Fiorenza L., Bruner E. 2017. Cranial shape variation in adult howler monkeys (Alouatta seniculus). Am. J. Primatol. [link]
Category Archives: Skull
Recently, Evin et al. 2016 have published a study comparing the accuracy of the three-dimensional reconstruction of five wolf crania using both photogrammetry and high-resolution surface scanner. For the photogrammetric images acquisition, they used an 8-megapixel (DSLR) Canon EOS 30D camera, mounted with a Canon EF 24–105mmf/4 L IS USMlens. The scanner-based 3D models were created using a Breuckmann StereoScan structured light scanner (http://www.breuckmann.com). The resulting 3D models were compared first through visual observation, and second with the computation of a mesh-to-mesh deviation map. The pairs of models were spatially aligned (using a least-square optimisation best-fit criterion), followed by a 3D landmark-based geometric morphometric approach using corresponding analyses. The results show that photogrammetric 3D models are as accurate in terms of coloration, texture, and geometry, as the highest-end surface scanners. Minimal differences between photogrammetric 3D models and surface scanner-based models have been only identified on intricate topological regions, such the tooth row. Photogrammetry is becoming a common tool in archaeological and anthropological research. The major advantage of this technique is the speed and ease of image acquisition and reconstruction. Photogrammetry is an equally good alternative and less expensive than other more common techniques, such as structured light or surface scanners. In terms of archaeological samples conservation, photogrammetry could be in the future an excellent alternative to provide accurate replica models that can be widely accessible for research, without affecting the original collections.
Gizéh Rangel de Lázaro
In a recent paper, Fabbri et al analyzed the relationship between brain and cranial vault shape in the transition from reptiles to birds. To assess the evolution of this relationship they used a broad sample including Aves, Lepidosauria, Crocodylia, Archosauria, and Reptilia. To assess developmental differences they included an ontogenetic sample of Alligator mississipiensis and Gallus gallus. The results showed that the relationship between the vault bones and the brain is conserved across these taxa, with the frontal bone positioned over the forebrain and the parietal bone over the midbrain or over midbrain and posterior forebrain. Nonetheless, they observed some shape variations, namely on the relative sizes of the frontal and parietal bones and in the position of the fronto-parietal suture relative to the forebrain-midbrain boundary. These two structures are significantly correlated, with the fronto-parietal suture being either anterior to (e.g. stem reptiles) or nearly aligned with (e.g. crown birds) the forebrain-midbrain boundary. In terms of ontogeny, chickens have a shorter ontogenetic trajectory than alligators, as the brain and skull of embryos are similar to the adult ones. The brain and skull of alligators develop with negative allometry, with the brain relatively large in early stages but becoming relatively small during growth. Conversely, the skull and brain of chicken grow with positive allometry, and the authors suggest the brain should be considered peramorphic in Aves. Overall the results stress the important role of the brain in shaping the cranial vault. The authors wonder whether the intimate relationship between brain and frontal and parietal bones is the key for the conservation of the cranial vault across vertebrates.
We have just published a new study on the spatial relationship between visual and endocranial structures in adult modern humans, chimpanzees, and fossil humans. The survey was conducted in collaboration with Michael Masters from Montana Tech (USA), who previously hypothesized that, in modern humans, the positioning of the orbits below the frontal lobes coupled with a reduced face could result in spatial conflict among ocular, cerebral, and craniofacial structures. This could lead to vision problems, such as myopia. In addition, another study evidenced that eye and orbit dimensions have a stronger correlation with the frontal lobes, rather than with the occipital lobes, indicating that the ocular structures can be more constrained by spatial (physical) than by functional (vision) relationships. In this study we used geometric morphometrics to investigate the longitudinal (antero-posterior) spatial relationships between orbito-ocular and endocranial structures. First, we addressed the the position of the eye relatively to the frontal and temporal cortex, on a sample of 63 modern humans’ MRIs. Second, we addressed the spatial relationship between orbital and endocranial structures on a CT sample comprising 30 modern humans, 3 chimpanzees, and 3 fossil humans (Bodo, Broken Hill 1, Gibraltar 1).
The results of the MRI analysis show that in adult modern humans the main pattern of shape variation deals with the antero-posterior position of the eye relative to the temporal lobes. Individuals which eyes are closer to the temporal lobes exhibit rounder frontal outline and antero-posterior shorter eyes, indicating a possible physical constraint associated with the spatial contiguity between the eye and the middle cranial fossa. A second pattern describes the supero-inferior position of the eye, relatively to the frontal lobe. Also in this case, proximity is apparently associated with slight changes in eye form. Individuals with larger volumes of the frontal and temporal lobes tend to have eyes located more posteriorly, closer to the temporal lobe, although with no apparent change in the shape of the eye. These results partially support Master’s hypothesis, suggesting reciprocal spatial patterns influencing brain and eye form.
When analyzing orbits and braincase through CT data, the main intra-specific variation among modern humans concerns the orientation of the orbit, not the position. Nonetheless, analyzing humans, apes, and fossil hominids all together, the main differences deal with the distance between orbits and braincase: they are separated in chimps, overlapped in modern humans, and in intermediate position in fossils. In this case, fossils belong to the hypodigm of Homo heidelbergensis. Modern humans are characterized by larger temporal lobes when compared with other living primates, and longer middle cranial fossa. The proximity with the eyeballs due to face reduction can stress further a morphogenetic spatial conflict between orbits and brain. Next step: 3D analyses, ontogenetic series, and vision impairment.
Virtual anatomy and inner structural morphology,
from head to toe
A tribute to Laurent Puymerail
Comptes Rendus Palevol 16 (2017)
The Finite Element (FE) method has increasing application to biological sciences but frequently lacks proper validation by robust experimental research. One aspect of particular biological and bio-mechanical importance is growth of the human infant skull. Specific local changes during growth of the infant skull are largely unknown with only the general rate of cranial increase from 25% at birth to 65% of the adult size by age six. The potential adverse effects of any abnormalities in infant skull growth is difficult to approximate if the isolated local areas likely to be most impacted are not accurately known. If properly validated, computer simulated modelling such as Finite Element methods would be invaluable in surgical settings. A new comprehensive study focusing on human infant cranial vault expansion utilized robust laboratory experiments of a fetal skull (ex-vivo), replicate physical model (in-vitro), several FE models (in-silico) and a sample of micro-CT infant skulls (in-vivo). The first validation tested a physical model against a FE model (A) in which the cranial base and facial bones formed a single structure with only the cranial vault comprising individual bones. The FE model (A) over-predicted size changes to the anterior of the skull especially near the orbits and mediolateral expansion of the skull. The second validation tested in-vivo models against an FE model (B) in which the only the facial bones formed a single structure while the vault and cranial base comprised individual bones. All analyses associated discrepancy between the FE model (B) and the in-vivo models with age-related changes. As age increased, the regions under-predicted by the FE model (B) were first the orbits and upper vault before tending toward the cranial base, while the regions over-predicted by the FE model (B) were focused on the anterior and posterior fontanelles.
This validation study showed that FE modelling could be used to approximate growth in the human skull with only small discrepancies. The differences between the predicted ranges of growth (FE models) and the observed growth (in-vivo models) was explained by assumption of isotropic brain expansion which simplified the highly complex and uneven growth rates in real brain expansion. The artificial construction of a single structure representing the facial bones added further constraints. The development of more advanced simulations could narrow the discrepancy between expected and observed growth patterns allowing a more accurate representation of human skull growth.
The human brain is the most expensive and costly organ in terms of energetic resources and management. However, the current understanding of its sophisticated thermal control mechanisms remains insufficient. Wang et al., 2016, have reviewed the most recent studies on brain thermoregulation and examined the anatomical and physiological elements associated with selective brain cooling. Modern humans have a brain that is approximately three times larger than a primate with a similar body size, which uses 20%– 25% of the total body energy compared with a maximum of 10% in other primates and 5% in other mammals. The evolution of a large and expensive brain in modern humans effectively influences critical factors such as temperature, and functional limits can affect cerebral complexity and neural processes. Brain thermoregulation depends on many anatomical components and physiological processes, and it is sensitive to various behavioral and pathological factors, which have specific relevance for clinical applications and human evolution. The anatomical structures protecting the brain, such as the human calvaria, the scalp, and the endocranial vascular system, act as a thermal interface, which collectively maintains and shield the brain from heat challenges, and preserves a stable equilibrium between heat production and dissipation. Future advances in biomedical imaging techniques would allow a better understanding of the physiological and anatomical responses related to the cerebral heat management and brain temperature in modern humans.
Gizéh Rangel de Lázaro