Evidence that the early postnatal restriction of the cells of origin of the callosal projection is due to the elimination of axonal collaterals rather than to the death of neurons

[125I]alpha-bungarotoxin binding marks primary sensory area developing rat neocortex

The postnatal ontogeny of [125I]alpha-bungarotoxin (alpha-Btx) binding distribution in rat neocortex was described and quantified using autoradiography of in vitro labeled brain sections. During the first two weeks, distinctive transitory radial and laminar patterns emerged. Dense columnar bands of alpha-Btx binding extended through the depth of primary sensory cortex, including somatosensory, visual and auditory areas. An association of alpha-Btx binding with thalamic input zones was further demonstrated within developing somatosensory cortex, where discrete radial bands appeared over the whisker barrels around the time that ingrowing thalamocortical fibers segregate as they selectively innervate the barrels. The early laminar distribution of alpha-Btx binding also resembled that of developing thalamocortical afferents. From P12 to P20, alpha-Btx radial distinctions faded and the laminar pattern changed further to achieve the adult distribution. The spatiotemporal ontogeny of alpha-Btx binding suggests a role for alpha-Btx binding sites in the development of cortical connectivity.

Do cortical areas emerge from a protocortex?

The adult mammalian neocortex consists of numerous 'areas' distinguished from one another largely on the basis of distinctions in cytoarchitecture and connections. The developing neocortex, though, lacks many of these area-specific distinctions, and is more uniform across its extent. This less differentiated structure, here termed the 'protocortex' undergoes considerable modification after neurogenesis which results in the emergence of well-defined neocortical areas. To what extent, then, are neocortical areas predetermined? This issue is considered in the context of recent findings on the generation of the neocortex and its subsequent parcellation into distinct areas.

Cerebrocortical microdysgenesis with anomalous callosal connections: a case study in the rat

A spontaneously occurring area of cerebrocortical microdysgenesis, resembling microgyria, and having its origin during development was noted in the left auditory cortex of a rat which had received a callosal section in adult life. Adjoining sections were prepared with cresyl violet for visualization of cell bodies, and by the Fink-Heimer method for degenerating axonal terminals. The microdysgenetic region was characterized by fused molecular layers containing ectopic neurons and by abnormal cortical lamination. The architecture of terminal degeneration of callosal axons within this region was abnormal with a notable absence of the usual lamination pattern and the presence instead of dense terminations in what would normally be layers I-IV and in connecting bridges running across layer V. Abnormal developmental processes that might account for the anomalous projection patterns in cortical malformations are discussed in the light of knowledge about normal corticogenesis. The behavioral implications for some childhood learning disorders are also considered.

Anatomical asymmetries of the human cerebral cortex

This paper summarizes current knowledge concerning the anatomical asymmetries of the cerebral cortex and presents the main hypotheses proposed so far to account for these asymmetries. The main question raised by the presence of these asymmetries is that of their functional significance and has been explored by attempting to correlate handedness, the most immediate feature of functional brain specialization, with neuroradiological evidence of brain asymmetry using either carotid angiograms or CT scan. These methods, however, yielded controversial or incomplete results. The recent advent of magnetic resonance imaging now makes it possible to perform direct evaluation and measurements of cortical asymmetries. Preliminary results of a personal study using this approach are presented. They show in particular a good correlation between the planum surface and handedness, and confirm the previously reported statistical correlation between handedness and callosal surface. These results are discussed in the light of modern theories about brain development.

Postnatal development of the efferent innervation of the rat cochlea

The postnatal development of the efferent innervation of the rat cochlea was studied by intracochlear injection of the fluorescent retrograde neuronal traces Diamidino yellow and Fast blue. Injections were performed on adult rats and on neonatal rats ranging from 0 to 8 postnatal days. It was found that the total number of neurones labelled in the brainstem after intracochlear injection was not significantly different in the newborn rat, compared to the adult. On the basis of cell body location and laterality of projections, there was a clear separation into lateral and medial efferent systems at the earliest postnatal age studied (PO). Evidence was also found in the newborn for a tonotopicity in the lateral system projection similar to that in the adult. Differences between the newborn and adult were a slight but significantly greater number of bilaterally-projecting cells in the newborn, and the presence in the newborn of a small number of cells located in the lateral superior olivary nucleus contralateral to their target cochlea. These were extremely rare in the adult brainstem. Evidence was found for the occurrence of postnatal neuronal death in nuclei of origin of both efferent systems. It is suggested that although the overall extent and general organization of the efferent projection to the cochlea in the rat appears to be established at birth, regressive changes are occurring during the postnatal shaping and maturation of this brainstem-to-cochlea pathway.

Frontal cortical and left temporal glutamatergic dysfunction in schizophrenia

Glutamatergic mechanisms have been investigated in postmortem brain samples from schizophrenics and controls. D-[3H]Aspartate binding to glutamate uptake sites was used as a marker for glutamatergic neurones, and [3H]kainate binding for a subclass of postsynaptic glutamate receptors. There were highly significant increases in the binding of both ligands to membranes from orbital frontal cortex on both the left and right sides of schizophrenic brains. The changes are unlikely to be due to antemortem neuroleptic drug treatment, because no similar changes were recorded in other areas. A predicted left-sided reduction in D-[3H]aspartate binding was refuted at 5% probability, but not at 10%. Previously reported high concentrations of dopamine in left amygdala were strongly associated with low concentrations of D-[3H]aspartate binding in left polar temporal cortex in the schizophrenics. The findings are compatible with an overabundant glutamatergic innervation of orbital frontal cortex in schizophrenia. The results also suggest that schizophrenia may involve left-sided abnormalities in the relationship between temporal glutamatergic and dopaminergic projections to amygdala.

N-methyl-D-aspartate induces recurrent synchronized burst activity in immature hippocampal CA3 neurones in vitro

Slices of hippocampus prepared from rats aged 1-10 days have been used to examine the chemosensitivity of CA3 pyramidal neurones to N-methyl-D-aspartate (NMDA). Superfusion of NMDA excited all neurones tested at all ages including the first day postnatal. In the majority of neurones this excitation was associated with the induction of a period of burst firing which disappeared on removal of NMDA. These bursts took the form of paroxysmal depolarizing shifts (PDSs) with a large amplitude depolarization and a high frequency discharge of spikes. The amplitude but not the frequency of occurrence of the PDSs was influenced by changes in the membrane potential and they could be abolished by either a high divalent cation medium or tetrodotoxin. Their occurrence was synchronous with an extracellularly recorded discharge. The NMDA induced excitation and the induction of the PDSs was attenuated by selective NMDA receptor antagonists D-aminophosphonovalerate (10-50 microM) and D,L-aminophosphonoheptanoate (20-30 microM). The results indicate that chemosensitivity to NMDA develops prenatally and that activation of NMDA receptors can in immature CA3 pyramidals induce recurrent synchronized burst activity.

Development of the thalamocortical system: transient-crossed projections to the frontal cortex in neonatal rats

The developmental remodeling of thalamic projections to frontal and prefrontal cortical fields was investigated in the rat by using a double retrograde tracing technique. Bilateral cortical injections of fluorescent tracers were made either in neonatal (first or second postnatal day) or in adult animals. In neonates, the cell populations retrogradely labeled from each cortical injection overlapped in a medial thalamic region that included the midline nuclei and the medial part of the mediodorsal nucleus, ventral medial nucleus, and nucleus gelatinosus. In adults, the overlap region was confined within the boundaries of the midline nuclei. Quantitative analysis showed that this overlap area was three times as wide in neonates as in adults. The neurons located in this region projected unilaterally both in neonatal and adult animals; bilaterally projecting cells were virtually absent. In neonates, a second set of contralaterally projecting neurons was found in more lateral thalamic regions. This population consisted of cell clusters in the dorsal part of the central lateral nucleus and in the lateral part of the ventral medial nucleus; scattered cells were also observed throughout other nuclei. This second cell population was represented in part by neurons bifurcating bilaterally. In adult animals, neurons projecting contralaterally were observed only occasionally in the lateral thalamus. The present results demonstrate that the bilaterality of thalamocortical projections undergoes a reduction during postnatal development. The mechanisms underlying this remodeling and the possible functional role of the transient-crossed thalamocortical system are discussed.

Retinal ganglion beta cells project transiently to the superior colliculus during development

In adult cats, retinal ganglion cells of the beta class project almost exclusively to the lateral geniculate nucleus rather than to the superior colliculus (SC). We have examined whether this target specificity is present during early development. To identify ganglion cells that send axons to the SC in development, rhodamine-labeled microspheres were deposited in the SC at embryonic day (E) 38, E43, or postnatal day (P) 4. Retinae were then removed between E56 and P32 and kept alive in a tissue-slice chamber so that ganglion cells that had been retrogradely labeled with microspheres could be injected intracellularly with Lucifer yellow to reveal their morphological class. Many beta cells could be retrogradely labeled by microspheres injected into the SC at E38 or E43. They were indistinguishable from beta cells projecting to the lateral geniculate nucleus and were found even when a single injection was restricted to the caudal portion of the SC. In contrast, beta cells could not be retrogradely labeled by microspheres injected into the SC at P4. The disappearance of a beta-cell projection to the SC cannot be explained entirely by cell death since as late as P32, well after the major period of ganglion cell death, many beta ganglion cells labeled with microspheres at E38 were still present. These observations suggest that many beta cells initially extend an axon collateral to the SC that is subsequently lost some time after E43. Thus, to achieve the remarkable specificity present in the adult visual system, beta cells must withdraw axon collaterals from an entire target nucleus. Similar collateral elimination may give rise to the specificity of afferent connections in other sensory systems.

Neurotrophic and behavioral effects of occipital cortex transplants in newborn rats

Cell suspensions of embryonic occipital cortex were transplanted into newborn rats with large unilateral visual cortex lesions. When the animals were adults, they were tested on a difficult visual discrimination, and subsequently their brains were analyzed for possible neurotrophic effects of the transplants on nonvisual cortical areas which normally form connections with the occipital cortex. Behaviorally, animals with lesions and transplants learn to discriminate between columns and rows of squares at a rate which is identical to normal rats while animals with lesions and no transplants are impaired. Volume and cell-density measures show that the transplants also rescue neurons in cortical area 8 that would normally degenerate following the cortical lesion. No such neurotrophic effect of the transplants is found in cortical area 24 or area 17 contralateral to the lesion. In rats with lesions and no transplants, there is a significant correlation between the amount of area 8 remaining after the lesion and trials to criterion on the columns-rows discrimination, a relationship that does not exist in transplant animals because of their normal learning curve and the consistent sparing of area 8. Injections of HRP into the visual cortex contralateral to the lesion result in variable numbers of labeled cells within the transplant. However, there is no consistent relationship between the number of transplant cells which project to the opposite hemisphere and learning of the discrimination. It is suggested that the learning deficit following the lesion is largely attentional and that the sparing of cortical area 8 (which in rats may include the analog of the frontal eye fields present in the primate cortex) contributes to the sparing of function.

A quantitative analysis of the development of the pyramidal tract in the cervical spinal cord in the rat

A quantitative electron microscopic analysis was undertaken of the development of the pyramidal tract, at the level of the third cervical spinal segment, in rats ranging in age from the day of birth to three months old. The axon number was calculated as the product of axon density, determined in a systematic random sample of electron micrographs, and tract area. During the first postnatal week the tract contains thin unmyelinated axons and growth cones. Growth cones are abundant in neonatal rats, but can still be observed occasionally at the end of the first postnatal week, indicating a continuous addition of pyramidal tract axons during the first postnatal week. Myelination starts around P10. By the end of the first postnatal month approximately 50% of the axons have already been myelinated. Myelination proceeds during further maturation, but in the three month old rat 28% of the axons are still unmyelinated. The total number of axons increases rapidly after birth up to 153,000 at the fourth postnatal day. Subsequently, the number of axons is reduced by nearly 50% to 79,000 in the adult rat. The axon loss is most prominent during the second postnatal week, when 32,000 axons are eliminated, but continues for several weeks at a slower rate.

Somatotopically organized transient projections from the primary somatosensory cortex to the cerebellar cortex

The organization of transient projections from the primary somatosensory cortex (S-I) to the cerebellar cortex in neonatal kittens was examined using orthograde intraaxonal labeling techniques. Tritiated amino acid injections into face, forelimb and hindlimb areas of representation in S-I labeled mossy fiber-like terminals of cerebrocerebellar axons in different areas of the cerebellar cortex bilaterally. The hindlimb area of S-I projected to lobules I-IV in the anterior lobe and to ventral folia of the paramedian lobule (PML). Injections into forelimb areas of S-I labeled terminals in lobules IV and V and in intermediate and dorsal folia of the PML. The face area of S-I projected to the lobules V and VI, to medial folia in the ansiform and simplex lobules and to dorsal PML folia. Labeled terminals were more numerous in the cerebellar cortex contralateral to the S-I injections, except in lobules I and II and the ventral PML where the density of hindlimb input was approximately the same on both sides. These observations were supplemented by findings that small wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections into the dorsal or ventral PML resulted in retrogradely labeled layer V pyramidal neurons in lateral (face and forelimb) and medial (hindlimb) areas of S-I respectively. The somatotopic organization of transient S-I cerebrocerebellar projections is very similar to the topography of cerebellar somatosensory afferent pathways in adult cats.

The role of competition in the refinement of the projections of sympathetic neurons to the rat eye during development

Within the iris, the extent of the nerve plexuses derived from the sympathetic neurons of the superior cervical ganglion (SCG) and the sensory, substance P (SP) neurons of the trigeminal ganglion depend on competition for target tissue derived growth factors. During the postnatal period when these plexuses are initially established, many sympathetic neurons are known to extend transient collateral projections to various targets within the eye. We have investigated the role of neuronal competition in the withdrawal of these transient projections by removing the sensory SP fibres using neonatal capsaicin treatment (50 mg/kg) on days 2, 10 and 17, or day 2 only. At 2, 4 and 7 weeks, fast blue was injected into the anterior chamber or posteriorly into the vitreous to retrogradely label sympathetic neurons in the SCG. Capsaicin treatment resulted in a transient retention of the projections of supernumerary neurons to the eye early during development and a maintenance of the collaterals of some of these sympathetic neurons to adulthood, but only when rats received multiple capsaicin injections. The retention of collaterals in these animals was reflected in an increase in the density of the sympathetic nerve plexus within targets such as the iris. Immunohistochemistry for SP showed that a single injection of capsaicin was less effective than multiple injections in removing the SP-containing nerve fibres from the iris and in causing long-lasting changes to the sympathetic projections. We conclude that some form of interaction between different neuronal populations within the eye plays an important role in the refinement of collateral projections of sympathetic neurons, but has no long-term effect in influencing the final number of neurons which project to the eye.

Transient direct connection of vestibular mossy fibers to the vestibulocerebellar Purkinje cells in early postnatal development of kittens

Postnatal development of mossy fiber afferents from the vestibular and the visual system to the vestibulocerebellum was studied electrophysiologically and morphologically. In kittens anesthetized with pentobarbital sodium and N2O plus halothane, extracellular simple and complex spikes of Purkinje cells were recorded in the flocculus, nodulus and uvula. In the flocculus, stimulation of the VIIIth, but not the optic nerve, evoked simple spike responses with a latency of 16 ms at the day of birth which decreased to 5 ms by day 15 (short latency group). On the other hand, another group of simple spike responses with much longer latencies (50-80 ms) began to be elicited on day 7 via both the optic and VIIIth nerves. The latency decreased to 24 ms by day 15 and 10 ms on day 30. These latencies further shortened with development to the adult latency value (3-5 ms). Simple spike responses of the short latency group were also evoked in the nodulus and uvula from the VIIIth nerve with a slightly longer latency than that in the flocculus (23 ms on day 3 and 12 ms on day 17). Because of the immaturity of granule cells in early postnatal days, short latency simple spike responses from the VIIIth nerve suggested the direct synaptic connection of vestibular mossy fibers with Purkinje cells. Horseradish peroxidase was injected into the white matter of the flocculus, nodulus and uvula in slice preparations. Mossy fibers labeled with horseradish peroxidase showed fine branches extending to reach Purkinje cell somata from mossy swellings in the internal granular layer during days 2-20. Electron microscopy showed that the labeled mossy fibers made intimate contacts with Purkinje cell somata and the terminals contained many round synaptic vesicles. Pre and postsynaptic densities were occasionally found. After day 20, direct mossy fiber connections with Purkinje cells could not be observed. During days 7-20, these direct connections, as well as mossy fiber-granule cell connections could be observed. It was demonstrated that during early postnatal development, vestibular mossy fibers temporarily make direct contact with Purkinje cells, through which impulses could be transmitted to elicit simple spikes in Purkinje cells.

Interhemispheric connections differ between symmetrical and asymmetrical brain regions

Coronal sections from the brains of male Wistar rats that underwent corpus-callosectomy in adulthood were stained with Cresyl Violet for Nissl substance or by the Fink-Heimer method for terminal axonal degeneration. Measurements of volumetric asymmetry of neocortical region SM-I were made, and the per cent of terminal degeneration computed. As in previous studies, there was a negative correlation between asymmetry coefficient and total (right plus left) architectonic volume, indicating that symmetrical brain regions are larger than the average of the corresponding regions in asymmetrical brains. It was also found that as volumetric asymmetry increased, the per cent of axonal termination decreased, partly as a result of a decrease in the number of patches of callosal axonal termination. These results are interpreted in the light of what is known about the ontogenesis of callosal connectivity, and mechanisms for the development of architectonic asymmetry in the cerebral cortex are postulated.

Sequential development of connections between striate and extrastriate visual cortical areas in the rat

In these experiments we have asked whether the projection from the rat's primary visual cortex, area 17, to the extrastriate visual cortical area 18a is formed in a sequence and whether that sequence resembles the pattern of inside-out cortical neurogenesis. For this purpose fluorescent retrograde tracers were injected into area 18a at different postnatal ages (P1, P5, adult). Animals survived until 3-4 weeks of age, after migration is complete and neurons have arrived at their final laminar location. In the ipsilateral cortex, P1 injections retrogradely labeled cells in layers 5 and 6 of area 17. Labeling after P5 injections extended into more superficial layers and included the bottom of layer 2/3 and layers 4-6. After P5, more labeled cells were found at the top of layer 2/3, producing the adult laminar pattern, where the projection originates predominantly from layer 2/3. A similar sequence of laminar labeling was observed in the transcallosal connection of area 18a. This sequence of labeling, deep layers before superficial, resembles the pattern in which cortical neurons are born and indicates that axons arrive at their cortical targets in the order the cells were generated.

Transient projections from the lateral geniculate to the posteromedial lateral suprasylvian visual cortex in kittens

The postnatal maturation of the projection from the lateral geniculate nucleus to the posteromedial lateral suprasylvian visual cortex (PMLS) was studied with injections of fluorescent dyes into the PMLS at various postnatal ages. Labeled neurons projecting to the PMLS were present in all laminae of the ipsilateral lateral geniculate on the day of birth. However, there was a conspicuous change in the distribution of labeled geniculo-PMLS neurons by 11 days of age: now very few labeled neurons were present in lamina A, indicating a loss of geniculo-PMLS connections. The loss of connections began at the peripheral margins of lamina A and proceeded through other laminae toward laminae C1-3. By adulthood, labeled geniculo-PMLS neurons were largely confined to laminae C1-3; they were never observed in lamina A or A1 and were rarely observed in lamina C. To determine whether the lateral geniculate neurons survived after their projections to PMLS were lost, injections of fast blue were made at 1 or 2 days postnatally and the animals were allowed long postinjection survival times. Labeled neurons were found in all lateral geniculate laminae, thereby indicating that for many neurons the loss of connections could be attributed to a loss of their axon collaterals rather than to the death of the neurons themselves. After injections of fast blue into the PMLS and diamidino yellow dihydrochloride into area 17 shortly after birth, many double-labeled neurons were present in all laminae, indicating that they have collaterals to both targets. Thus, the survival of many of the geniculo-PMLS neurons contributing to the transient geniculo-PMLS projection seems to be due to sustaining collateral projections to area 17 or other cortical targets.

Transient GABA-like immunoreactive axons in the corpus callosum of perinatal rats

During the early postnatal development of the rat, gamma-aminobutyric acid (GABA)-like immunoreactive axons were contained within the subcortical white matter. Some of the immunoreactive axons crossed the midline, while others followed a vertical or longitudinal trajectory within the lateral part of the corpus callosum. Growth cones were occasionally observed. The immunoreactive axons were transitory, and were last detected at postnatal day 6.

The development of the corpus callosum in cats: a light- and electron-microscopic study

Changes in the size and shape of the corpus callosum (CC)--and in number, size, and structure of callosal axons--between embryonic day 38 (E38) and postnatal day 150 (P150) were studied by light and electron microscope in 25 kittens. The development of the CC was divided into three phases: 1. Embryonic development (E38, 53, 58): At E38, only part of the body of the CC was formed. At E53 and E58, the CC was still very short, but its different parts (genu, body, and splenium) had formed. The cross-sectional callosal area (CCA) was 5.4 mm2 at E53 and 5.6 mm2 at E58. The CC contained 46.3 and 56.4 million axons at E53 and E58 respectively. Mean axon diameters were 0.26 micron at E53 and 0.27 micron at E58. 2. Early postnatal development (P4, 9, 15, 18, 21, 26): The CC at P4 was much longer than at E58 and still slightly elongated during this phase; CCA reached 8.55 mm2 at P4 and 8.88 mm2 at P26. There was a substantial axonal loss (66.8 million at P4 and 52.6 million at P26). From P15 onward, premyelinated and myelinated axons were seen. Mean axon diameter increased from 0.30 micron at P4 to 0.33 micron at P26. 3. Late postnatal development (P39, 57, 92, 107, 150). The CC grew dramatically in both length and thickness, the latter especially in the genu. CCA was 10.1 mm2 at P39 and 15.3 mm2 at P150. The number of axons still decreased (46.5 million at P39 and 31.9 million at P150). The growth of the CCA paralleled the increase of myelinated axons (0.5% at P26 and 29.6% at P150 and in the mean axon diameters (0.34 micron at P39 and 0.42 micron at P150). A number of axonal ultrastructural peculiarities (electron-dense bodies, large vacuoles, lamellated bodies, etc., including those mentioned below) were noticed; their frequency at different ages was estimated as the percent of total axons. Interestingly, accumulations of vesicles inside axons increased from 4.1% at E53 to 8.9% at P26, dropped to 0.2% at P39, and remained below 1% thereafter. Swollen mitochondria increased from 0.2% at E53 to 0.9% at P26 and dropped to 0.06% (on the average) from P39 onward. Accumulations of vesicles and swollen mitochondria increased during the phase of rapid axonal elimination; thus, they may indicate axonal retraction and/or degeneration. Microglia-gitter cells and astrocytes showing signs of phagocytosis were found during the embryonic and early postnatal development and may be involved in axon elimination.

In vivo visualization of callosal pathways: a novel approach to the study of cortical organization

I describe here the successful visualization of interhemispheric callosal connections in the live mammalian cortex. The development of this method was prompted by the finding that fluorescent tracer labeling of groups of cortical neurons, when done under optimal conditions, is sufficiently intense to be visible even in the whole brain preparation. The new approach could provide a useful tool for enhanced precision in localizing cortical modules in vivo. In a typical experiment, rats had their left cortical hemisphere extensively injected with the fluorescent tract-tracer bisBenzimide (BB). After appropriate survival, the right cortical hemisphere was illuminated with UV light and the fluorescing callosal pattern could be discerned under the network of blood vessels even with the unaided eye. The pattern, although diffuse, was grossly similar to the pattern of callosal connections as seen in flattened, sectioned cortex. Features that could be discerned were: the main callosal band straddling the lateral border of area 17, several rings and bands in extrastriate areas 18a and 18b, and a major band straddling the lateral border of area 3. The vitally visualized callosal pattern was used to guide injections of either wheat germ agglutinin conjugated to HRP (WGA-HRP) or rhodamine-labeled microspheres (RLM) into precisely localized sites in occipital cortex. There were numerous instances of doubly labeled neurons stained both with BB and WGA-HRP or RLM, suggesting that uptake of BB combined with UV exposure did not hinder the ability of stained neurons to take up and transport a second tracer. It is suggested that vital tract tracing be used as a tool for enhanced precision in studies of cortical connectivity.

Developmental interactions between the corpus callosum and the visual system in cats

The neonatal corpus callosum is involved in the development of pathways or mechanisms that coordinate the inputs from the two eyes. Several related visual functions are permanently altered by the absence of the callosum during early development. As determined behaviorally and by visual evoked potentials, the normal amount of 90 degrees of the visual field in which both eyes respond to stimulation is almost completely eliminated. Also, there is a reduced number of binocular cells in striate cortical regions representing most of the visual field. In addition, behaviorally measured visual acuity is reduced. Changes in acuity and striate binocularity only result when a callosal section occurs during a critical period of the first postnatal month, and the earlier the surgery, the greater the changes. The lack of myelination during the first postnatal month indicates that the conduction properties of the callosum are poorly developed during its critical period. The pattern of callosal connectivity is probably significant for its role in development, but not all callosal fibers are necessary for normal visual development. Developmental plasticity of callosal connections has been demonstrated for striate cortex, but now it has also been demonstrated for the claustrum. Thus, the callosal role in regions representing both central and peripheral visual field, in neocortical and non-neocortical brain areas should be reassessed.

Control of cell number in the developing neocortex. II. Effects of corpus callosum section

To determine if cell death participates in the regulation of cell number between interconnecting populations of the neocortex, we sectioned the corpus callosum of neonatal hamsters, thus depriving callosally projecting cells of their normal targets and callosally-recipient cells of their normal afference. The numbers of neurons per unit column in two areas of the cortex which have heavy callosal projections (the 17-18a border and area 6) and one area that is relatively acallosal (area 3) were compared in animals with early corpus callosum sections and controls. No differences were found, either for a 'unit cortical column,' or for the callosally-projecting layers (II-III and V). Mean soma sizes in layers II-III and V of all three areas were likewise unchanged. In area 6 and part of area 3, however, the distribution of soma sizes in callosally projecting and recipient laminae was significantly altered. The change in size distribution without change in mean soma area suggests that the cortex responds to the elimination of the callosal pathway in more than one way. Since no role for cell death in removal of diffuse connectivity or in target regulation of neuron number has yet been found, a new hypothesis for the function of cell death in local cytoarchitectural differentiation of the cortex is proposed.

Control of cell number in the developing neocortex. I. Effects of early tectal ablation

Target availability is an important factor in the early control of neuron number in many structures in the developing vertebrate nervous system. In early neocortical development, the role of target availability in the survival of subcortically projecting neurons is not yet understood, particularly because these cells' axons are widely distributed and highly branched. In this study, we have looked for alterations in the pattern of early cell death, adult cell density and adult morphology of pyramidal cells in layer V of visual cortex consequent to removal of one of their principal targets, the ipsilateral superior colliculus. After neonatal tectal ablation, there was no difference in the incidence of pyknotic cells in the cortex overall, or in layer V during the period of normal cell death in the cortex. Neither in adulthood, nor at any point in development did the density of layer V cells or cortical cell density overall differ from normal in Nissl material. Soma size of cells in layer V overall did not differ from normal in Nissl material. In addition, the soma size of the subpopulation of cells labelled with horseradish peroxidase (HRP) from midbrain injections was unaltered. In summary, this cell population appears unresponsive in both number and morphology to deletions of a major component of its target pool. This observation has some interesting implications for reasons of constancy of cell number in layer V across cytoarchitectonic areas.

Characterization of transient cortical projections from auditory, somatosensory, and motor cortices to visual areas 17, 18, and 19 in the kitten

We have examined the anatomical features of ipsilateral transient cortical projections to areas 17, 18, and 19 in the kitten with the use of axonal tracers Fast Blue and WGA-HRP. Injections of tracers in any of the three primary visual areas led to retrograde labeling in frontal, parietal, and temporal cortices. Retrogradely labeled cells were not randomly distributed, but instead occurred preferentially at certain loci. The pattern of retrograde labeling was not influenced by the area injected. The main locus of transiently projecting neurons was an isolated region in the ectosylvian gyrus, probably corresponding to auditory area A1. Other groups of transiently projecting neurons had more variable locations in the frontoparietal cortex. The laminar distribution of neurons sending a transient projection to the visual cortex is characteristic and different from that of parent neurons of other cortical pathways at the same age. In the frontoparietal cortex, transiently projecting neurons were located mainly in layer 1 and the upper part of layers 2 and 3. In the ectosylvian gyrus, nearly all the neurons are located in layers 2 and 3. In addition, a few transiently projecting neurons are found in layer 6 and in the white matter. Transiently projecting neurons have a pyramidal morphology except for the occasional spindle-shaped cell of layer 1 and multipolar cells observed in the white matter. Anterograde studies were used to investigate the location of transient fibers in the visual cortex. Injections of WGA-HRP at the site of origin of transient projections gave rise to few retrogradely labeled cells in areas 17, 18, and 19, demonstrating that transient projections to these areas are not reciprocal. Although labeled axons were found over a wide area of the posterior cortex, they were more numerous over certain regions, including areas 17, 18, and 19, and absent from other more lateral cortical regions. Transient projecting fibers were present in all cortical layers at birth. Plotting the location of transient fibers in numerous sections and at all ages showed that these fibers are not more plentiful in the white matter than they are in the gray matter. We found no evidence that the white/gray matter border constituted a physical barrier to the growth of transient axons. Comparison of the organization of this transient pathway to that of other transient connections is discussed with respect to the development of the cortex.

Sex and environmental influences on the size and ultrastructure of the rat corpus callosum

A contested report of sex differences in the size of the splenium of the corpus callosum in humans prompted the present examination of the corpus callosum in the rat. We have previously found that sex differences can vary with the rearing environment. Consequently, male and female rats were raised from weaning to 55 days of age in either a complex or an isolated environment. There were no sex differences in the size of the corpus callosum in sagittal cross section in these rats; however, rats of both sexes had a larger posterior third of the corpus callosum if they were raised in the complex environment. Because the corpus callosum continues to grow in size past 55 days of age, we examined socially housed rats at 113 days and again found no sex differences. The splenium was examined with electron microscopy in complex and isolation reared rats at 55 days of age. The ultrastructural analysis revealed differences that were not apparent from gross size measures. Females had more unmyelinated axons regardless of environment, and females from the complex environment had more myelinated axons than comparably housed males. In contrast, males in the complex environment had larger myelinated axons than females. Rats of both sexes from the complex environment had larger and more unmyelinated axons than isolated rats. In addition in myelinated axons, plasticity in the females occurred through changes in axon number and in males, through axon size. Thus sex differences exist in axonal number and size and the environment influences these differences.

Presence of crossed corticorubral fibers and increase of crossed projections after unilateral lesions of the cerebral cortex of the kitten: a demonstration using anterograde transport of Phaseolus vulgaris leucoagglutinin

Brain lesions made during early developmental stages produce more prominent remodeling of synaptic organization than those made in adults. This difference in the extent of neuronal or synaptic plasticity between immature and mature animals may be due to difference in the capacity for axonal elongation. Alternatively, it could be due to the prevention of retraction of exuberant projections present only in the early developmental stages. Aberrant crossed corticorubral projections seen after neonatal hemispherectomy have been ascribed to collateral sprouting. To determine whether these results from the prevention of retraction of crossed fibers, we studied the corticorubral pathway in normal kittens and compared it with that observed after unilateral cortical lesion, using the plant lectin Phaseolus vulgaris leucoagglutinin (PHA-L). One to two weeks after injection of PHA-L, many immunocytochemically labelled fibers were observed in the red nucleus (RN) ipsilateral to the cortical injection. Although very few, labelled fibers were also seen in the RN contralateral to the injection in normal kittens. By contrast, many labelled fibers were seen in the RN contralateral to the injection in lesioned animals. Many growth-cone like axonal endings were also observed. The abundant crossed corticorubral fibers seen in lesioned animals may be ascribed to the increase in the number of fibers crossing the midline towards the contralateral RN or they could be due to increased branching of pre-existing crossed fibers.

Topography of interhemispheric connections in neocortex of mice with congenital deficiencies of the callosal commissure

Normally, axons within the corpus callosum are ordered according to the cortical regions from which they originate, and callosal cells and terminations form elaborate cortical patterns related to the underlying topographic representations of the sensory periphery. About 30% of mice of the BALB/c strain show congenital deficiencies of the callosal commissure which range from total absence of the corpus callosum to a moderate reduction in the size of this commissure. In the light of current theories about the origin of these callosal deficiencies, it seems likely that fibers crossing the midplane in mutant mice have to circumvent local disturbances along their migration path. Since these disturbances in fiber trajectory may, in turn, alter the overall pattern of callosal projections, we set out to investigate whether the distribution of callosal connections in mice with marked deficiencies of the corpus callosum is as ordered as in normal mice. In groups of normal and mutant mice, we used multiple injections of horseradish peroxidase to reveal the overall distribution of callosal connections and restricted injections of horseradish peroxidase conjugated with wheat germ agglutinin to reveal finer aspects of the organization of the callosal pathway in these animals. Our results show that the number of labeled cells is reduced in mice with a small corpus callosum and that no labeled cells are present in the neocortex of acallosal mice. Furthermore, the topographic distribution of fibers within the corpus callosum of mutant mice can be significantly less ordered than in normal mice. However, even in mice with extreme deficiencies of the corpus callosum, callosal fibers originate from and terminate in all major areas of the cortex, and, within these areas, callosal cells and terminations are distributed according to the normal plan. The laminar distribution of callosal cells also appears normal in these mice. These findings indicate that gross developmental anomalies of the corpus callosum do not prevent normal specification of the callosal pattern during development. Within the context of current theories about the origin of congenital callosal deficiencies, our findings suggest that callosal fibers are able to establish appropriate contralateral connections in spite of alterations of their migration route. They also suggest that fiber topography within the corpus callosum does not play an important role in guiding migrating axons to their correct contralateral targets. Finally, our failure to find labeled fibers within the anterior commissure indicates that this commissure does not serve as an alternative route for deviated callosal axons.

The origin of projections from the medullary reticular formation to the spinal cord, the diencephalon and the cerebellum at different stages of development in the North American opossum: studies using single and double labeling techniques

We have employed the retrograde transport of horseradish peroxidase alone or conjugated to wheat germ agglutinin, to label neurons within the medullary reticular formation which project to the spinal cord, the diencephalon and the cerebellum at different stages of development in the North American opossum. At selected ages, the fluorescent markers Fast Blue and Diamidino Yellow were also used in double-labeling experiments to determine if single neurons innervate both the spinal cord and diencephalon or the spinal cord and cerebellum, presumably via axonal collaterals. The opossum was employed because it is born in a very immature state, 12 days after conception, and is thus available for injections at early stages of development. At all ages studied, the location of retrograde labeling within the medullary reticular formation after spinal, diencephalic or cerebellar placements of horseradish peroxidase or its conjugate appeared similar to that obtained in the adult animal. Such results suggest that the origin of projections from the medullary reticular formation to the areas injected is specified early in development. At some ages, however, the labeling density appeared greater than in the adult animal. When either Fast Blue or Diamidino Yellow was injected into the spinal cord and the other marker was placed into the diencephalon at such ages, relatively few neurons of the medullary reticular formation were double-labeled. When one marker was injected into the spinal cord and the other was placed within the cerebellum, no double-labeled neurons were found. These results indicate that at the ages studied, relatively few neurons of the medullary reticular formation provide collateral innervation to either the spinal cord and diencephalon or the spinal cord and cerebellum. Similar conclusions have been reached previously for the adult opossum. We have interpreted our results to suggest that the organization of reticular projections, at least to the areas injected, may not be shaped by the selective elimination of axonal collaterals as in certain other areas of the brain.

Specificity of sensory projections to the spinal cord during development in bullfrogs

Sensory neurons in dorsal root ganglia of frogs project to areas of the spinal cord they do not normally innervate following removal of adjacent ganglia at tadpole stages (Frank and Westerfield, J. Physiol. (Lond.) 324:495-505, '82b). A possible explanation of this phenomenon is that sensory neurons project to wider areas of the spinal cord in tadpoles than in adult frogs and that partial deafferentation causes the retention of these widespread projections. Therefore, the specificity of sensory projections to the spinal cord in tadpoles was assessed by staining individual dorsal roots with horseradish peroxidase. Thoracic sensory neurons project to thoracic segments of the spinal cord and to the brainstem in tadpoles, like thoracic sensory neurons in adult frogs. They rarely arborize in the brachial region even at stages when no other sensory fibers arborize at this level. Furthermore, their projections are restricted to the dorsal horn at all stages. Conversely, hypoglossal sensory neurons, which project into the intermediate gray matter in the adult, also project to this area in tadpoles. The finding that sensory neurons in tadpoles only project to areas of the spinal cord that they innervate in the adult suggests that the novel projections observed following partial deafferentation of the spinal cord are actually induced by the operation. An additional finding was that forelimb afferents, which project to an area extending from the obex to midthoracic levels in adult frogs, arborize at rostral spinal levels and at thoracic levels several stages before they form projections to the region around their own dorsal root. These differences in the stages at which projections to different levels of the spinal cord develop suggest that local properties of the spinal cord may control the timing of sensory fiber arborization.

A fibronectin-like molecule is present in the developing cat cerebral cortex and is correlated with subplate neurons

The subplate is a transient zone of the developing cerebral cortex through which postmitotic neurons migrate and growing axons elongate en route to their adult positions within the cortical plate. To learn more about the cellular interactions that occur in this zone, we have examined whether fibronectins (FNs), a family of molecules known to promote migration and elongation in other systems, are present during the fetal and postnatal development of the cat's cerebral cortex. Three different anti-FN antisera recognized a single broad band with an apparent molecular mass of 200-250 kD in antigen-transfer analyses (reducing conditions) of plasma-depleted (perfused) whole fetal brain or synaptosome preparations, indicating that FNs are present at these ages. This band can be detected as early as 1 mo before birth at embryonic day 39. Immunohistochemical examination of the developing cerebral cortex from animals between embryonic day 46 and postnatal day 7 using any of the three antisera revealed that FN-like immunoreactivity is restricted to the subplate and the marginal zones, and is not found in the cortical plate. As these zones mature into their adult counterparts (the white matter and layer 1 of the cerebral cortex), immunostaining gradually disappears and is not detectable by postnatal day 70. Previous studies have shown that the subplate and marginal zones contain a special, transient population of neurons (Chun, J. J. M., M. J. Nakamura, and C. J. Shatz. 1987. Nature (Lond.). 325:617-620). The FN-like immunostaining in the subplate and marginal zone is closely associated with these neurons, and some of the immunostaining delineates them. Moreover, the postnatal disappearance of FN-like immunostaining from the subplate is correlated spatially and temporally with the disappearance of the subplate neurons. When subplate neurons are killed by neurotoxins, FN-like immunostaining is depleted in the lesioned area. These observations show that an FN-like molecule is present transiently in the subplate of the developing cerebral cortex and, further, is spatially and temporally correlated with the transient subplate neurons. The presence of FNs within this zone, but not in the cortical plate, suggests that the extracellular milieu of the subplate mediates a unique set of interactions required for the development of the cerebral cortex.

Axon elimination in the developing corticospinal tract of the rat

Quantitative ultrastructural analysis of the corticospinal tract (CST) at the mid-thoracic spinal level in a series of early postnatal and young adult rats reveals that the tract is initially composed primarily of morphologically immature axon shafts, growth cones, and pale neuroglial processes. The total number of axons in the tract rises quickly to a peak level up to 90% greater than that present in the adult tract; it then declines, contemporaneously with the restriction of the areal extent of the set of spinally projecting cells in the cerebral cortex. During the time of axon elimination, axons remain small and morphologically immature, and small numbers of growth cones persist. Glial cells take on more mature forms within the tract several days before axon outgrowth ceases and myelination begins at the end of the second postnatal week. The fully mature CST retains a large complement of small, unmyelinated axons.

Topographic sequence of outgrowth of corticospinal axons in the rat: a study using retrograde axonal labeling with Fast blue

The retrogradely transported dye, Fast blue, was injected into cervical or lumbar segments of the spinal cord of rats during the first days of life in order to label the cell bodies of origin of the corticospinal tract which is growing down the cord during that period. The first corticospinal axons arrive at cervical levels immediately after birth and all arise from a circumscribed group of layer V pyramidal cells in a small region of dorsal parietal cortex. This same cell group provides the corticospinal projection to lumbar segments of the spinal cord, the axons reaching those segments at the end of the first postnatal week. The area of lumbar projecting cells undergoes relatively little expansion and no diminution during subsequent weeks and into adulthood. The area occupied by cortical cells projecting to the spinal cord expands during the first postnatal week, but the axons of all these additional cells do not appear to invade the lower sequents of the spinal cord. By the end of the first week, corticospinal cells can be labeled in a continuous sheet throughout most of the extent of the frontal, parietal and cingulate cortex. During the second and third postnatal weeks, the area sending axons to the upper levels of the spinal cord diminishes and large areas bereft of retrogradely labeled corticospinal cells appear: laterally, in lateral frontal and lateral parietal cortex; dorsally, at the border of frontal and parietal cortex; medially, in medial frontal and cingulate cortex. The more restricted adult pattern is established at the end of the third week. Hence, the first cortical axons to advance down the spinal cord are those that will innervate the lumbar segments in the adult. Later addition of corticospinal axons involves only those projecting to upper cord segments. Within this group there are those which will establish persistent connections from appropriate cortical areas and others that will shortly be eliminated from inappropriate areas.

Development and decision-making in the mammalian cerebral cortex

One of the fundamental tasks of neurobiology is to understand how the precision and specificity of the adult nervous system is achieved during development. This paper reviews the progress that has been made toward this end in studies of the developing mammalian cerebral cortex. Particular attention is focused on the problem of how cortical neurons make decisions during development: the correlation between a neuron's 'birthday' and its final laminar destination and projection patterns has raised the possibility that young neurons may be committed to their adult fates very early on in development, perhaps prior to migration. Indeed, several lines of evidence reviewed here suggest that at least some of the decisions made by cortical neurons are intrinsic properties of the cell itself. These studies include experiments on the reeler mouse mutant, and more recent attempts to manipulate developmental fates by pharmacological interventions and transplantation techniques. It is concluded that early commitment events in the cerebral cortex may specify a neuron's laminar position and restrict the range of potential axonal projections that the cell may form, but that local positional cues direct neurons to select (or maintain) only certain of the possible projections.

Brainstem projections to spinal motoneurons: an update

1. The existence of direct projections to spinal motoneurons and interneurons from the raphe pallidus and obscurus, the adjoining ventral medial reticular formation and the locus coeruleus and subcoeruleus is now well substantiated by various anatomical techniques. 2. The spinal projections from the raphe nuclei and the adjoining medial reticular formation contain serotonergic and non-serotonergic fibres. These projections also contain various peptides, several of which are contained within the serotonergic fibres. Whether still other transmitter substances (e.g. acetylcholine) are present in the various descending brainstem projections to motoneurons remains to be determined. 3. The spinal projections from the locus coeruleus and subcoeruleus are mainly noradrenergic, but there also exists a non-noradrenergic spinal projection. 4. Pharmacological, physiological and behavioural studies indicate an overall facilitatory action of noradrenaline and serotonin (including several peptides) on motoneurons. This may lead to an enhanced susceptibility for excitatory inputs from other sources. 5. The brainstem areas in question receive an important projection from several components of the limbic system. This suggests that the emotional brain can exert a powerful influence on all regions of the spinal cord and may thus control both its sensory input and motor output.

Major role for neuronal death during brain development: refinement of topographical connections

The precision of the topographic projection from the isthmo-optic nucleus (ION) to the retina has been examined in chicken embryos and chicks by the retrograde transport of a fluorescent carbocyanine dye from restricted retinal sites. At all ages, the labeled neurons are most numerous in the topographically appropriate part of the ION, but in younger embryos up to 49% of them are found outside this region. The distribution of these "aberrantly" projecting neurons is variable, but they generally occur throughout the entire ION. They all die during the ION's period of neuronal death, accounting for most of the 60% cell loss that then occurs. We therefore suggest that a major role of neuronal death during brain development is to reduce the imprecision of neuronal connections.

An anterograde tracer study of the developing corticospinal tract in the rat: three components

Light microscopic analysis of anterogradely transported wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) has been used to study the developing corticospinal tract (CST) in the rat. This study was carried out to examine the relationship between the site of injection within the cortex and the pattern of labelling of the developing CST in the spinal cord from postnatal day 1 (P1) through postnatal day 10 (P10). For this purpose the cortex was subdivided into 3 equal areas along the rostrocaudal axis: anterior, intermediate and posterior. After the operation the animals were allowed to survive for 24 h. The caudal extension of labelled CST axons originating in the anterior cortical area was restricted (L1 at P7 or P10) as compared with that of the CST fibres originating in the intermediate cortical area (S3 at P10). The axons of the posterior corticospinal (CS) neurones reach their most caudal extension in the spinal cord (T5) at P7 but then gradually disappear up till P14. Quantitative analysis of the amount of label along the length of the outgrowing CST fibres revealed the formation of a large stable peak at the level of the cervical enlargement after labelling of either the anterior or the intermediate cortical area. The formation of a second 'running' peak which moves caudally from mid-thoracic levels at P5 to mid-lumbar levels at P10 was only accomplished by labelling the intermediate cortical area and is probably caused by the accumulation of label in the growth cones at the distal ends of the outgrowing CST fibres. After labelling the posterior cortical area, no peaks could be detected, neither at the cervical nor at the lumbar intumescence. The major spinal grey termination field of the anterior CS neurones appeared to be the cervical intumescence, whereas the major spinal grey termination field of the intermediate CS neurones is the lumbar enlargement. By contrast, axons of posterior CS neurones never showed any outgrowth into the spinal grey matter at any level. Concluding, the developing CST in the rat consists of 3 components: the first having its originating neurones in the anterior part of the cortex and its termination field in the cervical intumescence; the second with its originating neurones in the intermediate part of the cortex and its termination field predominantly in the lumbar enlargement, and a third transient one, originating in the posterior cortex and gradually disappearing from spinal cord levels. Research using anterograde tracing techniques in combination with electron microscopy is necessary to further analyse these 3 different components.

The development of the corticotectal pathway in the albino rat: transient projections from the visual and motor cortices

In rats ranging in age from the second postnatal day (23rd postconceptional day-23 PCD) to adulthood, we have studied the distribution of corticotectal terminals labelled anterogradely by unilateral injections of horseradish peroxidase (conjugated with wheat germ agglutinin) into the visual or motor cortices. No projection to the contralateral superior colliculus (SC) was observed. The earliest age at which the labelled axons and/or terminals from the visual cortex were observed in the ipsilateral SC was 25 PCD. At this stage the projection only involves the optic layer. From 28 to 34 PCD, the projection involves the optic layer, the intermediate layers and the deep part of superficial gray layer. Between 34 and 40 PCD the projection becomes restricted to the superficial laminae (i.e. adultlike). On the 23 PCD (the earliest age examined) we observed a projection from the motor cortex to the intermediate laminae and to a lesser extent the optic layer of the ipsilateral SC. By 34 PCD only the adult-like projection extending from the brachium to the periaqueductal gray (PAG) is apparent. The disappearance of the transient projections to the intermediate collicular laminae may be the result of withdrawal of 'misprojecting' axonal collaterals.

Changes in the numbers of retinal ganglion cells and optic nerve axons in the developing albino rabbit

In albino rabbits aged from the 16th postconceptional day (16PCD) to adulthood, the number of axons in the optic nerves were estimated from sample areas totalling 1-12% of the cross-sectional area of the nerve. On the 16PCD there are about 20,000 axons in the optic stalk. The number of axons in the retrobulbar part of the optic nerve reaches a peak value of 766,000 on the 23PCD, and then decreases to about 350,000 by the 32PCD (the day of birth). The number of axons does not change between the 32PCD and 50PCD, but thereafter it slowly decreases, reaching the adult number (294,000) by the 84PCD. A similar trend is apparent in pigmented animals. Thus, on the 25PCD there are 736,000 axons in the retrobulbar part of the optic nerve and the number decreases to 428,000 by the 31PCD. In the adult pigmented rabbit there are 280,000 axons in the optic nerve. In animals younger than the 32PCD, growth cones are present, and the number of axons in the prechiasmal part of the optic nerve was 8-22% lower than in the retrobulbar part of the same nerve. These observations suggest that there is a continued outgrowth of axons from the eye towards the target nuclei. By the 32PCD, the numbers of axons in the retrobulbar and prechiasmal parts of the nerve were very similar, suggesting that by this age all axons had reached the chiasm. The numbers of retinal ganglion cells (RGCs) labelled by massive injections of horseradish peroxidase into the retino-recipient nuclei were estimated in albino rabbits aged from the 24PCD to adulthood. RGCs were counted in evenly spaced sample areas totalling 4-11% of the retinal area. On the 24PCD, the number of labelled RGCs (500,000) was lower than the number of axons in the optic nerve (probably because not all RGC axons had reached their target nuclei by this age). However, by the 27PCD the number of labelled RGCs (550,000) was very similar to the number of prechiasmal axons (568,000). At all ages thereafter, the numbers of both RGCs and axons were very similar, with adult RGC numbers (about 291,000) being reached by the 85PCD. We conclude that axon loss in the rabbit optic nerve after the 27PCD is almost certainly due to the elimination (presumably death) of the parent RGCs, and we suggest that RGC death is also the most likely cause of axon loss prior to the 27PCD.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for dopaminergic innervation on kitten retinal ganglion cells

Effects of iontophoretically applied dopamine and its receptor antagonists on physiologically identified retinal ganglion cells were studied in the optically intact eye of pentobarbitone anaesthetised kittens (7-9 weeks of age) and the results were compared with the effects obtained from adult cats (18-22 weeks of age). In both the adult and the kitten, dopamine had an inhibitory effect on visually evoked and spontaneous activity of the retinal ganglion cells, irrespective of cell type. However, unlike in the adult, the effects of dopamine in kittens were variable and not dependent on retinal eccentricity. In adult cells, only L-sulpiride (a potent D2 receptor antagonist) reduced the inhibitory effect of exogenous dopamine, whereas in kitten cells, both alpha-flupenthixol (a potent D1 receptor antagonist) and L-sulpiride did so. When applied alone, neither alpha-flupenthixol nor affected the activity of ganglion cells in adults, but in kittens both antagonists produced an excitatory effect in some cells. Physiologically active dopaminergic innervation seemed, therefore, to be present on the immature ganglion cells, but was subsequently 'eliminated' during the course of postnatal development. Furthermore, in immature cells, both D1 and D2 type receptors are present but only D2 receptors remain in adult. Therefore, there is a mismatch between dopamine receptors and dopamine in the adult retina and this mismatch may be the consequence of a developmental event.

Interhemispheric and subcortical collaterals of medial prefrontal cortical neurons in the rat

Efferent neurons of rat medial prefrontal cortex, projecting to subcortical structures and contralateral homotypical areas, were analyzed using anatomical and electrophysiological methods. Anterograde labelling with radioactive amino acids demonstrated the pathways of these efferents in the rostral part of the brain; terminal fields in contralateral cortical areas and localization of fibers projecting subcortically were particularly examined. The laminar location of cells projecting to the contralateral prefrontal cortex or through the striatum was investigated by means of the retrograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injected in these two structures. Cells innervating the contralateral prefrontal cortex were distributed in layers II-III and V, whereas injection of WGA-HRP in the striatum labelled cells in layer V. The antidromic activation technique was used to identify the cortical neurons which innervate the ipsilateral and contralateral subcortical structures as well as contralateral homotypical cortical areas. Among 743 recorded neurons, 282 neurons were antidromically driven from at least one of the stimulated sites (e.g. right striatum (RS), left striatum (LS), and contralateral prefrontal cortex (L-Cx]. The mean conduction velocities were 0.6 m/s and 0.8 m/s for subcortical and cortical efferents, respectively. The reciprocal collision test provided evidence for the existence of branched axons for 35% of the antidromically activated cells. All the possible branching patterns were found. The results of this study thus demonstrate the existence of single neocortical neurons that send axon collaterals to contralateral cortex and subcortical structures.

Ontogeny of the serotonergic projection to rat neocortex: transient expression of a dense innervation to primary sensory areas

The development of serotonergic innervation to rat cerebral cortex was characterized by immunohistochemical localization of serotonin combined with autoradiographic imaging of serotonin-uptake sites. In neonatal rat, a transient, dense, serotonergic innervation appears in all primary sensory areas of cortex. In somatosensory cortex, dense patches of serotonergic innervation are aligned with specialized cellular aggregates called barrels. The dense patches are not apparent after 3 weeks of age, and the serotonergic innervation becomes more uniform in adult neocortex. This precocious neonatal serotonergic innervation may play a transient physiologic role in sensory areas of cortex or may exert a trophic influence on the development of cortical circuitry and thalamocortical connections.

The cholinergic nuclei of the basal forebrain of the rat: normal structure, development and experimentally induced degeneration

The normal morphology and distribution of the cholinergic neurones of the basal forebrain of the rat have been studied qualitatively and quantitatively after staining immunohistochemically with a monoclonal antibody to choline acetyl transferase (ChAT). This was done in order to provide an adequate control for the changes found in these cells on both sides of the brain in the experimental investigation of the reaction of the cells to damage of their axons. The cholinergic cells form a more or less continuous anteroposterior band, but they can be subdivided into distinct nuclear groups on the basis of the size and form of the cell bodies and dendrites, their position and arrangement. these nuclei conform closely to previous descriptions of Nissl-stained material: the medial septal nucleus, the vertical and horizontal nuclei of the diagonal band and the basal nucleus. Quantitative measurements of the cross-sectional areas of the cells in the different nuclei confirmed the conclusions drawn from the qualitative examination. Measurements of the ChAT cells at different ages showed that in all nuclei they are significantly larger in size in infancy than in the adult, and they shrink to the mature size by 46 days. The cells in the various cholinergic nuclei show distinctly different reactions to damage of their terminal axonal fields. After removal of a large part of the neocortex by removal of the overlying pia-arachnoid mater the cells in the basal nucleus in the operated hemisphere underwent retrograde cellular degeneration, being swollen and paler-staining up to 14 days, and thereafter shrinking by 20-30% (as compared with those in the brains of age- and sex-matched littermate controls). The degree of shrinkage was appreciably greater when the animals were operated upon at the neonate stage. No cell loss was found, qualitatively or quantitatively, in the basal nucleus. After removal of the hippocampus there is marked loss of cholinergic neurones in the medial septal nucleus and in the vertical nucleus of the diagonal band, and with severe shrinkage of the remaining cells. Removal of the olfactory bulb results in only slight shrinkage of the cells, and no cell loss, in the horizontal nucleus of the diagonal band.(ABSTRACT TRUNCATED AT 400 WORDS)

Intrinsically directed pruning as a mechanism regulating the elimination of transient collateral pathways

In neonatal cats, neurons in frontoparietal areas of the cerebral cortex have axons which branch, some collaterals project transiently to the cerebellum, whereas others project by way of the pyramidal tract to the brainstem and spinal cord and persist into the adult. If cerebrocerebellar collaterals are eliminated simply because they are exuberant, then experimentally removing the collaterals in the pyramidal tract should cause the normally ephemeral projections to the cerebellum to persist. To test this hypothesis, the pyramidal tract was cut unilaterally at the pontomedullary junction in 5-9-postnatal-day-old (PND) cats, and 35-68 days later the frontoparietal cortex ipsilateral to the pyramidotomy was injected with tritiated amino acids. From the end of the lesioned pyramidal tract, labeled axons were traced into pathways that descended aberrantly into the caudal medulla and spinal cord, but there was never any transported label in the cerebellum. In a second series of experiments, the fluorescent dye Fast blue (FB) was injected into the spinal cord (2-5 PND) prior to cutting the contralateral pyramidal tract (9-12 PND) to determine if the pyramidotomy caused the axotomized cortical neurons to die. There were no neurons labeled with FB in the frontoparietal cortex on the side of the pyramidotomy, but many retrogradely labeled neurons were present contralaterally in the cortex, suggesting that the pyramidotomy caused the death of all axotomized cortical neurons. In a final set of experiments, FB was injected into the spinal cord and the cerebellar cortex was ablated (2-3 PND) prior to cutting the pyramidal tract (9-72 PND). Cerebellar decortication results in the persistence of cerebrocerebral projections to the partially deafferented deep nuclei, therefore injections of Nuclear yellow (NY) or Diamidino yellow (DY) were made later (32-86 PND) into the cerebellar nuclei on the side of the decortication to determine if these projections persist in pyramidotomized cats. After pyramidotomies at 9 PND, there were no neurons labeled with fluorescent dyes in the ipsilateral frontoparietal cortex, indicating that the cerebrocerebellar collaterals, even under experimental conditions which normally cause them to persist, could not sustain the axotomized cortical neurons. Pyramidotomies at 24 PND or later did not cause all axotomized neurons to die since neurons labeled with FB were present in the ipsilateral cortex. These findings suggest that during development of corticosubcortical pathways there is a hierarchical.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that selective collateral elimination during postnatal development results in a restriction in the distribution of locus coeruleus neurons which project to the spinal cord in rats

Experiments utilizing retrogradely transported fluorescent tracers in rats reveal that coeruleospinal cells are present throughout the locus coeruleus just after birth, but are confined to its ventral portion by the end of the fourth postnatal week. This change in distribution is not brought about by cell death, since neurons retrogradely labeled through their spinal axon following an injection of tracer shortly after birth are still present in the dorsal locus coeruleus even if the animal is not killed until the end of the fourth postnatal week. Thus the dorsal coeruleospinal neurons in newborn rats do not die but rather lose their spinal collateral.

Distribution of visual callosal projection neurons in hamsters subjected to transection of the optic radiations on the day of birth

The optic radiations of hamsters were transected on the day of birth and visual callosal projections in these animals were traced using retrograde transport of either horseradish peroxidase (HRP) or the fluorescent tracers True blue (TB) or Diamidino yellow (DY) when the animals reached maturity (greater than 45 days of age). In the hemisphere ipsilateral to the neonatal lesion, the distribution of callosal cells was markedly altered. These neurons were almost completely restricted to a continuous band in lower lamina V and the upper portion of layer VI. Anterograde HRP transport to the deafferented hemisphere also revealed an abnormal distribution of callosal terminals. The band of labelling that is located along the 17-18a border in the normals was much broader than is normally the case. In the hemisphere contralateral to the lesion, the distributions of callosal cells and terminals were essentially normal. Labelled neurons were located in the infragranular layers (primarily lower layer V and the upper part of lamina VI) throughout area 17 and also in layers II-IV in the 17-18a border region. Anterograde labelling was visible in layers V and VI throughout the mediolateral extent of the dorsal posterior neocortex and supragranular labelling was restricted to the lateral portion of area 17 and medial 18a. These results suggest that the normal thalamic projection to the visual cortex is necessary for the establishment of the strip of supragranular callosal projection neurons which is normally located in the 17-18a border region, but not for the establishment (or maintenance) of callosal projections by large numbers of neurons in the infragranular laminae. They show further that neonatal transection of the optic radiations results in reduction in the correspondence between the distributions of callosal cells and terminals in the deafferented hemisphere.

Peripheral nerve injury in developing rats reorganizes representation pattern in motor cortex

We investigated the effect of neonatal nerve lesions on cerebral motor cortex organization by comparing the cortical motor representation of normal adult rats with adult rats that had one forelimb removed on the day of birth. Mapping of cerebral neocortex with electrical stimulation revealed an altered relationship between the motor cortex and the remaining muscles. Whereas distal forelimb movements are normally elicited at the lowest threshold in the motor cortex forelimb area, the same stimuli activated shoulder and trunk muscles in experimental animals. In addition, an expanded cortical representation of intact body parts was present and there was an absence of a distinct portion of motor cortex. These data demonstrate that representation patterns in motor cortex can be altered by peripheral nerve injury during development.

Development and plasticity in hamster trigeminal primary afferent projections

At birth (gestational day 16), the hamster infraorbital nerve projects to the appropriate portion of the brainstem, though the projection lacks adult-like internal organization (patchiness). Infraorbital nerve damage at this time does not produce appreciable transganglionic atrophy in the central projections of the infraorbital nerve, but it does result in a failure to develop normal infraorbital primary afferent patches. Such damage also produces a more widespread central projection of spared mandibular afferents into regions occupied by 'regenerate' infraorbital terminals (J. Comp. Neurol., 235 (1985) 129-143). In the present study, transganglionic transport techniques were again used to show that, by postnatal day 5 (gestational day 21), rostrocaudally continuous aggregates of horseradish peroxidase-labelled infraorbital terminals are visible throughout the trigeminal brainstem nuclear complex. This aggregation pattern is nearly adult-like and isomorphic with the distribution of the mystacial vibrissae on the face. A similar infraorbital lesion performed on postnatal day 5, however, markedly decreased the density of the adult central projection of the infraorbital nerve to subnuclei principalis, oralis, interpolaris, and the magnocellular laminae of caudalis. The projection to superficial laminae of caudalis and the cervical dorsal horn was maintained. A postnatal-day-5 infraorbital lesion also failed to produce a more widespread central projection from spared mandibular primary afferents. These data suggest a relationship between the postnatal maturity of trigeminal primary afferents and the response of damaged and undamaged trigeminal afferents to infraorbital nerve transection in hamster. The similarity in the central primary afferent response to lesions at equivalent gestational times (postnatal days 5 and 0, respectively) in hamster and rat, suggests that this plasticity gradient may be a general characteristic of mammalian trigeminal primary afferents.