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

A combination of Golgi impregnation and fluorescent retrograde labeling

Central nervous system structures containing neurons labeled by the fluorescent tracers Fast blue (FB), Diamidino yellow dihydrochloride (DY), Rhodamine B isothiocyanate (RITC) and Rhodamine-labeled latex microspheres (RLM) were processed with the Golgi method. The goal was to improve the visualization of the fluorescent labeled neurons and to allow their ultrastructural examination. While the fluorescence of FB and RITC is greatly attenuated by the Golgi method, RLM and DY are still visible in Golgi-impregnated neurons. However, it is usually necessary to remove the silver precipitate by gold-toning.

Fasciculation in the early mouse trigeminal nerve is not ordered in relation to the emerging pattern of whisker follicles

Detailed reconstructions of lengths of the embryonic mouse maxillary nerve were made from serial light and electron microscope sections to determine whether there is any correspondence between the arrangement of fasciculi in the developing nerve and the emerging pattern of whisker follicles on the snout. There was neither correspondence nor obvious pattern in the arrangement of fasciculi at either E11, when trigeminal nerve fibers first contact the presumptive whisker pad, or at E12, when the pattern of developing whisker follicles becomes apparent. Fasciculi merged and branched to form an intricate plexus. Furthermore, the ratio of the number of nerve fibers in one fasciculus to the number in another prior to their merger differed significantly from the ratio of fiber numbers in the two fasciculi after their separation, which indicates that nerve fibers are freely exchanged between fasciculi. Our findings suggest that the somatotopic representation of the whisker follicle pattern in the brainstem does not develop by nerve fiber growth augmenting an initially ordered pattern of fasciculi.

Contralateral corticorubral fibers induced by neonatal lesions are not collaterals of the normal ipsilateral projection

Unilateral neonatal cortical ablation induces the development of a bilateral corticorubral projection from the remaining sensorimotor cortex. The retrograde fluorescent tracers Fast blue (FB) and Nuclear yellow (NY) were used to determine if the aberrant contralateral projection arises from axon collaterals of the normal uncrossed projection. Six to 8 weeks after unilateral cortical ablation in neonatal rats, the red nuclei were injected with FB on one side and NY on the other to study the source of the normal and aberrant afferents from the cerebral cortex. In control animals, many neurons in layer V of the sensorimotor cortex were retrogradely labeled with the tracer that had been injected into the ipsilateral red nucleus. In animals with unilateral ablations, many neurons throughout the remaining sensorimotor cortex were retrogradely labeled with FB or NY. No cortical neurons were doubly labeled. In addition to demonstrating the bilaterality of the corticorubral projection in animals which had received neonatal lesions, these results indicate that the aberrant contralateral corticorubral projection does not consist of axon collaterals of the normal ipsilateral fibers.

Ascending tract neurons survive spinal cord transection in the neonatal rat

Retrograde axonal transport was used to determine which ascending nerve tracts from the lumbosacral spinal cord are present in the cervical spinal cord of the newborn rat and if their cell bodies survive axotomy. A pledget of true blue was applied to a low cervical spinal transection in the newborn rat (N = 4). After a 5-day survival period, neurons were labeled in the laminae of origin of all ascending nerve tracts throughout the lumbosacral spinal cord. Neurons labeled in the same way survived for at least 1 month postoperatively when the spinal cord was transected at a midthoracic level at 5 days of age (N = 4). No neurons in the lumbosacral spinal cord were labeled if the midthoracic spinal cord was transected at the same time as application of the dye to cervical spinal cord (N = 2). Therefore, neurons labeled with true blue from cervical spinal cord during the neonatal period are likely to have been axotomized by thoracic injury made at 5 days of age. Three months after midthoracic spinal transection of newborn rats, HRP was injected or a pledget was applied to the first spinal segment caudal to this lesion (N = 8). The same population of neurons was labeled as in adult rats receiving application of HRP to an acute midthoracic spinal transection (N = 4). Neurons were seldom labeled in adult rats in which HRP was injected and ascending nerve tract axons not damaged (N = 4). These results suggest that most ascending nerve tract axons are present in cervical spinal cord during the neonatal period (by 4 to 5 days of age).(ABSTRACT TRUNCATED AT 250 WORDS)

A transient pyramidal tract projection from the visual cortex in the hamster and its removal by selective collateral elimination

During the early postnatal development of the neocortex in rats there is an axonal projection from the occipital cortex (which includes the visual cortex) to the spinal cord which is subsequently completely removed through a process of selective collateral elimination. In order to determine whether a similar phenomenon occurs during the development of the hamster cortex, we have injected the retrogradely transported fluorescent dye Fast Blue (FB) into the pyramidal decussation of hamsters at various ages. In adult hamsters such an injection results in a band of labeled neurons confined to layer V and to about the rostral two-thirds of the neocortex; no labeled cells are seen in the occipital cortex. However, a similar FB injection made during the first postnatal week results after a 4-day survival in a continuous band of FB-labeled layer V neurons spread throughout the tangential extent of the neocortex, including the occipital cortex. A similar continuous band of FB labeled layer V neurons is seen throughout the tangential extent of the neocortex including the occipital region in hamsters injected during the first postnatal week but allowed to survive until the fourth week (i.e., after the restriction of the widespread neonatal pattern has occurred). Injections of the anterograde tracer wheat germ agglutinin conjugated to horseradish peroxidase made into the occipital cortex, or for comparison, into more rostral cortical regions in hamsters ranging in age from neonates to adults, reveal that the extension of pyramidal tract axons is staggered along the anterioposterior axis of the cortex such that axons originating from the posterior regions lag behind those arising from more rostral areas. The transient occipital projection appears to reach a maximum around the end of the first postnatal week: a large number of labeled occipital axons is seen in the medullary pyramidal tract, and some of these can be followed through the pyramidal decussation and into the dorsal funiculus of the spinal cord. Injections into the occipital cortex on P16 label only a few fibers in the medullary pyramidal tract, and none is labeled in hamsters injected as adults.(ABSTRACT TRUNCATED AT 400 WORDS)

Topographic organization of peripheral trigeminal ganglionic projections in newborn rats

Retrograde transport of fluorescent tracers (true blue and diamidino yellow) was employed to delineate the topography of the peripheral projections of trigeminal ganglion cells in newborn (less than 12 h of age) rats. Identical injections were made in adult animals for comparison. In neonates, both inter- and intradivisional topography of ganglionic projections were adult-like. Neurons which innervated mandibular fields were located posterolaterally while cells with ophthalmic or maxillary projections were restricted to the anteromedial and central parts of the ganglion, respectively. An adult-like topographic representation of the mystacial vibrissae follicles was also evident in the neonates.

Numbers of rat dorsal root axons and ganglion cells during postnatal development

The present study demonstrates that T4 and S2 rat dorsal root axons decrease significantly from birth to adulthood with almost all of the decrease occurring in the first two weeks of life. Dorsal root ganglion cell numbers do not change during this time period. This is thus an example of postnatal axon elimination not associated with death of the cells that give rise to the axons. Presumably this regressive process is important in the formation of the normal adult nervous system. In addition, these findings raise the possibility that certain types of neonatal denervation may increase adult axon numbers by stopping a regressive process, the loss of axons, rather than initiating a progressive process, the formation of new axons.

A quantitative analysis of frog optic nerve regeneration: is retrograde ganglion cell death or collateral axonal loss related to selective reinnervation?

The present study was designed to assess whether axon collateral formation and loss or retrograde cell death contribute to selective reinnervation during optic nerve regeneration in the frog, Rana pipiens. The right optic nerve was crushed in 18 frogs, and samples were taken near the optic disc (retinal segment) and near the optic chiasm (brain segment). These samples were studied quantitatively with the electron microscope at various postoperative survival times (1, 2, 3, 4, 6, 12 weeks, 6 months, 1 year; N = 2). The number and size of axons in each segment were estimated from a series of electron micrographs taken at intervals across the transverse extent of each nerve and compared with normal nerves (N = 4). Results show that there are 5.3 +/- 1.8 X 10(5) (S.D.) unmyelinated and 2.3 +/- .5 X 10(4) myelinated axons in the normal nerve. One week post-crush (p.c.) there is a 27% decrease in the number of axons in the retinal segment (4.1 +/- 1.4 X 10(5)), indicating early retrograde axonal loss. As expected, there is a greater loss of axons at this time in the brain segment (3.0 +/- 1.3 X 10(5)). Between 2 and 6 weeks p.c. the number of axons increases in the retinal segment to over twice the normal number of axons increases in the retinal segment to over twice the normal number (12.3 +/- 3.8 X 10(5)) and to over four times this number in the brain segment (20.0 +/- 3.0 X 10(5)), showing collateral axon formation results from this injury. A large loss in the number of axons occurs in both nerve segments between 6 and 12 weeks p.c. (4.3 +/- 1.5 X 10(5)) and an additional loss at 20 weeks p.c. (2.2 +/- .98 X 10(5)). Subsequently, the number remains constant, approximately 40% of normal. Visual recovery was seen in the two frogs tested one year after optic nerve crush that were used for optic axon counts. Autoradiography in these same animals showed the optic nerve projections normally seen after regeneration. Besides axonal loss, our results also indicate that the size of both myelinated and unmyelinated axons is significantly above normal at chronic postoperative periods. This increase in axonal size is interpreted to be related to the increased territory each remaining optic axon must fill to restore the optic projections.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal loss of axons in normal rat sciatic nerve

Myelinated and unmyelinated axons were counted in sciatic nerves of newborn, 5-day-old, 14-day-old, and adult rats. Myelinated axons increase from essentially none at birth to approximately 8,000 in adulthood, but total axon numbers decrease steadily from 33,954 at birth to 22,872 in adulthood. Thus there is a significant postnatal loss of axons from rat sciatic nerve. This loss is, in our opinion, not associated with the death of the cells that give rise to these axons. This is thus an example of a regressive event that probably is of importance in normal neural development, namely the postnatal elimination of axons unaccompanied by death of the neurons that give rise to axons. These findings presumably imply a considerable amount of proximal peripheral axon branching, and the postnatal elimination of axons in the sciatic nerve presumably results from a reduction of this branching. Thus postnatal elimination of processes on, for example, somatic muscle cells may be at least partially the result of long axon elimination rather than local withdrawal of presynaptic processes, as is usually thought to be the case. In addition, an increased number of axons resulting from early postnatal manipulations may indicate cessation of axon loss rather than formation of new axons.

Ontogenetic change in the distribution of callosal projection neurons in the postcentral gyrus of the fetal rhesus monkey

In the postcentral gyrus of the mature rhesus monkey the distribution of callosal projection neurons is discontinuous. The density of callosal projection neurons, which are mainly located in the supragranular layers, varies both within and across cytoarchitectonic areas (Killackey et al., '83). In the present study, we investigated the ontogeny of corpus callosum projections of the postcentral gyrus in five fetal rhesus monkeys, ranging in age from embryonic day (E) 108 to E 133. Multiple large injections of horseradish peroxidase that involved the underlying white matter were made into the postcentral gyrus of one hemisphere and the distribution of labeled neurons in the ipsilateral thalamus and the other hemisphere was determined. The pattern of thalamic label indicated that the tracer was effectively transported from all portions of the postcentral gyrus. We found that the areal distribution pattern of labeled callosal projection neurons varied at the different fetal ages. At early fetal ages (E 108, E 111, and E 119) callosal projection neurons were continuously distributed throughout the postcentral gyrus. As in the adult animal, the vast majority of labeled callosal projection neurons were found in the supragranular layers, although a few labeled cells were located in the infragranular layers. From the earliest age, there was regional variation in the width of the band of labeled supragranular callosal projection neurons. The difference between the precentral and postcentral gyrus was most obvious, but there was also a difference between anterior and posterior portions of the postcentral gyrus. The first indication of some discontinuity in the distribution of callosal projection neurons was noted at E 126. By E 133, approximately 1 month before birth, the distribution of callosal projection neurons appeared remarkably mature. On E 119 aggregations of anterograde label could be detected in restricted portions of the posterior postcentral gyrus beneath the cortical layers. By E 133 anterograde label was found within the cortical layers (most densely in layer IV) in these regions of the postcentral gyrus. Thus, the emergence of the discrete pattern of callosal projection neurons appears to be temporally correlated with the ingrowth of callosal afferents. On the basis of these observations, as well as those of others (discussed in the text), we propose that the ontogenetic changes in the distribution of callosal projection neurons reflect the unique strategy employed by cortical projection neurons in establishing their patterns of connectivity. It is hypothesized that this strategy may involve multiple processes.

Dose-dependent occurrence of the aberrant longitudinal bundle in the brains of mice born acallosal after prenatal gamma irradiation

Two groups of pregnant mice were gamma-irradiated at the 16th gestation day with doses of 2 or 3 Gy. All litters were born acallosal, but while the 3-Gy mice showed a severely hypotrophic neocortex without the aberrant longitudinal bundle typical of early disconnected rodents, in the 2-Gy group the cortex was less deranged and the aberrant bundle appeared consistently underneath the white matter.

Collateralization in the mammalian nervous system

After reviewing the loci of origin for neurons with collateralized axons, some hypotheses on their distribution in the mammalian nervous system, on their functional contributions and on their significance in the course of encephalization are discussed. In principle, the distribution of collateralized neurons seems to be restricted to anatomical circuits subserving unspecific activation of forebrain regions and controlling body balance and movements. Concerning the limbic system, a minor degree of collateralization seems to exist only in less encephalized species. Based on a number of anatomical and functional arguments, it is assumed that the significance of collateralization fades in the course of encephalization.

The development of the dentate area and the hippocampal mossy fiber projection of the rat

The development of the dentate area and the hippocampal mossy fiber system of the rat has been investigated at the light microscopic level by using fluorescent tracing, Nissl, and Timm's histochemical methods. Although the cytoarchitectonic development of the dentate granular layer is mainly a postnatal phenomenon, the initial events take place before birth. The aggregation and maturation of the cells in the granular layer proceed in a graded fashion from the lateral to the medial and from the superficial to the deep aspects of the layer. The earliest-formed granule cells are probably derived directly from the cells of the ventricular zone. They start to form mossy fibers prenatally, either during the relatively long period of migration to the granular layer or soon after their arrival. However, most of the granule cells are derived from a secondary proliferative center in the hilus. They start to produce mossy fibers postnatally a while after arriving at the granular layer. The total complement of granule cells starts to grow mossy fibers in a sequence that is related to the final position of the cells of origin within the granular layer. This sequence also proceeds in a graded fashion from the lateral to the medial and from the superficial to the deep aspects of the layer. In the beginning the mossy fibers elongate relatively rapidly. Already at birth the Timm-stained mossy fiber zone occupies the anterolateral part of the hilus and the adjacent suprapyramidal parts of the regio inferior. Once the mossy fibers have reached the distal end of the regio inferior they elongate along the longitudinal axis of the hippocampus more slowly. At the same time the Timm-stainability of the mossy fiber zone, which, during the first postnatal week, is weaker toward the regio superior, develops a mature pattern in which the distal part of the zone stains most intensely. Throughout development, fibers from the granule cells that form first are longer and diverge more in the septotemporal dimension than fibers from later-forming granule cells. In contrast to other axonal systems which appear to be sculptured from a diffuse set of connections the results presented here provide evidence that the topographic relationships of the mossy fiber system develop in a stepwise fashion.

Neonatal hemidecortication and bilateral cutaneous stimulation in rats

In humans, a dominant somatosensory consequence of extensive unilateral neocortex damage is "simultaneous extinction," which is an interhemispheric perceptual interaction that is operationally distinguishable from neglect. A tactile stimulus presented on the contralateral side of the body is detected when presented singly, but is completely masked during bilateral stimulation. Analogous tests designed to calibrate somatosensory asymmetries in rats were used to determine the long-term effects of hemidecortication sustained on postnatal Day 1. These data were compared with that observed in adult operated rats at a comparable postoperative period. In one respect the neonatal brain was more vulnerable than the adult brain. That is, unlike adult operated rats which were tested at 3 postoperative months, a sensory asymmetry appeared to be permanent in the neonatally operated rats, at least for the duration of testing (3-9 months). Further analysis suggested that in another way the neonates were more resistant to the effects of hemidecortication than were the adults. Neonatally operated rats appeared to be capable of processing input from both sides of the body simultaneously, even during markedly asymmetrical input. In other words, the early occurrence of brain damage may have spared them from a condition reminiscent of "simultaneous extinction." Finally, the adult operated and neonatally operated rats both displayed a subtle motor abnormality. Thus, depending on the test used, the neonatal operation yielded more severe, less severe, or comparable behavior deficits.

Postnatal development of GABA- and glycine-mediated inhibition of feline retinal ganglion cells in the area centralis

Intraretinal iontophoresis in the optically intact eye of adult cats (18-22 weeks of age) and kittens (7-9 weeks of age) under pentobarbitone anaesthesia was performed. Studies were concentrated on retinal ganglion cells of the sustained (X) type in the area centralis under photopic conditions. In both the adult and kitten, gamma-aminobutyric acid (GABA) and muscimol inhibited the visually induced excitation, and bicuculline blocked the visually induced inhibition of on-cells. On the other hand, glycine inhibited the excitation and strychnine blocked the inhibition of off-cells. However, a greater current of GABA (muscimol) and glycine was required to produce total inhibition in kitten's on- and off-cells respectively when compared with the adult's. Furthermore, a smaller current of bicuculline and strychnine was needed to abolish the visually induced inhibition of kitten on- and off-cells respectively when compared with the adult's. In the adult, GABA and glycine did not affect the responses of off- and on-cells respectively, but in the kitten GABA inhibited off-cells and glycine inhibited on-cells to some extent. In neither the adult nor the kitten did bicuculline have any effect upon off-cells or strychnine any effect upon on-cells. Thus, the sustained on- and off-cells in the kitten area centralis exhibit: a reduced selectivity to inhibitory transmitters; a reduced sensitivity to exogenously applied inhibitory transmitter agonists; but a greater sensitivity to inhibitory transmitter antagonists, in comparison with the sustained on- and off-cells in the adult area centralis. The observed differences between the kitten and adult cat in transmitter actions on retinal ganglion cells appear to be analogous to those found in the postnatal development of functional synapses at the neuromuscular junction and sympathetic ganglia.

An exuberant retinocollicular pathway in Siamese kittens: effects of competition and abnormal activity on its maturation

Retinocollicular pathways were studied in normally pigmented and Siamese adult cats and newborn kittens. In addition, retinocollicular pathways were studied in Siamese cats which were unilaterally enucleated on the day of birth and in Siamese cats which were reared in a stroboscopically illuminated environment. In adult Siamese cats the ipsilateral retinocollicular pathway is spatially less extensive than it is in adult normally pigmented cats. In contrast, the ipsilateral retinocollicular pathway in newborn Siamese kittens is widespread, while that of newborn normally pigmented kittens is restricted, as it is in normally pigmented adults. This comparison indicates that the spatial restriction of the retinocollicular pathway occurs after birth in Siamese cats. After enucleation or stroboscopic rearing the ipsilateral retinocollicular pathway in Siamese cats remains widespread. These results demonstrate the importance of interactions with afferents from other sources and the requirement for appropriate neural activity in the normal maturation of this initially exuberant pathway.

Development of the spinal-medullary projection from the mouse barrel field

Neurons in layer V of the murine posteromedial barrel subfield (PMBSF) project to structures at or caudal to the spinal-medullary junction. During postnatal development a reduction occurs in the density of the neurons which form this projection. In principle, three processes might be expected to contribute to this reduction: cell death, tissue growth, and axon pruning. Three different paradigms in which cells of origin of the projection are labeled retrogradely with True Blue, injected into the spinal-medullary junction, taken together with an estimate of the relative growth of layer V, provide separate estimates of the magnitude and rate of reduction consequent to these different processes during the first 3 postnatal weeks. The density of neurons in an index sector of layer V of the PMBSF which contribute to the projection at varied ages is estimated by injections made at a range of ages from postnatal day 1 (P1) to P16, with a survival of 4 days in each instance. Overall reduction in density is 80%. The component due primarily to axon pruning is estimated to be 50% by injections delivered at graded ages from P1 to P16 with survival to P20 in each instance. The component of the reduction attributable to increase in volume is estimated at 30% by a series of injections delivered at P1 with graded survival times from P5 through P20. A reduction due to cell death is not identified. The reduction in density due to tissue growth is essentially linear through the interval P5-P11. At all ages, neuronal somata of origin of the spinal-medullary projection are located within layer V. Subsequent to P15 they are confined to sublayer Vb; at earlier ages somata in Va and Vc also contribute axons to the projection. Although volume increase due to growth of the neuropil reduces the density of the population contributing to the projection equally in all three sublayers, final elimination of all contributions from Va and Vc depends upon axon pruning.

Organization and postnatal development of callosal connections in the visual cortex of the rat

The distribution of callosal cells and terminals was studied in the posterior neocortex of pups whose ages ranged from 3 to 16 days and in adult rats 2 months of age or older. Callosal cells and terminations were revealed using retrograde (horseradish peroxidase) and anterograde (horseradish peroxidase; tritiated proline) tracing techniques, respectively, and the distribution of callosal connections was analyzed in tangential or coronal histological sections. In agreement with previous studies, we observed that the pattern of callosal connections in areas 17 and 18 of adult rats contains the following features: (1) a dense band of callosal cells and terminations separating the interiors of areas 17 and 18a, (2) a ringlike configuration anterolateral to area 17, (3) a region of dense labeling lateral to area 18a, (4) several narrow bands of labeling that bridge area 18a at different anteroposterior levels, and (5) one or more labeled regions in area 18b. In all these callosal regions, labeled cells and terminations are densely aggregated in layers II-III, Va, and Vc-VIa, and less densely in layer IV and the remaining portions of layers V and VI. High densities of isotope-labeled fibers are also observed in the lower half of layer I. Throughout the interiors of areas 17 and 18a, a significant number of labeled cells are observed in layers Vc-VIa. In contrast to adult rats, in neonates no distinct tangential pattern of callosal connections is apparent. Instead, labeled cells are densely aggregated in two continuous horizontal bands located in cortical layers Va and Vc-VIa, and callosal axons are largely restricted to white matter. During the first 2 postnatal weeks there is a progressive loss of callosal cells in regions that normally have few callosal cells in the adult (e.g., interiors of areas 17 and 18a) and an increase in the number of cells in layers II-IV in regions that are densely callosal in the adult (e.g., callosal regions at the 17/18a border, lateral border of area 18a, and in area 18b). The decrease in the number of callosal cells in the interiors of areas 17 and 18a is more severe in the upper than in the lower band of the immature labeling pattern, and our data from tangential sections indicate that this loss of callosal neurons occurs synchronously across the interiors of these areas. During this period there is also a localized invasion of labeled callosal axons into those regions of gray matter where they will be found in adult life.(ABSTRACT TRUNCATED AT 400 WORDS)

The morphology and phased outgrowth of callosal axons in the fetal rat

The growth of axons of the corpus callosum was studied in fetal and early postnatal rats by means of anterograde and retrograde transport of horseradish peroxidase (HRP) applied to the developing cerebral cortex in the frontal and presumptive sensory motor regions. In the sensorimotor regions, the first axons to reach the midline at E18 arise from two separated groups of cells situated medially near the superior sagittal sinus and laterally just above the rhinal sulcus. Each group forms a stratum just beneath the cortical plate. Axons from cells in intervening regions arrive at the midline approximately one day later. By the first postnatal day (P0), a second stratum of callosally projecting cells can be identified superficial to the first. Callosal axons grow out from this stratum in the same sequence as those from the deeper stratum, axons from medial and lateral regions preceding those from intervening regions. [3H]thymidine labeling of animals later injected with HRP, indicates that callosal cells in the deep stratum enter their final mitosis at E15 and those in the superficial stratum at E16. Growing callosal axons have identifiable growth cones and filopodia at their tips but, as far as they could be traced, the axons do not branch. They grow orthogonal to radial glial processes of the cerebral hemisphere and diverge early from simultaneously outgrowing corticofugal axons directed to subcortical sites, as though following separate cues. Callosal axons advancing from one side grow directly into the path taken by those advancing from the other side.

The transient corticospinal projection from the occipital cortex during the postnatal development of the rat

The transient occipital cortical component of the pyramidal tract which we previously had identified during the postnatal development of the rat (Stanfield et al., '82) has been studied with anterograde as well as retrograde techniques. A continuous band of retrogradely labeled layer V neurons which spans the entire cortex including the occipital cortex is seen following injections of the fluorescent marker Fast Blue into the pyramidal decussation during the first postnatal week. No labeled cells are found in the occipital cortex following similar injections made on postnatal day 20 (P20), although such injections label many neurons in the more rostral cortical fields. However, if the Fast Blue injection is made on P2 and the animal is allowed to survive until P25 a large number of Fast Blue-labeled layer V neurons is found in the occipital cortex, even though an acute, second injection of the retrograde tracer Nuclear Yellow made into the pyramidal decussation shortly before the animal is killed results in no occipital cortical labeling. When Fast Blue injections confined to the mid- or upper-cervical spinal cord are made on P4 and the animals are killed on P9, again many retrogradely labeled neurons are found in the occipital cortex. Further, when injections of 3H-proline or wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) confined to the occipital cortex are made during the first 2 postnatal weeks, anterogradely transported label is seen within the pyramidal tract. At all stages examined the amount of such label and its caudal extent are less than that seen following similar injections into the parietal or frontal cortex. The greatest extent of the labeled occipital cortical fibers is reached at about the end of the first postnatal week and the number of these fibers seems to peak at about this same time. At this stage many of these labeled axons extend for a considerable distance down the spinal cord with some reaching as far caudal as lower lumbar levels, and at this stage some of these labeled occipital corticospinal fibers enter into the spinal gray. Over the next week the number of occipital cortical fibers in the pyramidal tract rapidly decreases and by P17 occipital cortical injections of 3H-proline or WGA-HRP result in virtually no transported label caudal to the pons.(ABSTRACT TRUNCATED AT 400 WORDS)

Occipital cortical neurons with transient pyramidal tract axons extend and maintain collaterals to subcortical but not intracortical targets

During the early postnatal development of the rat large numbers of pyramidal tract neurons are present in layer V of the occipital cortex, but by the end of the third postnatal week the distribution of pyramidal tract neurons becomes restricted to the more rostral cortical areas. This restriction is brought about by selective collateral elimination rather than by cell death. We have found, by using retrogradely transported fluorescent dyes as either short-term or long-term markers, that occipital cortical neurons which had transiently extended pyramidal tract axons maintain subcortical axonal connections to either the superior colliculus or the pons, and, at least in the case of the corticotectal projection, that the maintained collateral is present prior to the elimination of the transient pyramidal tract collateral. Further, it appears that at no time during postnatal development do the occipital pyramidal tract neurons form either callosal or ipsilateral cortico-cortical collaterals. Thus in the early postnatal occipital cortex the neurons which project through the pyramidal tract constitute a population of cells which is separate from neurons which make cortico-cortical connections, but which largely overlaps with the population of corticotectal and corticopontine neurons.

Convergence of intra- and interhemispheric cortical afferents: lack of collateralization and evidence for a subrhinal cell group projecting heterotopically

The distribution of corticocortical projecting neurons in the rat's brain was investigated with fluorescent dyes and the retrograde transport of horseradish peroxidase. Although the fluorescent techniques especially revealed the existence of a considerable number of neurons interconnecting the limbic areas (sub- and perirhinal cortex, prefrontal, cingulate, and retrosplenial cortex) both intra- and interhemispherically, only a negligibly small number of neurons with collateralized axons could be detected. In the rat's anterior dorsolateral cortex an area is described whose neurons are organized in a columnlike fashion and project intra- and interhemispherically to the limbic areas examined. The density of efferent connections differed between areas, with an especially high density found in a small region of the subrhinal cortex. Injections of horseradish peroxidase into different regions of the cingulate and retrosplenial cortex confirmed the existence of widespread heterotopic interhemispheric connections originating from this defined subrhinal area, though the number of retrogradely labeled cells remained consistently smaller than that obtained following the injection of fluorescent dyes. Among the regions studied with horseradish peroxidase injections, those into the retrosplenial cortex showed the highest density of labeled cells within this subrhinal area. A more detailed examination of the subrhinal region containing the densely labeled neurons (projecting to the contralateral hemisphere) made use of Nissl stains and revealed a morphologically separable area which was characterized by medium-sized, dark-staining neurons whose long axons were oriented mediolaterally. The region includes portions of the insular-perirhinal, entorhinal, and piriform cortex. It is suggested that the corticocortical projections are basically noncollateralized in the rat. However, there apparently is a dense interhemispheric interconnectivity between the limbic areas. Functional evidence for the defined subrhinal area suggests a prominent role of its neurons in cognitive information processing. The present evidence for considerable interhemispheric cortical projections may provide a new impetus for studying the intercommunication of the two sides of the brain with both anatomical and behavioral methods.

Transient uncrossed corticospinal fibres in the newborn rat

A small number of uncrossed corticospinal fibres were demonstrated in a 1-day-old rat by injection of horseradish peroxidase into one hemisphere of the cerebral cortex. These fibres were not demonstrable after the age of 7 days.

Neuronal death and synapse elimination in the olivocerebellar system. I. Cell counts in the inferior olive of developing rats

A transient multiple innervation of cerebellar Purkinje cells by climbing fibers has been described during postnatal development of the rat. The aim of the present study was to determine if the regression of redundant synapses is related to the loss of presynaptic cells in the inferior olivary nucleus (ION), which is the sole source of climbing fibers in rodents. To this end, the population size of the ION was evaluated by counting healthy cells of the four main subnuclei in rats from birth to adulthood. The cell population at birth was found to be very similar to that of the adult animal (27,655 versus 28,385), but a loss of 25% of the cells occurred in the first five days, presumably through their death since degenerating cells were observed over the same period. Although cell loss was found throughout the whole nucleus, it was more pronounced in the medial accessory olive. A subsequent apparent increase of the cell population was observed so that the adult value was again reached at 15 days. The evolution of the ION population is then characterized by a period of moderate cell death which takes place before the peak of polyneuronal innervation of Purkinje cells by olivary axons is attained. This strongly suggests that the removal of the redundant synaptic contacts established by climbing fibers onto Purkinje cells during development is caused by a progressive reduction of the branching of olivary axons rather than by degeneration of the presynaptic cells.

The onset and development of descending pathways to the spinal cord in the chick embryo

The ontogenetic development of afferent (supraspinal and propriospinal) as well as efferent (ascending) fiber connections of the spinal cord was examined following the injection of horseradish peroxidase (HRP) or wheat germ agglutinin HRP (WGA-HRP) into the cervical and lumbar spinal cords (or brains) of embryos ranging in age from 4 to 14 days of incubation. A few cells were first reliably retrogradely labelled in the pontine reticular formation on embryonic day (E) 4 and E5 following the injection of WGA-HRP into the cervical and lumbar spinal cord, respectively. Propriospinal projections to the lumbar spinal cord, originating from brachial spinal cord, were found by E5, and from the cervical spinal cord by E5.5. Ascending fibers arising from neurons in the lumbar spinal cord could be followed to rostral mesencephalic levels in E5 embryos. Thus, the earliest supraspinal, propriospinal, and ascending fiber connections appear to be formed almost simultaneously. Retrogradely labelled cells were found in the raphe, reticular, vestibular, interstitial, and hypothalamic nuclei in E5.5 embryos following lumbar injections of WGA-HRP. Except for neurons in cerebellar nuclei, all the cell groups of origin that project to the cervical spinal cord of posthatching chicks were also retrogradely labelled by E8. There was a delay in the time of appearance of the projections from various regions of the brain stem to the lumbar versus the cervical spinal cord, ranging from 0.5 to 7 days, but typically of about 3 days duration. A large number of cells located in the ventral hypothalamic region, just dorsal to the optic chiasma, were found to be labelled following cervical HRP injection between E6 and E10. These cells may represent transient projections that are present only during embryonic stages since no labelled cells were found in this region in the newly-hatched chick.

Axonal regeneration after crush injury of rat central nervous system fibres innervating peripheral nerve grafts

Recent experimental studies in adult rodents indicate that neurons in many regions of the brain and spinal cord are capable of extensive axonal growth along peripheral nerve grafts inserted into the C.N.S. To explore further the capacity of damaged intrinsic C.N.S. neurons to initiate and sustain fibre growth we have studied the regenerative response of brain stem and spinal neurons to the crushing of their axons after such axons had already grown across peripheral nerve 'bridges' linking both these levels of the neuraxis. In adult rats, an autologous segment of sciatic nerve approximately 4 cm long was used to connect the medulla oblongata and the lower cervical spinal cord. After 6-42 weeks, when C.N.S. axons are known to have regenerated across these 'bridges', the nerve grafts were crushed near both their rostral and caudal insertions into the C.N.S. Axonal regeneration beyond the sites of injury was investigated 4-11 weeks after crush by retrogradely labelling C.N.S. neurons with horseradish peroxidase (HRP) applied 1 cm away from the injured site, along the assumed course of the C.N.S. fibres regrowing across the graft. The number and distribution of HRP-labelled neurons was found to be similar to that in rats with uncrushed grafts. To prove that such axonal regrowth from spinal and brain stem nerve cells did originate from injury of central nerve fibres innervating the graft and not by sprouting from undamaged C.N.S. neurons at both ends of the 'bridge', we first labelled with the fluorescent dye Fast Blue (FB) the cells whose axons were interrupted by the crush and, after two weeks, applied a second dye, Nuclear Yellow (NY) 1 cm beyond the site of injury. The presence of FB and NY double-labelled C.N.S. neurons in these animals, together with the results of the HRP-labelling experiments, suggest that central neurons whose axons innervate peripheral nerve grafts are capable of renewed growth after axonal injury. Under such experimental conditions these intrinsic C.N.S. neurons respond to axonal interruption in a manner that resembles the responses of cells that normally project along peripheral nerves. We believe this to be an additional indication of the powerful role in regeneration of interactions between neurons and the axonal environment.

Growth and restriction of the ipsilateral retinocollicular projection in the opossum

The distribution of optic nerve fibers and terminals in the superior colliculus (SC) was followed throughout its development in pouch young opossums in order to establish the normal sequence of events leading to the formation of mature patterns. Up to 7 days of life in the pouch, labeled fibers can be followed only as far as the rostral aspect of the optic tract. The earliest evidence for crossed retinal projections in the SC is found at 10 days of age. In parasagittal sections, the label extends along the rostrocaudal tectal axis from the rostral border to the presumptive caudal pole of the SC. Unequivocal evidence for ipsilateral retinocollicular projection is found at 15 days extending to all but the caudal 5th of the rostrocaudal extent of the SC. The projections from both eyes overlap extensively in the SC at 22 days and after this age significant changes occur, mostly at the ipsilateral side: a sub-pial tier of fine label develops excluding both rostral and caudal collicular poles; a deeper tier of coarse label extends from the rostral to the caudal pole and a third, patchy tier of label is found at the prospective strata griseum superficiale and griseum intermediate. By 47 and 60 days the tangential distribution of the projections is virtually indistinguishable from the adult pattern although laminar segregation does not seem as sharp as in the adult. Comparisons of the changeable patterns of ipsilateral retinocollicular projections from 22 to 34 days with the invariant, aberrant pattern in adult animals submitted to uniocular enucleation at either age suggests that the preservation of a juvenile pattern does not provide a comprehensive explanation for the formation of aberrant projections.

An anterograde HRP-WGA study of aberrant corticorubral projections following neonatal lesions of the rat sensorimotor cortex

Anterograde transport of horseradish peroxidase - wheat germ agglutinin (HRP-WGA) was used to examine the effect of unilateral neonatal ablation of the sensorimotor cortex on the remaining corticofugal projections to the midbrain in the rat. In unlesioned animals, the sensorimotor cortical efferents to the midbrain were entirely ipsilateral, terminal labeling being evident in the red nucleus, the midbrain reticular formation, the periaqueductal gray, the intermediate gray layer of the superior colliculus, the nucleus parafascicularis prerubralis and the perilemniscal area. Corticorubral fibers were seen to reach the midbrain through the thalamus or the cerebral peduncle. In the red nucleus, terminal labeling was essentially restricted to the parvocellular region. In neonatally lesioned adults, aberrant corticofugal fibers crossed the midline to terminate in the contralateral red nucleus, the midbrain reticular formation, the periaqueductal gray, the nucleus parafascicularis prerubralis and the intermediate gray layer of the superior colliculus. The aberrant projections maintained the topographic specificity of the normal ipsilateral projections. This was most evident in the corticorubral projection, where the aberrant contralateral fibers terminated in the parvocellular area of the red nucleus.

Evidence for self-absorption of terminals by developing axons of retinal ganglion cells in the chick

The appearance of membrane-bound degenerative organelles in chick optic nerve axons was studied at the electron microscopic level. A semiquantitative analysis revealed a sharp increase in the number of axons containing accumulations of such organelles during the second day after hatching. In dark-reared chicks this increase was retarded, suggesting the presence of a light-influenced event in the early post-hatch period.

Subcortical projections of area 17 in the anophthalmic mouse

Anterograde and retrograde tracing methods were used to compare the subcortical projections of area 17 in ZRDCT-an, anophthalmic mice with those of sighted C57BL mice. In both groups, area 17 projected to the dorsocaudal striatum, the reticular, lateral and lateral posterior nuclei, the dorsal and vental lateral geniculate nuclei, the zona incerta, the anterior and posterior pretectal nuclei, the stratum griseum superficiale of the superior colliculus and the dorsolateral pons. Occasional labeled fibers in both groups were also seen in the ventrobasal nucleus, but it was not clear whether or not any axons terminated in this region. The projections to the superior colliculus and dorsal lateral geniculate nucleus were analyzed in greater detail. In both normals and blind mice the striate corticotectal projection arose from cells in layer V and that to the geniculate from neurons in lamina VI. The topographic organizations of these projections in the two groups were indistinguishable. The striate corticotectal projection to the colliculus in the anophthalmic mice did appear to terminate more dorsally in the stratum griseum superficiale than that in sighted animals. These data demonstrate that a signal from the retina is not required for either the restriction of 'visual' cortical axons to their normal subcortical targets or the achievement of normal topography within those target nuclei.

The development of commissural connections of somatic motor-sensory areas of neocortex in the North American opossum

The North American opossum does not have a corpus callosum; neocortical commissural axons are contained entirely within the anterior commissure. We have used axonal transport techniques to study the origin and distribution of commissural axons from somatic motor-sensory cortex in developing and adult opossums. Neocortical axons grow into the anterior commissure by postnatal day (PND) 12, the contralateral external capsule by approximately PND 19, the area deep to the contralateral homotypic cortex by approximately PND 26 and the cortex proper by approximately PND 35. Commissural neurons were first demonstrated at about PND 26, when they form a fairly continuous band in the cortical subplate (presumptive layers V-VI). By at least PND 37, commissural neurons are also present in layers II and III, where they form a continuous band, and in layer IV, where they are sparse. In older pouch young and adult opossums the bands of commissural neurons, especially in layers V-VI, are interrupted, and commissural neurons are rare in layer IV. In general, commissural axons in both pouch-young and adult opossums innervate areas containing commissural neurons as well as layer I. In the acallosal opossum as well as in the callosal rat, the development of commissural connections from somatic motor-sensory cortex is characterized by pauses during the growth of axons into the opposite cortex, by a general inside-out-gradient, and by a transition from continuous bands to patchy, radial columns of commissural neurons and axons. This suggests that similar mechanisms govern the formation of commissural connections in the two species.

The organization of immature callosal connections

In newborn kittens, the anterograde transport of horseradish peroxidase, alone or bound to wheat-germ agglutinin, indicates that callosal axons have entered selectively the restricted portions of the neocortical gray matter (e.g., the area 17/18 border) which receive callosal afferents in adults. The callosal axons do also reach regions where they lack in the adult, but there they seem not to penetrate far into the gray matter. Neonatal injections of retrograde fluorescent tracers restricted to the gray matter in areas 17, 18, and posteromedial lateral suprasylvian area (PMLS) label neurons in the contralateral hemisphere only when the tracers were directed into regions known to receive callosal axons. In particular, injections near the 17/18 border label neurons in the contralateral hemisphere at the homologous site and at restricted, retinotopically corresponding locations in other visual areas: a pattern similar to the adult one. In contrast, an injection reaching the white matter of areas 17 or 18 labels a wider, continuous territory extending mediolaterally over most visual areas from 17 to posterolateral lateral suprasylvian area (PLLS) and including regions which later become acallosal; in addition, labeled neurons are found in the limbic cortex medial to area 17 and in the auditory cortex lateral to PLLS, none of which is known to project to either 17 or 18 in the adult. In flattened reconstructions of the cortex, the shape of the territory labeled by each of these injections is characteristically, although somewhat irregularly, crescent shaped; its rostrocaudal position varies with that of the injection. An injection extending into the white matter of more lateral visual areas (19, 21a, PMLS) labels callosal neurons over a similar territory, which extensively overlaps that labeled by the 17/18 border injections and likewise includes regions which are acallosal in the adult. In spite of the overlapping distribution of labeling obtained from separate injection sites, as in adults, each cytoarchitectonically (or retinotopically) defined area seems to receive from a different set of neurons, although a few neurons send bifurcating axons to more than one area. In conclusion, injections restricted to the cortical gray matter reveal a topographic organization of juvenile callosal connections similar to that of the adult. In contrast, injections extending into the white matter and adequate to reach the transitory callosal axons which appear to be confined there reveal what appears to be an earlier organization. These two organizations probably reflect different morphogenetic factors.

The transience of cerebrocerebellar projections is due to selective elimination of axon collaterals and not neuronal death

Fluorescent dyes were used to determine firstly if the transience of cerebrocerebellar projections in neonatal kittens is due to the selective elimination of axon collaterals or to neuronal death; and secondly, if the cerebrocerebellar projection neurons lived, did any maintain a projection to the brainstem or spinal cord. Injections of Fast Blue were made into the cerebellar cortex and deep nuclei in 7-9 postnatal days old kittens, the age in which cortical axons grow into the cerebellum. Later, at 31-71 postnatal days of age, when the transient cerebrocerebellar projections have disappeared, injections of Nuclear Yellow were made into the brainstem or the spinal cord. In the frontoparietal cortex, numerous neurons were labeled with Fast Blue suggesting that the disappearance of cerebrocerebellar projections is due primarily to the selective elimination of axon collaterals and not neuronal death. Moreover, many of the cortical neurons labeled with Fast Blue also were labeled with Nuclear Yellow which shows that many of the cortical neurons with transient collateral projections to the cerebellum in the neonate maintain a projection to brainstem or spinal targets in older animals.

Thalamocortical connections in newborn mice

Thalamocortical axons reach the developing neocortex and become distributed within the cortical subplate during the third week of gestation. The present study is an analysis of the organization of connections that link thalamus and cortical subplate (corresponding to future layers V and VI) at birth. This age antedates the ascent of thalamic axons to contact cells of the supragranular layers, their principal targets in the adult cortex. At birth thalamic nuclear subdivisions are explicit; field-characteristic cytoarchitectonic features, relating principally to the infragranular layers, delineate the majority of neocortical fields. The projection of principal relay nuclei upon the majority of fields of the cerebral convexity has been mapped by means of retrograde transport of HRP. Nucleus-to-field interrelationships as well as topologic order of the overall thalamic projection prove to be identical to that in the adult animal. The neonatal projection appears to be somewhat more divergent than that of the adult.

Regressive events in neurogenesis

The development of most regions of the vertebrate nervous system includes a distinct phase of neuronal degeneration during which a substantial proportion of the neurons initially generated die. This degeneration primarily adjusts the magnitude of each neuronal population to the size or functional needs of its projection field, but in the process it seems also to eliminate many neurons whose axons have grown to either the wrong target or an inappropriate region within the target area. In addition, many connections that are initially formed are later eliminated without the death of the parent cell. In most cases such process elimination results in the removal of terminal axonal branches and hence serves as a mechanism to "fine-tune" neuronal wiring. However, there are now also several examples of the large-scale elimination of early-formed pathways as a result of the selective degeneration of long axon collaterals. Thus, far from being relatively minor aspects of neural development, these regressive phenomena are now recognized as playing a major role in determining the form of the mature nervous system.

Developmental sequence in the origin of descending spinal pathways. Studies using retrograde transport techniques in the North American opossum (Didelphis virginiana)

The origin of descending pathways to thoracic and cervical levels of the spinal cord has been investigated with retrograde tracing techniques in a series of pouch young and adult opossums. The opossum was chosen because it is born in a very immature state, 12-13 days after conception, and has a protracted development in an external pouch. A few neurons in the pontine reticular formation and nucleus coeruleus were labeled by horseradish peroxidase (HRP) injections of the thoracic cord as early as postnatal day (PND) 3. By PND 5, similar injections labeled neurons in the same areas as well as in the medullary reticular formation, the raphe nuclei of the caudal pons and medulla, the spinal trigeminal nuclei, the vestibular complex, the accessory oculomotor nuclei and the interstitial nucleus of Cajal. When Nuclear Yellow (NY) was employed, neurons were also labeled in the red nucleus, the hypothalamus and possibly in the nucleus of the solitary tract. Regardless of the technique employed, neurons in the dorsal column nuclei were not labeled by thoracic injections until at least PND 14. Axons from the nucleus ambiguus, the fastigial and interposed nuclei of the cerebellum as well as the intermediate and deep layers of the superior colliculus reach cervical levels of the cord, where they are specifically targeted, by at least PND 17. They do not significantly overgrow those levels during development. Corticospinal axons are the last of the major descending pathways to innervate the spinal cord. Cortical neurons cannot be labeled by cervical injections of either HRP or NY until at least PND 30. Evidence for transient brainstem-spinal and corticospinal projections was obtained.

Variability in the distribution of callosal projection neurons in the adult rat parietal cortex

Previous reports have shown that the barrel field area of the parietal cortex of the adult rat contains relatively few callosal projection neurons, even though callosal projection neurons are abundant in this cortical region in the neonatal rat. Furthermore, it has been shown that many of the callosal neurons which seem to disappear as the animal matures do not die, but project to ipsilateral cortical areas. These findings rely on the ability of retrograde transport techniques which utilize injections of horseradish peroxidase (HRP) or of fluorescent dyes into one hemisphere. We now show that several technical modifications of the HRP technique yield a wider distribution of HRP-containing neurons in the contralateral barrel field area of the adult rat than previously reported. These include implants of HRP pellets into transected axons of the corpus callosum, the addition of DMSO and nonidet P40 to Sigma VI HRP, wheat germ agglutinin HRP and the use of tetramethyl benzidine as the chromogen in the reaction procedure. Our findings have implications for transport studies in general and for the development of the cortical barrel field in particular.

Growth of the mouse corpus callosum

Brains of BALB/cCF inbred mice were examined at 15 ages ranging from 16.5 to 50.5 days from conception and cross-sectional areas of major forebrain fibre tracts at the midsagittal plane were measured. The anterior commissure appeared prior to the corpus callosum (CC), which was first seen at midplane at 17.0 days, and both tracts underwent a very rapid increase in size in the prenatal and early postnatal period, reaching the adult range of size at about 1 week after birth or several days prior to the onset of myelination. The growth spurt of these fibre tracts was much more pronounced than that of whole brain. By comparing BALB/c mice with hybrid mice that always have normal CC, it was found that some BALB/c mice at 18.5 days of age which have very small or absent CC do so because the growth of the whole brain is retarded whereas others have CC that is small for the brain size. Evidence also suggested that many mice with no CC but normal brain size at 18.5 days prenatally do eventually acquire at least a small CC. Observations at 3 postnatal ages of BALB/c mice weighed and marked at birth revealed that the 'runts' with low birth weight, which were presumably retarded prenatally, either die or catch up with mice of normal birth weight and do not have unusually small adult CC.