Callosal connectivity of areas V1 and V2 in the newborn monkey

Role of Nrp1 in controlling cortical inter-hemispheric circuits

Axons of the corpus callosum (CC) mediate the interhemispheric communication required for complex perception in mammals. In the somatosensory (SS) cortex, the CC exchanges inputs processed by the primary (S1) and secondary (S2) areas, which receive tactile and pain stimuli. During early postnatal life, a multistep process involving axonal navigation, growth, and refinement, leads to precise CC connectivity. This process is often affected in neurodevelopmental disorders such as autism and epilepsy. We herein show that in mice, expression of the axonal signaling receptor Neuropilin 1 (Nrp1) in SS layer (L) 2/3 is temporary and follows patterns that determine CC connectivity. At postnatal day 4, Nrp1 expression is absent in the SS cortex while abundant in the motor area, creating a sharp border. During the following 3 weeks, Nrp1 is transiently upregulated in subpopulations of SS L2/3 neurons, earlier and more abundantly in S2 than in S1. In vivo knock-down and overexpression experiments demonstrate that transient expression of Nrp1 does not affect the initial development of callosal projections in S1 but is required for subsequent S2 innervation. Moreover, knocking-down Nrp1 reduces the number of S2L2/3 callosal neurons due to excessive postnatal refinement. Thus, an exquisite temporal and spatial regulation of Nrp1 expression determines SS interhemispheric maps.

Developmental refinement of visual callosal inputs to ferret area 17

Corticocortical connections link visual cortical areas in both the ipsilateral and contralateral hemispheres. We studied the postnatal refinement of callosal connections linking multiple cortical areas with ferret area 17 during the period from just before eye opening (4 weeks) to 10 weeks of age. We aimed to determine (1) whether callosal projections from multiple visual cortical areas to area 17 refine with a similar rate and (2) whether the refinement of callosal projections parallels that of intrahemispheric cortical circuits. We injected the bidirectional tracer CTb into area 17, and mapped the areal and laminar distribution of labeled cells in visual areas of the contralateral hemisphere. Like intrahemispheric projections, callosal inputs to area 17 before eye opening are dominated by Suprasylvian area Ssy (with lesser and comparable input from areas 17, 18, 19, and 21), but within 2 weeks of eye opening are jointly dominated by area 18 and Ssy inputs; however, there are fewer labeled cells in the contralateral hemisphere. Unlike intrahemispheric projections, there is no laminar reorganization of callosal inputs; in all visual areas and at all ages studied, the greatest proportion of callosal projections arises from the infragranular layers. Also, unlike intrahemispheric projections, the peak density of callosal cells in each area projecting to area 17 declines more modestly. These results reveal important similarities and differences in the postnatal reorganization of inter- and intrahemispheric projections to area 17.

Development and plasticity of the corpus callosum

The corpus callosum (CC) connects the cerebral hemispheres and is the major mammalian commissural tract. It facilitates bilateral sensory integration and higher cognitive functions, and is often affected in neurodevelopmental diseases. Here, we review the mechanisms that contribute to the development of CC circuits in animal models and humans. These species comparisons reveal several commonalities. First, there is an early period of massive axonal projection. Second, there is a postnatal temporal window, varying between species, in which early callosal projections are selectively refined. Third, sensory-derived activity influences axonal refinement. We also discuss how defects in CC formation can lead to mild or severe CC congenital malformations.

Transient callosal projections of L4 neurons are eliminated for the acquisition of local connectivity

Interhemispheric axons of the corpus callosum (CC) facilitate the higher order functions of the cerebral cortex. According to current views, callosal and non-callosal fates are determined early after a neuron's birth, and certain populations, such as cortical layer (L) 4 excitatory neurons of the primary somatosensory (S1) barrel, project only ipsilaterally. Using a novel axonal-retrotracing strategy and GFP-targeted visualization of Rorb+ neurons, we instead demonstrate that L4 neurons develop transient interhemispheric axons. Locally restricted L4 connectivity emerges when exuberant contralateral axons are refined in an area- and layer-specific manner during postnatal development. Surgical and genetic interventions of sensory circuits demonstrate that refinement rates depend on distinct inputs from sensory-specific thalamic nuclei. Reductions in input-dependent refinement result in mature functional interhemispheric hyperconnectivity, demonstrating the plasticity and bona fide callosal potential of L4 neurons. Thus, L4 neurons discard alternative interhemispheric circuits as instructed by thalamic input. This may ensure optimal wiring.

The Mouse Cortical Connectome, Characterized by an Ultra-Dense Cortical Graph, Maintains Specificity by Distinct Connectivity Profiles

The inter-areal wiring pattern of the mouse cerebral cortex was analyzed in relation to a refined parcellation of cortical areas. Twenty-seven retrograde tracer injections were made in 19 areas of a 47-area parcellation of the mouse neocortex. Flat mounts of the cortex and multiple histological markers enabled detailed counts of labeled neurons in individual areas. The observed log-normal distribution of connection weights to each cortical area spans 5 orders of magnitude and reveals a distinct connectivity profile for each area, analogous to that observed in macaques. The cortical network has a density of 97%, considerably higher than the 66% density reported in macaques. A weighted graph analysis reveals a similar global efficiency but weaker spatial clustering compared with that reported in macaques. The consistency, precision of the connectivity profile, density, and weighted graph analysis of the present data differ significantly from those obtained in earlier studies in the mouse.

Convergence of Cortical, Thalamocortical, and Callosal Pathways during Human Fetal Development Revealed by Diffusion MRI Tractography

There has been evidence that during brain development, emerging thalamocortical (TC) and corticothalamic (CT) pathways converge in some brain regions and follow each other's trajectories to their final destinations. Corpus callosal (CC) pathways also emerge at a similar developmental stage, and are known to converge with TC pathways in specific cortical regions in mature brains. Given the functional relationships between TC and CC pathways, anatomical convergence of the two pathways are likely important for their functional integration. However, it is unknown (1) where TC and CT subcortically converge in the human brain, and (2) where TC and CC converge in the cortex of the human brain, due to the limitations of non-invasive methods. The goals of this study were to describe the spatio-temporal relationships in the development of the TC/CT and CC pathways in the human brain, using high-angular resolution diffusion MR imaging (HARDI) tractography. Emerging cortical, TC and CC pathways were identified in postmortem fetal brains ranging from 17 gestational weeks (GW) to 30 GW, as well as in vivo 34-40 GW newborns. Some pathways from the thalami were found to be converged with pathways from the cerebral cortex as early as 17 GW. Such convergence was observed mainly in anterior and middle regions of the brain until 21 GW. At 22 GW and onwards, posterior pathways from the thalami also converged with cortical pathways. Many CC pathways reached the full length up to the cortical surface as early as 17 GW, while pathways linked to thalami (not only TC axons but also including pathways linked to thalamic neuronal migration) reached the cortical surface at and after 20 GW. These results suggest that CC pathways developed earlier than the TC pathways. The two pathways were widespread at early stages, but by 40 GW they condensed and formed groups of pathways that projected to specific regions of the cortex and overlapped in some brain regions. These results suggest that HARDI tractography has the potential to identify developing TC/CT and CC pathways with the timing and location of their convergence in fetal stages persisting in postnatal development.

A hierarchical, retinotopic proto-organization of the primate visual system at birth

The adult primate visual system comprises a series of hierarchically organized areas. Each cortical area contains a topographic map of visual space, with different areas extracting different kinds of information from the retinal input. Here we asked to what extent the newborn visual system resembles the adult organization. We find that hierarchical, topographic organization is present at birth and therefore constitutes a proto-organization for the entire primate visual system. Even within inferior temporal cortex, this proto-organization was already present, prior to the emergence of category selectivity (e.g., faces or scenes). We propose that this topographic organization provides the scaffolding for the subsequent development of visual cortex that commences at the onset of visual experience.

Consciousness and hallucinations in schizophrenia: the role of synapse regression

Consciousness takes two forms, transitive and intransitive. Transitive consciousness is a matter of being conscious of something or other whereas intransitive consciousness has no object, as being conscious or awake. Of the different forms of transitive consciousness, perceptual, somatic, kinaesthetic and so on, cognitive neuroscience has concentrated on determining the neural concomitants of perceptual consciousness. To be conscious of a percept is to be aware of it and this requires attending to it. This work sets out a hypothesis as to what brain areas are involved in a schizophrenia subject attending and becoming aware of hallucinations. First, the different areas of cortex that support different visual and auditory illusions of percepts are considered. Next it is argued that endogenous activity in these areas of cortex give rise to hallucinations of percepts that are similar to the percepts that these same areas support during illusions. The basis of such endogenous activity, it is suggested, is to be found in the paucity of afferent synapses to these cortical areas. This may occur as a consequence of loss and regression of synapses due to a degenerative disease or because of abnormal synapse formation and regression during childhood and adolescence, as is likely to be the case in schizophrenia. Finally the neural basis of attention and awareness of these hallucinations are considered for subjects suffering from schizophrenia, and a set of important questions posed that await elucidation through future experimental studies.

Genetic determination of neurophysiological mechanisms of cortical-subcortical integration of bioelectrical brain activity

The contribution of genetic factors to the formation of the neurophysiological mechanisms of cortical-subcortical integration was studied in 12 pairs of monozygotic and five pairs of dizygotic twins (aged 18-25 years). Intrapair similarity of the nature of spatial interactions between bioelectrical activity in the cerebral cortex, assessed from different combinations of statistical interactions of EEG from 16 monopolar recordings, was assessed in each pair of twins (and among 544 non-related pairs of subjects in both groups). The results suggest a high level of general population invariance and relatively small inherited and phenotypic variability in the morphofunctional systems making up the major neurophysiological mechanisms of brain integration as a whole. The ontogenetic formation of stem and subcortical regulatory structures, which have a leading role in the systems combination of different parts of the brain into a single formation, appears to occur in all individuals by the same principle, as disturbance can apparently affect the fundamental monomorphic features of the species. In turn, we might expect to find large interindividual variability in the establishment of interregional connections of the neocortex, the role of inherited and environmental factors being different in the processes forming long and relatively short intercortical interactions.

Phenotypic characterisation of respecified visual cortex subsequent to prenatal enucleation in the monkey: development of acetylcholinesterase and cytochrome oxidase patterns

Prenatal bilateral enucleation induces cortex, which normally would have become striate cortex, to follow a default developmental pathway and to take on the cytoarchitectonic appearance of extrastriate cortex (default extrastriate cortex, Dehay et al. [1996] J. Comp. Neurol. 367:70-89). We have investigated if this manipulation influences the cortical expression of acetylcholinesterase (AChE) and cytochrome oxidase (CO). Early enucleation (before embryonic day 81; E81) had only minor effects on the distribution of AChE and CO in the striate cortex. In animals that underwent operation, the striate cortex CO blobs were significantly more closely spaced on the operculum compared with the calcarine. After early enucleation, there was a periodic distribution of CO dense patches in default extrastriate cortex. These CO patches had a center-to-center spacing that was considerably smaller than that of CO stripes in normal area V2, but was somewhat larger than that of the CO blobs in striate cortex. Although the CO stripes characteristic of normal area V2 could not be detected, there were some high-frequency CO patches, similar to those found in default extrastriate cortex. Early enucleation caused a failure to form the transient AChE bands running perpendicular to the striate border, which are normally present in the fetus and early neonate. Late enucleation did not alter AChE expression in extrastriate cortex. The relatively minor effects of early enucleation in the reduced striate cortex contrast with the changes in expression of these enzymes in extrastriate cortex, which accompany large shifts in the location of the striate border. This suggests a massive reorganisation of cortical phenotype in extrastriate cortex.

Role of directed growth and target selection in the formation of cortical pathways: prenatal development of the projection of area V2 to area V4 in the monkey

In experiments combining retrograde tracers and histochemistry, we have looked at the prenatal development of the cortical pathway linking areas V2 and V4. Transient expression of acetylcholinesterase in fetal area V2 reveals the separate compartments that project to V4 (temporal directed pathway) and V5 (parietal directed pathway). During early stages of pathway formation, V2 neurons projecting to area V4 are clustered in the appropriate compartments. During the phase of rapid axonal growth, there is a selective increase of connections originating from the appropriate compartments leading to a strongly clustered organization at the peak of connectivity. During this phase, injections involving the white matter also showed clustering, but this was somewhat reduced in comparison to that of gray matter injections. The growth phase is followed by an elimination phase during which there is a tendency for a preferential loss of intercluster connections, which may sharpen the early formed pattern. These results demonstrate the primary role of axonal guidance and target recognition mechanisms followed by a limited extent of selective elimination during the formation of functional cortical pathways in the primate isocortex. Compared to previous findings, these results suggest that the developmental restriction of callosal connections is not a universal model of cortical development. In the present report, the directed growth and early specification of feed-forward connections contrast with the prolonged remodelling of monkey feedback projections, suggesting two distinct developmental strategies of pathway formation in the monkey.

Transient subcortical connections of inferior temporal areas TE and TEO in infant macaque monkeys

As part of a long-term study designed to examine the ontogeny of visual memory in monkeys and its underlying neural circuitry, we have examined the subcortical connections of the inferior temporal cortex in infant monkeys and compared them to those previously described in adult monkeys (Webster et al. [1993] J. Comp. Neurol. 335:73-91). Inferior temporal areas TEO and TE were injected with wheat germ agglutinin conjugated to horseradish peroxidase and tritiated amino acids, respectively, or vice versa, in 1-week-old (N = 6) and 3-4-year-old (N = 6) Macaca mulatta, and the distributions of labeled cells and terminals were examined in subcortical structures. Although the connections of inferior temporal cortex with subcortical structures were found to be similar in infant and adult monkeys, several projections appear to undergo refinement during development. Quantitative analysis showed that 1) whereas the projection from TE to the superior colliculus is consistent (5 of 5 cases) and widespread in infants, it is less reliable (2 of 7 cases) and limited in areal extent in adults; 2) although the projections from TE to nucleus medialis dorsalis and the tail of the caudate are present in infants and adults, they are reduced in adults; and 3) TEO receives input from the dorsal lateral geniculate nucleus in both infants and adults, but the number of cells giving rise to this projection is lower in adults. There was also a suggestion that TE projects to nucleus paracentralis in infants (2 of 5 cases) but not in adults (0 of 7 cases). No differences between infants and adults were apparent in other subcortical connections, including those with the pulvinar, reticular nucleus, claustrum, and putamen.

Visual callosal connections and strabismus

Strabismus is a condition that exists when the visual axes of the two eyes fail to intersect at the fixation point under binocular viewing conditions. When it occurs in mammals during the critical period which corresponds to the period of maximal plasticity early in life, strabismus is known to induce both morphological anomalies and abnormal connections from the retina to the cortex; it further leads to binocular neural changes and to spatial vision deficits, especially at the cortical level. After a brief review of the already known data about the consequences of early strabismus in cats, monkeys and humans on the development of the visual system and of visual perception, new data are presented here concerning interhemispheric connections in the cat. In normally-reared kittens, visual callosal transfer is shown to be almost adult-like as soon as 12 days after birth: it is almost limited to the 17/18 border of the visual cortex when using visual stimulations in spite of the presence of still numerous juvenile exuberant callosal projections. In contrast, callosal transfer of visual information is extended to both areas 17 and 18 after strabismus, leading to the conclusion that at least some juvenile exuberant callosal projections are not only anatomically but also functionally stabilized after such an oculomotor disease. The possibility that similar abnormalities might be present in monkeys and humans is discussed.

Interhemispheric connections in neonatal owl monkeys (Aotus trivirgatus) and galagos (Galago crassicaudatus)

Interhemispheric connections were studied by injecting a mixture of horseradish peroxidase (HRP) and wheatgerm agglutinin conjugated with horseradish peroxidase (WGA-HRP) into multiple sites in dorsolateral occipital and parietal cortex of one cerebral hemisphere of three galagos (Galago crassicaudatus) and two owl monkeys (Aotus trivirgatus) within seven days of birth. Cortex was either separated from the rest of the brain, flattened and cut parallel to the surface to aid reconstructing surface-view patterns of labeled neurons and processes, or cut in standard coronal or parasagittal planes to better reveal laminar patterns of connections. In both primate species, the surface-view pattern of callosal connections in infants was remarkably adult-like. In infant owl monkeys, callosal connections were concentrated along the margin of area 18 with area 17, and only a few labeled cells were found within area 17. Other visual areas including the second visual area, V-II, and the middle temporal visual area, MT, had patchy distributions of labeled neurons that extended over large parts of the visual field representations. Primary motor, auditory, and somatosensory fields also had patchy distributions of labeled neurons, with regions of areas 3b and adjoining somatosensory fields having few callosal connections in portions that appeared to correspond with representations of the hand and foot. Results were very similar in galagos, except that newborn galagos, as in adults, had a patchy distribution of callosally projecting neurons that extended well within area 17. Furthermore, the labeled neurons were concentrated in patches that aligned with the cytochrome oxidase blobs of area 17. Finally, callosal connections were concentrated in cytochrome oxidase poor regions of area 3b.

Developmental changes in the distribution of acetylcholinesterase in the extrastriate visual cortex of the monkey

In the fetal and neonatal monkey, periodically organized regions of high activity of acetylcholinesterase were found in the visual cortical area V2 (Area 18). The acetylcholinesterase bands, like the thin and thick stripes of cytochrome oxidase, were found to run orthogonal to the area 17/18 border. During neonatal development these bands progressively narrow and finally disappear shortly after four months of age.

Postnatal development of area 17 callosal connections in Tupaia

The goal of the present study was to investigate the pattern of maturation of callosal projecting neurons in a well-studied mammalian visual system with unique structural and functional properties. Studies of the distribution pattern of interhemispheric connections in the adult tree shrew primary visual cortex reveal not only a high concentration of labeled neurons along the area 17/18 border, as in standard experimental animals such as the cat and monkey, but also numerous callosal projecting neurons in the adjacent dorsal part of area 17, which largely corresponds to the binocular visual field (Kretz and Rager, Exp. Brain Res. 82:271, '90). Callosal projections were anatomically traced in 11 tree shrews (Tupaia belangeri) at various ages between postnatal day 7 (7, 9, 10, 13, 15, 17, 19, and 26 days old) and adulthood (107 days old). In each animal, four injections of wheat germ agglutinin conjugated to horseradish peroxidase were made in a standard configuration into the striate cortex of one hemisphere. In young tree shrews only 7 and 9 days old, heavily labeled terminal axon structures could be seen in the white matter and in layer VI of the opposite hemisphere. Only a few labeled neurons, however, were detected in layer III. The small number of labeled neurons indicated that early in postnatal development, only a few callosal axons had invaded the upper cortical layers. By 10 days of age, the number of supragranular neurons was increasing and the maximal value was counted in a 13-day-old tree shrew. A sharp decline in the number of labeled supragranular neurons was noticed--about 94% in our case--between days 13 and 15. In animals more than 15 days old, the distribution pattern and the density of the neurons looked like the pattern seen in the adult Tupaia brain. The labeled cells were mostly concentrated in layers II and III. The majority of neurons resembled typical pyramidal cells. However, some of the neurons in sublayer IIIc had elongated cell bodies oriented parallel to the laminar boundaries. In contrast to the supragranular cells found in all stages investigated, small populations of labeled cells in layer VI were observed in 9- to 17-day-old tree shrews only. In young postnatal animals 7 to 13 days old, a peculiar cell type was labeled on the ipsilateral side. In coronal sections these cell bodies formed a continuous band that extended from the ventricular wall to the subcortical white matter. These cells might belong to a population of cells still in migration.

Visual callosal projections in the adult ferret

The laminar and tangential organization of visual callosal projections of areas 17 and 18 were investigated in the adult ferret, using histochemical methods to visualize axonally transported horseradish peroxidase (HRP). Normal adult ferrets were given injections of HRP throughout one visual cortex or had gelfoam soaked in HRP applied to the transected corpus callosum. The ferret callosal cell distribution has a greater tangential extent in area 18 than in area 17. In addition, the radial organization of callosal cells in areas 17 and 18 differs: three times as many infragranular cells are present in area 18 than in area 17, although the number of supragranular cells is similar for both areas 17 and 18. Since the projections of alpha retinal ganglion cells are reported to be exclusively contralateral in the ferret (Vitek et al., 1985), callosal projections may make a major contribution to the binocularity of neurons in area 18.

Organization of callosal connections in the visual cortex of the rabbit following neonatal enucleation, dark rearing, and strobe rearing

The organization of visual callosal projections was studied in (1) normal adult rabbits; (2) adult rabbits which had undergone monocular enucleation (ME) or binocular enucleation (BE) at birth; and (3) adult rabbits which had been deprived of normal visual experience during development by dark rearing (DR) or strobe rearing (SR). Previously published observations (Murphy and Grigonis, Behav Brain Res 30:151, 1988) on callosal organization in adult rabbits in which retinal ganglion cell activity was eliminated during development by intraocular tetrodotoxin (TTX) injections, are also summarized for comparison with these data. The tangential extent of the callosal cell zone was significantly larger than normal in DR, TTX, and ME rabbits, was unchanged in BE rabbits, and was significantly reduced in SR rabbits. An analysis of the laminar distribution of the callosal cells revealed a significant increase in the percentage of callosal cells in lamina IV in ME, DR, and TTX animals. Measurements of density of callosal cells showed a significant increase in the density of the callosal projection in ME and SR rabbits and a decrease in density in BE rabbits compared with normal. The data suggest that the mechanisms involved in the development of the tangential and laminar organization of the callosal cell zone are different. In addition, the data suggest that the mechanisms involved in the maintenance of callosal projections are different from the mechanisms involved in the elimination of callosal projections during development. The effects of these developmental manipulations on callosal organization in other mammals are reviewed and compared with the effects in rabbits. The data suggest that species differences in the degree of maturity of the visual system at birth and in the extent of callosal development at the time of eye opening, may underlie species differences in the effects of these manipulations on the organization of visual callosal projections during development.

Thalamic projections to sensorimotor cortex in the newborn macaque

In the present experiments thalamocortical projections to different functional areas of the newborn (or prematurely delivered) macaque's sensorimotor cortex were labeled using retrogradely transported fluorescent dyes. Several dyes were used in each animal to (1) enable the direct comparison of the soma distributions of different thalamocortical projections within thalamic space, and (2) identify by double labeling neurons shared between these distributions. The projection patterns in the newborn macaque were compared with those of the mature animal reported by Darian-Smith et al. (J. Comp. Neurol. 1990;298:000-000). The main observations were (1) all thalamocortical projections to the sensorimotor cortex of the mature macaque are well established by embryonic days 146-150, as was shown by labeling these pathways in infants delivered by cesarean section, (2) a significant number of thalamocortical neurons in the newborn were double-labeled following dye injections into different pre- or postcentral areas, and where the margins of the dye uptake zones were separated by 3-8 mm, and (3) extensive projections from the anterior pulvinar nucleus to the motor and premotor cortex, and to the supplementary motor cortex were labeled in the newborn macaque. Both the exuberant terminal arborizations, and the precentral pulvinar projections were diminished by the 6th postnatal month, and absent in the mature macaque. The role of epigenetic determinants of these postnatal events is briefly considered.

Transient projection from the superior temporal sulcus to area 17 in the newborn macaque monkey

Area 17 in the neonate of numerous species receives projections from cortical areas that do not project to area 17 in the adult. To investigate if this were the case in the developing primate, we have made injections of retrograde tracers in area 17 of newborn monkeys (Macaca irus) and examined the areal distribution of labeled neurons. Neurons projecting to area 17 were found to be restricted to those cortical regions that project to area 17 in the adult. The projection to area 17 in the neonate did appear to be very different in that in the superior temporal sulcus there was a large contingent of labeled neurons in supragranular layers. This constitutes a transient projection because in the adult area 17 projecting neurons in this cortical region originate almost exclusively from infragranular layers. To test if a change in the laminar distribution of area 17 projecting neurons in extrastriate cortex is a general feature of postnatal development, we have computed in neonates and adults the proportion of area 17 afferent neurons in infra- and supragranular layers for each cortical region that projects to area 17. This revealed (i) that in the adult the laminar distribution of area 17 afferents is characteristic for each cortical area and (ii) that this distribution emerges during development from an immature state in which labeled neurons are more numerous in supragranular layers. These results show that there is an extensive remodeling of the neuronal circuitry connecting visual cortical areas during postnatal development in the monkey and that the transient connectivity of primate area 17 is very different from that observed in other mammalian species.

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.

Maturation and connectivity of the visual cortex in monkey is altered by prenatal removal of retinal input

In several species, the peripheral input from the eyes partly determines the pattern of interconnections between the visual areas of the two cerebral hemispheres through the fibre tract termed the corpus callosum. In the macaque monkey, the neurons projecting through the callosum are largely restricted to area 18 throughout ontogeny, whereas area 17 is characterized by few or no callosal projections. Here, we show that suppressing the peripheral input by prenatal removal of the eyes leads to a marked reduction in the extent of area 17, resulting in a large shift in the position of the histologically identifiable boundary between the two areas. Even so, the boundary continues to separate an area rich with callosal connections (area 18) from one poor in such projections (area 17), indicating there is no effect on the callosal connectivity of area 17. In contrast, in area 18, eye removal results in many more neurons with callosal projections than in normal animals. The results suggest that the factors that determine the parcellation of cortical areas also specify their connectivity.

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.

The maturational status of thalamocortical and callosal connections of visual areas V1 and V2 in the newborn monkey

Cytochrome oxidase (CytOx) is known to preferentially stain those regions of the visual cortex which receive direct projections from the thalamus. The pattern of CytOx stain has been used to investigate the maturation of thalamic input to areas V1 and V2 in the newborn monkey. In both areas, the intensity of CytOx activity was similar in newborns and adults. The distribution of CytOx in area V2 was not found to vary with age. In area V1, the only difference in CytOx activity in newborns was a relative immaturity of staining in layer 4C. The callosal connections of visual areas V1 and V2 were investigated by the axonal transport of wheat germ agglutinin conjugated to horseradish peroxidase and free horseradish peroxidase. In the adult, V1 was found to be reciprocally callosally connected for a distance of 1-2.5 mm from the V1/V2 border, whilst V2 was connected for a distance of 3-8 mm from the border. In both areas, callosal connections showed a certain degree of clustering, particularly in V2 which contained 97-98% of the total number of callosal connections of these two areas. In the newborn, the number, tangential extent and clustered distribution of callosal connections were as in the adult. In the newborn, as in the adult, callosal connections coincided with regions of high CytOx activity in area V2. The results showing a relative maturity of the tangential distribution of callosal projecting neurons on the one hand, and an immaturity of thalamic projections on the other, are discussed in terms of: (1) the maturational status of the newborn monkey compared to other mammals at the moment of birth and (2) the possible role of visual experience in shaping cortical connections.

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.

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.