The brain growth pattern in humans is distinctive among primates. In the past decades several hypotheses have been proposed to analyze cranial ontogenetic changes (i.e shape and size variations) in humans (e.g. Moss and Young, 1960; Lieberman et al., 2002; Bruner, 2004; Neubauer et al., 2009). The recent study conducted by García Gil et al. (2015) presents a preliminary approach to the histological variations of the vault bones in three individuals of different ages (child, adolescent and young adult). According to their results, it is possible to identify three different histological phases of cranial growth. In the child, vault bones are primarily composed of avascular lamellar bone (widely vascularized). In contrast, the adolescent bones show a larger extension of mineralized regions (highly remodeled areas) and low levels of vascularization, with a much reduced diploe. In the adult, the vault bone is highly vascularized and the diploe is largely expanded. The authors suggest that the sealing of the cranial bone surfaces helps to minimize the bone porosity while increases bone expansion (during childhood) and thickness (during youth). This “sealing process” could play a main role controlling head thermoregulation until the brain finishes its maturation. When confirmed on larger samples, these results can introduce new perspectives in functional craniology.
Gizéh Rangel de Lázaro
Cranial vault consists of two cortical tables (inner and outer) sandwiching a layer of trabecular bone (diploe). Cranial vault thickness (CVT) is the distance between endocranial and ectocranial surfaces of vault bones. Several studies have pointed out that CVT differs not only between hominids but also among modern human populations. This morphological trait is mainly influenced by systemic and local stimuli, such as brain growth and development, mechanical forces (at the muscles attachments), circulating hormone levels, formation of sutures etc. Marisol Anzelmo and colleagues have recently published a study of ontogenetic changes in CVT in a modern sample of Homo sapiens. They tested age differences in CVT and if these changes are associated with changes in endocranial volume (EV), which reflect brain size. CT cranial images of 143 individuals (males and females) from 0 to 31 years were used to obtain, among others, a thickness mean measure (TMM), a measure of endocranial volume (EV), and a 3D topographic mapping of CVT, which indicates thickness distribution at different regions through a chromatic scale. A topographic mapping is very useful for picturing differences across vault regions in every age group, and it also reveals development of the regions during ontogeny and the onset of adulthood. The results of this study show that TMM increases during ontogeny without sex differences. Most accelerated growth rates of TMM occur during the first 6 years of life. Also, association between TMM and EV was significant only in this period (infants and children). Furthermore, adult pattern of thickness distribution seems to begin early in ontogeny. Increase of CVT in early ontogeny is directly linked to brain protection. However several mechanisms are involved in CVT formation, such as sutures patterning and vessels development. The vault bones dynamic in later ontogeny and in adulthood may be then influenced by different type of muscular activity and mechanical demands or, most commonly, by systemic factors associated with hormones, physical activity, nutrition.
During normal cranial morphogenesis, most neurocranial sutures remain open till advanced adult stages, closing long after the brain is fully grown and developed. In some cases, however, sutures may close prematurely causing a pathological condition known as craniosynostosis which is associated with different neurocranial malformations depending on the suture involved. Apart from its aesthetic consequences, craniosynostosis is relevant from a biomedical perspective as malformations due to premature fusion of sutures may result in limited or abnormal brain growth and development, higher intra-cranial pressure, as well as respiratory and visual impairments. Moreover, a complete understanding on craniosynostotic processes is also relevant for evolutionary biologists, as species-specific cranial and cerebral morphologies may be somewhat influenced by changes on the particular tempo and sequence of suture closure (see for example the case of the australopithecine metopic suture).
From the above lines, it seems clear that an early and accurate diagnosis of the disease is essential for the correct management and prevention of possible clinical complications, as well as proper planning of surgical interventions. In this context, Carlos S. Mendoza and colleagues have recently published an automated pipeline that allows untrained observers to diagnose early craniosynostotic individuals with a 96% accuracy. In brief, their novel methodology is based on the automated computation of suture fusion indices and on the quantitative characterization of local deformation and curvature changes of bones and sutural areas. For the former task, the proposed pipeline incorporates a dedicated algorithm based on graph-cut techniques which is capable for automatically detecting and labeling both bones and sutures. After the identification of skeletal and sutural components, open and fused sutures are detected following a ‘contact degree’ criteria and fusion indices are computed as the number of voxels belonging to different bones that are in contact. On the other hand, normal neurocranial shape variation is modeled using a novel landmark-free methodology and principal component analysis (PCA). The resultant shape space is then used as a multi-atlas reference to compare and evaluate any potential patient that is to be diagnosed. Although this methodology may show some limitations, validation analyses presented by Mendoza and colleagues have demonstrated that by considering together fusion indices and shape descriptors it is possible to automatically discriminate between normal synostotic patients, as well as between different phenotypes of craniosynostosis. Moreover, this methodology not only enables the fast and accurate diagnosis of particular clinical cases (with all its associated medical advantages), but it also provides a methodological framework for the statistical analysis of large datasets, which in turn could provide important information regarding the structural and functional role of sutures during ontogeny and phylogeny.
José Manuel de la Cuétara