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We present an image of diffuse intrinsic pontine glioma (DIPG) showing its relationship to white matter structures based on reconstructed tractography (figure). An 8-year-old girl presented with diplopia, right abducens nerve palsy, and mild ataxia. Reconstruction showed at the level of the pontine decussation a right-sided disruption of the pontocerebellar fibers anterior to the descending corticospinal fibers involving the pontine nuclei region. Understanding the white matter tract anatomy of DIPG in relation to surrounding structures critical for function by this method may have the potential to assist in minimizing functional deficit risk during biopsy, but requires future validation.

We report on the conventional and diffusion tensor imaging (DTI) findings of a 2-year-old child with clinical presentation of Joubert's Syndrome (JS) and brainstem structural abnormalities as depicted by neuroimaging.Conventional magnetic resonance imaging (MRI) showed a “molar tooth” configuration of the brainstem. A band-like formation coursing in an apparent axial plane anterior to the interpeduncular fossa was noted and appeared to partially cover the interpeduncular fossa.DTI maps and three-dimensional (3D) tractography demonstrated a prominent red-encoded white matter bundle anterior to the midbrain. Probable aberrant course of the bilateral corticospinal tracts (CST) was also depicted. Absence of the decussation of the superior cerebellar peduncles and elongated thickened, horizontal superior cerebellar peduncle (SCP) reflecting the molar tooth sign were also shown.Our report and the review of the published cases suggest that DTI and tractography may be very helpful to differentiate between interpeduncular heterotopias and similarly located white matter bundles corroborating the underlying etiology of axonal guidance disorders in the complex group of ciliopathies including JS. Our case represents an important additional puzzle piece to explore the variability of these ciliopathies.

In the current study, we detail, through the analysis of immunohistochemically stained sections, the morphology and nuclear parcellation of the serotonergic neurons present in the brainstem of a lar gibbon and a chimpanzee. In general, the neuronal morphology and nuclear organization of the serotonergic system in the brains of these two species of apes follow that observed in a range of Eutherian mammals and are specifically very similar to that observed in other species of primates. In both of the apes studied, the serotonergic nuclei could be readily divided into two distinct groups, a rostral and a caudal cluster, which are found from the level of the decussation of the superior cerebellar peduncle to the spinomedullary junction. The rostral cluster is comprised of the caudal linear, supralemniscal, and median raphe nuclei, as well as the six divisions of the dorsal raphe nuclear complex. The caudal cluster contains several distinct nuclei and nuclear subdivisions, including the raphe magnus nucleus and associated rostral and caudal ventrolateral (CVL) serotonergic groups, the raphe pallidus, and raphe obscurus nuclei. The one deviation in organization observed in comparison to other primate species is an expansion of both the number and distribution of neurons belonging to the lateral division of the dorsal raphe nucleus in the chimpanzee. It is unclear whether this expansion occurs in humans, thus at present, this expansion sets the chimpanzee apart from other primates studied to date.

Objective Joubert syndrome (JS) is a rare syndrome characterized by ataxia and the molar tooth sign (MTS) on imaging. The present study aims to explore the clinical and radiological features in a cohort of patients with JS.Methods This was a retrospective chart review of patients with JS evaluated by movement disorder specialists.Results Nine patients were included in the study. All patients had facial dysmorphism and ocular abnormalities, and 4 patients had dystonia. Ocular tilt reaction and alternate skew deviation (66%) were the most common ocular abnormalities. Horizontally aligned superior cerebellar peduncles were observed in all four patients with diffusion tensor imaging, with a lack of decussation in three. Exome sequencing performed in four patients revealed novel variants in the MKS1, CPLANE1, and PIBF1 genes.Conclusion Facial dysmorphism, ocular abnormalities and classical imaging findings were observed in all patients with JS. Apart from ataxia, dystonia and myoclonus are other movement disorders observed in JS.

Albinism is a group of rare inherited disorders arising from impairment of melanin biosynthesis. The reduction of melanin synthesis leads to hypopigmentation of the skin and eyes. A wide range of ophthalmic manifestations arise from albinism, including reduction of visual acuity, nystagmus, strabismus, iris translucency, foveal hypoplasia, fundus hypopigmentation, and abnormal decussation of retinal ganglion cell axons at the optic chiasm. Currently, albinism is incurable, and treatment aims either surgically or pharmacologically to optimize vision and protect the skin; however, novel therapies that aim to directly address the molecular errors of albinism, such as l-dihydroxyphenylalanine and nitisinone, are being developed and have entered human trials though with limited success. Experimental gene-based strategies for editing the genetic errors in albinism have also met early success in animal models. The emergence of these new therapeutic modalities represents a new era in the management of albinism. We focus on the known genetic subtypes, clinical assessment, and existing and emerging therapeutic options for the nonsyndromic forms of albinism.

Variations of the pectoralis major muscle are commonly seen. However, during the routine dissection of an adult male cadaver, an apparently uncommon muscular variant was identified. In this cadaver, the sternal heads of the left and right pectoralis muscles crossed the midline and interdigitated with one another. In addition, the clavicular heads of both pectoralis major muscles were separated from the remaining parts of the muscles. Such anatomical variants such be kept in mind by clinicians and surgeons during patient evaluation and treatment.

Neuro-Behçe disease (NBD) has a predilection for affecting the parenchyma of the upper brainstem; however, involvement of the fourth nerve nucleus or fascicle by NBD has not been previously described. We report a case of a young man with acute right fourth nerve palsy and history of Behçet disease with an enhancing lesion in the left caudal midbrain corresponding to the left trochlear nerve nucleus/fascicle. This is the first described case of NBD producing nuclear/fascicular fourth nerve palsy. It also demonstrates an important clinicoanatomical correlate of decussation of fourth nerve fibers to the opposite side after exiting the midbrain.

Binocular vision depends on retinal ganglion cell (RGC) axon projection either to the same side or to the opposite side of the brain. In this article, we review the molecular mechanisms for decussation of RGC axons, with a focus on axon guidance signaling at the optic chiasm and ipsi- and contralateral axon organization in the optic tract prior to and during targeting. The spatial and temporal features of RGC neurogenesis that give rise to ipsilateral and contralateral identity are described. The albino visual system is highlighted as an apt comparative model for understanding RGC decussation, as albinos have a reduced ipsilateral projection and altered RGC neurogenesis associated with perturbed melanogenesis in the retinal pigment epithelium. Understanding the steps for RGC specification into ipsi- and contralateral subtypes will facilitate differentiation of stem cells into RGCs with proper navigational abilities for effective axon regeneration and correct targeting of higher-order visual centers.

Forehead aesthetics have a major contribution to the youthful appearance of the face. Restoration of the upper facial aesthetics is important to counteract the changes related to the ageing process. The interaction between the frontalis and its antagonists' muscles contributes to the overall aesthetic balance of the forehead. In this study, we evaluated the gross anatomy of the frontalis and classified the muscle according to the morphological appearance. 26 cadavers of Caucasian and South East Asian origin were dissected. The frontalis muscle was dissected without mobilisation, and the gross anatomy and variations were analysed on the backdrop of gender and ethnicity. Our dissection studies revealed three main variations of the muscle based on the extent of interdigitation between the two bellies in the midline. The average length of the muscle was 10.9 cm in males and 9.1 cm in females. The width of the muscle in females was 4 cm at the origin and 6.5 cm at the insertion and in males 4.4 cm at the origin and 5.8 cm at the insertion. In six specimens, the muscle attached only up to the medial two-thirds of the eyebrows. This was predominantly observed in the Caucasians cadavers and may contribute to the constitutional downward slanting eyebrows in some individuals. This study provides an in-depth analysis and classification of the frontalis muscle. Understanding the morphological variation of the muscle helps to amend clinical application and treatment protocols. Evaluation of the patterns of decussation of the frontalis muscle may assist with non-surgical interventions using botulinum toxin in the treatment of forehead rhytids. This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

Albinism is a congenital disorder where misrouting of the optic nerves at the chiasm gives rise to abnormal visual field representations in occipital cortex. In typical human development, the left occipital cortex receives retinal input predominantly from the right visual field, and vice-versa. In albinism, there is a more complete decussation of optic nerve fibers at the chiasm, resulting in partial representation of the temporal hemiretina (ipsilateral visual field) in the contralateral hemisphere. In this study, we characterize the receptive field properties for these abnormal representations by conducting detailed fMRI population receptive field mapping in a rare subset of participants with albinism and no ocular nystagmus. We find a nasal bias for receptive field positions in the abnormal temporal hemiretina representation. In addition, by modelling responses to bilateral visual field stimulation in the overlap zone, we found evidence in favor of discrete unilateral receptive fields, suggesting a conservative pattern of spatial selectivity in the presence of abnormal retinal input.

Background Migraine surgery is an increasingly popular treatment option for migraine patients. The lesser occipital nerve is a common trigger point for headache abnormalities, but there is a paucity of research regarding the lesser occipital nerve and its intimate association with the spinal accessory nerve. Methods Six cadaver necks were dissected. The lesser occipital, great auricular, and spinal accessory nerves were identified and systematically measured and recorded. These landmarks included the longitudinal axis (vertical line drawn in the posterior), the horizontal axis (defined as a line between the most anterosuperior points of the external auditory canals) and the earlobe. Mean distances and standard deviations were calculated to delineate the relationship between the spinal accessory, lesser occipital, and great auricular nerves. Results The point of emergence of the spinal accessory nerve was determined to be 7.17 ± 1.15 cm lateral to the y axis and 7.77 ± 1.10 caudal to the x axis. The lesser occipital nerve emerges 7.5 ± 1.31 cm lateral to the y axis and 8.47 ± 1.11 cm caudal to the x axis. The great auricular nerve emerges 8.33 ± 1.31 cm lateral to the y axis and 9.4 ±1.07 cm caudal to the x axis. The decussation of the spinal accessory and the lesser occipital nerves was found to be 7.70 ± 1.16 cm caudal to the x axis and 7.17 ± 1.15 lateral to the y axis. Conclusion Understanding the close relationship between the lesser occipital nerve and spinal accessory nerve in the posterior, lateral neck area is crucial for a safer approach to occipital migraine headaches, occipital neuralgia, and new daily persistent headaches and other reconstructive or cosmetic operations.

Horizontal gaze palsy with progressive scoliosis (HGPPS) is a rare, autosomal recessively inherited disorder characterized by a congenital absence of conjugated horizontal eye movements with progressive scoliosis developing in childhood and adolescence. HGPPS is caused by mutations of the ROBO3 gene that disrupts the midline crossing of the descending corticospinal and ascending lemniscal sensory tracts in the medulla. We present two siblings, 5-year-old and 2-year-old boys with HGPPS, from non-consanguineous parents. The older brother was brought for the evaluation of moderate psychomotor retardation. He had bilateral horizontal gaze palsy with preserved vertical gaze and convergence. Scoliosis was absent. Cranial MRI showed brainstem abnormalities, and diffusion tensor imaging showed absent decussation of cortico-spinal tracts in the medulla. Clinical diagnosis of HGPPS was confirmed by sequencing of ROBO3 gene, IVS4–1G > A (c.767–1G > A) and c.328_329delinsCCC (p.Asp110Profs*57) compound heterozygous variations were found, and segregated in parents. The younger boy was first reported at 16 months of age and had the same clinical and neuroradiological findings, unlike mild psychomotor retardation. ROBO3 gene analysis showed the same variants in his brother. Our cases show the importance of evaluating eye movements in children with neurodevelopmental abnormalities and looking for brainstem abnormalities in children with bilateral horizontal gaze palsy.

BACKGROUND: The precise pathogenesis of anophthalmia/microphthalmia remains unknown. Prenatal observation of the optic chiasm in fetuses with this malformation would assist in understanding the embryonic development of the condition. The present study aimed to establish the normal fetal size ranges of decussation of the optic chiasm, optic nerves, and optic tracts in the axial plane using two-dimensional transabdominal ultrasound throughout gestation and to compare these ranges to the corresponding values in fetuses with anophthalmia/microphthalmia METHODS: In total, 310 normal fetuses and 16 fetuses with anophthalmia/microphthalmia were included in this study. The widths of the decussation of the optic chiasm, optic nerves, and optic tracts of normal fetuses at 19–40 weeks' gestation were measured in the axial plane by two-dimensional transabdominal ultrasound. The same widths were retrospectively measured in the axial plane using three-dimensional ultrasound in fetuses with anophthalmia/microphthalmia and compared to the results from the normal fetuses. RESULTS: The decussation, optic nerves, and optic tracts of 310 normal fetuses were measured. The normal widths of the decussation of the optic chiasm, optic nerves, and optic tracts increased linearly with gestational age. The interobserver and intraobserver reproducibility was excellent for the decussation but relatively low for the optic nerves and optic tracts. The optic nerve width of fetuses with anophthalmia/microphthalmia was significantly smaller than that of normal fetuses (P

Until recently, there had been little attempt in the literature to identify and quantify the underlying mechanics of tooth durability in terms of materials engineering concepts. In humans and most mammals, teeth must endure a lifetime of sustained occlusal mastication—they have to resist fracture and wear. It is well documented that teeth are resilient, but what are the unique features that make this possible? The present article surveys recent materials engineering research aimed at addressing this fundamental question. Elements that determine the mechanics and micromechanics of tooth fracture and wear are analysed: at the macrostructural level, the geometry of the enamel shell and cuspal configuration; and at the microstructural level, interfacial weakness and property gradients. Inferences concerning dietary history in relation to evolutionary pressures are discussed.

Mechanical properties of enamel are known to vary across molar crowns in some primates, but the association of this variation with phylogeny, structural properties and tribological behaviour is not well understood. In this study, 20 molars from a range of primate taxa (n=15) were studied using nanoindentation, micro-CT scanning, and SEM imaging. After micro-CT scanning, teeth were sectioned in the lingual-buccal plane through the mesial cusps. Five positions (buccal lateral, buccal cuspal, occlusal middle, lingual cuspal, lingual lateral) were studied in three locations (inner, middle, outer enamel regions). The results show middle enamel had the highest hardness and elastic modulus values in all positions. ‘Non-functional’ molar sides (lingual in lower molars and buccal in upper molars) had higher hardness values than their ‘functional’ counterparts. Increase in prism size was associated with a decrease in hardness in some tooth positions, and mineral density showed a significant relationship with elastic modulus values. Variation in enamel structure variation (e.g., enamel Schmelzmuster, Hunter-Schreger band thickness), may also be crucial in explaining variation in mechanical properties, with decussation zones associated with higher mechanical properties values. Primate enamel is not a homogeneous material, with variation in mechanical and structural properties across the crown likely associated with functional differences and variation in force distribution. Overall structural and mechanical patterns were similar in the primate species studied despite substantial differences in diet, suggesting these properties are potentially evolutionary conserved.

Lateral medullary syndrome is a common brainstem stroke associated with a classical triad of Horner’s Syndrome, ipsilateral ataxia and hypalgesia and thermoanasthesia of ipsilateral face. We report a case of a 49-year-old diabetic, non-hypertensive, postmenopausal female who presented with symptoms involving the left dorsal medulla along with right sided hemiparesis and left UMN-type facial palsy. Contralateral hemiparesis was explained by caudal extension of infarct involving the pyramids before decussation at the medulla, known as Babinski–Nageotte Syndrome. UMN-type facial palsy was attributed to involvement of hypothetical supranuclear aberrant corticobulbar fibres of facial nerve which descend down in the contralateral ventromedial medulla, decussate at level of upper medulla and then ascend in the dorsolateral medulla to reach the facial nerve nucleus. Association of these two entities with Wallenberg’s Syndrome have been reported separately in literature, but not together as in this case.

Background The course of the corticobulbar tract (CBT) to the facial nucleus has been investigated by some previous studies. However, there are some unclear points of the course of the CBT to the facial nucleus. This study aimed to elucidate the detailed course of the CBT to the facial nucleus through the analysis of lateral medullary infarction (LMI) cases. Methods The neurological characteristics and magnetic resonance imaging findings of 33 consecutive patients with LMI were evaluated. The location of the lesions was classified rostro-caudally (upper, middle, or lower) and horizontally. Further, we compared the neurological characteristics between the groups with and without central facial paresis (FP). Results Eight (24%) patients with central FP ipsilateral to the lesion were identified. Dysphagia and hiccups were more frequently observed in the group with central FP than in the group without central FP. In patients with central FP, middle medullary lesions and those including the ventral part of the dorsolateral medulla were more frequently observed. Contrastingly, patients with lesions restricted to the lateral and dorsal regions of the dorsolateral medulla did not present with central FP. Conclusion The results of this study indicate that the CBT to the facial nucleus descends with the corticospinal tract at least to the middle portion of the medulla, and then ascends to the facial nucleus through the medial and ventral areas of the dorsolateral medulla after decussation.

Objective Projections from the dentate nucleus (DN) follow a certain organized course to upper levels. Crossing and noncrossing fibers of the dentatorubrothalamic (DRT) tract terminate in the red nucleus and thalamus and have various connections throughout the cerebral cortex. We aimed to establish the microsurgical anatomy of the DN in relation to its efferent connections to complement the increased recognition of its surgical importance and also to provide an insight into the network-associated symptoms related to lesions and microsurgery in and around the region. Methods The cerebellum, DN, and superior cerebellar peduncle (SCP) en route to red nucleus were examined through fiber dissections from the anterior, posterior, and lateral sides to define the connections of the DN and its relationships with adjacent neural structures. Results The DN was anatomically divided into 4 areas based on its relation to the SCP; the lateral major, lateral anterosuperior, posteromedial, and anteromedial compartments. Most of the fibers originating from the lateral compartments were involved in the decussation of the SCP. The ventral fibers originating from the lateral anterosuperior compartment were exclusively involved in the decussation. The fibers from the posteromedial compartment ascended ipsilaterally and decussated, whereas most anteromedial fibers ascended ipsilaterally and did not participate in the decussation. Conclusions Clarifying the anatomofunctional organization of the DN in relation to the SCP could improve microneurosurgical results by reducing the complication rates during infratentorial surgery in and around the nucleus. The proposed compartmentalization would be a major step forward in this effort.

Split cord malformation (SCM) is a term used for all double spinal cords. It represents 3.8%–5% of spinal dysraphisms. Pang et al.’s embryological theory proposes the formation of an “accessory neurenteric canal” that communicates with the yolk sac and amnion. To the authors’ knowledge, only three cases of diastematobulbia (basicranial SCM) associated with a spur or dermoid have been reported in the literature.The case patient is a newborn girl with an occipitocervical meningocele and dermal sinus associated with an anomaly of notochordal development in the transition between the medulla oblongata and the spinal cord (diastematobulbia) without a bony septum or dermoid cyst. The patient also has agenesis of the atlas and an absence of corticospinal tract decussation. This patient underwent reconstruction of the occipital meningocele and dermal sinus excision.To the authors’ knowledge, this is the first described case of type II diastematobulbia (basicranial SCM), without a dermoid cyst. The authors analyzed the embryological errors present in the case patient and considered the option of further surgical treatment depending on the evolution of the patient’s condition. At the time of this report, the patient had shown normal psychomotor development. However, this fact may only be due to the patient’s young age. Considering that after initial untethering the patient remained clinically asymptomatic, conservative and close surveillance has been and continues to be the proposed treatment.

Multiple behavioral and biomechanical analyses have demonstrated that capuchin monkeys (Cebus and Sapajus) are specialized for breaking down hard-object foods as compared to other cebid monkeys. In addition to a complex suite of craniodental adaptations, it has specifically been demonstrated that capuchins possess highly complex dental enamel, with extensive Hunter-Schreger banding and other decussation, that likely serve as an adaptation to resist crack propagation during hard-object feeding. Furthermore, it has been demonstrated that robust capuchins (Sapajus spp., formerly Cebus apella) demonstrate further adaptation for hard-object feeding than other capuchins, routinely breaking down extremely mechanically challenging foods. However, there has been no comparison of dental enamel complexity in robust versus gracile capuchins, to assess whether the dental enamel in Sapajus follows this same pattern of further specialization. Therefore, this study compares dental enamel complexity in images of dental thin sections from a sample of robust versus gracile capuchins using image compression ratio (ICR) analysis. ICR is a variable that correlates with enamel complexity, such that higher ICR values are indicative of increased complexity in the form of enamel decussation. We found no significant difference between robust and gracile capuchins when assessing all teeth in our sample together, however, we did find that robust capuchins have significantly higher ICR values than gracile capuchins for canine teeth, specifically. Our results support prior studies suggesting that robust capuchins are specialized to generate increased masticatory loads with their anterior dentition, specifically, as compared to gracile species.

In mammals with frontal eyes, optic-nerve fibers from nasal retina project to the contralateral hemisphere of the brain, and fibers from temporal retina project ipsilaterally. The division between crossed and uncrossed projections occurs at or near the vertical meridian. If the division was precise, a problem would arise. Small objects near midline, but nearer or farther than current fixation, would produce signals that travel to opposite hemispheres, making the binocular disparity of those objects difficult to compute. However, in species that have been studied, the division is not precise. Rather, there are overlapping crossed and uncrossed projections such that some fibers from nasal retina project ipsilaterally as well as contralaterally and some from temporal retina project contralaterally as well as ipsilaterally. This increases the probability that signals from an object near vertical midline travel to the same hemisphere, thereby aiding disparity estimation. We investigated whether there is a deficit in binocular vision near the vertical meridian in humans and found no evidence for one. We also investigated the effectiveness of the observed decussation pattern, quantified from anatomical data in monkeys and humans. We used measurements of naturally occurring disparities in humans to determine disparity distributions across the visual field. We then used those distributions to calculate the probability of natural disparities transmitting to the same hemisphere, thereby aiding disparity computation. We found that the pattern of overlapping projections is quite effective. Thus, crossed and uncrossed projections from the retinas are well designed for aiding disparity estimation and stereopsis.

Decussation of axonal tracts is an important hallmark of vertebrate neuroanatomy resulting in one brain hemisphere controlling the contralateral side of the body and also computing the sensory information originating from that respective side. Here, we show that BMP interferes with optic chiasm formation and RGC pathfinding in zebrafish. Experimental induction of BMP4 at 15 hpf results in a complete ipsilateral projection of RGC axons and failure of commissural connections of the forebrain in part being the result of an interaction with shh signaling, transcriptional regulation of midline guidance cues and an affected optic stalk morphogenesis. Experimental induction of BMP4 at 24 hpf, resulting in only a mild repression of forebrain shh ligand expression but in a broad expression of pax2a in the diencephalon, does not per se prevent RGC axons from crossing the midline. It nevertheless shows severe pathologies of RGC projections e.g. the fasciculation of RGC axons with the ipsilateral optic tract resulting in the innervation of one tectum by two eyes or the projection of RGC axons in the direction of the contralateral eye.

Spinal circuitry that enables wing flapping in birds evolved via genetic alternations of a guidance molecule., Flight in birds evolved through patterning of the wings from forelimbs and transition from alternating gait to synchronous flapping. In mammals, the spinal midline guidance molecule ephrin-B3 instructs the wiring that enables limb alternation, and its deletion leads to synchronous hopping gait. Here, we show that the ephrin-B3 protein in birds lacks several motifs present in other vertebrates, diminishing its affinity for the EphA4 receptor. The avian ephrin-B3 gene lacks an enhancer that drives midline expression and is missing in galliforms. The morphology and wiring at brachial levels of the chicken embryonic spinal cord resemble those of ephrin-B3 null mice. Dorsal midline decussation, evident in the mutant mouse, is apparent at the chick brachial level and is prevented by expression of exogenous ephrin-B3 at the roof plate. Our findings support a role for loss of ephrin-B3 function in shaping the avian brachial spinal cord circuitry and facilitating synchronous wing flapping.

The development of spinal axons is crucial to form, in the central nervous system, the functional circuits that are involved in the generation of locomotor patterns. The spinal axonal tracts transport the information from the motor cortex to an intricate circuit in spinal cord that allows to perform different types of motor tasks in mammals. For example, setting of fine and gross locomotor control, coordination and directing the voluntary movements and spinal reflexes are tasks dominated by spinal axon circuits. In humans, the corticospinal tract (CST), a type of axonal tract that constitutes the main motor pathway controlling voluntary movements, originates from pyramidal cells located in the primary motor and sensory cortices. CST axons begin to grow during the embryogenesis. The establishment of circuits in the final destination of the spinal cord, the refinement of CST projections to spinal gray matter, and the elimination of primitive reflexes occur during postnatal development. The three-dimensional navigation of CST axons to their final targets in the spinal cord is carefully directed by specific molecular signaling events. Alterations in CST structure can occur either during embryogenesis, with an abnormal decussation of CST or during adulthood, by mechanical trauma in the spinal cord. In both cases, the disruption of CST structure generates a pathological stage, with impairments in the locomotor activities.

A 39-year-old woman presented with acute visual loss in her right eye. Brain and orbit MRI demonstrated T2 hyperintensity along a long section of her right optic nerve, chiasm, and tract with no evidence of decussation of the inflammation. Subsequent seropositivity for the aquaporin 4 antibody confirmed a diagnosis of neuromyelitis optica. Posterior pathway involvement is typical in neuromyelitis optica and supports the hypothesis that the condition is an astrocytopathy. Furthermore, the absence of decussation in the condition may be a function of astrocyte localization within the chiasm.

Teeth, like other vertebrate skeletal structures, must retain their shapes and resist fracture during load-bearing. Teeth functionally differ from bones in three ways that makes satisfying these requirements more difficult: Teeth function primarily to concentrate stresses and therefore require greater stiffnesss; They routinely make direct contact with opposing teeth and external objects, causing wear; Fractured dental tissue is not reparable. These requirements are generally resolved at the tissue level by increased mineral density, especially in enamel. Hydroxyapatite is anisotropic in its rate of wear, with the slowest rates occurring in the direction parallel to the c-axis of the crystallite, which predicts that selection will have favored crystallite alignment parallel to the abrasive vector. This tendency has been found in several mammalian species. In these taxa, prism directions (and therefore average crystallite directions) on opposite sides of the tooth axis are asymmetric with respect to the axis but parallel with the abrasion vector. The very high mineral density of enamel makes it fundamentally very brittle, which has resulted in selection for mechanisms increasing its resistance to fracture. Anisotropic fracture responses of enamel prisms has led to prism decussation as a means of improving fracture resistance in Cenozoic mammals with high occlusal stresses. The greater weakness to fracture in directions parallel to prism long axes allows stresses perpendicular to the nominal tensile stess (which propagates a crack) to open small cracks normal to and ahead of the nominal crack tip, thus enlarging the tip and reducing stress concentration when the tip reaches those areas. Decussation forces cracks to pass through prism diameters, requiring higher tensile stresses and resulting in prism pull-out and deeply pitted surfaces that require additional work to form. Decussation may cause the fracture path to deviate for short distances from the nominal direction, thus also reducing stress magnitude at the crack tip. Evidence that the functional advantages of such mechanisms have effectively influenced the evolution of enamel fracture resistance include: The association of prism decussation with increased body size or with the appearance of specialized high-stress gnawing mechanisms; correlation of the directions of decussation planes with the direction of maximum tensile stress (as predicted by both stress analysis and 238by premortem crack directions); the association of wavy decussation planes in teeth subject to varying stress directions (carnivorous mammals and tusks of large South American astrapotheres); and the correlation of the degree of decussation with intra-tooth variations in relative stress intensity (early horse molars and hyena canines). Zahne mussen wie alle Skelettelemente der Vertebraten auch unter Belastung formbestandig und bruchfest sein. Zahne unterscheiden sich funktionell von Knochen in dreifacher Weise: In Zahnen werden Krafte konzentriert. Deswegen benotigen sie eine grosere Steifigkeit; Zahne haben regelmasigen Kontakt mit antagonistischen Zahnen und Materialien der Umwelt, die Abrieb verursachen; Bruche in Zahnen konnen nicht repariert werden. Diesen Erfordernissen wird im allgemeinen auf der Ebene des Werkstoffes durch einen hoheren Mineralisationsgrad entsprochen, was besonders fur den Schmelz gilt. Hydroxyapatit verhalt sich anisotrop in bezug auf den Abrieb, der parallel zur c-Achse des Kristallits am geringsten ist. Daraus ergibt sich eine bevorzugte Ausrichtung der Kristallite senkrecht zur Abriebrichtung. Eine entsprechende Anordnung wurde bei mehreren Saugetieren nachgewiesen. Bei diesen Formen ist die Prismenrichtung und damit die Ausrichtung der meisten Kristallite auf den gegenuberliegenden Seiten eines Zahnes assymmetrisch im bezug auf die Zahnachse, aber parallel zum ‘Abriebsvektor’. Der hohe Mineralisationsgrad des Schmelzes macht den Zahnschmelz grundsatzlich sprode. Deswegen zielt die Selektion auf verschiedene Mechanismen Bruche zu verhindern. Die richtungsabhangige Gefahrdung des Bruchs von Schmelzprismen hat zu Prismenuberkreuzungen gefuhrt, die die Bruchgefahr bei kanozoischen Saugetieren mit hohen Kaudrucken erheblich herabsetzt. Die grosere Anfalligkeit fur Bruche entlang der Prismenrichtung fuhrt zu Spannungen senkrecht zu der nominalen Zugspannung, die den Ris vorantreibt. Durch diese zusatzlichen Krafte werden kleine Risse senkrecht zur Hauptrisrichtung und vor der Spitze des Hauptrisses geoffnet, die die Spannungskonzentration des Hauptrisses mindern, wenn er auf solche Querrisse stost. Prismenuberkreuzung zwingt Risse auch querliegende Prismen zu uberwinden, was hohere Zugkrafte erfordert, und was zu Oberflachen fuhrt, die wegen der ausgebrochenen Prismen stark gefurcht sind und deswegen zusatzliche Arbeit erfordern. Prismenuberkreuzungen konnen ebenfalls uber kurze Strecken die Richtung des Risses aus der Hauptrichtung ablenken, wodurch die Spannung an der Risspitze reduziert wird. Die funktionalen Vorteile dieser Mechanismen haben die Evolution der Bruchfestigkeit des Schmelzes nachhaltig beeinflust. Dies wird durch folgende Beobachtungen belegt: Das Zusammengehen von Prismenuberkreuzung und zunehmender Korpergrose oder dem Auftreten von spezialisierten Hochdruck-Nageapparaten; Korrelation der Uberkreuzungsebenen mit den Richtungen maximaler Zugspannungen, was von Spannungsanalysen ebenso wie aus der Analyse von pramortalen Bruchen abgelitet wurde; das Vorkommen von gewellten Uberkreuzungsebenen bei wechselnden Spannungsrichtungen (Raubtiere und Stoszahne von Sud-Amerikanischen Astrapotheria); und die Korrelation des Grades der Uberkreuzung mit der 239unterschiedliche konzentrieten Drucken innerhalb eines Zahnes (z.B. Molaren der fruhen Pferde und Eckzahne der Hyane).

A 56-year-old hypertensive man presented with left-sided weakness of 2-hour duration. He made complete recovery from right hemiparesis because of left parietal infarct. Examination showed dysarthria and left hemiparesis (NIHSS 8/42). MRI brain showed both left parietal acute infarct and gliosis from old infarct (Figure 1). He was successfully thrombolyzed with intravenous alteplase. Present stroke was diagnosed as ipsilateral hemiparesis, confirmed by diffusion tensor imaging (Figure 2). Ipsilateral hemiparesis, mostly seen with posterior fossa malformations and remote infarctions,1 results from injury to uncrossed corticospinal tract (CST) in patients of remote brain injury or with no decussation of CST or injury to ipsilateral extrapyramidal motor pathway.2

We describe herein a case with left-side ptosis induced by pure midbrain infarction in a 49-year-old woman. She also presented with diplopia and right-side cerebellar ataxia. MRI demonstrated new ischemic stroke of the left ventral paramedian midbrain. In this case, ischemia of the left oculomotor fascicles caused the left-side ptosis and diplopia, and ischemia of the left decussation of the superior cerebellar peduncle caused the right-side cerebellar ataxia. These symptoms resulted from inner superior medial mesencephalic branch infraction. This case offers an educational example that can be explained by models proposed in the past and requires knowledge of neuroanatomy and cerebrovasculature.

Most large fibers in central nervous system of vertebrates are decussated. The cause or the evolutionary force of this phenomenon is not convincingly described. Here we show a pattern for this nervous system property that simultaneously is found to a more basic rule in evolution.The idea here is the "function to anatomy" theorem. We claim that functions of physical, biochemical, enzymatic, etc., in lower scale of living organisms "emerge" to anatomy in larger scales. And reversely, the anatomy we see in multicelular organisms are/were functions in microscopic scales.We propose descriptions, predictions, and a probable road map to what this can implicate in biological and medical issues.

In humans, execution of unimanual movements requires lateralized activation of the primary motor cortex, which then transmits the motor command to the contralateral hand through the crossed corticospinal tract (CST). Mutations in NTN1 alter motor control lateralization, leading to congenital mirror movements. To address the role of midline Netrin-1 on CST development and subsequent motor control, we analyze the morphological and functional consequences of floor plate Netrin-1 depletion in conditional knockout mice. We show that depletion of floor plate Netrin-1 in the brainstem critically disrupts CST midline crossing, whereas the other commissural systems are preserved. The only associated defect is an abnormal entry of CST axons within the inferior olive. Alteration of CST midline crossing results in functional ipsilateral projections and is associated with abnormal symmetric movements. Our study reveals the role of Netrin-1 in CST development and describes a mouse model recapitulating the characteristics of human congenital mirror movements., This work was funded by grant ANR-18-CE16-0005-02 MOMIC, Fondation Desmarest, and Merz Pharma. We also acknowledge Sarujan Devakumaran (IBPS, France) for his contribution to experiments, France Lam for confocal imaging (IBPS, France), Morgane Belle (Institut de la Vision, France) for light-sheet imaging, Alexis Bemelmans (CEA, France) for producing the AAV9-CAG-tdTomato virus, Isabelle Caillé for reviewing the manuscript, and PHENO-ICMice (ICM, France) Core supported by 2 “Investissements d’avenir” (ANR-10- IAIHU-06 and ANR-11-INBS-0011-NeurATRIS), and the “Fondation pour la Recherche Médicale.”

Background Anomalies of pyramidal tract decussation are rare phenomena that can be caused by ectodermal dysplasia. Herein, we describe a patient with ichthyosis who exhibited ipsilateral hemiparesis after stroke and whose neuroimaging results showed evidence of motor control being provided by the ipsilateral motor cortex. Case presentation A 24-year-old right-handed man presented with skin abnormalities, sudden-onset left hemiparesis, and dysarthria. He exhibited a mild-to-moderate left-sided weakness (grade 4 on the Medical Research Council scale). Magnetic resonance imaging revealed an acute infarct in the left corona radiata. Diffusion tensor imaging revealed uncrossed corticospinal tracts. Next-generation sequencing identified heterozygous FLG mutations. The patient was diagnosed with cerebral infarction and ichthyosis vulgaris and was treated with aspirin (100 mg/d). His symptoms gradually dissipated. Conclusions This case suggests that pyramidal decussation anomalies can be associated with ichthyosis. Patients with ichthyosis should therefore be evaluated for nerve involvement.

Rodent enamel microstructure has been extensively investigated, primarily on the basis of 2D electronic microscopy data. The nature and dynamics of the ameloblasts (the enamel-secreting cells) have also been well studied. However, critical issues still remain surrounding exactly how the ameloblasts produce the astonishing microstructural complexity of enamel, and how this subtle architecture evolved through time. In this article, we used a new methodology based on confocal laser microscopy to reconstruct the enamel microstructure of rodent incisors in three dimensions (3D) with the ameloblasts in situ. We proposed interpretations regarding the possible relationships between the workings of the ameloblasts and the resulting enamel prisms, especially how the phenomenon of decussation is generated. Finally, we were able to represent the two main types of modern rodent incisor microstructures (uniserial and multiserial decussations), as a set of parameters that have been entered into the 3D enamel simulation software Simulenam to generate 3D models that can be digitally manipulated. Associating 2D data of incisor enamel microstructure of fossil rodents and Simulenam, it was then possible to better understand how the various decussation parameters evolved through time and gave rise to the two modern microstructure types from the same ancestral type (pauciserial). This study also confirmed that rodent and artiodactyl enamel do not share the same mechanism of decussation formation. Anat Rec, 302:1195-1209, 2019. © 2018 Wiley Periodicals, Inc.

Considerable descriptive information about the overall organization of mouse mandibular incisor enamel is available but almost nothing is known about the quantitative characteristics of enamel rod arrangement and distribution in these teeth. This has important implications concerning cell movement during the secretory stage because each ameloblast makes one enamel rod. Knowing how many enamel rods are cut open in a cross‐section of the enamel layer could provide insights into understanding the dynamics of how groups of ameloblasts form the enamel layer. In this study, cross‐sections of fully mineralized enamel were cut on 24 mandibular mouse incisors, polished and etched, and imaged by scanning electron microscopy in backscatter mode. Montaged maps of the entire enamel layer were made at high magnification and the enamel rod profiles in each map were color‐coded based upon rod category. Quantitative analyses of each color layer in the maps were then performed using standard routines available in imagej. The data indicated that that there were on average 7233 ± 575 enamel rod profiles per cross‐section in mandibular incisors of 7‐week‐old mice, with 70% located in the inner enamel layer, 27% located in the outer enamel layer, and 3% positioned near the mesial and lateral cementoenamel junctions. All enamel rod profiles showed progressive increases in tilt angles, some very large in magnitude, from the lateral to mesial sides of the enamel layer, whereas only minor variations in tilt angle were found relative to enamel thickness at given locations across the enamel layer. The decussation angle between alternating rows of rod profiles within the inner enamel layer was fairly constant from the lateral to central labial sides of the enamel layer, but it increased dramatically in the mesial region of the enamel layer. The packing density of all rod profiles decreased from lateral to central labial regions of the enamel layer and then in progressing mesially, decreased slightly (inner enamel, mesial tilt), increased slightly (outer enamel layer) or almost doubled in magnitude (inner enamel, lateral tilt). It was concluded that these variations in rod tilt angle and packing densities are adaptations that allow the tooth to maintain a sharp incisal edge and shovel‐shape as renewing segments formed by around 7200 ameloblasts are brought onto the occluding surface of the tooth by continuous renewal.

BACKGROUND: Pigs and minipigs are increasingly used as non-primate large animal models for preclinical research on nervous system disorders resulting in motor dysfunction. Knowledge of the minipig pyramidal tract is therefore essential to support such models.AIM AND METHODS: This study used 5 female Göttingen minipigs aging 11-15 months. The Göttingen minipig corticospinal tract was investigated, in the same animals, with in vivo neuronal tracing and with postmortem diffusion weighted MRI tractography to provide a thorough insight in the encephalic distribution of this primary motor pathway and its decussation at the craniocervical junction.RESULTS: The two methods similarly outlined the course of the pyramidal tract from its origin in the motor cortex down through the internal capsule to the craniocervical junction, where both methods displayed an axonal crossover at the pyramid decussation. The degree of crossover was quantified with unbiased stereology, where 81-93% of the traced corticospinal fibers crossed to the contralateral spinal cord. Accordingly, in the upper cervical spinal cord the corticospinal tract is primarily distributed in the contralateral lateral funiculus and in close relation to the gray matter, wherein some direct terminations on large ventral column gray matter neurons could be identified.DISCUSSION: The combination of neuronal tracing and tractography exploited the strengths of the respective methods to gain a better understanding of the encephalic distribution and craniocervical decussation of the Göttingen minipig corticospinal tract. Moreover, a quantification of the crossing fibers was obtained from the tracing data, which was not possible with tractography. Our data indicate that the porcine corticospinal system is quite lateralized down to the investigated upper cervical levels. However, further elucidation of this point will require a full examination of the corticospinal tracing pattern into the caudal spinal cord combined with an analysis of the direct versus indirect termination pattern on the lower motor neurons.

DLX3 is essential for tooth enamel development and is so far the only transcription factor known to be mutated in a syndromic form of amelogenesis imperfecta. Through conditional deletion of Dlx3 in the dental epithelium in mouse, we have previously established the involvement of DLX3 in enamel pH regulation, as well as in controlling the expression of sets of keratins that contribute to enamel rod sheath formation. Here, we show that the decussation pattern of enamel rods was lost in conditional knockout animals, suggesting that DLX3 controls the coordinated migration of ameloblasts during enamel secretion. We further demonstrate that DLX3 regulates the expression of some components of myosin II complexes potentially involved in driving the movement of ameloblasts that leads to enamel rod decussation.

Here, we describe the superficial appearance of the brain of the rarely studied tree pangolin. Phylogenetic analyses have placed the pangolins, order Pholidota, as a sister group to the order Carnivora. The majority of features visible on the surface of the tree pangolin brain, and its overall appearance can be described as typically mammalian. The pattern of sulci and gyri, while simple, appears very similar to that observed in carnivores. Two derived features of the Pholidota were observed, the first being the rostral decussation of the pyramidal tract, which instead of occurring at the spinomedullary junction, decussates at the level of the caudal pole of the facial nerve nucleus in the rostral medulla oblongata. This appears to be related to the need for voluntary control of the tongue, with a potentially enlarged corticobulbar tract ending in the hypoglossal nucleus. The second derived feature is the very short spinal cord, which terminates midway along the thoracic vertebrae before giving rise to a long and extensive cauda equina. This foreshortened spinal cord appears to be related to anisotropic growth of the somatic and neural elements following early development of the central nervous system. The olfactory system appears to be generally enlarged and is likely the predominant sense used in foraging. Vision and hearing do not appear specialized based on the relative size of the superior and inferior colliculi, but potential somatic specializations indicate that the somatosensory system is heavily relied upon for food consumption and prehensile tail usage.

Background and importance Lesions in the corticospinal tract above the decussation at the medullary pyramids almost universally produce contralateral deficits. Rare cases of supratentorial lesions causing ipsilateral motor deficits have been reported previously, but only ever found secondary to stroke or congenital pyramidal tract malformations. Clinical presentation Herein, we report a case of ipsilateral corticospinal tract innervation discovered incidentally with intraoperative monitoring during a microsurgical resection of a vestibular schwannoma. Intraoperative monitoring with electrical transcranial stimulation of the frontal scalp triggered motor-evoked potentials in the ipsilateral arms. The uncrossed pathways were later confirmed with MRI tractography using diffusion tensor imaging. Conclusion To the best of our knowledge, this is the first case of isolated ipsilateral motor innervation of the corticospinal tract discovered incidentally during a neurosurgical procedure. Given the increasing use of intraoperative monitoring, this case underscores the importance of cautious interpretation of seemingly discordant neurophysiological findings. Once technical issues have been ruled out, ipsilateral motor innervation may be considered as a possible explanation and neurosurgeons should be aware of the existence of this rare anatomic variant.

The ventral midline of the embryonic neural tube, the floor plate, has a profound role in guiding axons during embryonic development. Floor plate-derived guidance cues attract or repel axons, depending on the neuronal subtype and developmental stage. Netrin-1 and its receptor, Deleted in Colon Carcinoma (DCC), are the key constituents of commissurral axons guidance cues toward the floor plate. Recent studies have implicated Down Syndrome Cell Adhesion Molecule (Dscam) as an additional Netrin-1 receptor. In this study, we examined the role of Dscam in guiding defined spinal dorsal interneuron populations. In vivo knockdown and ectopic expression of Dscam were performed in the dorsal dI1, dI2 and dI3 interneurons of chick embryos, by separately increasing or decreasing Dscam expression in each of these three specific interneuronal populations. Neuron-specific gain and loss of function of Dscam had no effect on the axonal trajectories of dI1-3 neurons. The commissural neurons, dI1c and dI2, crossed the midline, and the ipsilaterally projecting neurons, dI1i and dI3, projected ipsilaterally. However, the fasciculation of dI1 axons was diminished when Dscam expression was attenuated. Dscam is not required for either attraction to or repulsion from the floor plate. In contrast, Dscam is required for the fasciculation of axons, probably via homophilic interaction.

Objectives Our aims were: (1) to perform an echoanatomic correlation study, in order to confirm that the structure identified as the optic chiasm (OC) on ultrasound (US) is indeed this anatomical structure; (2) to assess and compare the reproducibility of two- (2D) and three-(3D) dimensional US in measurement of the OC in normal fetuses; and (3) to assess whether the spatial orientation of the OC changes with increasing gestational age. Methods For the echoanatomic study, the OC was studied in a neonatal specimen, deceased at 29 + 4 weeks, by passing a suture around the OC and visualizing the supposed OC structure on US while pulling gently on the suture. The reproducibility study included 39 women with normal pregnancy at 20–33 weeks undergoing routine obstetric US examination. After the routine exam, the OC was visualized on 2D-US, and a 2D image and 3D volume dataset were stored for offline measurement. On the 2D images, the diameters of the OC decussation and the optic tract proximal to the transducer were measured. For the 3D volume dataset, multiplanar image correlation with volume contrast imaging (VCI) was used to measure both these diameters and the chiasmocallosal angle (CCA). Two operators each took two sets of measurements of the diameters on 2D- and 3D-US, and intra- and interoperator variability were analyzed using Cronbach's alpha intraclass correlation coefficient (ICC), while a single operator took two sets of CCA measurements for assessment of intraoperator variability. Differences in CCA with increasing gestational age were also analyzed by regression, and CCA measurements were divided into three groups according to gestational age and their means compared by one-way ANOVA. Results During the echoanatomic experiment, when the sling suture was pulled, the hyperechoic X-shaped structure just below the circle of Willis identified on 2D-US as the OC was displaced slightly and was eventually cut by the sling, confirming its identity as the OC. Intraoperator variability was low and almost identical for the two operators and the two imaging modalities for measurement of the decussation (ICC for 2D-US: 0.96 vs 0.95; 3D-US: 0.95 vs 0.96), but less so for the optic tract (ICC for 2D-US: 0.95 vs 0.91; 3D-US: 0.94 vs 0.83). Interoperator variability was low for the decussation (2D-US: 0.92; 3D-US: 0.92), but higher for the optic tracts (ICC for 2D-US: 0.80; 3D-US: 0.78). The difference between the mean measurement of the two operators was not statistically significantly different for the decussation, but it was for the optic tracts (P = 0.04). The CCA increased steadily between 20 and 30 gestational weeks and plateaued thereafter, at least until 33 weeks. Conclusions The hyperechoic structure evident on 2D- and 3D-US, just below the circle of Willis, is indeed the OC. 2D-US is apparently as good as 3D-US for visualization of the OC. However, only measurement of the decussation showed low intra- and interoperator variability, whereas measurement of the optic tract is of questionable variability. As gestation advances between 20 and 30 weeks, the OC becomes more oblique in orientation. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd.

El síndrome de Opalski es la presencia de hemiplejia ipsilateral asociada con síntomas del síndrome medular lateral. Diferentes estudios confirman que el infarto en tales casos ocurre a un nivel más bajo que el encontrado en el síndrome de Wallenberg. Se localiza debido a la presentación característica, el territorio único de suministro de sangre y el área pequeña de lesión. La implicación del tracto corticoespinal ipsilateral después de la decusación piramidal, o compresión de la decusación, podrían causarlo. Presentamos el caso de una mujer de 71 años que desarrolló hemiplejia izquierda y dificultad repentina para caminar, acompañada de diplopía, cefalea, disfagia y vómito recurrente. La imagen de resonancia magnética demostró un infarto lateral medular izquierdo. La angiografía reveló estenosis distal y proximal de la arteria vertebral izquierda e hipoplasia de la arteria vertebral derecha.

Francois Pourfour du Petit was a Parisian experimental neuro-anatomist, and ophthalmologist, who investigated his extensive wartime experiences of brain and spinal injuries and verified his conclusions by animal experiments. His results showed with great originality that brain injuries caused weakness or paralysis of the opposite limbs. He also clarified the anatomy of the spinal cord and decussation of the pyramidal tracts, and demonstrated the anatomy and clinical significance of the cervical sympathetic chain.

Purpose: To investigate the optimal procedures for multichannel visually evoked potentials (VEPs) to detect misrouting in albinism subjects. Methods: Investigations were done in a phenotypically heterogeneous group of 180 albinism subjects and 187 controls with and without ocular pathology. We retrospectively compared standard flash VEP (fVEP), high-frequency fVEP with a handheld device (hh fVEP), pattern-onset VEP (poVEP), and short-onset acuity sweep VEP. The diagnostic power of these stimuli were estimated by calculating the area under the curve (AUC). Subjects were divided in three age groups (0.3 logMAR). Results: The optimal stimulus was hh fVEP, standard fVEP, and poVEP 60' for subjects 0.3 logMAR. Remarkably, some albinism subjects showed misrouting on full-field stimulation but normal routing of the central retina, suggesting that not the whole line of decussation is shifted temporally.

Objective Overview the development of human tooth; enamel, dentoenamel junction and dentin in regard to hierarchical structure property relationships and how these component structures can serve as templates for the design of tough materials. Methods The dental, engineering and ceramic literature (PubMed, Science Direct, Google Scholar) covering the last 20 years was over viewed regarding enamel and dentin characterization, structure-property studies, as well as, publications related to bioinspired materials with relationship to tooth structure. Relevant publications were selected for inclusion. Results Enamel has been studied and modelled at 3 hierarchical levels, prism structure, parallel prism interactions and enamel decussation effects. Missing is a 4th level where the previous three hierarchies are combined with the 3D arrangement of these levels in enamel areas. Aspects of the enamel prism infrastructure and prism decussation have been used in 3D printing of Bouligand ceramic structures. The dento-enamel junction serves to arrest cracks and reduce the stress in enamel as a graded elastic modulus layer, leading to development of dental ceramics with increased strength and fatigue resistance. Dentin is a compliant structure that supports enamel mechanically and may, through providing interstitial fluid at the DEJ, allow repair of microcracks in enamel. Adequate models of dentin properties remain to be developed as it remains highly variable in tubule lumen size and the degree of mineral density around and between tubules. Significance The structure of teeth, particularly the 4 hierarchical levels of enamel, creates a vital, hard, tough damage tolerant system for inspiring new materials.

During neuronal development, the formation of neural circuits requires developing axons to traverse a diverse cellular and molecular environment to establish synaptic contacts with the appropriate postsynaptic partners. Essential to this process is the ability of developing axons to navigate guidance molecules presented by specialized populations of cells. These cells partition the distance traveled by growing axons into shorter intervals by serving as intermediate targets, orchestrating the arrival and departure of axons by providing attractive and repulsive guidance cues. The floor plate in the central nervous system (CNS) is a critical intermediate target during neuronal development, required for the extension of commissural axons across the ventral midline. In this review, we begin by giving a historical overview of the ventral commissure and the evolutionary purpose of decussation. We then review the axon guidance studies that have revealed a diverse assortment of midline guidance cues, as well as genetic and molecular regulatory mechanisms required for coordinating the commissural axon response to these cues. Finally, we examine the contribution of dysfunctional axon guidance to neurological diseases.