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

Dendritic Spines: Synaptogenesis and Synaptic Pruning for the Developmental Organization of Brain Circuits

Synaptic overproduction and elimination is a regular developmental event in the mammalian brain. In the cerebral cortex, synaptic overproduction is almost exclusively correlated with glutamatergic synapses located on dendritic spines. Therefore, analysis of changes in spine density on different parts of the dendritic tree in identified classes of principal neurons could provide insight into developmental reorganization of specific microcircuits.The activity-dependent stabilization and selective elimination of the initially overproduced synapses is a major mechanism for generating diversity of neural connections beyond their genetic determination. The largest number of overproduced synapses was found in the monkey and human cerebral cortex. The highest (exceeding adult values by two- to threefold) and most protracted overproduction (up to third decade of life) was described for associative layer IIIC pyramidal neurons in the human dorsolateral prefrontal cortex.Therefore, the highest proportion and extraordinarily extended phase of synaptic spine overproduction is a hallmark of neural circuitry in human higher-order associative areas. This indicates that microcircuits processing the most complex human cognitive functions have the highest level of developmental plasticity. This finding is the backbone for understanding the effect of environmental impact on the development of the most complex, human-specific cognitive and emotional capacities, and on the late onset of human-specific neuropsychiatric disorders, such as autism and schizophrenia.

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.

Refinement of the Primate Corticospinal Pathway During Prenatal Development

Perturbation of the developmental refinement of the corticospinal (CS) pathway leads to motor disorders. While non-primate developmental refinement is well documented, in primates invasive investigations of the developing CS pathway have been confined to neonatal and postnatal stages when refinement is relatively modest. Here, we investigated the developmental changes in the distribution of CS projection neurons in cynomolgus monkey (Macaca fascicularis). Injections of retrograde tracer at cervical levels of the spinal cord at embryonic day (E) 95 and E105 show that: (i) areal distribution of back-labeled neurons is more extensive than in the neonate and dense labeling is found in prefrontal, limbic, temporal, and occipital cortex; (ii) distributions of contralateral and ipsilateral projecting CS neurons are comparable in terms of location and numbers of labeled neurons, in contrast to the adult where the contralateral projection is an order of magnitude higher than the ipsilateral projection. Findings from one largely restricted injection suggest a hitherto unsuspected early innervation of the gray matter. In the fetus there was in addition dense labeling in the central nucleus of the amygdala, the hypothalamus, the subthalamic nucleus, and the adjacent region of the zona incerta, subcortical structures with only minor projections in the adult control.

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.

How Areal Specification Shapes the Local and Interareal Circuits in a Macaque Model of Congenital Blindness

There is little understanding of the structural underpinnings of the functional reorganization of the cortex in the congenitally blind human. Taking advantage of the extensive characterization of the macaque visual system, we examine in macaque the influence of congenital blindness resulting from the removal of the retina during in utero development. This effectively removes the normal influence of the thalamus on cortical development leading to an induced hybrid cortex (HC) combining features of primary visual and extrastriate cortex. Retrograde tracers injected in HC reveal a local, intrinsic connectivity characteristic of higher order areas and show that the HC receives a uniquely strong, purely feedforward projection from striate cortex but no ectopic inputs, except from subiculum, and entorhinal cortex. Statistical modeling of quantitative connectivity data shows that HC is relatively high in the cortical hierarchy and receives a reinforced input from ventral stream areas while the overall organization of the functional streams are conserved. The directed and weighted anophthalmic cortical graph from the present study can be used to construct dynamic and structural models. These findings show how the sensory periphery governs cortical phenotype and reveal the importance of developmental arealization for understanding the functional reorganization in congenital blindness.

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.

Postnatal refinement of interareal feedforward projections in ferret visual cortex

We studied the postnatal refinement of feedforward (FF) projections from ferret V1 to multiple cortical targets during the period around eye opening. Our goal was to establish (a) whether the developmental refinement of FF projections parallels that of feedback (FB) cortical circuits, and (b) whether FF pathways from V1 to different target areas refine with a similar rate. We injected the tracer CTb into V1 of juvenile ferrets, and visualized the pattern of labeled axon terminals in extrastriate cortex. Bouton density of FF projections to target areas 18, 19, and 21 declined steadily from 4 to 8 weeks postnatal. However, in area Ssy this decline was delayed somewhat, not occurring until after 6 weeks. During this postnatal period, mean interbouton intervals along individual FF axons to all visual areas increased, and we observed a concomitant moderate decrease in axon density in areas 18, 21, and Ssy. These data suggest that FF circuits linking V1 to its main extrastriate targets remodel largely synchronously in the weeks following eye opening, that FF and FB cortical circuits share a broadly similar developmental timecourse, and that postnatal visual experience is critical for the refinement of both FF and FB cortical circuits.

Subtype-Specific Genes that Characterize Subpopulations of Callosal Projection Neurons in Mouse Identify Molecularly Homologous Populations in Macaque Cortex

Callosal projection neurons (CPN) interconnect the neocortical hemispheres via the corpus callosum and are implicated in associative integration of multimodal information. CPN have undergone differential evolutionary elaboration, leading to increased diversity of cortical neurons-and more extensive and varied connections in neocortical gray and white matter-in primates compared with rodents. In mouse, distinct sets of genes are enriched in discrete subpopulations of CPN, indicating the molecular diversity of rodent CPN. Elements of rodent CPN functional and organizational diversity might thus be present in the further elaborated primate cortex. We address the hypothesis that genes controlling mouse CPN subtype diversity might reflect molecular patterns shared among mammals that arose prior to the divergence of rodents and primates. We find that, while early expression of the examined CPN-enriched genes, and postmigratory expression of these CPN-enriched genes in deep layers are highly conserved (e.g., Ptn, Nnmt, Cited2, Dkk3), in contrast, the examined genes expressed by superficial layer CPN show more variable levels of conservation (e.g., EphA3, Chn2). These results suggest that there has been evolutionarily differential retraction and elaboration of superficial layer CPN subpopulations between mouse and macaque, with independent derivation of novel populations in primates. Together, these data inform future studies regarding CPN subpopulations that are unique to primates and rodents, and indicate putative evolutionary relationships.

High precision systems require high precision "blueprints": a new view regarding the formation of connections in the Mammalian visual system

Abstract It is well established that early in development interconnections within the mammalian visual system are often more widespread and less precise than at maturity. The literature dealing with the formation of visual connections has largely ignored differences in developmental specificity among species differing in their phylogenetic status and/or the visual ecological niche that they occupy. Based on a review of the available evidence, we have formulated an hypothesis to account for the varying degrees of developmental specificity that characterize different visual systems. It is suggested that extremely precise systems required for high-acuity binocular vision exhibit fewer presumed developmental errors than do visual systems characterized by poorer acuity and relatively crude depth perception. The developmental implications of the hypothesis are considered, and specific experiments are proposed to further test its validity.

Retinal input influences the size and corticocortical connectivity of visual cortex during postnatal development in the ferret

Retinal input plays an important role in the specification of topographically organized circuits and neuronal response properties, but the mechanism and timing of this effect is not known in most species. A system that shows dramatic dependence on retinal influences is the interhemispheric connection through the corpus callosum. Using ferrets, we analyzed the extent to which development of the visual callosal pattern depends on retinal influences, and explored the period during which these influences are required for normal pattern formation. We studied the mature callosal patterns in normal ferrets and in ferrets bilaterally enucleated (BE) at postnatal day 7 (P7) or P20. Callosal patterns were revealed in tangential sections from unfolded and flattened brains following multiple injections of horseradish peroxidase in the opposite hemisphere. We also estimated the effect of enucleation on the surface areas of striate and extrastriate visual cortex by using magnetic resonance imaging (MRI) data from intact brains. In BEP7 ferrets we found that the pattern of callosal connections was highly anomalous and the sizes of both striate and extrastriate visual cortex were significantly reduced. In contrast, enucleation at P20 had no significant effect on the callosal pattern, but it still caused a reduction in the size of striate and extrastriate visual cortex. Finally, retinal deafferentation had no significant effect on the number of visual callosal neurons. These results indicate that the critical period during which the eyes influence the development of callosal patterns, but not the size of visual cortex, ends by P20 in the ferret.

Cortical and subcortical connections of V1 and V2 in early postnatal macaque monkeys

Connections of primary (V1) and secondary (V2) visual areas were revealed in macaque monkeys ranging in age from 2 to 16 weeks by injecting small amounts of cholera toxin subunit B (CTB). Cortex was flattened and cut parallel to the surface to reveal injection sites, patterns of labeled cells, and patterns of cytochrome oxidase (CO) staining. Projections from the lateral geniculate nucleus and pulvinar to V1 were present at 4 weeks of age, as were pulvinar projections to thin and thick CO stripes in V2. Injections into V1 in 4- and 8-week-old monkeys labeled neurons in V2, V3, middle temporal area (MT), and dorsolateral area (DL)/V4. Within V1 and V2, labeled neurons were densely distributed around the injection sites, but formed patches at distances away from injection sites. Injections into V2 labeled neurons in V1, V3, DL/V4, and MT of monkeys 2-, 4-, and 8-weeks of age. Injections in thin stripes of V2 preferentially labeled neurons in other V2 thin stripes and neurons in the CO blob regions of V1. A likely thick stripe injection in V2 at 4 weeks of age labeled neurons around blobs. Most labeled neurons in V1 were in superficial cortical layers after V2 injections, and in deep layers of other areas. Although these features of adult V1 and V2 connectivity were in place as early as 2 postnatal weeks, labeled cells in V1 and V2 became more restricted to preferred CO compartments after 2 weeks of age.

Ways of coloring: Comparative color vision as a case study for cognitive science

Different explanations of color vision favor different philosophical positions: Computational vision is more compatible with objectivism (the color is in the object), psychophysics and neurophysiology with subjectivism (the color is in the head). Comparative research suggests that an explanation of color must be both experientialist (unlike objectivism) and ecological (unlike subjectivism). Computational vision's emphasis on optimally “recovering” prespecified features of the environment (i.e., distal properties, independent of the sensory-motor capacities of the animal) is unsatisfactory. Conceiving of visual perception instead as the visual guidance of activity in an environment that is determined largely by that very activity suggests new directions for research.

Organizational effects of fetal testosterone on human corpus callosum size and asymmetry

Previous theory and research in animals has identified the critical role that fetal testosterone (FT) plays in organizing sexually dimorphic brain development. However, to date there are no studies in humans directly testing the organizational effects of FT on structural brain development. In the current study we investigated the effects of FT on corpus callosum size and asymmetry. High-resolution structural magnetic resonance images (MRI) of the brain were obtained on 28 8-11-year-old boys whose exposure to FT had been previously measured in utero via amniocentesis conducted during the second trimester. Although there was no relationship between FT and midsaggital corpus callosum size, increasing FT was significantly related to increasing rightward asymmetry (e.g., Right>Left) of a posterior subsection of the callosum, the isthmus, that projects mainly to parietal and superior temporal areas. This potential organizational effect of FT on rightward callosal asymmetry may be working through enhancing the neuroprotective effects of FT and result in an asymmetric distribution of callosal axons. We suggest that this possible organizational effect of FT on callosal asymmetry may also play a role in shaping sexual dimorphism in functional and structural brain development, cognition, and behavior.

Functional and evolutionary insights into human brain development through global transcriptome analysis

Our understanding of the evolution, formation, and pathological disruption of human brain circuits is impeded by a lack of comprehensive data on the developing brain transcriptome. A whole-genome, exon-level expression analysis of 13 regions from left and right sides of the mid-fetal human brain revealed that 76% of genes are expressed, and 44% of these are differentially regulated. These data reveal a large number of specific gene expression and alternative splicing patterns, as well as coexpression networks, associated with distinct regions and neurodevelopmental processes. Of particular relevance to cognitive specializations, we have characterized the transcriptional landscapes of prefrontal cortex and perisylvian speech and language areas, which exhibit a population-level global expression symmetry. We show that differentially expressed genes are more frequently associated with human-specific evolution of putative cis-regulatory elements. These data provide a wealth of biological insights into the complex transcriptional and molecular underpinnings of human brain development and evolution.

Development of callosal topography in visual cortex of normal and enucleated rats

In normal rats callosal projections in striate cortex connect retinotopically corresponding, nonmirror-symmetric cortical loci, whereas in rats bilaterally enucleated at birth, callosal fibers connect topographically mismatched, mirror-symmetric loci. Moreover, retina input specifies the topography of callosal projections by postnatal day (P)6. To investigate whether retinal input guides development of callosal maps by promoting either the corrective pruning of exuberant axon branches or the specific ingrowth and elaboration of axon branches at topographically correct places, we studied the topography of emerging callosal connections at and immediately after P6. After restricted intracortical injections of anterogradely and retrogradely transported tracers we observed that the normal, nonmirror-symmetric callosal map, as well as the anomalous, mirror-symmetric map observed in neonatally enucleated animals, are present by P6-7, just as collateral branches of simple architecture emerge from their parental axons and grow into superficial cortical layers. Our results therefore do not support the idea that retinal input guides callosal map formation by primarily promoting the large-scale elimination of long, nontopographic branches and arbors. Instead, they suggest that retinal input specifies the sites on the parental axons from which interstitial branches will grow to invade middle and upper cortical layers, thereby ensuring that the location of invading interstitial branches is accurately related to the topographical location of the soma that gives rise to the parental axon. Moreover, our results from enucleated rats suggest that the cues that determine the mirror-symmetric callosal map exert only a weak control on the topography of fiber ingrowth.

Prenatal development of retinogeniculate axons in the macaque monkey during segregation of binocular inputs

In the fetal monkey the projections from the two eyes are initially completely intermingled within the dorsal lateral geniculate nucleus (DLGN) before separating into eye-specific layers (). To assess the cellular basis of this developmental process, we examined the morphological properties of individual retinogeniculate axons in prenatal monkeys of known gestational ages. The period studied spanned the time from when binocular overlap has been reported to be maximum, circa embryonic (E) day 77 through E112, when the segregation process is already largely completed in the caudal portion of the nucleus. Retinogeniculate fibers were labeled by making small deposits of DiI crystals into the fixed optic tract. After adequate time was allowed for diffusion of the tracer, fibers were visualized by confocal microscopy, and morphometric measures were made from photomontages. This revealed that retinogeniculate fibers in the embryonic monkey undergo continuous growth and elaboration during binocular overlap and subsequent segregation. Importantly, very few side-branches were found along the preterminal axon throughout the developmental period studied. Thus, restructuring of retinogeniculate fibers does not underlie the formation of eye-restricted projections in the primate. Rather, the results support the hypothesis that binocular segregation in the embryonic monkey is caused by the loss of retinal fibers that initially innervate inappropriate territories ().

Change of expression of full-length and truncated TrkBs in the developing monkey central nervous system

We examined the expression of full-length TrkB (TrkBTK+) and truncated TrkB (TrkBTK-) in the central nervous system (CNS) of the macaque monkey (Macaca fascicularis) using a western blot analysis. At the adult stage, the levels of TrkBTK+ in cerebral cortices were higher than those in other structures of CNS and the expressions of TrkBTK+ in the association cortices (except area PE) were relatively lower than those in the primary cortices. In contrast, TrkBTK- in the hippocampus and the cerebellum was significantly higher than in other structures. In various developing cerebral cortices, TrkBTK+ was detected at the same levels from embryonic day 120 (E120) to the adult period. In contrast, the expression of TrkBTK- increased remarkably after the newborn stage (NB), reached the maximum level at postnatal day 60 (P60) and maintained the same level into adulthood. The peaks of TrkBTK- in the association cortices were more delayed than in the primary cortices. The expression of TrkBTK- occurred at a time that correlates with the elimination of axons and the down-regulation of neuropeptides. The present study suggests that TrkBTK- plays an important role in the axonal remodelling and that it may act as a negative effector of TrkBTK+ in the primate CNS, reducing responsiveness to BDNF and/or NT-4/5.

Judgements of visual precedence by strabismics

Strabismus induced early in the life of cats results in disruption of the normal development of the posterior corpus callosum. In human strabismic amblyopic subjects a similar disruption in callosal development may cause poor interhemispheric integration of simple visual information. To examine this possibility judgements of visual target onset precedence were made for targets presented in bilateral and unilateral viewing conditions using a method of constants psychophysical procedure. In one condition, with targets presented in opposite visual fields requiring integration between the two hemispheres, the subjects having strabismic amblyopia had significantly larger just noticeable difference values (JNDs) than those found for the non-strabismic controls. In a second condition, with targets presented within a visual field requiring integration within only a single hemisphere, the JNDs for the two groups were not significantly different. The results suggest that strabismic amblyopes have poor integration of visual information between the cerebral hemispheres, and that this reflects disruptions in the development of the posterior corpus callosum.

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.

Development of the visual callosal cell distribution in the rat: mature features are present at birth

In the present study, the early postnatal distribution and subsequent fate of visual callosal neurons were studied in neonatal rat pups. Previous studies had indicated that the adult pattern of visual callosal neurons was sculpted from an initially uniform distribution in the neonatal cortex. To reexamine this issue, we used a sensitive tracer, latex microspheres conjugated either to rhodamine or fluorescein, that was injected into the occipital cortex of one hemisphere in pups on the day of birth (PND 1), PND 6, or PND 12. Examination of the resulting retrograde labeling of cortical neurons in the opposite hemisphere indicates that features of the mature visual callosal pattern are present as early as PND 1. At all stages of postnatal development, the relative density of callosal projection cells varies consistently across the mediolateral extent of primary visual cortex-it is always highest in the region of the 17/18a border and lowest in the body of area 17. These data strongly suggest that, from the outset, visual cortical neurons in the region of the 17/18a border preferentially make connections with the opposite hemisphere. The results of experiments in which callosal neurons were labeled on the day of birth indicate that only those neurons that have migrated to their final cortical destinations have extended callosal axons into the vicinity of the visual cortex in the opposite hemisphere. The initial pattern of callosal neurons resembles a dense, compact version of the mature one, and the present study suggests that much of the remaining change in the appearance of this pathway may be accounted for by the decrease in the overall density of neurons that is due to expansion of the cortical gray matter during postnatal life. Taken together, these results suggest that the development of the visual callosal pathway in the rat may be more similar to that in the monkey than has been reported previously.

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.

The development of the anterior commissure in normal and hypothyroid rats

The development of axon number in the anterior commissure (AC) was analyzed in 39 normal and 37 hypothyroid rats using conventional electron microscopy. Hypothyroid rats underwent antithyroid treatment with methimazole from embryonic day (E) 14 onwards, followed in a fraction of the animals by thyroidectomy at postnatal day (P) 6. In normal rats, the midsagittal cross-sectional anterior commissure area (ACA) increased throughout their life; in hypothyroid rats, ACA was stationary from P4 onwards and at P174-180 it was reduced by 39% relative to normal rats. In normal rats, the number of AC axons increased rapidly from 168,500 at E18 to, on average, 1,049,000 from P4 onwards. Similarly, in hypothyroid rats, the number of axons increased from 135,000 at E18 to, on average, 1,052,000 from P4 onwards. At all ages, the number of axons was similar in normal and hypothyroid rats. During development of the AC, the evolution of axon number observed in normal and hypothyroid rats is different from what was reported for other telencephalic commissures, including the AC of the monkey, where an important fraction of the axons are eliminated postnatally. Antithyroid treatment dissociated ACA from total number of AC axons.

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.

Human strabismus: evaluation of the interhemispheric transmission time and hemiretinal differences using a reaction time task

Experimentally induced strabismus in visually immature cats leads to abnormal development of the posterior corpus callosum. This, in turn, should lead to abnormal interhemispheric integration of unilaterally presented visual information. To test whether strabismus produces deficits in the human commissural visual system, the interhemispheric transmission time (ITT) was compared in strabismic and normal subjects. Simple unimanual reaction times (RT) were tested in 30 subjects in response to a lateralized target presented monocularly at 4 degrees and 35 degrees nasally and temporally from the fovea along the horizontal meridian. This method was also used to examine the effect of strabismus on the central and peripheral portions of each hemiretina. The results showed that in strabismic subjects with or without amblyopia, the ITT did not differ significantly from normals at both eccentricities. In non-amblyopic strabismic patients, RTs in the central and peripheral portions of hemiretina were comparable to normals. However, a reduced speed of response was found in the central visual field (4 degrees) in the amblyopic eye. Our results suggest that the ITT is normal in strabismic subjects and that the longer RTs in the central portion of the nasal and temporal hemiretina of the amblyopic eye may be associated with the severe amblyopic condition.

Axon overproduction and elimination in the anterior commissure of the developing rhesus monkey

We have analyzed axon overproduction and elimination in the anterior commissure (AC) of 16 fetal, neonatal, and juvenile rhesus monkeys. Axons are added to the AC at an average rate of 115,000/day during the last two-thirds of gestation, and growth cones are present in a constant proportion to AC axons throughout this period. The peak number of approximately 11 million axons in the AC is reached at birth. Thereafter, axons are eliminated at a net rate of approximately 1 axon/sec during the first 3 postnatal months until the adult number of approximately 3.3 +/- 0.5 million axons is reached. Although there is considerable variability in AC axon number during the period of axon loss, the adult number of AC axons is relatively invariant among the eight adult rhesus monkeys examined. Increase in axon diameter and myelination begins before the major phase of axon elimination and is completed long after the adult number of axons is reached. Apparently, myelinated axons are not eliminated from the AC. Quantitative differences in the magnitude and timing of axon overproduction and elimination in the AC versus that in the corpus callosum (LaMantia and Rakic [1990] J. Neurosci. 10:2156) indicate specific modulation of the development of each commissure, perhaps reflecting differences in the developmental history and functional identity of the distinct cortical regions that give rise to them. This process of overproduction and elimination of AC axons during postnatal development in primates might contribute to individual variations in AC size correlated with a wide range of physical and behavioral differences.

Transient cortical pathways in the pyramidal tract of the neonatal ferret

Anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was used to study transient axons from the visual cortex in the pyramidal tract. Injections at birth restricted to the visual cortex labeled axons in the vicinity of the pontine nuclei. Two to eight days after birth, axons from the occipital cortex were found posterior to the pontine nucleus, their caudalmost stable target. Transient corticospinal axons from the presumptive primary visual cortex did not grow caudal to the pyramidal decussation. Innervation of more distal targets preceded innervation of proximal targets. Innervation of the pontine nucleus is initiated around 68 hours after birth, when the transient extension in the medullary pyramidal tract has attained its maximum caudal extent. Innervation of the superior colliculus begins 9 days after birth. Retrograde tracers were used to follow the developmental changes in the cortical distribution of the parent neurons giving rise to axons in the pyramidal tract. In the adult, labeled neurons following injection of retrograde tracer in the pyramidal tract occupied less than a third of the neocortex and were centred on the anterior part of the coronal and spleniocruciate gyri. In the immature brain, labeled neurons covered more than two-thirds of the neocortex. Areal density measurements in the neonate showed that peak labeling was centred in the anterior coronal and spleniocruciate gyri, where corticospinal cells in the adult are located. There was a marked rostral-caudal gradient so that labeled neurons were very scarce towards the occipital pole. These results, showing transient neocortical axons in the pyramidal tract in a carnivore, suggest that this may be a common feature of mammalian development. The finding that the adult pattern of corticospinal projections does not emerge from a uniform distribution is discussed with respect to the areal specification of cortical connectivity.

The ontogenesis of the forebrain commissures and the determination of brain asymmetries

We have reviewed the organization and development of the interhemispheric projections through the forebrain commissures, especially those of the CC, in connection with the development of brain asymmetries. Analyzing the available data, we conclude that the developing CC plays an important role in the ontogenesis of brain asymmetries. We have extended a previous hypothesis that the rodent CC may exert a stabilizing effect over the unstable populational asymmetries of cortical size and shape, and that it participates in the developmental stabilization of lateralized motor behaviors.

Prenatal formation of the normal mouse corpus callosum: a quantitative study with carbocyanine dyes

Judgment of abnormalities in fetal cortical axon development is more sensitive when a good standard of normal ontogeny is established. The recent availability of postmortem tract tracing methods has greatly improved the observation of axon extension and growth cone morphology in mouse fetuses, which allows much stronger statements about the timing of crucial steps in the formation of the corpus callosum in particular. The first outgrowth and crossing of midplane by axons of the corpus callosum (CC) were examined in 153 normal mouse embryos and fetuses of the hybrid cross B6D2F2/J with carbocyanine dyes applied to brains fixed by perfusion. In most brains a crystal of DiI was inserted into either frontal, parietal, temporal, or occipital cortex in one hemisphere, and a crystal of DiA was placed into a different site in the opposite hemisphere. Although dye diffusion obscured the emergence of axons, linear regression analysis revealed that the first callosal axons emerged from their cortical cells of origin at about 0.4 g body weight or 15.5 days after conception for all four sites. Subsequent axon growth rate was substantially faster for those from frontal cortex (3.2 mm/day) than occipital cortex (1.8 mm/day). Axons from frontal cortex crossed the cerebral midplane first (0.69 g, E16.3), followed by those from parietal (0.74 g), temporal (0.77 g) and occipital cortex (0.92 g, E16.9). Prior to crossing midplane, the pioneering CC axons were usually 200 microns or less in advance of the main bundle, but when they crossed midplane and encountered CC axons growing from homotopic sites in the opposite hemisphere, the pioneering axons were often 0.5 to 2.5 mm ahead of the main bundle. Growth cones were usually large and complex until they had crossed midplane and were thereafter smaller with simple and flat morphologies. The topography of axons in the CC at midplane was organized according to cortical region of origin from the very beginning, when the CC was only a small cap over the hippocampal commissure and dorsal septum. The quantitative results provide a convenient standard for normal callosal development in mice and should facilitate comparative studies.