Callosal Projection Neurons Research Articles

Prenatal specification of callosal connections in rhesus monkey

Anatomical tracing and quantitative techniques were used to examine the tempo and pattern of maturation for callosal projection neurons in the monkey prefrontal cortex (PFC) during fetal and postnatal development. Nineteen monkeys were injected with retrograde tracers (fluorescent dyes, horseradish peroxidase conjugated to wheat germ agglutinin [WGA-HRP] or HRP crystals) at various ages between embryonic day 82 (E82) and adulthood. The size of injection sites was varied in fetal, newborn, and adult cases. In adults, labeled neurons were found in greatest density in the homotopic cortex of the opposite hemisphere and considerable numbers were also observed in a constellation of heterotopic areas including the medial and lateral orbital cortex, the dorsomedial convexity, and the pregenual cortex. The majority of labeled neurons were consistently concentrated in the lower half of layer III in all areas. In cases with large injection sites, callosal neurons of layer III formed a continuous and uninterrupted band that extended over the entire lateral surface of the prefrontal cortex spanning both homotopic and heterotopic areas. In contrast, in cases with small injection sites, the labeling of layer III neurons exhibited discontinuities. Between embryonic ages E82 and E89, injections limited to the cortical layers labeled only a small number of neurons in the opposite hemisphere, indicating that few callosal axons have invaded the cortex by this age. However, by E111 comparable injections labeled a large number of callosal neurons and many features of their distribution were adult-like. The number and constellation of cytoarchitectonic areas that were labeled in the frontal cortex of the opposite hemisphere were the same as in adults and the majority of callosal neurons were found in supragranular layer III. Finally, in fetal animals beyond E111, labeled neurons extended as a nearly unbroken band over a wide expanse of the dorsolateral PFC, resembling the pattern seen in adult monkeys with large injections. The conclusion we draw from these results, together with our earlier findings (Schwartz and Goldman-Rakic: Nature 299:154, 1982), is that callosal neurons whose axons enter the cortical layers of the primate prefrontal cortex achieve their mature laminar and areal distribution prior to birth and do so largely by cumulative processes.

Synapses made by axons of callosal projection neurons in mouse somatosensory cortex: Emphasis on intrinsic connections

This is one of a series of papers aimed at identifying the synaptic output patterns of the local and distant projections of subgroups of pyramidal neurons. The subgroups are defined by the target site to which their main axon projects. Pyramidal neurons in areas 1 and 40 of mouse cerebral cortex were labeled by the retrograde transport of horseradish peroxidase (HRP) transported from severed callosal axons in the contralateral hemisphere. Terminals of the local axon collaterals of these neurons ("intrinsic" terminals) were identified in somatosensory areas 1 and 40, and their distribution and synaptic connectivity were examined. Also examined were the synaptic connections of "extrinsic" callosal axon terminals labeled by lesion induced degeneration consequent to the severing of callosal fibers. A post-lesion survival time of 3 days was chosen because by this time the extrinsic terminals were all degenerating, whereas the intrinsic terminals were labeled by HRP. Both intrinsic and extrinsic callosal axon terminals occurred in all layers of the cortex where they formed only asymmetrical synapses. Layers II and III contained the highest concentrations of both types of callosal axon terminal. Analyses of serial thin sections through layers II and III in both areas 1 and 40 yielded similar results: 97% of the extrinsic (277 total sample) and of the intrinsic (1215 total sample) callosal axon terminals synapsed onto dendritic spines, likely those of pyramidal neurons; the remainder synapsed onto dendritic shafts of both spiny and nonspiny neurons. Thus the synaptic output patterns of intrinsic vs. extrinsic callosal axon terminals are strikingly similar. Moreover, the high proportion of axospinous synapses formed by both types of terminal contrasts with the proportion of asymmetrical, axospinous synapses that occur in the surrounding neuropil where only about 80% of the asymmetrical synapses are onto spines. This result is in accord with previous quantitative studies of the synaptic connectivities of both extrinsic and intrinsic axonal pathways in the cortex (White and Keller, 1989: Cortical Circuits; Boston: Birkhauser): in all instances, axonal pathways are highly selective for the types of elements with which they synapse.

Callosal projections in rat somatosensory cortex are altered by early removal of afferent input.

During the first postnatal week, the distribution of callosal projection neurons in the rat somatosensory cortex changes from a uniform to a discontinuous pattern. To determine if this change is influenced by afferent inputs to the somatosensory cortex, the effect of both early unilateral infraorbital nerve section and unilateral removal of the dorsal thalamus on the distribution of callosal projections in rat somatosensory cortex was examined. One month after either of the above manipulations at birth, the tangential distribution of callosal projections in the somatosensory cortex was examined using the combined retrograde and anterograde transport of horseradish peroxidase. Both manipulations alter the distribution of callosal projection neurons and terminations in the somatosensory cortex. After infraorbital nerve section, the distribution of callosal projections is altered in the contralateral primary somatosensory cortex. The abnormalities observed are consistent with the altered distribution of thalamocortical projections. In addition, consistent abnormalities were observed in the pattern of callosal projections of the second somatosensory area of both hemispheres. Most notably, they are absent in a portion of the region that contains the representation of the mystacial vibrissae and sinus hairs in this area. Thalamic ablation resulted in highly aberrant patterns of callosal projections in the somatosensory cortex on the operated side, where abnormal bands and clusters of callosal projections were observed in apparently random locations. These results are interpreted as evidence that both peripheral and central inputs influence the maturational changes in the distribution of callosal projection neurons.

Comparison of human transcallosal responses evoked by magnetic coil and electrical stimulation

Human transcallosal responses (TCRs) were elicited by focal magnetic oil (MC) stimulation of homologous sites in contralateral frontal cortex and compared with those to focal anodic stimulation. With MC stimulation, the TCR consisted of an initially positive wave with an onset latency of 8.8–12.2 msec, a duration of 7–15 msec, and an amplitude which reached up to 20 μV, sometimes followed by a broad low amplitude negative wave. With anodic stimulation, a similar response was obtained in which the positive wave was similar in latency and maximum amplitude, but had a greater duration. With anodic stimulation, not only was the TCR threshold below that for contralateral movement, but it reached substantial size at intensities below motor threshold. With MC stimulation, contralateral arm movement and scalp corticomotor potentials were observed when the MC was displaced posteriorly towards the central sulcus. Unlike with anodic stimulation, the MC evoked TCR was usually not preceded by a prominent EMG potential from temporalis muscle and was not associated with subject discomfort. The TCR provides unique information concerning the functional integrity of callosal projection neurons, their axons and transsynaptic processes in recipient cortex. This information may prove useful in the evaluation of intrinsic cerebral mechanisms and in establishing cortical viability.

Callosal projection neurons in area 17 of the fetal rhesus monkey

We have studied the distribution of callosal projection neurons in area 17 of a fetal rhesus monkey which received large injections of horseradish peroxidase into the contralateral occipital cortex. In comparison to other cortical areas, area 17 contains few callosal projection neurons. Most of these cells are confined to a region extending tangentially about 2.5 mm from the 17/18 border, although a few neurons were noted as much as 5 mm from the border. Comparing the distribution of callosal projection neurons in the fetal monkey with what has been described in newborn and adult macaques, it is apparent that although some degree of refinement in striate callosal connections may occur during in utero development, the prenatal development of callosal connections in the macaque is inherently adult-like.

Process elimination underlies ontogenetic change in the distribution of callosal projection neurons in the postcentral gyrus of the fetal rhesus monkey.

During fetal development, the regional distribution of callosal projection neurons in the rhesus monkey's postcentral gyrus changes from a uniform to a discontinuous pattern. To determine if this developmental change reflects the retraction of transient callosal projections, two different fluorescent tracers were injected into the brain of fetal monkeys of known gestational ages. Fast blue was injected into the entire postcentral gyrus of one hemisphere, whereas a second tracer (rhodamine latex beads or diamidino yellow) was injected into the caudal portion of the postcentral gyrus of the other hemisphere. The rostral portion of the postcentral gyrus (contralateral to the hemisphere injected with fast blue) was subsequently examined for the presence of labeled cells. In animals injected early in fetal development, on embryonic day 110 or younger and sacrificed 4 weeks later, there were numerous cells labeled with both tracers. In contrast, very few double-labeled cells were found in fetuses injected at an older age, embryonic day 135. We interpret these findings as showing that early in fetal development, when callosal projection neurons in the postcentral gyrus show a continuous distribution pattern, single cells in the rostral portion of this gyrus possess at least two collaterals, one projecting to the contralateral hemisphere and the other to the caudal portion of the gyrus. Subsequently, many of these neurons retract callosal collaterals while maintaining ipsilateral projections. Thus, process elimination accounts for the establishment of the discontinuous distribution of callosal neurons found in the postcentral gyrus of the mature primate.

Absence of interhemispheric connections of area 17 during development in the monkey.

Our understanding of the development of cortical connectivity largely stems from studies of the ontogeny of interhemispheric pathways in carnivores, rodents and lagomorphs. Early in development, cortical neurons projecting to the contralateral hemisphere through the corpus callosum (callosal projection neurons) have a widespread distribution. As maturation proceeds, callosal projection neurons become restricted to those cortical regions that are connected in the adult. In newborn cats and rats, for example, callosal projection neurons are not restricted to the 17-18 border as in the adult, but are found throughout areas 17 and 18. The macaque monkey is an exception, because at birth it has an adult-like distribution of callosal projection neurons in area 18, with practically none in area 17. Here we show that whereas area 17 is devoid of interhemispheric connections throughout prenatal development, the distribution of callosal projection neurons in area 18 shows the common sequence of an early widespread distribution followed by regression. The absence of callosal projection neurons in area 17 throughout ontogeny may well be a feature unique to Old World primates.

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 (> 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.

Synaptic connections of callosal projection neurons in the vibrissal region of mouse primary motor cortex: an electron microscopic/horseradish peroxidase study.

Reciprocal axonal projections between homotypic areas of the vibrissal region of mouse primary motor cortex (MsI) (Porter and White: Neurosci. Lett. 47:37-40, '84) suggested the existence of reciprocal synaptic connections between callosal projection neurons and callosal afferents. In the present study, the retrograde transport of horseradish peroxidase (HRP) was combined with lesion-induced degeneration to identify synapses between callosal afferents and callosal neurons in the corresponding region of the contralateral cortex. The procedure was as follows: MsI was injected with HRP and aspirated on the following day. After 4 days, the animals were perfused and motor cortex was processed for HRP according to a variation of the Adams (Brain Res. 176:33-47,'77) technique, and postfixed in OsO4. The methods used consistently filled fine dendritic branches and spines with dense reaction product, thus allowing examination of synaptic contacts with these processes. All callosal projection neurons were identified as pyramidal neurons, having somata in cortical layers II/III and V. Labeled cells from each of the two levels were prepared for electron microscopy, and that part of each cell's apical dendrite that traversed the superficial cortical layers, where most callosal axons terminate, was cut in an unbroken series of thin sections. Micrographs were taken of all labeled profiles in each thin section, and tracings of the profiles were assembled to reconstruct the apical dendrites. Data on the distribution, type, and amount of callosal and other synapses with the shaft and spines of the apical dendrites were obtained by examining the reconstructions. In addition, profiles of basal dendrites of layer II/III cells were examined in thin sections to ascertain the numbers of callosal and other synapses formed with their shafts and spines. The proportion of synapses that each dendrite formed with callosal axon terminals was compared to the concentration of callosal afferents in the neuropil. Dendrites of both layer II/III and layer V pyramidal cells synapsed with callosal axon terminals. The apical and basal dendrites of layer II/III neurons formed a similar proportion of their synapses with callosal afferents, and this was similar to the concentration of callosal synapses in the surrounding neuropil. Segments of apical dendrites belonging to layer V and layer II/III neurons course through neuropil containing nearly the same concentration of degenerating callosal terminals, but the layer V cells form fewer callosal synapses.(ABSTRACT TRUNCATED AT 400 WORDS)

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 ssem 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 and 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.

Glia and the elimination of transient cortical projections

Over the past decade a number of studies have demonstrated that during the course of development, cortical projection neurons undergo major changes in their areal distribution. In particular, the distribution of neurons projecting to a given target is much wider in the infant than in the adult. This was first demonstrated for callosal projection neurons in the visual system of the cat 1 , but since then the observation has been extended to callosal projections of the somatosensory system of the cat 2 , rat 3 and monkey 4 . There is also evidence that similar changes occur in the distribution of cortical neurons which project sub-cortically 5–7 . Other evidence, obtained utilizing retrograde double labelling techniques, has suggested that these changes are accomplished, not by the elimination of neurons (‘cell death') 8.9 , but by the selective elimination of neuronal processes 10 . These findings, which were treated in detail in a recent review in TINS by Stanfield 11 , raise the intriguing question of how axonal processes are selectively eliminated during the course of development.

Termination of callosal afferents onto identified callosal projection neurons in the primary motor cortex of the mouse

Techniques utilizing the retrograde transport of horseradish peroxidase and lesion-induced degeneration have been used to identify neurons which form the callosal projection pathway and callosal axon terminals in the primary motor cortex of the mouse. Synapses occur between callosal projection neurons and degenerating callosal afferents, demonstrating a direct synaptic link between neurons which project to homotopic regions of the contralateral cortex.

The pattern of interhemispheric connections and its relationship to extrastriate visual areas in the macaque monkey

The distribution of interhemispheric connections was studied in extrastriate visual cortex of the macaque monkey. Callosal fiber terminations were identified by staining for anterograde degeneration following transection of the splenium of the corpus callosum. Retrogradely labeled cell bodies of callosal projection neurons were identified histochemically following application of horseradish peroxidase to the cut surface of the callosum. Results were displayed on unfolded, two-dimensional representations of the cortex, which permitted spatial and topological relationships between callosal recipient and callosal free cortical regions to be discerned readily. The overall pattern of callosal inputs to visual cortex can be subdivided into nine callosal recipient zones which surround seven callosal free regions in the occipital, temporal, and parietal lobes. This pattern provides reliable and useful landmarks for identifying the borders of at least five topographically organized extrastriate visual areas. There is a pronounced dorsoventral asymmetry in callosal projections, not recognized in previous studies, which reflects an asymmetry in the organization of visual areas in dorsal versus ventral halves of the occipital lobe. The pattern of callosal fiber terminations is mirrored by a very similar distribution of callosal projection neurons. There are significant regional differences in the laminar distribution of callosal projection cells, and these differences may reflect functionally distinct cortical subdivisions. A considerable degree of individual variability was found in the relationship of callosal connections to gyral and sulcal landmarks as well as in the fine structure of individual callosal recipient strips, suggesting that each animal has a unique "callosal fingerprint." These findings emphasize the usefulness of the callosal pathway in elucidating the functional organization of extrastriate visual cortex.

The ontogeny of the distribution of callosal projection neurons in the rat parietal cortex.

The ontogeny of callosal projection neurons in the rat parietal cortex was examined using the retrograde and anterograde transport of horseradish peroxidase (HRP), as well as Golgi and Nissl stains. From postnatal day 0 (PND 0) to early PND 4, the callosal projection neurons are distributed as two continuous horizontal bands of cells which extend throughout the subplate in layers Va and Vc-upper VIa. Neurons within the cortical plate (CP), however, do not transport HRP from a contralateral injection site until PND 3 to early PND 4, when a few cells at the lower CP border are generally labeled. However, by late on PND 4, and more consistently by PND 5, several changes in the distribution of callosal projection neurons take place. First, cells at all levels of the CP become labeled in a sequential fashion, from the lower border upward. Second, gaps, or areas devoid of HRP, become apparent in layer IV of the barrel field area. Third, in the cortical areas containing the gaps, as well as in other areas which are destined not to be callosally connected in the adult, there is a noticeable decrease in the number of cells labeled with HRP. This decrease continues through PND 15 and possibly into adulthood. The foregoing developmental events are compared to cortical maturation as seen in both Golgi- and Nissl-stained material. By PND 15, the basic adult pattern of callosal projection neurons is established. The neurons reside mainly in layers III and Va, with fewer in layers II and Vc-upper VIa, and fewer still in the other cortical layers. They are aligned in vertical arrays in discrete areas of the cortex.

Differential distribution of callosal projection neurons in the neonatal and adult rat

Differential distribution of callosal projection neurons in the neonatal and adult rat

Demonstration of geniculocortical and callosal projection neurons in the squirrel monkey by means of retrograde axonal transport of horseradish peroxidase

Demonstration of geniculocortical and callosal projection neurons in the squirrel monkey by means of retrograde axonal transport of horseradish peroxidase

PROJECTION OF THE RETINA ON TO STRIATE AND PRESTRIATE CORTEX IN THE SQUIRREL MONKEY, SAIMIRI SCIUREUS.

PROJECTION OF THE RETINA ON TO STRIATE AND PRESTRIATE CORTEX IN THE SQUIRREL MONKEY, SAIMIRI SCIUREUS.