Terminal arbors of axons that have formed abnormal connections

Reconstructing neural circuits using multiresolution correlated light and electron microscopy

Correlated light and electron microscopy (CLEM) can be used to combine functional and molecular characterizations of neurons with detailed anatomical maps of their synaptic organization. Here we describe a multiresolution approach to CLEM (mrCLEM) that efficiently targets electron microscopy (EM) imaging to optically characterized cells while maintaining optimal tissue preparation for high-throughput EM reconstruction. This approach hinges on the ease with which arrays of sections collected on a solid substrate can be repeatedly imaged at different scales using scanning electron microscopy. We match this multiresolution EM imaging with multiresolution confocal mapping of the aldehyde-fixed tissue. Features visible in lower resolution EM correspond well to features visible in densely labeled optical maps of fixed tissue. Iterative feature matching, starting with gross anatomical correspondences and ending with subcellular structure, can then be used to target high-resolution EM image acquisition and annotation to cells of interest. To demonstrate this technique and range of images used to link live optical imaging to EM reconstructions, we provide a walkthrough of a mouse retinal light to EM experiment as well as some examples from mouse brain slices.

Conversations with Ray Guillery on albinism: linking Siamese cat visual pathway connectivity to mouse retinal development

In albinism of all species, perturbed melanin biosynthesis in the eye leads to foveal hypoplasia, retinal ganglion cell misrouting, and, consequently, altered binocular vision. Here, written before he died, Ray Guillery chronicles his discovery of the aberrant circuitry from eye to brain in the Siamese cat. Ray's characterization of visual pathway anomalies in this temperature sensitive mutation of tyrosinase and thus melanin synthesis in domestic cats opened the exploration of albinism and simultaneously, a genetic approach to the organization of neural circuitry. I follow this account with a remembrance of Ray's influence on my work. Beginning with my postdoc research with Ray on the cat visual pathway, through my own work on the mechanisms of retinal axon guidance in the developing mouse, Ray and I had a continuous and rich dialogue about the albino visual pathway. I will present the questions Ray posed and clues we have to date on the still-elusive link between eye pigment and the proper balance of ipsilateral and contralateral retinal ganglion cell projections to the brain.

Rearrangement of retinogeniculate projection patterns after eye-specific segregation in mice

It has been of interest whether and when the rearrangement of neuronal circuits can be induced after projection patterns are formed during development. Earlier studies using cats reported that the rearrangement of retinogeniculate projections could be induced even after eye-specific segregation has occurred, but detailed and quantitative characterization of this rearrangement has been lacking. Here we delineate the structural changes of retinogeniculate projections in the C57BL/6 mouse in response to monocular enucleation (ME) after eye-specific segregation. When ME was performed after eye-specific segregation, rearrangement of retinogeniculate axons in the dorsal lateral geniculate nucleus (dLGN) was observed within 5 days. Although this rearrangement was observed both along the dorsomedial-ventrolateral and outer-inner axes in the dLGN, it occurred more rapidly along the outer-inner axis. We also examined the critical period for this rearrangement and found that the rearrangement became almost absent by the beginning of the critical period for ocular dominance plasticity in the primary visual cortex. Taken together, our findings serve as a framework for the assessment of phenotypes of genetically altered mouse strains as well as provide insights into the mechanisms underlying the rearrangement of retinogeniculate projections.

The CRE/CREB pathway is transiently expressed in thalamic circuit development and contributes to refinement of retinogeniculate axons

The development of precise connections in the mammalian brain proceeds through refinement of initially diffuse patterns, a process that occurs largely within critical developmental windows. To elucidate the molecular pathways that orchestrate these early periods of circuit remodeling, we have examined the role of a calcium- and cAMP-regulated transcriptional pathway. We show that there is a window of CRE/CREB-mediated gene expression in the developing thalamus, which precedes neocortical expression. In the LGN, this wave of gene expression occurs prior to visual experience, but requires retinal function. Mutant mice with reduced CREB expression show loss of refinement of retinogeniculate projections. These results suggest an important role of the CRE/CREB transcriptional pathway in the coordination of experience-independent circuit remodeling during forebrain development.

Dendritic development in the dorsal lateral geniculate nucleus of ferrets in the postnatal absence of retinal input: a Golgi study

In order to determine the ongoing role of retinal fibers in the development of dorsal lateral geniculate nucleus (dLGN) neurons during postnatal development, the development of dLGN neurons in the postnatal absence of retinal input was studied in pigmented ferrets using the Golgi-Hortega technique. The development of four dLGN cell classes, defined on the basis of somatic and dendritic morphology, was described previously in normal ferrets (Sutton and Brunso-Bechtold, 1991, J. Comp. Neurol. 309:71-85). The present results indicate that the morphological development of dLGN neurons is strikingly similar in normal and experimental ferrets. The exuberant dendritic appendages that appear after eye opening in normal ferrets are overproduced and eliminated in the postnatal absence of retinal input; however, the final reduction of these transient appendages is delayed. Because exuberant appendages develop in the absence of retinal input, their production cannot depend upon visual experience. Differences in cell body size between normal and experimental ferrets are apparent only after neurons can be classified at the end of the first postnatal month. Cell body size is markedly reduced for class 1 neurons; class 2 cells also are reduced in size but to a far lesser extent. As there is a general trend for class 1 neurons to have the functional properties of Y-cells, it is likely that the dLGN neurons most affected by the absence of retinal input also are Y-cells.

Anatomical plasticity of the tectospinal tract after unilateral lesion of the superior colliculus in the neonatal rat

After unilateral ablation of the superior colliculus (SC) in neonatal or adult rats, the reorganization of the tectospinal tract (TST) was examined using the technique of anterograde transport of horseradish peroxidase to which wheat germ agglutinin had been conjugated (WGA-HRP). In neonatally lesioned rats, aberrant labeled terminals of TST axons were found on the ipsilateral side of the spinal cord. Postnatal development of the TST was then studied by retrograde transport of HRP to determine whether the aberrant tectospinal projections resulted from normally transient ipsilateral projections that persisted in operated rats or were due to collateral sprouting of projections to the contralateral projection field. The results failed to show an ipsilateral projection from the SC to spinal cord in normal neonatal rats. However, in neonatally lesioned rats, aberrant labeled fibers were observed recrossing the midline of the cervical spinal cord. Therefore, the increase in labeled terminals on the ipsilateral side following unilateral SC ablation appeared to originate from collateral sprouting at the spinal cord level of TST fibers from the intact pathway.

Climbing Fibre Plasticity in the Cerebellum of the Adult Rat

The present paper reports an example of collateral sprouting, and provides a detailed morphological description of newly formed axonal branches, terminal arborizations, and synapses in the central nervous system of an adult mammal. By means of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L), we demonstrate that the climbing fibres, which survive a subtotal lesion of the inferior olive in the rat, emit collateral sprouts and develop new terminal plexuses forming new synaptic contacts on Purkinje cell dendrites. Adult climbing fibres thus show striking plastic capabilities that represent the morphological basis of the functional reinnervation of Purkinje cells.

Reorganization of neuronal connections following CNS trauma: principles and experimental paradigms

The present review summarizes how the nervous system responds to trauma. The goal is to provide an introduction to the problems, techniques, experimental paradigms, current issues, and future promise. The review is especially designed for basic scientists and clinicians who are not currently involved in research on CNS reorganization, and for students just entering the field. The review characterizes the secondary degenerative events that occur after trauma, and the types of growth that commonly occur. A standard terminology is set forth with criteria for differentiating between related phenomena. Experimental methods are described that can be used documenting reorganization of circuitry. The principles that determine whether a given process will or will not occur are summarized, and some of the factors that may regulate the nature and extent of growth are considered. Research strategies are outlined that have been used to evaluate whether reorganization of circuitry is functionally significant. Finally, future directions in research and clinical application are discussed, focusing especially on the efforts to facilitate regeneration, and the work on transplants of CNS tissue to facilitate growth of surviving connections, and to replace tissue destroyed by trauma.

Axon arbors of X and Y retinal ganglion cells are differentially affected by prenatal disruption of binocular inputs

In the mammalian visual system, the terminal arbors of retinal ganglion cell axons from the two eyes are restricted to mutually exclusive territories within their thalamic target, the lateral geniculate nucleus (LGN). Here we have investigated some of the factors that determine the adult morphology of terminal arbors in the cat's retinogeniculate system. Removal of one eye during prenatal life at a time when retinogeniculate axons from the two eyes are extensively intermixed within the LGN perturbs the subsequent morphological development of some but not all axons from the remaining eye. The presence of terminal arbors qualitatively normal in size, shape, and location within the LGN suggests that for some retinal axons, ongoing binocular interactions throughout prenatal life are not needed for the development of normal arbor morphology. However, many of the axons form arbors of abnormal size or location, suggesting that such features of axon morphology are not intrinsically determined for these axons but may be susceptible to external influences. Electrophysiological studies reveal that the abnormal arbors all belong to the functionally distinct Y class of retinal ganglion cells, whereas the normal arbors all belong to X cells. The different responses of X and Y axons to prenatal enucleation demonstrate that during development subsets of a single neuronal population projecting to the same target in the central nervous system can be under different developmental controls for axon arbor differentiation.

Morphological plasticity in the chick ventral lateral geniculate nucleus: temporal parameters

The retinogeniculate projection in the chick undergoes apparent augmentation following lesions in the optic tectum. Using autoradiographic tracing techniques we determined that the alteration of the retinal projection required a minimum of 4 days to be detected if tectal lesions were made at hatching and could be produced by lesions placed up to 1.5 years posthatch.

Organization of the dorsal lateral geniculate nucleus in a cat with congenital microphthalmia

The effects of congenital monocular microphthalmia on the development of the lateral geniculate nucleus were examined in a 10-week-old cat. The left eye and optic nerve in this animal appear normal. The right eye is about 30% smaller in volume than the left and the optic nerve from this eye has a cross-sectional area that is only 15% that of the left. In addition, this nerve contains few, if any, large myelinated axons. Both lateral geniculate nuclei are abnormal and the abnormality differs rostrally and caudally. The caudal portion most closely resembles the normal nucleus. Retinal input from both eyes is segregated into cellular laminae that are separated from each other by cell sparse interlaminar zones. However, the input from the microphthalmic eye seems to be sparse and patchy and it does not support normal cell growth. All neurons, including glutamic acid decarboxylase-positive (GAD+) neurons, in laminae innervated by the small eye are reduced in size in a pattern similar to that seen following the removal of retinal input. In comparison, the rostral portion of the nucleus receives very little input from the microphthalmic eye. Instead the normal eye densely innervates nearly the entire nucleus. In this region, interlaminar zones fail to form but the input from the normal eye is able to support cell growth including the growth of GAD+ neurons.

Role of competitive interactions in the postnatal development of X and Y retinogeniculate axons

The cat's retinogeniculate pathway is largely composed of X and Y axons, which represent two distinct neuronal streams organized in parallel. Our earlier data, summarized in the previous paper, suggest that the postnatal development of retinogeniculate axon arbors is characterized by competitive interactions between the X and Y axons. Thus, during development, X arbors in lamina A or A1 are initially broad or exuberant before the Y arbors begin to develop adultlike arbors; the X arbors then shrink to their adult form as the Y arbors grow and establish their mature complement of connections; monocular lid suture prevents the rapid growth of Y arbors, which in turn prevents the pruning of X arbors; and monocular enucleation at birth allows X arbors from the remaining eye to retain their exuberance although completely confined to their appropriate lamina A or A1, whereas the Y arbors develop aberrant extensions into adjacent, previously denervated laminae. We now provide additional evidence for the role of competition between retinogeniculate X and Y axons during development. The addition of visual deprivation by lid suture of the remaining eye to monocular enucleation at birth causes no apparent change in the morphology of X arbors in laminae A and A1. In contrast, the Y arbors of such cats continue to form extensive translaminar sprouts in the previously denervated laminae despite severely reduced terminations in the lamina A or A1 normally innervated by the remaining eye. We interpret these new data, in conjunction with our earlier data, as follows. If retinogeniculate X and Y arbors complete for synaptic space during postnatal development, terminations of Y axons can be affected by lid suture only in geniculate laminae where terminations of X axons are also present. Thus, Y axon arbors are severely reduced in deprived lamina A or A1 following lid suture whether or not the other eye is removed. Where X arbors are not present, such as in lamina C or the laminae inappropriate for the remaining eye after removal of the other, the lid suture has no obvious effect on development of the Y arbors.

Morphology of retinogeniculate X and Y axon arbors in monocularly enucleated cats

We examined the terminal arbors of single, physiologically identified retinogeniculate X and Y axons from the remaining retinas of adult cats raised from birth with monocular enucleation. These were compared with arbors of X and Y axons in normally reared cats. We used intra-axonal injections of horseradish peroxidase to label each axon after recording its response properties. While the axons in monocularly enucleated cats exhibited normal response properties, both X and Y axons in these cats had abnormally large terminal arbors. Each of the hypertrophied X arbors appeared to be completely confined to the single geniculate lamina A or A1 appropriate to its eye of origin (i.e., lamina A for the contralateral retina and lamina A1 for the ipsilateral retina). In contrast, in addition to their normal terminations, most of the Y arbors seemed to extend well into laminae normally innervated only by the retina that was removed. Thus most or all of the translaminar sprouting previously reported for monocularly enucleated cats appears to reflect extensions of Y axon arbors. These data, in addition to earlier, analogous data from young kittens and cats reared with monocular lid suture, suggest the following sequelae during postnatal development: the retinogeniculate X arbors mature first and develop exuberant arbors that are later competitively pruned as the Y axons expand their innervation of the lateral geniculate nucleus; monocular lid suture prevents the Y axons from succeeding in this competition, so they fail to establish normal arbors and cannot reduce the exuberant X arbors; monocular enucleation offers a less resistant path in the denervated laminae for the rapidly growing Y arbors from the remaining eye, and the expansion of these arbors there reduces the competitive pressure on the exuberant X arbors. Thus, in monocularly enucleated cats, sprouting is limited to Y axons, either because only they possess the capacity to sprout or because they are in the midst of a period of relatively rapid growth at the time of the neonatal enucleation. The X axon arbors are also abnormally large within their appropriate laminae. This occurs presumably because they are able to maintain their immature exuberance, although we cannot rule out the possibility that they are pruned and later regrow to the final size seen in our experiments.

Competition and the dynamics of axon arbor growth in the cricket

The growth of an identified axonal arborization in the cricket cercal sensory system was studied under conditions that vary the number of neighboring axonal arborizations. The cell studied is one of a small number of neurons that arborize bilaterally and is called the X-neuron. Normally the axonal arborization of X is distributed roughly symmetrically about the midline. Seven days after the birth of this neuron, as soon after its birth as it can be stained, the axonal arbor is more than half the normal size and it exhibits the normal degree of bilaterality. During the remaining 50 days of postembryonic development, the arbor grows to its adult size, maintaining the bilateral distribution. The relationship between the growth of this axonal arbor and the presence or absence of its neighbors was studied by the removal of one of the sensory appendages. The removal of a cercus removes those neighbors near one half of the X-neuron's arbor and causes a dramatic shift in the bilateral distribution of X's axonal processes; after treatment nearly all of the varicosities were found in the deafferented region (Figs. 2, 6). Thus, neuron X responds to the loss of some to its neighbors by removing synaptic material from the area with normal neighbors and inserting additional material in the region with fewer neighbors. This effect is age dependent. Removal of neighbors early in life, during the initial period of synaptogenesis, causes a very rapid change, while similar treatment later in life causes a much slower response (Fig. 7). Thus the flexibility of the neuron is correlated with its growth rate; it is very flexible during its early, rapid growth phase and less flexible when the growth rate slows later in life. This continuing flexibility of the axonal arbor could also be demonstrated by allowing the amputated cercus to regenerate (Figs. 8, 9). Under these conditions the growth of X's arbor returned toward the normal bilateral distribution. However, regeneration of the neighboring arbors never restored the arbor to complete normality because a bias in the structure had already been imposed. In effect, regeneration arrested the change but could not reverse it. In brief, an identified sensory neuron's growth has been demonstrated to depend in part on the presence or absence of neighbors. This dependence extends throughout the life of the animal and fits the definition of a competitive interaction.

Mode of reinnervation in neonatally denervated lamina of the cat's lateral geniculate nucleus

Physiological properties of relay cells were studied in the lateral geniculate nucleus (LGN) of neonatally one-eye-removed adult cats. Most of the relay cells recorded from the denervated lamina (lamina A1) were still classifiable into either Y- or X-cells, like those recorded from the non-denervated lamina (lamina A). However, receptive field centers of both Y- and X-cells in the denervated lamina were slightly over twice as large as those of the corresponding classes in the non-denervated lamina. These data indicate that reinnervated relay cells may receive excessive convergent inputs from either Y or X type retinal axons which have grown translaminarly.

Regulation of axon number in primate optic nerve by prenatal binocular competition

We report here that in mature rhesus monkeys in which one eye was removed during the first half of gestation, the optic nerve of the remaining eye is larger and contains significantly more retinal axons than in age-matched control animals. Such supernumerary fibres in monocularly enucleated monkeys probably result from an arrest in the normal process of elimination of excess embryonic optic axons. Although the function of retained supernumerary optic axons is unknown, this finding demonstrates that (1) competition between the two eyes for synaptic territory occurs prenatally, before visual experience and (2) an early lesion in one brain area can adjust or enhance the size and perhaps the performance of other synaptically related structures.

A light and electron microscopic study of regrowing pyramidal tract fibers

Autoradiographic and EM techniques were used to study the regenerative capacity of severed axons in the mammalian CNS. In infant and adult hamsters the pyramidal tract was severed unilaterally in the medulla several millimeters rostral to the decussation. After survival to adulthood, the animals received injections of [3H] proline in the sensorimotor cortex ipsilateral to the lesion. Autoradiography showed that labeled pyramidal tract axons in the medulla did not cross the lesion site. Instead, in animals with infant lesions there was massive new axonal growth arising from the severed pyramidal tract several millimeters rostral to the cut. Most of these labeled fibers crossed to the contralateral brainstem, coalesced into a compact bundle, descended just medial to the spinal trigeminal nucleus, and grew caudally for 6-7 mm. Although the trajectory of the regrowing axons was completely abnormal, their pattern of termination in the dorsal column nuclei and dorsal horn of the cervical spinal cord was normal. Synapse formation by the anomalous regrowing pyramidal tract axons in their appropriate terminal areas was confirmed by electron microscopy of terminal degeneration in animals with infant pyramidotomies followed by adult cortical lesions. Autoradiographic labeling of the new pathway at short postlesion survival times showed that the fibers grew out rapidly at about 1 mm/day, a rate somewhat slower than normal (2-4 mm/day). There was a dramatic difference in the capacity of the pyramidal axons to regrow in animals operated as infants vs. those operated as adults. The regrowth was maximal with lesions at 4-8 days of age. Capacity for new growth declined sharply thereafter such that after 20 days of age, pyramidal tract lesions elicited no new growth but instead a progressive axon degeneration retrograde to the lesion. These results, in contrast to many previous findings, show that significant regrowth of severed axons can occur in the neonatal CNS. Most importantly pyramidal tract fibers regrowing by anomalous routes can nevertheless establish synaptic connections in appropriate terminal areas and thus, as we show in the following paper, play a functional role in maintaining normal motor behavior.

Development of terminal arbors of retino-geniculate axons in the kitten--II. Electron microscopical observations

Individual retino-geniculate axon arbors from kittens of 1 to 8 postnatal weeks were densely filled with horseradish peroxidase. The light-microscopical appearance of various types of immature growing tips was correlated with their synaptic relations as seen in the electron-microscope. During the first 2 postnatal weeks, the broad irregular bases of spray-like terminal extensions as well as foliate growth-cone-like terminals contain the round synaptic vesicles and pale mitochondria characteristic of adult retinal terminals. They form elementary glomerular arrangements in which they are the central profile synapsing with dendritic profiles. Many postsynaptic profiles contain large electron-lucent vesicles characteristic of growing neurites, whereas the immature retinal terminations do not. The more numerous finger-like growing tips also contain round synaptic vesicles and make short en passage contacts onto dendrites and outgrowths of perikarya of geniculate cells. The latter type of contact is not seen in the adult. After 3 weeks, retinal terminals are larger and make contacts in distinctly glomerular arrangements. Glial sheaths are evident for the first time. By 6-8 weeks, the terminals which appear crenulated in the light-microscope are always the central profile in a mature glomerulus. Smaller rounder terminals make simple axo-dendritic contacts as in the adult. The few immature terminal structures seen at 8 weeks also contain synaptic vesicles, but their contacts are not adult-like, nor are their synaptic arrangements entirely surrounded by glia. These findings, along with those of the previous paper, demonstrate that (a) many retino-geniculate terminals establish synaptic contacts long before attaining their adult form; (b) during the period optimal for induction of translaminar axon sprouting (1-2 weeks postnatal), only a few immature terminal forms participate in elementary glomeruli; (c) the decreasing malleability of these axons after 3 weeks is accompanied by an increase in crenulated terminals that enter glomeruli surrounded by glial sheaths. It is suggested that ensheathing of synaptic arrangements by astrocytic glia may be one factor which subsequently impedes axon sprouting in older animals.

Development of terminal arbors of retino-geniculate axons in the kitten--I. Light microscopical observations

The maturation of terminal arbors of retino-geniculate axons was studied in normal kittens from 1 to 8 postnatal weeks. Horseradish peroxidase injected into the optic tract rostral to the lateral geniculate nucleus gave a dense fill of cut axons and their terminals, resembling results obtained by the Golgi methods. At 1 and 2 weeks postnatal, the overall size and extent of axon arbors is not significantly different than in the adult. However, terminal branches of axon arbors at this age give rise to high variable endings. They terminate in finely divided sprays of finger-like extensions and filopodia bearing small spikes rather than the characteristic clusters and strands of crenulated terminals of adult axons. The bases of these sprays are broad and irregular in contour, with foliate growth-cone-like structures occurring at the ends of some branches. At 3 weeks postnatal, terminal swellings become thicker and more crenulated in contour. By 5 to 6 weeks, axon arbors have adult-like terminals with respect to their size, shape and arrangement, although entire branches may still be immature with irregular terminal swellings. Small slightly indented terminals are also seen for the first time. By 8 weeks, axon arbors are generally mature, but occasionally have a bizarre and immature arrangement of fine extensions or growth-cone-like tips. Although these observations do not establish whether reduction of branches or of terminals takes place during postnatal maturation, they demonstrate that kitten retino-geniculate axon terminal arbors are highly immature and undergo considerable changes during the period optimal for induction of sprouting by eye enucleation. The morphogenetic maturation of terminal branches that begins at 3 weeks also marks a decline in their sprouting capacity, even though remodeling of terminals is not complete until after 8 weeks of age.

Effect of neonatal unilateral enucleation on the development of orientation selectivity in the primary visual cortex of normally and dark-reared kittens

The developmental properties of 573 neurones have been investigated in the primary visual cortex of eight binocularly intact and twelve unilaterally enucleated kittens. It is shown that removal of one eye at birth alters the development of orientation selectivity observed in the presence or absence of visual experience. In 6-week-old deprived kittens, there remain significantly more orientation selective cells in enucleated than in binocularly deprived kittens. These deprivation-resistant cells respond preferentially to horizontal or vertical orientations and are recorded mainly in the cortex contralateral to the remaining eye. In six-week-old kittens with visual experience, the process of tuning maturation appears to be unaffected by unilateral enucleation at birth. However, a larger over-representation of horizontal and vertical orientation preferences is observed in uniocular kittens than in binocularly intact kittens, suggesting that the development of oblique orientation preference depends upon the presence of binocular afferents in the visual pathway.

Short- and long-term effects of neonatal and adult visual cortex lesions on the retinal projection to the pulvinar in cats

An increased retinal projection to th pulvinar occurred in cats following neonatal ablation of visual cortical areas 17, 18, and 19. After unilateral lesions, the retinopulvinar projection visualized with autoradiography was larger and denser on the lesioned side than on the intact side. This increased projection was first recognized when the lesioned kittens were 1 week old, when increased labeling in the pulvinar was first detected. The retinopulvinar projection on the intact side of the brain decreased in size with age over the first 3 months of life. This decrease in size was not observed on the lesioned side of the brain. The absence of this decrease on the lesioned side may represent a failure of retraction of exuberant connections, but because of the active invasion by retinal fibers immediately after the lesion, observations of the normal retraction may simply be obscured. Severe cellular degeneration in the dorsal lateral geniculate occurred a few days before the changes in the retinopulvinar projection began; this loss of a postsynaptic target for many retinal axons may trigger the reorganization of input to the pulvinar retino-recipient zone (pulivnar-RRZ). After adult visual cortex lesions, no reorganization of the retinopulvinar projection took place. The modification of the projection from the retina to the pulvinar that occurs after ablation of visual cortex in the newborn cat may contribute to the sparing of visual abilities seen after these lesions.

Abnormal axonal growth in the dorsal lateral geniculate nucleus of the cat

Retino-geniculate axons in the cat were induced to grow abnormally by cutting one optic nerve in kittens. Surviving optic tract axons that had grown into the denervated regions were then filled in the adults with horseradish peroxidase to reveal the terminal arbors of individual axons. Two types of abnormal axonal growth are described--translaminar growth and monocular segment growth. Translaminar growth is the most common and occurs between laminae in the binocular part to the nucleus. Axons giving rise to translaminar growth do not branch as they pass through the denervated regions of the nucleus. Instead, the abnormal branches originate from portions of the terminal arbor within the normal target lamina. These axons look like normal retino-geniculate axons in terms of their branching patterns, cytological features, and patterns of synaptic contacts except that parts of their terminal arbors have expanded to innervate inappropriate laminae. The distribution of translaminar branches overlaps the distribution of a restricted group of surviving large neurons that have not undergone denervation atrophy. Monocular segment growth invades the lateral pole of the nucleus directly from the optic tract. These branches arise from axons passing through or near the denervated region and appear to represent the formation of new terminal arbors. The synaptic swellings arising from these branches have cytological features like the synaptic swellings arising from translaminar branches and they form similar patterns of synaptic contacts. However, monocular segment branches degenerate more rapidly when damaged and they are not associated with surviving large neurons.

Synapse formation after injury in the adult rat brain: preferential reinnervation of denervated fimbrial sites by axons of the contralateral fimbria

The dorsolateral quadrant of the lateral septal nucleus receives projections from both the ipsilateral and the contralateral fimbria. In the adult rat the effect of fimbrial lesions on synapse formation has been studied by a quantitative electron microscopic analysis of the various types of synapses present, using electron-dense degeneration to identify fimbrial fibre terminals. In this area, the fimbrial axons from both sides together account for about 30% of the total number of synapses and they terminate mainly on dendritic spines. The ipsilateral fimbria forms twice as many synapses as the contralateral fimbria. When one fimbria is cut and time left for the degeneration to be removed, the numbers of synapses are restored to normal levels and the remaining fimbria acquires, on both sides of the septum, a number of synapses equal to the sum of the two individual fimbria, This suggests that the axons of the surviving fimbria have completely reinnervated the denervated postsynaptic sites formerly occupied by the cut fimbria of the other side, effectively excluding non-fimbrial axon terminals, even though the latter constitute the majority (70%) of the synaptic terminals in the region. When both fimbria are cut the numbers of synapses are once again restored to normal levels. However, since there are now no fimbrial axons left, the denervated fimbrial postsynaptic sites must this time have been reinnervated by non-fimbrial axons. Reinnervation by non-fimbrial axons is numerically equally effective in reclaiming the denervated sites, although when compared to the reinnervation by fimbrial axons, the removal of degenerating terminals is somewhat slower, and among the reinnervating terminals there is a much higher incidence of axon terminals making more than one synaptic contact in the plane of section. Thus, fimbrial axons, when present, have the ability to exclude the reinnervation of denervated fimbrial sites by non-fimbrial axons, despite the fact that the latter are both more numerous and also clearly capable of reinnervating those sites when no fimbrial axons are present. Two possible mechanisms are discussed: a spatial preference based on the geometrical arrangements in the neuropil, and a temporal preference based on the relative rates of response of the fimbrial vs the non-fimbrial axons.

Absence of sprouting by retinogeniculate axons after chronic focal lesions in the adult cat retina

Focal lesions were placed in the retina of adult cats in order to denervate partially the laminae of the lateral geniculate nucleus (LGN). Retinogeniculate projections were assessed after survival times of from 5 days to 2 years by means of either reduced silver staining for degeneration or autoradiographic labelling. Filling of the lesion-denervated zones by 'sprouts' from the intact retinofugal fibers was not observed, even in the brains of animals with long-term lesions. It was concluded that the retinogeniculate projection in adult cat does not display any significant ability to sprout into denervated regions.

Regrowth of severed axons in the neonatal central nervous system: establishment of normal connections

When pyramidal tract axons are cut in the adult hamster, fibers degenerate in both anterograde and retrograde directions from the lesion. If the same operation is performed on infant hamsters, however, there is massive regrowth of the severed axons via a new brainstem pathway to their appropriate terminal sites in the medulla and spinal cord. In contrast to previous studies, these results suggest that axons in the mammalian central nervous system damaged early in life may regenerate in a functionally useful way.