Abnormalities of Corpus Callosum and Other Interhemispheric Commissures

Epidemiology

Complete agenesis of the CC has been seen in up to 7 infants per 1,000 live births.^21,22 The incidence in those with developmental delay may be as high as 3%.^22,23 About 25% of all patients with agenesis of the CC have other idiopathic anomalies, and another 25% have chromosomal, monogenic, or teratogenic syndromes.^24–27 Inherited conditions may include complete or partial chromosomal anomalies, autosomal dominant, autosomal recessive, or X-linked monogenic disorders, and these could develop de novo in some cases.²³ In about half of all patients, agenesis of the CC has been believed to be an isolated anomaly or in conditions limited to the central nervous system (CNS).²⁴

Structural Abnormalities

Many patients show isolated, complete agenesis of the CC (Figs 4 to 11). The widely spaced lateral ventricles due to the agenesis of the CC has been described as the “racing car sign” (Fig. 5E). Appearances on axial magnetic resonance imaging (MRI) or computed tomography (CT) are reminiscent of a Formula One car seen from above.

Others may have associated abnormalities in other midline structures or adjacent parts of the forebrain.^28,29 The subcallosal sling, the midline glial populations and pioneering axons are known to work together to guide axons across the midline, and several abnormalities may result from altered balance between guidance molecules, and/or that between chemorepellent molecules and cognate receptors.^30–33 These changes can restrict the development of commissural axons with failure of these tracts to cross the midline.

Complete agenesis of the CC can be associated with altered development of the anterior and hippocampal commissures.³⁴ Migrational disturbances (e.g., schizencephaly) (Fig. 11) and callosal lipomas (Figs 10 and 12) have also be seen to interfere with the normal callosal development.³⁵ The combined absence of multiple commissures may represent a severe form of cerebral malformation;³⁴ there may also be altered morphology of the cerebral commissures and associated malformations of the midline, cortical development, white matter, and of the differentiating diencephalon and rhombencephalon.¹⁷ They may need diffusion tensor imaging studies with adequate number of subjects to assess the impact of absence/anomalous course of white matter tracts/commissures.³⁴ The importance of the absence/hypoplasia of the AC and/or HC along with CC agenesis on neurocognitive outcomes also needs review.^22,36

Corpus callosum agenesis/hypogenesis can be seen in association with complex telencephalic, diencephalic, or rhombencephalic malformations in the cortex such as heterotopia (Fig. 13) or abnormal sulcation, reduced cerebral hemispheric white matter volume, non-callosal midline anomalies in the anterior or hippocampal commissures, interhemispheric cysts (Figs 7 and 8), and lipomas (Figs 10 and 12).¹⁷ There might also be associated abnormalities of the cerebellum or brainstem.³⁷ Corpus callosum agenesis/hypogenesis may not be distinct disorders but might just represent a larger dysgenetic spectrum. In some infants, CC damage could also be a secondary phenomenon related to infarction, hemorrhage, trauma, and some metabolic diseases.³⁸

Associated Systemic Anomalies

Commissural abnormalities are frequently associated with abnormalities in the musculoskeletal, central nervous, cardiovascular, urogenital, and digestive systems. In syndromic conditions, neurological anomalies, such as hydrocephalus, cerebellar hypoplasia, periventricular nodular heterotopia, polymicrogyria, microgyria, and lissencephaly have been seen.^25–27 Infants with hydrocephalus could have radiological markers such as altered frontal-occipital horn ratio, apparent diffusion coefficient, and cerebral blood flow indices.

Multicentric Data from the Agenesis of the CC (ACC) Network

The ACC network is a national support organization; Hetts et al.¹⁷ analyzed 66,736 MRI examinations from the period 1985–2003 and queried these for the keywords “callosum” and “hypogenesis” or “dysgenesis” or “agenesis; 198 patients were identified. After excluding cases with incomplete records, 142 were reviewed. Their scans were evaluated for commissural anomalies, interhemispheric cysts, malformations of cortical development; altered cerebral ventricles, and anomalies of white matter such as reduced volume with its location and state of myelination. Corpus callosum agenesis or dysgenesis was seen in 167 (0.25%); 82 had agenesis and 60 had hypogenesis of the CC. After excluding patients who had normal commissures, known multiple congenital anomalies, or technically limited studies, the remaining 142 patients were studied. The mean age (± standard deviation, SD) of identification of agenesis and hypogenesis of CC was 4.0 ± 7.1 (range, 1 day-39 years) and 7.8 ± 13.7 years (range, 2 days-68 years; p = 0.04), respectively. There were no significant differences in gender.

Seventy-three had cortical malformations such as heterotopia and abnormal sulcations. The AC was absent in 48 and was abnormal in size in 46 (10 enlarged and 36 small). Overall, 71% of the infants with agenesis of the CC and 67% of those with hypogenesis also had AC abnormalities. The hippocampal commissure was absent in 107 and abnormal in size in 4. Cerebral ventricles were abnormal in 128 patients, white matter volume was reduced in 134, and myelination was delayed in 32. Cerebral ventricles were more frequently abnormal in patients with agenesis (96%) than in hypogenesis of CC (83%, p < 0.01). Patients with agenesis of CC showed abnormal olfactory sulci more frequently than those with abnormalities in other commissures (33% anterior and 15% hippocampal commissure; p = 0.03). The size of the centrum semiovale was compared with age-matched controls to assess differences in white matter. Cortical malformations, such as pachygyria, polymicrogyria, and oversulcation; the size and folial pattern of the cerebellum; and the size and location of the 4th ventricle were assessed qualitatively.

Data from one hospital in this network showed a higher frequency of abnormal ACs (78% tertiary hospital, 63% ACC Network; p = 0.05), cortical malformations (64% tertiary hospital, 45% ACC Network; p = 0.027), abnormal ventricles (82% tertiary hospital, 95% ACC Network; p = 0.013), moderately to markedly reduced white matter volume (76% tertiary hospital, 56% ACC Network; p = 0.016), abnormal (delayed or incomplete) myelination (32% tertiary hospital, 17% ACC Network; p = 0.024), and anomalies of the cerebellar vermis (42% tertiary hospital, 24% ACC Network; p = 0.027) and brainstem (34% tertiary hospital, 17% ACC Network; p = 0.023). Probst bundles, however, were far more common among ACC Network prospective cohort patients (66%) than among patients from one hospital (18%, p < 0.001).

Many patients with absent/abnormal cerebral commissures have associated abnormalities. Byrd et al.³⁹ performed a retro-/prospective analysis of 105 children with agenesis of the CC. Twenty-six had isolated ACC, but 8 of these 26 had Aicardi’s syndrome with heterotopia, polymicrogyria, and other characteristic features. Thirty-five had interhemispheric cysts, and 31 communicated with the ventricles (type I). Four did not communicate (type II). Twenty (14%) with agenesis of the CC had interhemispheric cysts, of which 11 communicated with the ventricles and 9 did not. Interhemispheric lipomas were seen in 3% of patients in both studies.

Patients with commissural anomalies frequently show malformations in cortical development.¹⁷ In one study, gray matter heterotopias were seen in 29% of subjects with agenesis and 21% of those with hypogenesis of the CC. Barkovich and Norman⁴⁰ reported a lower frequency of 2/68 patients. Byrd et al.³⁹ noted 18/105 patients with “migration disorder.” They documented polymicrogyria in 17 and abnormal sulcation in 35. Classic lissencephaly, cobblestone lissencephalies, polymicrogyria, schizencephaly, heterotopias, meningoceles, and other malformations (Figs 13 to 20) can be seen in conjunction with anomalies of the cerebral commissures.^17,41 Many patients with CC agenesis/hypogenesis have altered gyral patterns that do not fit into one of the classic categories (30/35 with abnormal sulcation).^22,42 In addition to the expected eversion of the cingulum and radial orientation of paramedian gyri associated with CC agenesis, sulcation abnormalities ranging from overly shallow olfactory sulci to notable hemispheric dysplasia can also be seen.¹⁷ Abnormal sulcation, in association with commissural anomalies, could reflect a more generalized developmental disorder of the cerebral white matter.^17,43

Most patients had decreased extracallosal white matter volumes in the supratentorial zone.¹⁷ Only 1/81 with agenesis and 5/59 patients with hypogenesis of the CC had normal white matter volume. Reduced extracallosal white matter may represent a primary dysplasia or hypogenesis with fewer axons forming during development, or a secondary regression due to retraction of axons that cross the midline to synapse with their homologues and gain the necessary neurotrophic support. Alternatively, the number of axons in white matter tracts that did not cross midline could be similar, but these may have relatively less myelin than in normal commissures.

Cerebral hemispheres have lateralized functions, which are particularly pronounced in humans. The CC is critical for integrating interhemispheric information; a developmental agenesis of the CC affects about 1 in 4,000 live births.^8,34 Interestingly, there is some preserved interhemispheric connectivity as seen in behavioral assessment and in resting-state functional MRI studies. These distinct functional outcomes for different ages of callosal loss were first noted by Roger Sperry, and are known as the “Sperry paradox.”⁴⁴ This age-dependent plasticity may result from some compensatory rewiring through alternative routes.

Probst Bundles

One notable structural feature in these patients with agenesis or hypogenesis of the CC are the PBs, which are longitudinal, rostrocaudally oriented coiled white matter fascicles running alongside the lateral ventricles into the tapetum. These were named as the PBs, after Moriz Probst (1867–1923), an Austrian psychiatrist and neuroanatomist. The presence or absence of these PBs can affect the clinical presentation and outcome of these patients.^{8,22,40,45–64} Some of these fibers show a ventromedial projection toward the fornix, where a few may cross the hippocampal commissure to the contralateral hemisphere.

Probst bundles indicate a failure of the callosally projecting neurons to extend axons across the midline due to anomalous axonal guidance in forming the CC. The inability of these axons to cross the midline results in front-to-back projections within each hemisphere, rather than connecting in the CC between the hemispheres. The inner fibers arise from the rostral pole and outer ones from the caudal pole, and these may project in either direction. These findings have been confirmed in multiple reports at various stages of development.^8,45–60 More recently, these anomalies are detected in even more detail by MRI-based diffusion tensor imaging (Fig. 6).

Pathophysiology of PB Development

Probst bundles development is still being explored. There are two possibilities:

1. (a) Probst bundles may develop from mispositioning of midline glial structures.⁸ In primates, callosal axons get positioned according to the guidance cues from specialized glia-rich guideposts at the midline: the indusium griseum, the glial wedge, and the midline zipper glia.⁸ In agenesis of the CC with PB, these structures are frequently mispositioned; the guidance cues drives PB directionality in aberrant locations.^8,34 Midline glia frequently get situated within and around the PB structure itself. These might provide guidance signals to the axons within. However, alternative or additional mechanisms may exist.

   The development of interhemispheric cysts in the interhemispheric fissure that communicate with the ventricular system can disrupt the development of CC.⁶⁵ Midline lipomas are also an important cause; these form within the intradural space and may develop in the pericallosal region in the interhemispheric fissure.⁶⁶ In murine models, early surgical callosotomy conducted either embryonically, on embryonic day (E)16, or on the first postnatal day consistently produces PBs.⁶⁷ Surgical lesioning of the glial populations that contribute to remodeling of the midline seems to be the primary cause.

2. (b) Probst bundles might hijack axon guidance systems of other fiber tracts in neighboring regions that are spared in agenesis of the CC.³² One mechanistic possibility is that axon guidance ligands in existing association tracts encourage axon growth and guidance along the alternative paths. In some cases, pioneering populations of axon tracts rely on guidance signals, while these signals are less necessary for follower axons. Follower axons may therefore be able to indiscriminately follow in the pre-existing tracts.

   Probst bundles most likely develop along a physical scaffold and/or concentrations gradients of molecular cues.⁸ The putative physical scaffolds are referred to as the cingulum bundles – these are believed to be longitudinal, bilateral tracts that bidirectionally interconnect diverse areas that might be prelimbic, anterior cingulate, retrosplenial, and occipital cortex, as well as extracortical areas, such as the hippocampus, thalamus, and brainstem.⁵⁸ Cingulum bundles are also present in agenesis of CC brains dorsomedial to the PBs. Other hypothesized longitudinal tracks include the inferior longitudinal fasciculus, interior fronto-occipital fasciculus, and the superior fronto-occipital fasciculus.^68–70

Hetts and coworkers¹⁷ noted PBs in 66 of their patients. These tracts crossed the midline in some infants with a partial or intact CC. Interhemispheric cysts were seen in 20 (11 patients had cysts in communication with the ventricular system and 9 had cysts that did not communicate. Interhemispheric lipomas were noted in 3 patients. Anomalies of the cerebellum, brainstem, orbits, and olfactory apparatus were also evident in a few patients. Microcephaly, anomalies of the cerebellum, brainstem, orbits, and the olfactory apparatus may also be associated. There was no difference in the frequency of interhemispheric cysts, interhemispheric lipomas, abnormalities of the anterior and the hippocampal commissures, cortical malformations, and gray matter heterotopias. Similarly, the incidence of abnormal orbits, pituitary, white matter myelination, cerebellar hemispheres, cerebellar vermis, and/or structural abnormalities of the brainstem did not differ. There was no difference in the volume of extracallosal white matter.

Patients with prominent PBs frequently showed altered cortical development, rhombencephalic and diencephalic anomalies, and were also seen in multiple congenital anomaly syndromes such as Aicardi’s syndrome.²⁵ Other associated minor anomalies can also be seen. Midline anomalies such as primarily cysts, lipomas, and anomalies of the anterior or hippocampal commissures are frequently associated supratentorial abnormalities. Most patients with agenesis of CC and other midline anomalies (22/25) had PBs, but that was not the case with those with hypogenesis of CC and other midline anomalies. Rhombencephalic or diencephalic abnormalities (14/61) were less likely to be associated with PBs (53/81).

At a genetic level, PBs have been associated with nearly 115 different gene mutations in mice and humans.⁸ Most PBs contain morphologically similar fibers with a predictable stereotypical orientation in specific locations of the brain.⁸ These also show functional similarities with both faciliatory and detrimental functions. Unlike other developmental anomalies where aberrant axons appear pruned, PBs are preserved intact into adulthood;⁸ the longitudinal ridge seems to persist as an underdeveloped CC.⁷¹ The lateral callosal PBs indent the superomedial aspect of the lateral ventricles and may represent the axons that would have crossed the midline.⁷² Probst bundles are seen about twice as frequently in patients with agenesis (59%) than in hypogenesis (30%) of the CC.²² Byrd et al.³⁹ described PBs in patients with isolated agenesis of the CC. However, the histomorphology still needs to be characterized. Further studies are needed to outline the role(s) of these stereotypical ectopic tracts; plastic anatomical changes might be adaptive, maladaptive, or neutral.⁸ MRI/diffusion tensor tractography⁷³ and neurodevelopmental follow-up are needed.

Axons composing PBs originate from neurons arising from cortical projections that resemble those in the CC in neurotypical brains.⁸ In the CC, the axon density is highest at the rostral end, it tapers through the mid-callosum, and then increases again caudally at its posterior pole.⁷⁴ In contrast, the neuronal density in PBs is highest at the rostral end but it progressively decreases toward the caudal end.⁸ The origin of these fibers likely decreases anteroposteriorly from the frontal, parietal, and the occipital cortex. Questions remain whether these rostrocaudal differences in brains with PBs originate in neuronal proliferation/migration during development or in cell death related to developmental pruning.⁸

The seemingly tortuous configuration of PBs might contain a consistent topographic arrangement.⁸ Probst bundles seem to be orientated along the rostrocaudal axis, although the exact directionality is not always clear. During early development, axons stretch longitudinally in both a rostral and caudal direction. The longitudinal directionality of PB may be mixed in earlier developmental stages, but may get pruned later in different regions into a primarily rostral or caudal trajectory. Probst bundles may run dorsally and medially to the lateral ventricles and extend into the caudal tapetum. These patterns may cause morphological indenting of the rostral lateral ventricles and dilation of the caudal portions. The interpretation needs some caution as the appearance of PBs varies across species and might get altered with various stimuli. “PB-like” fibers have been noted as halted at the midline in arrested prenatal growth.

There are important differences in the neurons contributing to the CC or the PB, but the tracts are similar in the initial stages of development;^8,75,76 the morphological differences become more evident when callosal axons begin to traverse the midline. The CC axons cross the midline through a glia-rich permissive midline substrate but those from the PB make a sharp longitudinal turn as they approach the midline in response to cues such as disrupted midline guide-post cells, spatial and physical constraints, altered axon guidance molecules, and axon contacts with a retained/unmodeled interhemispheric fissure.⁸ Corpus callosum axons may transiently bifurcate before reaching midline targets, depending on the location in the cortex.⁷⁷ The bifurcation prior to midline crossing might promote some axons to take alternative routes and differentiate to get integrated into the PB. Future studies are needed to dissect these differentiation pathways. Both form fasciculated axon tracts and share many callosal axon guidance and maturation programs.

In both tracts, spatially symmetrical bilateral activity might be necessary for normal contralateral callosal targeting. However, it is unclear if both differentiate from the same precursors. Cortical neurons in both project axons medially and rostrocaudally; these trajectories diverge as these approach the midline glia in the indusium griseum, glial wedge, and midline zipper glia, which displace the leptomeningeal fibroblasts and consequently shorten the interhemispheric fissure.^8,32 In the developing PB, the midline glia often appear malformed or mispositioned, or are functionally unable to secrete guidance cues and consequently, are not able to intercalate at the midline. The elongated axons take an ipsilateral U-turn in the same hemisphere. Some of these target ipsilateral structures such as the anterior septum. Other targets of PBs are still unknown.

Some of the PB fibers may project out of the fasciculated tract to other locations in the CNS.^8,78 However, the precise PB connectome and its degree of variability remains unclear. Current studies suggest that most PB fiber largely project to ipsilateral areas innervated by the CC.⁸ Human diffusion tensor imaging studies show similar ipsilateral PB projection pattern consistent with a neurotypical CC projecting contralaterally to the frontal, parietal, occipital and temporal lobes.^8,79 The PB fibers do seem to preferentially project to more rostral frontal lobes and more paramedial cortical regions. This rostral and rostrocaudal axis of PBs may contribute to a higher rostrocaudal connectivity in absent agenesis of CC. Some of these anatomical features of PBs resemble those of the neurotypical cingulate bundle, which runs rostrocaudally over the CC to connect ipsilateral cortical hubs along the midline (regions that are also heavily connected interhemispherically by the CC), as well as extracortical regions that include the thalamus, basal forebrain, hippocampal formation, and other limbic regions.

In brains with agenesis of CC, some axons may project ventrally from the PB into the ipsilateral anterior septum. The cell bodies of origin might be in the cingulate cortex, where the first pioneering exons in the CC originate. These ectopic septal neurons in caudal regions of the brain are misplaced glutamatergic neurons and Sema3C cells.⁸⁰ Such ectopic projections to the septum have also been reported in brains with agenesis of the CC without PBs; the two structures are not necessarily always linked. Some patients with absent CC can be unexpectedly more functional than expected;⁸¹ these tasks require bilateral integration and the PBs could contribute to other interhemispheric connections.⁸²

Some PB fibers may project ventromedially to join the fornix.^83–85 The fibers cross at the level of the hippocampal commissure⁸⁶ and may maintain connectivity within cortical regions and for projecting to subcortical targets.⁸ In animal models, a virtual Probstomy can alter the connections between cortical and subcortical regions.⁸⁷ Further work is needed to understand these circuits.⁸⁷

PBs in Partial Agenesis of the CC

Probst bundles may also be seen with partial agenesis of the CC;⁸ these patients have a callosal remnant along the rostro-caudal axis of the CC (partial hypogenesis) or a hypoplastic CC (thinning along the dorsoventral axis). In one study, the authors studied 113 reports of partial agenesis of the CC in experimental animals and humans and found adequate data for analysis in 43.⁸ They noted 19 cases with hypogenesis of the CC, 31 with partial agenesis, and 1 who could not be classified. Of the 19 cases with hypogenesis, PBs were noted dorsal to (16/19) and ventral to (3/19) a thin callosum.⁸ In these infants, the tract thinned out along its dorsocaudal axis, and can include both dorsal and ventral fibers. In the 31 infants with partial hypogenesis, PBs were seen most frequently posterior to callosal remnants (20 cases); anterior in 9, and in the mid-callosum in 2.⁸

Another type of an ectopic tract, the sigmoid bundle, has been reported in some cases of partial hypogenesis of the CC.⁸⁸ These aberrant fiber bundles asymmetrically connect the frontal lobe to the contralateral parieto-occipital cortex via the callosal remnant.⁸ These tracts could possibly contribute to functional connectivity. The contribution of PBs to the sigmoid bundle is unclear.

Genetic Etiologies of Altered Development of Commissures

Genetic factors are an important cause of agenesis of the CC. Nearly, 85% of human cases have associated PBs.^8,22,34 In contrast, the specific genes have not been identified as a cause of agenesis of the CC without PBs. The formation of PBs has been associated with underexpression of several genes; there is a possibility that altered development of the midline structures may lead to agenesis of CC with PB formation. In one study, nearly all human MRIs with agenesis of CC identified disrupted midline territories.⁸⁸ About 115 unique genes have been identified in human/mouse agenesis of CC that did or did not lead to PB formation.⁸ Ninety-one of these genes are known mediators in PB formation (79%); the rest have been associated with an absence of PBs. There are some differences in the genes identified in humans vs murine models.⁸ There could also be other polygenic/environmental factors that influence gene expression and biological processes and/or the identification of genetic mutations that may not be involved in PB development, but can often be implicated in agenesis of CC.

Lynton and coworkers⁸ used the Database for Annotation, Visualization, and Integrated Discovery (DAVID) to identify the human or mouse genes associated with or without PB formation shared common ontologies, such as the cellular component where the gene is typically enriched. These analyses did not show major differences in the cellular component ontologies of genes in either species. In both mice and humans, 3 genes showed a strong association with PB formation. The gene deleted in colorectal cancer (DCC)/Dcc, tubulin beta-3 class III (TUBB3)/Tubb3, and tubulin alpha 1a (TUBA1A)/Tuba1a.^31,89,90 Five genes were identified as associated with PB formation/absence in humans (L1 cell adhesion molecule (L1CAM), ectopic P-granules 5 autophagy tethering factor (EPG5), chromodomain helicase DNA-binding protein 7 (CHD7), TUBB3, and TUBA1A).^8,90,91 The Tables 1 and 2 have been modified based on the work of these experts.

Several genes associated with absence of CC and PBs have been associated with callosal axon-crossing in the brain midline.⁸ Fibroblast growth factor 8 (Fgf8) may play an important role in an astrogliogenic program of midline tissue remodeling.⁸ The transcription factors, nuclear factor I/A (Nfia) and nuclear factor I B (Nfib) may be important upstream regulators. Probst bundles have also been seen in several other murine models,^8,22 such as those involving the secreted protein Draxin and its axon guidance receptor Dcc.⁴⁶ These show altered axon guidance system(s) involved in midline crossing and CC formation.⁹² Genetic causes of absent CC with PB formation are related to changes in midline remodeling.⁸ Further studies are needed.

Clinical Manifestations of Altered Development of Commissures with PBs

Agenesis of the CC may first present with seizures during infancy, beginning as early as the first few weeks after birth.²² Many infants have feeding problems, neurodevelopmental delay, impaired hand-eye coordination, visual and/or auditory impairment, and deficits in memory acquisition.⁹³ Delayed acquisition of motor milestones is seen very frequently. Some patients develop hydrocephalus.⁹⁴ Some of these patients can benefit from intracranial pressure monitoring.^95,96 In other infants with milder manifestations, the clinical features might be delayed for many years,²² and may present with seizures, headaches, motor abnormalities, or speech abnormalities.^97,98 Finally, a subset of patients remains asymptomatic.⁹⁹

Mowat-Wilson syndrome¹⁰⁰ may be noted with micro- or macrocephaly. There may be dysmorphic features, such as ocular hypertelorism, pre-auricular skin tags, a small nose with anteverted nostrils, abnormally shaped pinnae, laryngeal abnormalities, loose skin on the neck, short hands, and digital abnormalities such as camptodactyly. Congenital heart defects and symptoms of Pierre-Robin syndrome could be seen. Some patients may only have growth failure.

Aicardi syndrome, an X-linked dominant disorder, may show agenesis of the CC.¹⁰¹ These infants may present with infantile spasms; eye abnormalities in the choroid and the retinae; seizures; and developmental delay.

Andermann syndrome is another multisystem genetic disorder that can include agenesis of CC. These infants often show developmental delay and progressive sensorimotor neuropathy.¹⁰² Many patients with this condition originate from the Charlevois County and the Saguenay-Lac St. Jean area of Quebec, Canada. The causative gene has been identified as the SLC12A6.

XLAG (X-linked lissencephaly with ambiguous genitalia)¹⁰³ is a rare genetic disorder in which males have abnormal cerebral gyration (lissencephaly), abnormal genital development such as with microphallus, severe developmental delay, and intractable seizures. The causative gene has been identified as ARX. Females can present with only agenesis of the CC.

Other Etiologies of Altered Development of CC with PBs

Some environmental and infectious causes have been identified to be associated with altered development of CC and PBs. Gamma irradiation in mice and Zika virus infection in a human fetus have been associated with agenesis of CC with PB formation.^45,104 Gamma irradiation resulted in altered midline remodeling with the absence of midline glia. Zika viruses disrupted midline glia, local microvasculature, and cortical development. There were fewer proliferating cortical cells, intermediate progenitors, and SATB2⁺ neurons.¹⁰⁵ Viral-mediated neurodevelopmental deficits may be caused by disruption of the CC but formation of PBs when structural disorganization is less extreme.

Agenesis of CC without PBs

To understand the physiological importance of PBs, one group of investigators reviewed a total of 39 reports of human subjects and 30 of murine models of agenesis of CC who did not show PBs.⁸ Most of these cases involved major nervous system malformations such as meningomyelocele/Chiari II malformations or classic holoprosencephaly. Reported mechanisms included deficits in genes encoding growth factors, such as Insulin-like growth factor-1 (Ifg1), and tubulins and proteins associated with cellular metabolism such as pyruvate dehydrogenase (PDH). These findings suggested that deficiencies in growth, and axonal outgrowth and metabolism could contribute to cerebral disorganization without the CC and PBs. However, there are 3 case reports where neurodevelopmental malformations that lacked CC but showed PBs, including syntelencephaly, myelomeningocele, and Chiari II malformations, suggesting that PBs could form in these conditions. We clearly need additional studies to better understand the significance of the PBs.⁸

Probst bundles are seen in nearly all cases of agenesis of CC where the structure of the cerebral cortex is not significantly altered.⁸ However, some murine models lacking the CC show contrary evidence; axons do not form PBs but appear halted in the hemispheres. Genotypes lacking slit guidance ligand 2 (Slit2), roundabout guidance receptor 1 (Robo1), or special AT-rich sequence binding protein 2 (Satb2) are notable, although the impact on axon growth is uncertain.⁸⁸ There is also a possibility that the axons could have been re-routed through a separate, non-callosal interhemispheric tract.^8,32 More studies are needed to understand the mechanisms involved in the formation of ectopic bundles or re-routing through existing commissures and tracts.

Clinical Significance of Altered Development of Commissures and PBs

The function of PBs is still unclear. One possibility could be related to less axonal elimination during development but these could also contain more functional connections.⁸ Features such as the myelination patterns resemble those in mature brain structures. The functionality of PB fibers can be identified in glucose uptake, EEG coherence, fetal connectome, and electrophysiology. Probst bundles function could well be compensatory, neutral, or maladaptive to cognitive outcome.⁸ These could promote neurodevelopmental outcomes and performance, but these findings are difficult to confirm as these structural findings are often associated with other gross malformations.

Compared with infants with complete absence of CC, those with partial absence have fewer anatomical changes in the brain and might have better behavioral outcomes.¹⁰⁶ However, existing data suggest that the contrary might be the case. In one connectome study, the functional connectivity patterns in patients with complete absence of the CC resembled those in controls. In contrast, those with callosal hypoplasia showed abnormal structural and functional connectivity patterns; disorganized cortical neurons projected inconsistently into the ectopic PB.⁸ This high variability in connectivity could have explained the variability in behavioral and cognitive performance. Another explanation could be rooted in a higher frequency of minor brain abnormalities in these patients. More studies are required to confirm these findings.

Probst bundles might help maintain interhemispheric communication, and consequently, promote certain behavioral phenotypes.^8,92 Individuals with absence of CC may still show preserved interhemispheric connectivity on behavioral and resting-state functional MRI studies.¹⁰⁷ Virtual lesions also suggest that PBs may contribute to interhemispheric communication. Individuals born with absence of CC frequently do not display the disconnection syndrome, the PBs could well contribute to interhemispheric communication such as through the hippocampal commissure or subcortical routes.⁸ Augmented ipsilateral connectivity with cortical hubs may enhance polysynaptic communication via other pre-existing interhemispheric circuits.⁸

The functional importance of PBs may well extend beyond interhemispheric communication.⁸ These commissures have been associated with “syndromic” diagnoses such as the autism spectrum that also have altered structural and functional connectivity of different parts of the brain and reduced callosal volume.³⁴ The developmental plasticity of brain connections might help in understanding altered neurodevelopment and regulation of compensatory plasticity.¹⁰⁸

Corpus callosum helps maintain communication between the left and right sides of the body.¹⁰⁹ Despite existing information that patients with agenesis of the CC may not always display a disconnection syndrome,¹¹⁰ the morphological substrates facilitating intact interhemispheric communication in those infants remain unclear. Probst bundles seem to contain tangled and dysfunctional axons.⁸ However, some anatomical consistency has been maintained in these topographic patterns through evolution, indicating that there could be some functional roles.¹¹¹ There might be some cognitive compensation that has maintained interhemispheric communication in the absence of CC.¹¹² Focused studies on developmental abnormalities in axon tracts may be helpful in understanding axon plasticity and connectivity disorders.