Here, we propose a new concept linked to neuroplasticity and cognitive reserve: 'Connectomic reserve'. It is suggested and discussed as a hypothesis, aiming to stimulate experimental testing, taking as an example the exuberant circuits formed during development by commissural neurons and their projections. Upon reviewing the developmental strategies of the formation of callosal circuits, from axonal emission by cortical cell bodies to the choice of trajectories within the ipsi and contralateral hemispheres, we identified the formation of many ectopic and heterotopic projections. Ectopic projections were first described in cases of congenital malformations but later shown to exist in typical adult brains as well. Heterotopic projections are usually presumed to be eliminated and give way to the more restricted set of connections of adult brains. However, many experiments have shown that they, in fact, remain 'hidden' in the brain throughout life in typical subjects of many species. Morphological shaping (pruning or enlargement) and physiological modulation (inhibition or enhancement) of these circuits would be resources for optimizing the function of the alternative bundles and preterminal arbors typical of neuroplasticity. The former mechanisms would take place during development, the latter in adulthood. 'Connectomic reserve', thus, would consist of a pervasive set of connections broadly distributed throughout the brain, susceptible to selective neuroplastic shaping or modulation depending on the requirements of the external or internal environment. Of course, being a hypothesis, this requires experimental testing in brain circuits other than commissural tracts and functional validation using emerging high-resolution imaging techniques.

Brain commissures are antiparallel white matter fiber bundles that connect both hemispheres across the midsagittal plane at different anatomical levels. In the mammalian brain, the best-known commissures are the corpus callosum, the anterior and the posterior commissures. Recent studies have identified a new white matter pathway, the thalamic commissures (TCs), that connect the cortex to the contralateral thalamus in rodents and primates. However, the specific cortical regions that project via the TCs and their target thalamic nuclei are still unknown. To thoroughly and accurately map this projectome, we utilized the mouse Allen Brain Institute connectivity atlas. Our results indicate that not all cortical regions project via the TCs, with most of these regions concentrated in the rostral portion of the brain. In addition, our data suggest that the TCs projections are very numerous, although less robust than the ipsilateral counterpart. Furthermore, these projections had a high degree of bilateral symmetry. We did not find any projections from the thalamus to the contralateral cortex, suggesting that the TCs are not reciprocal. Our findings reinforce the concept of the thalamus as an interhemispheric connectivity hub and suggest that the TCs might play a role in interhemispheric functional modulation.

Cortical neurons of eutherian mammals project to the contralateral hemisphere, crossing the midline primarily via the corpus callosum and the anterior, posterior, and hippocampal commissures. We recently reported and named the thalamic commissures (TCs) as an additional interhemispheric axonal fiber pathway connecting the cortex to the contralateral thalamus in the rodent brain. Here, we demonstrate that TCs also exist in primates and characterize the connectivity of these pathways with high-resolution diffusion-weighted MRI, viral axonal tracing, and fMRI. We present evidence of TCs in both New World (Callithrix jacchus and Cebus apella) and Old World primates (Macaca mulatta). Further, like rodents, we show that the TCs in primates develop during the embryonic period, forming anatomical and functionally active connections of the cortex with the contralateral thalamus. We also searched for TCs in the human brain, showing their presence in humans with brain malformations, although we could not identify TCs in healthy subjects. These results pose the TCs as a vital fiber pathway in the primate brain, allowing for more robust interhemispheric connectivity and synchrony and serving as an alternative commissural route in developmental brain malformations.

Congenital Zika syndrome (CZS) is a set of birth defects caused by Zika virus (ZIKV) infection during pregnancy. Microcephaly is its main feature, but other brain abnormalities are found in CZS patients, such as ventriculomegaly, brain calcifications, and dysgenesis of the corpus callosum. Many studies have focused on microcephaly, but it remains unknown how ZIKV infection leads to callosal malformation. To tackle this issue, we infected mouse embryos in utero with a Brazilian ZIKV isolate and found that they were born with a reduction in callosal area and density of callosal neurons. ZIKV infection also causes a density reduction in PH3+ cells, intermediate progenitor cells, and SATB2+ neurons. Moreover, axonal tracing revealed that callosal axons are reduced and misrouted. Also, ZIKV-infected cultures show a reduction in callosal axon length. GFAP labeling showed that an in utero infection compromises glial cells responsible for midline axon guidance. In sum, we showed that ZIKV infection impairs critical steps of corpus callosum formation by disrupting not only neurogenesis, but also axon guidance and growth across the midline.

The corpus callosum (CC), the largest brain commissure and the primary white matter pathway for interhemispheric cortical connectivity, was traditionally viewed as a predominantly homotopic structure, connecting mirror areas of the cortex. However, new studies verified that most callosal commissural fibers are heterotopic. Recently, we reported that ~75% of the callosal connections in the brains of mice, marmosets, and humans are heterotopic, having an essential role in determining the global properties of brain networks. In the present study, we leveraged high-resolution diffusion-weighted imaging and graph network modeling to investigate the relationship between heterotopic and homotopic callosal fibers in human subjects and in a spontaneous mouse model of Corpus Callosum Dysgenesis (CCD), a congenital developmental CC malformation that leads to widespread whole-brain reorganization. Our results show that the CCD brain is more heterotopic than the normotypical brain, with both mouse and human CCD subjects displaying highly variable heterotopicity maps. CCD mice have a clear heterotopicity cluster in the anterior CC, while hypoplasic humans have strongly variable patterns. Graph network-based connectivity profile showed a direct impact of heterotopic connections on CCD brains altering several network-based statistics. Our collective results show that CCD directly alters heterotopic connections and brain connectivity.

The corpus callosum (CC) is the largest white matter structure and the primary pathway for interhemispheric brain communication. Investigating callosal connectivity is crucial to unraveling the brain’s anatomical and functional organization in health and disease. Classical anatomical studies have characterized the bulk of callosal axonal fibers as connecting primarily homotopic cortical areas. Whenever detected, heterotopic callosal fibers were ascribed to altered sprouting and pruning mechanisms in neurodevelopmental diseases such as CC dysgenesis (CCD). We hypothesized that these heterotopic connections had been grossly underestimated due to their complex nature and methodological limitations. We used the Allen Mouse Brain Connectivity Atlas and high-resolution diffusion-weighted imaging to identify and quantify homotopic and heterotopic callosal connections in mice, marmosets, and humans. In all 3 species, we show that ~75% of interhemispheric callosal connections are heterotopic and comprise the central core of the CC, whereas the homotopic fibers lay along its periphery. We also demonstrate that heterotopic connections have an essential role in determining the global properties of brain networks. These findings reshape our view of the corpus callosum’s role as the primary hub for interhemispheric brain communication, directly impacting multiple neuroscience fields investigating cortical connectivity, neurodevelopment, and neurodevelopmental disorders.

The human cerebral cortex is one of the most evolved regions of the brain, responsible for most higher-order neural functions. Since nerve cells (together with synapses) are the processing units underlying cortical physiology and morphology, we studied how the human neocortex is composed regarding the number of cells as a function of sex and age. We used the isotropic fractionator for cell quantification of immunocytochemically labeled nuclei from the cerebral cortex donated by 43 cognitively healthy subjects aged 25-87 years old. In addition to previously reported sexual dimorphism in the medial temporal lobe, we found more neurons in the occipital lobe of men, higher neuronal density in women's frontal lobe, but no sex differences in the number and density of cells in the other lobes and the whole neocortex. On average, the neocortex has ~10.2 billion neurons, 34% in the frontal lobe and the remaining 66% uniformly distributed among the other 3 lobes. Along typical aging, there is a loss of non-neuronal cells in the frontal lobe and the preservation of the number of neurons in the cortex. Our study made possible to determine the different degrees of modulation that sex and age evoke on cortical cellularity.

Background: Education is believed to contribute positively to brain structure and function, as well as to cognitive reserve. One of the brain regions most impacted by education is the medial temporal lobe (MTL), a region that houses the hippocampus, which has an important role in learning processes and in consolidation of memories, and is also known to undergo neurogenesis in adulthood. We aimed to investigate the influence of education on the absolute cell numbers of the MTL (comprised by the hippocampal formation, amygdala, and parahippocampal gyrus) of men without cognitive impairment. Methods: The Isotropic Fractionator technique was used to allow the anisotropic brain tissue to be transformed into an isotropic suspension of nuclei, and therefore assess the absolute cell composition of the MTL. We dissected twenty-six brains from men aged 47 to 64 years, with either low or high education. Results: A significant difference between groups was observed in brain mass, but not in MTL mass. No significant difference was found between groups in the number of total cells, number of neurons, and number of non-neuronal cells. Regression analysis showed that the total number of cells, number of neurons, and number of non-neuronal cells in MTL were not affected by education. Conclusions: The results indicate a resilience of the absolute cellular composition of the MTL of typical men to low schooling, suggesting that the cellularity of brain regions is not affected by formal education.

Chronic social stress is a significant risk factor for several neuropsychiatric disorders, mainly major depressive disorder (MDD). In this way, patients with clinical depression may display many symptoms, including disrupted social behavior and anxiety. However, like many other psychiatric diseases, MDD has a very complex etiology and pathophysiology. Because social isolation is one of the multiple depression-inducing factors in humans, this study aims to understand better the link between social stress and MDD using an animal model based on social isolation after weaning, which is known to produce social stress in mice. We focused on cellular composition and white matter integrity to establish possible links with the abnormal social behavior that rodents isolated after weaning displayed in the three-chamber social approach and recognition tests. We used the isotropic fractionator method to assess brain cellularity, which allows us to robustly estimate the number of oligodendrocytes and neurons in dissected brain regions. In addition, diffusion tensor imaging (DTI) was employed to analyze white matter microstructure. Results have shown that post-weaning social isolation impairs social recognition and reduces the number of neurons and oligodendrocytes in important brain regions involved in social behavior, such as the anterior neocortex and the olfactory bulb. Despite the limitations of animal models of psychological traits, evidence suggests that behavioral impairments observed in patients might have similar biological underpinnings.

Neocortex development comprises of a complex series of time- and space-specific processes to generate the typical interconnected six-layered architecture of adult mammals. Axon growth is required for the proper establishment of cortical circuits. Malformations in axonal growth and pathfinding might lead to severe neuropathologies, such as corpus callosum dysgenesis. Cenpj, a microcephaly gene, encodes a scaffold protein that regulates centrosome biogenesis and microtubule stabilization. During corticogenesis, Cenpj regulates progenitor division and neuronal migration. Since microtubule stabilization is crucial for axon extension, we investigated the role of Cenpj in axon growth during cortical development in a mouse model. Through loss- and gain-of-function assays ex vivo and in utero, we quantified callosal axonal length, branching, and growth cone size compared to controls. We observed that silencing Cenpj results in an increased axonal length. Ex vivo, we assessed the number of branches, the area of growth cones and the stability of microtubules. In silenced Cenpj axons, there were more branches, larger growth cone area, and more stable microtubules. Rescue experiments confirmed that neurons present axonal length comparable to controls. Here we propose that Cenpj regulates axon growth by destabilizing microtubules during cortical development. Finally, our findings suggest that Cenpj might be a novel target for axonal regeneration.

The corpus callosum (CC), the anterior (AC), and the posterior (PC) commissures are the principal axonal fiber bundle pathways that allow bidirectional communication between the brain hemispheres. Here, we used the Allen mouse brain connectivity atlas and high-resolution diffusion-weighted MRI (DWI) to investigate interhemispheric fiber bundles in C57bl6/J mice, the most commonly used wild-type mouse model in biomedical research. We identified 1) commissural projections from the primary motor area through the AC to the contralateral hemisphere; and 2) intrathalamic interhemispheric fiber bundles from multiple regions in the frontal cortex to the contralateral thalamus. This is the first description of direct interhemispheric corticothalamic connectivity from the orbital cortex. We named these newly identified crossing points thalamic commissures. We also analyzed interhemispheric connectivity in the Balb/c mouse model of dysgenesis of the corpus callosum (CCD). Relative to C57bl6/J, Balb/c presented an atypical and smaller AC and weaker interhemispheric corticothalamic communication. These results redefine our understanding of interhemispheric brain communication. Specifically, they establish the thalamus as a regular hub for interhemispheric connectivity and encourage us to reinterpret brain plasticity in CCD as an altered balance between axonal reinforcement and pruning.

The corpus callosum (CC) is a major interhemispheric commissure of placental mammals. Early steps of CC formation rely on guidance strategies, such as axonal branching and collateralization. Here we analyze the time-course dynamics of axonal bifurcation during typical cortical development or in a CC dysgenesis mouse model. We use Swiss mice as a typical CC mouse model and find that axonal bifurcation rates rise in the cerebral cortex from embryonic day (E)17 and are reduced by postnatal day (P)9. Since callosal neurons populate deep and superficial cortical layers, we compare the axon bifurcation ratio between those neurons by electroporating ex vivo brains at E13 and E15, using eGFP reporter to label the newborn neurons on organotypic slices. Our results suggest that deep layer neurons bifurcate 32% more than superficial ones. To investigate axonal bifurcation in CC dysgenesis, we use BALB/c mice as a spontaneous CC dysgenesis model. BALB/c mice present a typical layer distribution of SATB2 callosal cells, despite the occurrence of callosal anomalies. However, using anterograde DiI tracing, we find that BALB/c mice display increased rates of axonal bifurcations during early and late cortical development in the medial frontal cortex. Midline guidepost cells adjacent to the medial frontal cortex are significant reduced in the CC dysgenesis mouse model. Altogether these data suggest that callosal collateral axonal exuberance is maintained in the absence of midline guidepost signaling and might facilitate aberrant connections in the CC dysgenesis mouse model.

Developmental malformations (dysgenesis) of the corpus callosum lead to neurological conditions with a broad range of clinical presentations. Investigating the altered brain connectivity patterns is crucial to understanding both adaptive and maladaptive neuroplasticity in corpus callosum dysgenesis patients. Here, we acquired structural diffusion-weighted and resting-state functional MRI data from a cohort of 11 corpus callosum dysgenesis patients (five with agenesis and six with hypoplasia) and compared their structural and functional connectivity patterns to healthy subjects selected from the Human Connectome Project. We found that these patients have fewer structural inter- and intra-hemispheric brain connections relative to healthy controls. Interestingly, the patients with callosal agenesis have a scant number of inter-hemispheric connections but manage to maintain the full integrity of functional connectivity between the same cortical regions as the healthy subjects. On the other hand, the hypoplasic group presented abnormal structural and functional connectivity patterns relative to healthy controls while maintaining the same total amount of functional connections. These results demonstrate that acallosal patients can compensate for having fewer structural brain connections and present functional adaptation. However, hypoplasics present atypical structural connections to different brain regions, leading to entirely new and abnormal functional brain connectivity patterns.

You might not expect it, but the brain is always changing—it adapts itself to good and bad life experiences. We call this neuroplasticity. Although neuroplasticity usually helps us cope with problems, sometimes things go wrong, and the changes produced by neuroplasticity are harmful to the person. In this article, we cover two cases of neuroplasticity, one positive and one harmful. The positive one is about changes that happen when we are young and have good social experiences. The harmful one is about phantom limb syndrome, a strange phenomenon felt by amputees in their limbs and organs … that no longer exist!

Corpus callosum dysgenesis (CCD) is a developmental brain condition in which some white matter fibers fail to find their natural course across the midplane, reorganizing instead to form new aberrant pathways. This type of white matter reorganization is known as long-distance plasticity (LDP). The present work aimed to characterize the Balb/c mouse strain as a model of CCD. We employed high-resolution anatomical MRI in 81 Balb/c and 27 C57bl6 mice to show that the Balb/c mouse strain presents a variance in the size of the CC that is 3.9 times higher than the variance of normotypical C57bl6. We also performed high-resolution diffusion-weighted imaging (DWI) in 8 Balb/c and found that the Balb/c strain shows aberrant white matter bundles, such as the Probst (5/8 animals) and the Sigmoid bundles (7/8 animals), which are similar to those found in humans with CCD. Using a histological tracer technique, we confirmed the existence of these aberrant bundles in the Balb/c strain. Interestingly, we also identified sigmoid-like fibers in the C57bl6 strain, thought to a lesser degree. Next, we used a connectome approach and found widespread brain connectivity differences between Balb/c and C57bl6 strains. The Balb/c strain also exhibited increased variability of global connectivity. These findings suggest that the Balb/c strain presents local and global changes in brain structural connectivity. This strain often presents with callosal abnormalities, along with the Probst and the Sigmoid bundles, making it is an attractive animal model for CCD and LDP in general. Our results also show that even the C57bl6 strain, which typically serves as a normotypical control animal in a myriad of studies, presents sigmoid-fashion pattern fibers laid out in the brain. These results suggest that these aberrant fiber pathways may not necessarily be a pathological hallmark, but instead an alternative roadmap for misguided axons. Such findings offer new insights for interpreting the significance of CCD-associated LDP in humans.

Deafferentation is an important determinant of plastic changes in the CNS, which consists of a loss of inputs from the body periphery or from the CNS itself. Although cortical reorganization has been well documented, white matter plasticity was less explored. Our goal was to investigate microstructural interhemispheric connectivity changes in early and late amputated rats. For that purpose, we employed diffusion-weighted magnetic resonance imaging, as well as Western blotting, immunohistochemistry, and electron microscopy of sections of the white matter tracts to analyze the microstructural changes in the corticospinal tract and in the corpus callosum (CC) sector that contains somatosensory fibers integrating cortical areas representing the forelimbs and compare differences in rats undergoing forelimb amputation as neonates, with those amputated as adults. Results showed that early amputation induced decreased fractional anisotropy values and reduction of total myelin amount in the cerebral peduncle contralateral to the amputation. Both early and late forelimb amputations induced decreased myelination of callosal fibers. While early amputation affected myelination of thinner axons, late amputation disrupted axons of all calibers. Since the CC provides a modulation of inhibition and excitation between the hemispheres, we suggest that the demyelination observed among callosal fibers may misbalance this modulation.