A group coordinated by Dr. Vera Weisbecker examined whether the evolution of mammalian brain partitions follows conserved developmental constraints, causing the brain to evolve as an integrated unit in which the partitions scale with brain size. According to this ‘late equals large’ hypothesis, the timing of neurogenesis predicts the size of the partition such that later and more extended neurogenesis produces larger partitions due to the production of more neural precursors. In order to investigate the impact of neurogenesis on patterns of brain partition growth, the volumes of the whole brain and major partitions were reconstructed from soft-tissue diceCT scans of three marsupial species, including individuals with ages ranging from 1 day to adulthood. They tested three hypotheses consistent with a conserved brain partition growth: H1 postulates that partition scaling during development reflects the evolutionary partition scaling, and thus growth patterns should be uniform between species; H2 assumes that a neurogenesis-driven pattern of partition scaling is predictable from adult brain size, i.e. brain partitions scale with brain size; and H3 states that growth with age might differ between species according to brain size and/or neurogenetic events. Regressions of log partition volume against log rest-of-the-brain volume (whole-brain volume minus partition volume) showed significant interspecific differences in slopes and intercepts of most brain partitions, indicating diverse scaling patterns between species, which could not be predicted by adult brain size, as the smallest-brained species had intermediate slope to the other two. Growth curves of log partition volume against age were similar in all partitions within-species, but differed between species, particularly in growth rates, with the species with intermediate brain size having slower rates than the other two. Differences in growth patterns do not seem to be related to neurogenetic schedule as largest partitions are not especially late in their development and important maturation processes, like eye opening, occur closer to the end of the growth phase. Thus, none of the hypotheses are supported by these results, challenging the conserved neurogenetic schedules behind the evolution of mammal brain partitions. Moreover, the authors found high phylogenetic signal in brain partition scaling, revealing that a large part of the scaling relationship between brain and partition volumes is explained by phylogeny, which is more in agreement with a mosaic evolution of brain partition sizes, stressing its biological meaning and the level of mammalian brain plasticity. However, the intraspecific regular partition growth curves led the authors to contemplate the existence of an early brain partition pattern regulated by regional gene expression, and propose that further studies of brain partition evolution should integrate developmental neuromere expression models, neuron density, and patterns of neuron migration.
A study on covariation between parietal bone and endocranial base …
The Finite Element Method (FEM) was developed within the framework of Engineering but has become a popular tool in bio-mechanical studies. It is natural that computational bio-mechanics and Finite Element Analysis (FEA) became increasingly promising in fossil studies where there are no examples of some taxa still living. To study the bio-mechanical responses of fossil hominids, modern humans and non-human primates are often used as comparative samples for which there are already known values. Despite this, precisely how accurately computational bio-mechanics compares with physical studies is still not well understood. The biological composition of bone and dentition is hard to replicate in computational terms with the cranium a mixture of trabecular and cortical bone while teeth comprise variable layers of enamel and dentine. The resolution required from Computed Tomography (CT) scans to accurately capture these finer biological compositions is not feasible for the heavy demands on software to analyze such FEA models with flow-effects for the number of specimens that can be included into any single study.
Godinho et al investigated the validity and sensitivity of Finite Element (FE) models using a direct comparison with a human cadaver. Results were particularly affected if the model was simplified by assigning all materials as cortical bone, including dentition and trabecular bone components. Results showed that the real and virtual skull showe d no differences in strain magnitude; differences in strain pattern (high or low strain distribution) were only partially different; simplifying the virtual model decreased the strain magnitude; simplifying the virtual model partially affected the strain pattern with the regions near the dentition, particularly the alveolar ridge, most affected.
For bio-mechanical studies, by not simplifying virtual models and attempting to designate dental and bone tissues properly acknowledges the underpinning biology of the cranium while potentially revealing sensitive adaptations of this biological structure. By adopting these changes, new variations between living and fossil humans, that have so-far been obscured by less time-consuming computational methods, could reveal unique adaptational trends that have real significance for human evolution.
A surface analysis on the frontal lobes in archaic humans …
[A post] [The Paper]
Dimitri Neaux and colleagues have published a series of comprehensive analyses on the influence of the cranial base in facial morphology of humans and apes. In one of the papers, they assessed the integration between the face and the two basicranial modules: the sagittal and the lateral cranial base. They tested the covariation between the three sets of 3D landmarks (face vs. midline base and face vs. lateral base) on modern humans and chimpanzees, separately. Only the correlation between the face and the lateral cranial base was significant, confirming the important role of the lateral cranial base in facial morphology. Though the levels of covariation were comparable, the patterns differed between the two species, as taller faces were associated with wider and shorter cranial fossae in chimps and with longer and narrower cranial fossae in humans. In another article, they assessed the relationship between cranial base flexion, facial orientation, and facial shape in modern humans, chimpanzees, and gorillas. Using 3D landmark analysis, they evaluated the within-species patterns of covariation, confirming the intraspecific relationship between facial structures and base flexion. Base flexion is associated with downward rotation of the facial block in both humans and chimps (confirming previous works) but not in gorillas. On the other hand, an upward rotation of the facial block is associated with anterior face vertical elongation on the three species. In humans, facial elongation is also associated with base flexion, which might have been selected during evolution to match the elongation of the nasomaxillary complex, as proposed before. The authors further tested whether increased base flexion is associated with the shortening of the facial length or with the decrease of facial projection. The relationship between base flexion and facial length was only observed in chimps, while facial projection was not related with cranial base flexion in chimpanzees and gorillas. In humans, contrary to what was expected, basicranial flexion was associated with increasing facial projection, which the authors attribute to sexual dimorphism, as males have increased base flexion and facial projection. Again, the main patterns of correlation differed between the species. Cranial base angle is negatively correlated with facial projection in modern humans, with facial length in chimps, and with the angle between the posterior-maxillary plane and the anterior facial plane in gorillas. As the authors conclude, these differences in the patterns of integration might reflect changes in the structural relationships between the face and the cranial base during hominoid evolution.
The cranial vault is composed by three bone layers (inner table, diploe, outer table), and its principal function is to safeguard the brain from impacts. Bone thickness is a crucial parameter to understand the biomechanical factors contributing to skull deformations and fractures after head injury. It is therefore important to establish an accurate measurement system to quantify its variation. Lillie et al., 2015 analyzed microCT scans of two cadavers to evaluate the accuracy of the estimated cortical thickness from clinical CT data. Microscans were acquired at 25-microns, while CT scans had a resolution of 0.48-0.62 mm. The skull average thickness in both cases was below 4 mm. Cortical thickness measurements obtained from CT scans are more accurate compared with traditional physical methods, although results are comparable with those available in literature. The average cortical thickness discrepancy between microCT scans (higher resolution) and CT scans (lower resolution) is 0.078+ 0.58 mm. Such methodological validation is necessary when dealing with age-related changes in distribution of the skull cortical thickness, and to identify species-specific or population differences.
Gizéh Rangel de Lázaro
Lynn Copes has posted online 705 cranial CT scans of modern humans for free download. The dataset is part of her PhD sample which comes from archaeological sites in Alaska, Greenland and Oceania, from the Terry Collection (a cadaver-based collection from the United States). The original skulls are deposited in the National Museum of Natural History (Washington, DC), and the Point Hope Alaska Collection at the American Museum of Natural History (New York, NY). Lynn Copes is on Google Scholar and Twitter.
Gizéh Rangel de Lázaro
(click the image to enlarge)
Virtual Reconstruction and Computational Biomechanics Workshop
The Centre of Human Anatomy Education is organising a workshop in the application of Virtual Reconstruction and Finite Element methods in human biology at Monash University on 20th and 21st October 2016. The workshop will focus on computer-based biomechanical modelling of teeth and bones, virtual reconstructions of fragmented fossil remains, and on simulated kinematic analyses of the masticatory system.
[Info & Webpage]
Studying anatomical variability in paleontological and archaeological context is a challenge to look behind (and beyond) the bones. In the case of cranial remains we are able to make inferences not only on bone morphology but also on part of the vascular system. With computed tomography we can observe the diploic channels inside the bone matrix, and the imprints of the middle meningeal vessels on the endocranial surface of the vault. In the parietal bone both networks are particularly developed, most of all in modern humans. This month we have published a new study focusing the size and morphology of these vascular imprints in adult humans, and on their relationship with bone size and thickness. Our aim was to reveal possible influences between vascular and bone morphology. Vessels and bones share morphogenetic processes, and there can be shared functional and structural relationships between angiogenesis and osteogenesis. Shared growth factors can generate a positive correlation between bones and vessels dimensions or, conversely, biomechanical constraints between bone matrix and its embedded soft tissues can generate an inverse relationship between their volumes. We used CT data of human adult crania to measure cranial size, parietal bone thickness, and lumen size of these vascular traces. We provide a metric description of the size variation and size distribution of the diploic channels and meningeal imprints, for different orders of branches. The diploe largely influences the overall thickness of the bone. The upper part of the parietal bone shows the thickest values. The lumen size of the diploic channels and meningeal imprints is very similar, with no patent sexual or hemispheric differences. The correlation analysis did not revealed any clear relationship between vessels size, cranial size, and cranial thickness. Therefore, these results do not support the hypothesis of a reciprocal influence between bone and vascular morphology, which are likely to respond to different factors. Actually, although some vascular changes may be described in extreme cases of cranial deformation, also according to a previous survey on the endocranial vascular pattern in normal variation there is no apparent correspondence between gross cranial form and craniovascular traces.