end of the first postnatal month there are still many immature glia of indeterminate morphology. Myelination tends at first to be concentrated in the region adjacent to the hippocampus, and only reaches completion by the end of the second month.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗
PubMed1994

Extrusion transplantation of Schwann cells into the adult rat thalamus induces directional host axon growth.

In a previous study we found that Schwann cells microtransplanted into the central nervous system rapidly dispersed from the transplantation site and became intimately associated with host grey and white matter. We have now investigated whether this migratory behavior of the donor Schwann cells is compatible with the production of stable, continuous anatomical cell tracks and whether such tracks can induce directional host axon growth. During the gradual withdrawal of a micropipette, highly purified suspensions of cultured adult peripheral nerve Schwann cells were continuously extruded to form a vertical column of cells extending for up to 4 mm through the thalamus and across the choroid fissure into the hippocampus of adult rat hosts. The donor Schwann cells were identified by immunohistochemistry for low-affinity nerve growth factor receptor, vimentin, and Rat 401. Although donor Schwann cells migrated into the host tissues, a large number remained along the axis of the injection track to form a column which was maintained for up to 3 weeks. From 4 days, increasing numbers of parallel, unbranching host RT97-positive axons entered the Schwann cell column in alignment with the long axis of the Schwann cells in the vertical tracks. The axons did not fasciculate directly with each other, but mingled diffusely with the Schwann cells. The Schwann cell tracks were able to convey host axons out of the dorsal thalamus, across the extracellular space of the choroid fissure, and into the ventral hippocampus. Thus, Schwann cells, transplanted in the form of elongated tracks, can establish bridges across boundary membranes in the brain and carry substantial numbers of nerve fibers from one area to another.

Animals↗
PubMed1994

Schwann cells induce sprouting in motor and sensory axons in the adult rat spinal cord.

Circumscribed lesions were made within either the corticospinal tract or the ascending dorsal column tracts at the upper cervical level in adult rats. The responses of the tract axons were studied by orthograde transport from injections of horseradish peroxidase or biocytin. At 2 d, the ends of the cut axons were swollen, and the lesions induced en passant varicosities in the adjacent uncut axons. Although there have been reports of retraction, we found that even after several weeks, large numbers of cut axons still persisted in the central lesion area (where there was complete tissue destruction and intense macrophage infiltration), and also in the adjacent regions of the tract. The cut ends were expanded into a variety of shapes--large, complex, bulbous, and recurved--and many had profuse local branches with or without small, terminal-type varicosities. A suspension of Schwann cells cultured from neonatal sciatic nerve was injected by a minimally traumatic air pressure microinjection technique so as to form a bolus, comparable in size to the lesions, in either the corticospinal or the ascending dorsal column tracts at the upper cervical level. Despite previous findings that corticospinal axons do not elongate into peripheral nerve grafts, we found that both corticospinal and ascending dorsal column axons sprouted in response to contact with the transplanted Schwann cells. The response to the Schwann cells was much more rapid than to the lesions. By 2 d, in both descending and ascending tracts, both the axons that had been severed at the time of injection and also the adjacent uncut axons had already given rise to the branches that (unlike the localized sprouting seen after the long-term lesions) extended for considerable distances parallel to and fasciculating with each other and with the uncut tract axons. In addition, a mass of fine, tortuous, varicose branches invaded the superficial parts of the Schwann cell grafts, where they formed aborizations with small bead-like expansions resembling presynaptic boutons; as in their normal terminal fields, the aborizations formed by the corticospinal axons were smaller and finer than those formed by the ascending axons.

Animals↗
PubMed1994

Long interfascicular axon growth from embryonic neurons transplanted into adult myelinated tracts.

In a previous study we used the species-specific marker M6 to demonstrate that transplanted mouse embryonic hippocampal neurons grow axons at a rate of at least 1 mm/d for a distance of at least 10 mm along the longitudinal axis of the fimbria in immunosuppressed adult rat hosts. We now show that hippocampal neurons are able to grow comparably long interfascicular axons in two other myelinated adult fiber tracts, the corpus callosum and the cingulum. Moreover, suspensions of cells from embryonic neocortex and superior colliculus transplanted into each of these three adult host sites also give interfascicular axon growth whose speed, intensity, and pattern of distribution are identical to those of transplanted hippocampal neurons. The axons of the donor cells grow in both directions along the longitudinal axis of the host tracts, where they are interspersed in parallel among the normal host axons, the rows of host interfascicular glial nuclei, and the longitudinal processes of host tract astrocytes. Serial section analysis through the complex trajectories of the host fiber bundles of the fimbria and corpus callosum shows that the course of the donor axons conforms to the underlying orientation of the axonal and glial structures of the host fiber tract. These observations indicate that long interfascicular axon growth can occur in several different adult myelinated fiber tracts. The donor axons become integrated with the host tract fibers and glia, and they respect intertract boundaries. Growth is not restricted to the types of axons normally present in the tracts.

Animals↗