An amazing article has been published in Nature Physics. Brain cortical folding is influenced by genetic and physiological factors, but there are also many hypotheses concerning the possible role of mechanical forces associated with the cerebral tissues. These hypotheses are largely based on theoretical approaches and numerical simulations, integrating geometry and biomechanics. Because of the mechanical properties of cells and tissues, growth forces can be redistributed within and among the elements of the anatomical system, channeling morphogenesis and shaping the spatial organization of the anatomical components. This month Tuomas Tallinen and colleagues provide a further mathematical model of the growing cortex, introducing constraints associated with the sulcal pattern. But, more incredibly, they provide an extremely elegant and efficient experimental evidence. After MRI imaging, they prepare a physical model of the fetal brain with two gel components. The outer thin layer (simulating the cortex) swells when in contact with a solvent, undergoing a tangential expansion. When this happens, the growing outer surface and the stable inner volume must properly interact in terms of physical forces and distribution of the surface to volume adjustments. The result is amazing, because it really mimics the human cortical folding! There is an incredible correspondence between the real and simulated folding pattern, in terms of topology and degree of convolution. No programming here except the growing schedule, just physical properties, structural interaction, and forces redistribution.
“Morphology is not only a study of material things and of the forms of material things, but has its dynamical aspect, under which we deal with the interpretation, in terms of force, of the operations of energy.”
(D’Arcy Wentworth Thompson – On Growth and Form, 1942)
The skull has represented, since ever, a crystal ball to investigate history and geography of past and present human populations. Considering the reciprocal influences between brain and braincase, we can wonder whether the brain can also provide traces of that long run. It looks like it does, according to a recent study which evidences a correlation between cortical patterns and genetic ancestry. It seems not a matter of size or surface area, but of cortical organization and sulcal geometry. If confirmed, these results are extremely interesting. Taking into account these differences among human groups, the authors of the study cast some doubts on the possibility to obtain robust information from fossil species, questioning the relationships between brain shape changes and specific volumetric variations of the brain districts. I must confess I can’t really see an antagonistic relationship between these results and the paleoneurological data. Intra-specific and inter-specific variations do not necessarily undergo the same rules and patterns. Most importantly, paleoneurological evidence is also aimed at considering specific changes of surface and volume proportions, beyond sulcal appearance. And, as recently described for the precuneus, larger size of a brain element may generally mean larger cortex of that element. Brain and braincase share a lot of morphogenetic mechanisms, but of course they are also influenced by independent factors. Their boundaries may vary according to different rules, but the intimate relationships between their respective surfaces allow at least a gross quantification of spatial organization, volumetric changes, and relative proportions among brain areas. We know that morphological changes as shown on endocasts are only a part of the story, and we know they are not always associated with neural (or even cognitive) changes. But reductionists approaches should be avoided in any fields, including genetics or neuroimaging, not only in paleoneurology. Conversely, a genetic signal on the sulcal pattern may promote further interest in brain shape variation. Last but not least, the study seems to support, rather than contradict, the information available from bones: it turns out that the conclusions of the analysis match the results of the Howells‘ craniometric studies. Good.