The alx3 gene shapes the zebrafish neurocranium by regulating frontonasal neural crest cell differentiation timing.

During craniofacial development, different populations of cartilage- and bone-forming cells develop in precise locations in the head. Most of these cells are derived from pluripotent cranial neural crest cells and differentiate with distinct developmental timing and cellular morphologies. The mechanisms that divide neural crest cells into discrete populations are not fully understood. Here, we use single-cell RNA sequencing to transcriptomically define different populations of cranial neural crest cells. We discovered that the gene family encoding the Alx transcription factors is enriched in the frontonasal population of neural crest cells. Genetic mutant analyses indicate that alx3 functions to regulate the distinct differentiation timing and cellular morphologies among frontonasal neural crest cell subpopulations. This study furthers our understanding of how genes controlling developmental timing shape craniofacial skeletal elements.

A detailed redescription of the mesoderm/neural crest cell boundary in the murine orbitotemporal region integrates the mammalian cranium into a pan-amniote cranial configuration.

The morphology of the mammalian chondrocranium appears to differ significantly from those of other amniotes, since the former possesses uniquely developed brain and cranial sensory organs. In particular, a question has long remained unanswered as to the developmental and evolutionary origins of a cartilaginous nodule called the ala hypochiasmatica. In this study, we investigated the embryonic origin of skeletal elements in the murine orbitotemporal region by combining genetic cell lineage analysis with detailed morphological observation. Our results showed that the mesodermal embryonic environment including the ala hypochiasmatica, which appeared as an isolated mesodermal distribution in the neural crest-derived prechordal region, is formed as a part of the mesoderm that continued from the chordal region during early chondrocranial development. The mesoderm/neural crest cell boundary in the head mesenchyme is modified through development, resulting in the secondary mesodermal expansion to invade into the prechordal region. We thus revealed that the ala hypochiasmatica develops as the frontier of the mesodermal sheet stretched along the cephalic flexure. These results suggest that the mammalian ala hypochiasmatica has evolved from a part of the mesodermal primary cranial wall in ancestral amniotes. In addition, the endoskeletal elements in the orbitotemporal region, such as the orbital cartilage, suprapterygoid articulation of the palatoquadrate, and trabecula, some of which were once believed to represent primitive traits of amniotes and to be lost in the mammalian lineage, have been confirmed to exist in the mammalian cranium. Consequently, the mammalian chondrocranium can now be explained in relation to the pan-amniote cranial configuration.

Cranial cartilages: Players in the evolution of the cranium during evolution of the chordates in general and of the vertebrates in particular.

The present contribution is chiefly a review, augmented by some new results on amphioxus and lamprey anatomy, that draws on paleontological and developmental data to suggest a scenario for cranial cartilage evolution in the phylum chordata. Consideration is given to the cartilage-related tissues of invertebrate chordates (amphioxus and some fossil groups like vetulicolians) as well as in the two major divisions of the subphylum Vertebrata (namely, agnathans, and gnathostomes). In the invertebrate chordates, which can be considered plausible proxy ancestors of the vertebrates, only a viscerocranium is present, whereas a neurocranium is absent. For this situation, we examine how cartilage-related tissues of this head region prefigure the cellular cartilage types in the vertebrates. We then focus on the vertebrate neurocranium, where cyclostomes evidently lack neural-crest derived trabecular cartilage (although this point needs to be established more firmly). In the more complex gnathostome, several neural-crest derived cartilage types are present: namely, the trabecular cartilages of the prechordal region and the parachordal cartilage the chordal region. In sum, we present an evolutionary framework for cranial cartilage evolution in chordates and suggest aspects of the subject that should profit from additional study.

Embryonic pattern of cartilaginous head development in the European toad, Bufo bufo.

The craniofacial skeleton of vertebrates is a major innovation of the whole clade. Its development and composition requires a precisely orchestrated sequence of chondrification events which lead to a fully functional skeleton. Sequential information on the precise timing and sequence of embryonic cartilaginous head development are available for a growing number of vertebrates. This enables a more and more comprehensive comparison of the evolutionary trends within and among different vertebrate clades. This comparison of sequential patterns of cartilage formation enables insights into the evolution of development of the cartilaginous head skeleton. The cartilaginous sequence of head formation of three basal anurans (Xenopus laevis, Bombina orientalis, Discoglossus scovazzi) was investigated so far. This study investigates the sequence and timing of larval cartilaginous development of the head skeleton from the appearance of mesenchymal Anlagen until the premetamorphic larvae in the neobatrachian species Bufo bufo. Clearing and staining, histology, and 3D reconstruction enabled the tracking of 75 cartilaginous structures and the illustration of the sequential changes of the skull as well as the identification of evolutionary trends of sequential cartilage formation in the anuran head. The anuran viscerocranium does not chondrify in the ancestral anterior to posterior direction and the neurocranial elements do not chondrify in posterior to anterior direction. Instead, the viscerocranial and neurocranial development is mosaic-like and differs greatly from the gnathostome sequence. Strict ancestral anterior to posterior developmental sequences can be observed within the branchial basket. Thus, this data is the basis for further comparative developmental studies of anuran skeletal development.

The genetic basis of neurocranial size and shape across varied lab mouse populations.

Brain and skull tissues interact through molecular signalling and mechanical forces during head development, leading to a strong correlation between the neurocranium and the external brain surface. Therefore, when brain tissue is unavailable, neurocranial endocasts are often used to approximate brain size and shape. Evolutionary changes in brain morphology may have resulted in secondary changes to neurocranial morphology, but the developmental and genetic processes underlying this relationship are not well understood. Using automated phenotyping methods, we quantified the genetic basis of endocast variation across large genetically varied populations of laboratory mice in two ways: (1) to determine the contributions of various genetic factors to neurocranial form and (2) to help clarify whether a neurocranial variation is based on genetic variation that primarily impacts bone development or on genetic variation that primarily impacts brain development, leading to secondary changes in bone morphology. Our results indicate that endocast size is highly heritable and is primarily determined by additive genetic factors. In addition, a non-additive inbreeding effect led to founder strains with lower neurocranial size, but relatively large brains compared to skull size; suggesting stronger canalization of brain size and/or a general allometric effect. Within an outbred sample of mice, we identified a locus on mouse chromosome 1 that is significantly associated with variation in several positively correlated endocast size measures. Because the protein-coding genes at this locus have been previously associated with brain development and not with bone development, we propose that genetic variation at this locus leads primarily to variation in brain volume that secondarily leads to changes in neurocranial globularity. We identify a strain-specific missense mutation within Akt3 that is a strong causal candidate for this genetic effect. Whilst it is not appropriate to generalize our hypothesis for this single locus to all other loci that also contribute to the complex trait of neurocranial skull morphology, our results further reveal the genetic basis of neurocranial variation and highlight the importance of the mechanical influence of brain growth in determining skull morphology.

Sequence of chondrocranial development in basal anurans-Let's make a cranium.

BACKGROUND: The craniofacial skeleton is an evolutionary innovation of vertebrates. Due to its complexity and importance to protect the brain and aid in essential functions (e.g., feeding), its development requires a precisely tuned sequence of chondrification and/or ossification events. The comparison of sequential patterns of cartilage formation bears important insights into the evolution of development. Discoglossus scovazzi is a basal anuran species. The comparison of its chondrocranium (cartilaginous neuro- & viscerocranium) development with other basal anurans (Xenopus laevis, Bombina orientalis) will help establishing the ancestral pattern of chondrification sequences in anurans and will serve as basis for further studies to reconstruct ancestral conditions in amphibians, tetrapods, and vertebrates. Furthermore, evolutionary patterns in anurans can be studied in the light of adaptations once the ancestral sequence is established. RESULTS: We present a comprehensive overview on the chondrocranium development of D. scovazzi. With clearing and staining, histology and 3D reconstructions we tracked the chondrification of 44 elements from the first mesenchymal Anlagen to the premetamorphic cartilaginous head skeleton and illustrate the sequential changes of the skull. We identified several anuran and discoglossoid traits of cartilage development. In D. scovazzi the mandibular, hyoid, and first branchial arch Anlagen develop first followed by stepwise addition of the branchial arches II, III, and IV. Nonetheless, there is no strict anterior to posterior chondrification pattern within the viscerocranium of D. scovazzi. Single hyoid arch elements chondrify after elements of the branchial arch and mandibular arch elements chondrify after elements of the branchial arch I. CONCLUSIONS: In Osteichthyes, neurocranial elements develop in anterior to posterior direction. In the anurans investigated so far, as well as in D. scovazzi, the posterior parts of the neurocranium extend anteriorly, while the anterior parts of the neurocranium, extend posteriorly until both parts meet and fuse. Anuran cartilaginous development differs in at least two crucial traits from other gnathostomes which further supports the urgent need for more developmental investigations among this clade to understand the evolution of cartilage development in vertebrates.

Sexual dimorphism in shape and size of the neurocranium.

Sex identification is a primary step in forensic analysis of skeletal remains. The accuracy of sex estimation methods greatly depends on the sexual dimorphism manifested by the target anatomical region. The study aims to evaluate the sexual dimorphism in shape and size of the neurocranium and to compare the potential of shape and size of different cranial regions to classify correctly the male and female crania. The study was carried out on computed tomography images of 373 Bulgarian adults (161 males and 212 females). Three-dimensional coordinates of 32 landmarks were acquired. The landmarks were arranged in 4 configurations: neurocranium, frontal bone, parietotemporal region, and occipital bone. For each configuration, the presence of significant sex differences in shape and size was tested. Principal component analysis (PCA) was applied to explore the shape variation. The classification power of size and shape was tested using discriminant analysis and k-means clustering. The neurocranium shows significant sex differences in shape and size. The parietotemporal region is the most dimorphic neurocranial part in size and the frontal bone is the most differing one in shape. The size of the parietotemporal region and frontal bone classifies correctly more than 80% of the crania. The discrimination ability based on shape is rather low as the highest values of about 70% are obtained for the frontal and occipital bone. The PCA plots show large overlapping of the male and female crania. It can be inferred that the sex-specific size differences in the neurocranium are more important than the shape differences.

Fractures of the neuro-cranium: sensitivity and specificity of post-mortem computed tomography compared with autopsy.

Post-mortem computed tomography (PMCT) is a routine tool in many forensic pathology departments as it is fast and non-destructive and allows less gruesome visualization than photographs, and the images are indefinitely storable. Several studies investigated congruence between PMCT and autopsy for skull fracture but registered only the presence or absence of fracture systems. The objective of this study was to determine location-specific sensitivity and specificity of PMCT for individual fracture lines in blunt force head trauma. Accurate 3D models based on PMCT data with all fracture lines visible are important for future studies on fractures, applying finite element analysis (FEA). We retrospectively sampled adult cases from 2013 to 2019 with skull fracture mentioned in the autopsy report. PMCT was on a Siemens 64-slice scanner and autopsy according to international guidelines. The location and direction of all fracture lines at autopsy and at de novo interpretation of scans were registered and compared. Ninety-nine cases with 4809 individual findings were included. Age ranged from 18 to 100 years. The overall sensitivity was 0.58, and specificity was 0.91. For individual locations, sensitivity ranged from 0.24 to 0.85, and specificity ranged from 0.73 to 1.00. Intra-observer agreement was 0.74, and inter-observer agreement ranged from 0.43 to 0.58. In conclusion, PMCT is suited for detection of fracture systems, but not for detection of all individual fracture lines. Our results differed from the existing literature due to the methodological choices of registering individual fracture lines. Future studies utilising FEA must supplement PMCT with autopsy data.

Evidence for independent brain and neurocranial reorganization during hominin evolution.

Throughout hominin evolution, the brain of our ancestors underwent a 3-fold increase in size and substantial structural reorganization. However, inferring brain reorganization from fossil hominin neurocrania (=braincases) remains a challenge, above all because comparative data relating brain to neurocranial structures in living humans and great apes are still scarce. Here we use MRI and same-subject spatially aligned computed tomography (CT) and MRI data of humans and chimpanzees to quantify the spatial relationships between these structures, both within and across species. Results indicate that evolutionary changes in brain and neurocranial structures are largely independent of each other. The brains of humans compared to chimpanzees exhibit a characteristic posterior shift of the inferior pre- and postcentral gyri, indicative of reorganization of the frontal opercular region. Changes in human neurocranial structure do not reflect cortical reorganization. Rather, they reflect constraints related to increased encephalization and obligate bipedalism, resulting in relative enlargement of the parietal bones and anterior displacement of the cerebellar fossa. This implies that the relative position and size of neurocranial bones, as well as overall endocranial shape (e.g., globularity), should not be used to make inferences about evolutionary changes in the relative size or reorganization of adjacent cortical regions of fossil hominins.

Assessing the status of the KNM-ER 42700 fossil using Homo erectus neurocranial development.

Based on ontogenetic data of endocranial shape, it has been proposed that a younger than previously assumed developmental status of the 1.5-Myr-old KNM-ER 42700 calvaria could explain why the calvaria of this fossil does not conform to the shape of other Homo erectus individuals. Here, we investigate (ecto)neurocranial ontogeny in H. erectus and assess the proposed juvenile status of this fossil using recent Homo sapiens, chimpanzees (Pan troglodytes), and Neanderthals (Homo neanderthalensis) to model and discuss changes in neurocranial shape from the juvenile to adult stages. We show that all four species share common patterns of developmental shape change resulting in a relatively lower cranial vault and expanded supraorbital torus at later developmental stages. This finding suggests that ectoneurocranial data from extant hominids can be used to model the ontogenetic trajectory for H. erectus, for which only one well-preserved very young individual is known. However, our study also reveals differences in the magnitudes and, to a lesser extent, directions of the species-specific trajectories that add to the overall shared pattern of neurocranial shape changes. We demonstrate that the very young H. erectus juvenile from Mojokerto together with subadult and adult H. erectus individuals cannot be accommodated within the pattern of the postnatal neurocranial trajectory for humans. Instead, the chimpanzee pattern might be a better 'fit' for H. erectus despite their more distant phylogenetic relatedness. The data are also compatible with an ontogenetic shape trajectory that is in some regards intermediate between that of recent H. sapiens and chimpanzees, implying a unique trajectory for H. erectus that combines elements of both extant species. Based on this new knowledge, neurocranial shape supports the assessment that KNM-ER 42700 is a young juvenile H. erectus if H. erectus followed an ontogenetic shape trajectory that was more similar to chimpanzees than humans.

Modeling the effect of brain growth on cranial bones using finite-element analysis and geometric morphometrics.

PURPOSE: Brain expansion during ontogeny has been identified as a key factor for explaining the growth pattern of neurocranial bones. However, the dynamics of this relation are only partially understood and a detailed characterization of integrated morphological changes of the brain and the neurocranium along ontogeny is still lacking. The aim of this study was to model the effect of brain growth on cranial bones by means of finite-element analysis (FEA) and geometric morphometric techniques. METHODS: First, we described the postnatal changes in brain size and shape by digitizing coordinates of 3D semilandmarks on cranial endocasts, as a proxy of brain, segmented from CT-scans of an ontogenetic sample. Then, two scenarios of brain growth were simulated: one in which brain volume increases with the same magnitude in all directions, and other that includes the information on the relative expansion of brain regions obtained from morphometric analysis. RESULTS: Results indicate that in the first model, in which a uniform pressure is applied, the largest displacements were localized in the sutures, especially in the anterior and posterior fontanels, as well as the metopic suture. When information of brain relative growth was introduced into the model, displacements were also concentrated in the lambda region although the values along both sides of the neurocranium (parietal and temporal bones) were larger than under the first scenario. CONCLUSION: In sum, we propose a realistic approach to the use of FEA based on morphometric data that offered different results to more simplified models.

The early elasmobranch Phoebodus: phylogenetic relationships, ecomorphology and a new time-scale for shark evolution.

Anatomical knowledge of early chondrichthyans and estimates of their phylogeny are improving, but many taxa are still known only from microremains. The nearly cosmopolitan and regionally abundant Devonian genus Phoebodus has long been known solely from isolated teeth and fin spines. Here, we report the first skeletal remains of Phoebodus from the Famennian (Late Devonian) of the Maïder region of Morocco, revealing an anguilliform body, specialized braincase, hyoid arch, elongate jaws and rostrum, complementing its characteristic dentition and ctenacanth fin spines preceding both dorsal fins. Several of these features corroborate a likely close relationship with the Carboniferous species Thrinacodus gracia, and phylogenetic analysis places both taxa securely as members of the elasmobranch stem lineage. Identified as such, phoebodont teeth provide a plausible marker for range extension of the elasmobranchs into the Middle Devonian, thus providing a new minimum date for the origin of the chondrichthyan crown-group. Among pre-Carboniferous jawed vertebrates, the anguilliform body shape of Phoebodus is unprecedented, and its specialized anatomy is, in several respects, most easily compared with the modern frilled shark Chlamydoselachus. These results add greatly to the morphological, and by implication ecological, disparity of the earliest elasmobranchs.

Development of the chondrocranium of two caiman species, Caiman latirostris and Caiman yacare.

Little is known about the embryonic development and variation of the chondrocranium in Crocodylia and there are no works on any Caiman species. Due to the importance of cranial features in the systematics of this clade, investigating the development of the skull in embryonic stages is essential. In this study, we present for the first time the development of the cartilaginous skull of two extant Caiman species. Anatomical descriptions of the embryonic chondrocranium of Caiman latirostris and Caiman yacare were made, paying special attention to their inter- and intraspecific variation. For this purpose, pre-hatching ontogenetic cranial series of these two caiman species were prepared with a double staining and diaphanization technique. The main differences with other crocodylian species were observed in the palatoquadrate, and interspecific variation within the genus was recorded in the hyobranchial apparatus and larynx. Some characters may be distinctive of Caiman (posterior and ventral surface of the otic process of the palatoquadrate articulated with the dorsal process of the columella auris, and otic process articulated with the lateral wall of the auditory capsule), Alligatoridae (presence of an epiphanial foramen) or C. latirostris and C. yacare (Corpus hyoidei with different number and position of foramina and different shapes of its anterior contour and anterior and posterior notch, different degrees of broadening of the distal end of the Cornu branchiale I, and presence/absence of a notch in the posteroventral surface of the cricoid). Homologies of the elements belonging to the hyobranchial apparatus could not be confirmed. As in other tetrapods the trachea consists of incomplete cartilaginous rings. Morphological changes and dissimilarities found in this study are useful as a context to start studying phylogenetic constraints. Moreover, in a heterochronic context, variations may be involved.

The growth of the neurocranium: literature review and implications in cranial repair.

BACKGROUND: Postnatal growth of neurocranium is prevalently completed in the first years of life, thus deeply affecting the clinical presentation and surgical management of pediatric neurosurgical conditions involving the skull. This paper aims to review the pertinent literature on the normal growth of neurocranium and critically discuss the surgical implications of this factor in cranial repair. METHODS: A search of the electronic database of Pubmed was performed, using the key word "neurocranium growth", thus obtaining 217 results. Forty-six papers dealing with this topic in humans, limited to the English language, were selected. After excluding a few papers dealing with viscerocranium growth or pathological conditions not related to normal neurocranium growth 18 papers were finally included into the present review. RESULTS AND CONCLUSIONS: The skull growth is very rapid in the first 2 years of life and approximates the adult volume by 7 years of age, with minimal further growth later on, which is warranted by the remodeling of the cranial bones. This factor affects the outcome of cranioplasty. Thus, it is essential to consider age in the planning phase of cranial repair, choice of the material, and critical comparison of results of different cranioplasty solutions.

Neuromandibular integration in humans and chimpanzees: Implications for dental and mandibular reduction in Homo.

OBJECTIVES: Although the evolution of the hominin masticatory apparatus has been linked to diet and food processing, the physical connection between neurocranium and lower jaw suggests a role of encephalization in the trend of dental and mandibular reduction. Here, the hypothesis that tooth size and mandibular robusticity are influenced by morphological changes in the neurocranium was tested. MATERIALS AND METHODS: Three-dimensional landmarks, alveolar lengths, and mandibular robusticity data were recorded on a sample of chimpanzee and human skulls. The morphological integration between the neurocranium and the lower jaw was analyzed by means of Singular Warps Analysis. Redundancy Analysis was performed to understand if the pattern of neuromandibular integration affects tooth size and mandibular robusticity. RESULTS: There is significant morphological covariation between neurocranium and lower jaw in both chimpanzees and humans. In humans, changes in the temporal fossa seem to produce alterations of the relative orientation of jaw parts, while the influence of similar neurocranial changes in chimpanzees are more localized. In both species, postcanine alveolar lengths and mandibular robusticity are associated with shape changes of the temporal fossa. CONCLUSIONS: The results of this study support the hypothesis that the neurocranium is able to affect the evolution and development of the lower jaw, although most likely through functional integration of mandible, teeth, and muscles within the masticatory apparatus. This study highlights the relative influence of structural constraints and adaptive factors in the evolution of the human skull.

Reconstruction of composite defects of the scalp and neurocranium-a treatment algorithm from local flaps to combined AV loop free flap reconstruction.

BACKGROUND: Reconstruction of cranial composite defects, including all layers of the scalp and the neurocranium, poses an interdisciplinary challenge. Especially after multiple previous operations and/or radiation therapy, sufficient reconstruction is often only possible using microsurgical free flap transplantation. The aim of this study was to analyze the therapy of interdisciplinary cases with composite defects including the scalp and neurocranium. METHODS: From 2009 to 2017, 23 patients with 18 free flaps and 10 pedicled/local flaps were analyzed. First choices for free flaps were muscle flaps followed by fasciocutaneous flaps. RESULTS: Except for four patients, a stable coverage could be reached in the first operation. Three of these patients received a local scalp rotation flap in the first operation and needed an additional free flap because the local flap was no longer sufficient for coverage after wound healing deficiency or tumor relapse. The superficial temporal artery or external carotid artery served as recipient vessels. In special cases, venous grafts or an arteriovenous loop (AV loop) were used as extensions for the recipient vessels. CONCLUSIONS: In summary, an interdisciplinary approach with radical debridement of infected or necrotic tissue and the reconstruction of the dura mater are essential to reach a stable, long-lasting reconstructive result. Based on our experience, free flaps seem to be the first choice for patients after multiple previous operations and/or radiation therapy.

Benign fibrous histiocytoma arising at the temporal bone of an infant--case report and review of the literature.

PURPOSE: We report a very rare case of a fast-growing benign fibrous histiocytoma at the temporal bone. METHODS: A 1-year-old girl was referred for investigation of a right temporal mass that increased during 2-week observation. Imaging studies showed a lytic, loculated skull tumor at the left temporal bone. On magnetic resonance imaging scans, the tumor was isointense on T1- and relatively high-intense on T2-weighted images. The tumor was mostly homogeneously enhanced by gadolinium. On diffusion-weighted images, it was iso- to relatively low-intense; perfusion-weighted images revealed low perfusion. RESULTS: Complete macroscopic resection of the tumor was performed. Immunohistochemical analysis showed that the tumor was positive for CD68 and α-smooth muscle actin and negative for CD1a and CD34; the MIB-1 labeling index was 4 %. A diagnosis of primary benign fibrous histiocytoma of the skull was made. At 6-month follow-up, there were no clinical or radiological signs of tumor recurrence and/or metastasis. CONCLUSIONS: We review the clinical, radiological, and immunohistochemical characteristics of benign fibrous histiocytoma at the skull.

Ocular and craniofacial phenotypes in a large Brazilian family with congenital aniridia.

Congenital aniridia is a rare genetic disorder characterized by varying degrees of iris hypoplasia that are associated with additional ocular abnormalities. More than 90% of the causal mutations identified are found in the PAX6 gene, a transcription factor of critical importance in the process of neurogenesis and ocular development. Here, we investigate clinical, molecular, and craniofacial features of a large Brazilian family with congenital aniridia. Among the 56 eyes evaluated, phenotype variation encompassed bilateral total aniridia to mild iris defects with extensive variation between eyes of the same individual. PAX6 molecular screening indicated a heterozygous splice mutation (c.141 + 1G>A). Thus, we hypothesize that this splicing event may cause variation in the expression of the wild-type transcript, which may lead to the observed variation in phenotype. Affected individuals were more brachycephalic, even though their face height and cephalic circumference were not significantly different when compared to those of non-affected relatives. From this, we infer that the head shape of affected subjects may also be a result of the PAX6 splice-site mutation. Our data summarize the clinical variability associated with the ocular phenotype in a large family with aniridia, and help shed light on the role of PAX6 in neurocranial development.

Genetically induced abnormal cranial development in human trisomy 18 with holoprosencephaly: comparisons with the normal tempo of osteogenic-neural development.

Craniofacial malformations are common congenital defects caused by failed midline inductive signals. These midline defects are associated with exposure of the fetus to exogenous teratogens and with inborn genetic errors such as those found in Down, Patau, Edwards' and Smith-Lemli-Opitz syndromes. Yet, there are no studies that analyze contributions of synchronous neurocranial and neural development in these disorders. Here we present the first in-depth analysis of malformations of the basicranium of a holoprosencephalic (HPE) trisomy 18 (T18; Edwards' syndrome) fetus with synophthalmic cyclopia and alobar HPE. With a combination of traditional gross dissection and state-of-the-art computed tomography, we demonstrate the deleterious effects of T18 caused by a translocation at 18p11.31. Bony features included a single developmentally unseparated frontal bone, and complete dual absence of the anterior cranial fossa and ethmoid bone. From a superior view with the calvarium plates removed, there was direct visual access to the orbital foramen and hard palate. Both the eyes and the pituitary gland, normally protected by bony structures, were exposed in the cranial cavity and in direct contact with the brain. The middle cranial fossa was shifted anteriorly, and foramina were either missing or displaced to an abnormal location due to the absence or misplacement of its respective cranial nerve (CN). When CN development was conserved in its induction and placement, the respective foramen developed in its normal location albeit with abnormal gross anatomical features, as seen in the facial nerve (CNVII) and the internal acoustic meatus. More anteriorly localized CNs and their foramina were absent or heavily disrupted compared with posterior ones. The severe malformations exhibited in the cranial fossae, orbital region, pituitary gland and sella turcica highlight the crucial involvement of transcription factors such as TGIF, which is located on chromosome 18 and contributes to neural patterning, in the proper development of neural and cranial structures. Our study of a T18 specimen emphasizes the intricate interplay between bone and brain development in midline craniofacial abnormalities in general.

The human brain and face: mechanisms of cranial, neurological and facial development revealed through malformations of holoprosencephaly, cyclopia and aberrations in chromosome 18.

The study of inborn genetic errors can lend insight into mechanisms of normal human development and congenital malformations. Here, we present the first detailed comparison of cranial and neuro pathology in two exceedingly rare human individuals with cyclopia and alobar holoprosencephaly (HPE) in the presence and absence of aberrant chromosome 18 (aCh18). The aCh18 fetus contained one normal Ch18 and one with a pseudo-isodicentric duplication of chromosome 18q and partial deletion of 18p from 18p11.31 where the HPE gene, TGIF, resides, to the p terminus. In addition to synophthalmia, the aCh18 cyclopic malformations included a failure of induction of most of the telencephalon - closely approximating anencephaly, unchecked development of brain stem structures, near absence of the sphenoid bone and a malformed neurocranium and viscerocranium that constitute the median face. Although there was complete erasure of the olfactory and superior nasal structures, rudiments of nasal structures derived from the maxillary bone were evident, but with absent pharyngeal structures. The second non-aCh18 cyclopic fetus was initially classified as a true Cyclops, as it appeared to have a proboscis and one median eye with a single iris, but further analysis revealed two eye globes as expected for synophthalmic cyclopia. Furthermore, the proboscis was associated with the medial ethmoid ridge, consistent with an incomplete induction of these nasal structures, even as the nasal septum and paranasal sinuses were apparently developed. An important conclusion of this study is that it is the brain that predicts the overall configuration of the face, due to its influence on the development of surrounding skeletal structures. The present data using a combination of macroscopic, computed tomography (CT) and magnetic resonance imaging (MRI) techniques provide an unparalleled analysis on the extent of the effects of median defects, and insight into normal development and patterning of the brain, face and their skeletal support.