No abstract

Anatomical systems are organized through a network of structural and functional relationships among their elements. This network of relationships is the result of evolution, it represents the actual target of selection, and it generates the set of rules orienting and constraining the morphogenetic processes. Understanding the relationship among cranial and cerebral components is necessary to investigate the factors that have influenced and characterized our neuroanatomy, and possible drawbacks associated with the evolution of large brains. The study of the spatial relationships between skull and brain in the human genus has direct relevance in cranial surgery. Geometrical modeling can provide functional perspectives in evolution and brain physiology, like in simulations to investigate metabolic heat production and dissipation in the endocranial form. Analysis of the evolutionary constraints between facial and neural blocks can provide new information on visual impairment. The study of brain form variation in fossil humans can supply a different perspective for interpreting the processes behind neurodegeneration and Alzheimer’s disease. Following these examples, it is apparent that paleontology and biomedicine can exchange relevant information and contribute at the same time to the development of robust evolutionary hypotheses on brain evolution, while offering more comprehensive biological perspectives with regard to the interpretation of pathological processes.

Recent analyses indicate that the precuneus is one of the main centres of integration in terms of functional and structural processes within the human brain. This neuroanatomical element is formed by different subregions, involved in visuo-spatial integration, memory and self-awareness. We analysed the midsagittal brain shape in a sample of adult humans (n = 90) to evidence the patterns of variability and geometrical organization of this area. Interestingly, the major brain covariance pattern within adult humans is strictly associated with the relative proportions of the precuneus. Its morphology displays a marked individual variation, both in terms of geometry (mostly in its longitudinal dimensions) and anatomy (patterns of convolution). No patent differences are evident between males and females, and the allometric effect of size is minimal. However, in terms of morphology, the precuneus does not represent an individual module, being influenced by different neighbouring structures. Taking into consideration the apparent involvement of the precuneus in higher-order human brain functions and evolution, its wide variation further stresses the important role of these deep parietal areas in modern neuroanatomical organization.

The parietal bones approximately cover the extension of the underlying parietal lobes. Although the boundaries of these two anatomical elements do not coincide, during morphogenesis the growth of the parietal bones is largely induced by the pressure exerted by the parietal lobes. Modern humans display larger parietal chords and arcs compared with non-modern human species. However, the variation of these variables have not been analyzed before according to the covariation with the general endocranial diameters. When the curvature of the parietal bones is regressed onto the main neurocranial distances, modern humans show larger relative values, suggesting not only an absolute enlargement but a definite allometric change. Taking into account the morphogenetic relationships with the parietal lobes, these results further support previous hypotheses suggesting a relative enlargement of these cortical areas in Homo sapiens, by using simple and reliable homologous neurocranial arcs. Anat Rec, 294:1548Rec, 294: -1556Rec, 294: , 2011. V V C 2011 Wiley-Liss, Inc.

The corpus callosum displays considerable morphological variability between individuals. Although some characteristics are thought to differ between male and female brains, there is no agreement regarding the source of this variation. Biomedical imaging and geometric morphometrics have provided tools to investigate shape and size variation in terms of integration and correlation. Here we analyze variations at the midsagittal outline of the corpus callosum in a sample of 102 young adults in order to describe and quantify the pattern of covariation associated with its morphology. Our results suggest that the shape of the corpus callosum is characterized by low levels of morphological integration, which explains the large variability. In larger brains, a minor allometric component involves a relative reduction of the splenium. Small differences between males and females are associated with this allometric pattern, induced primarily by size variation rather than genderspecific characteristics.

The evidence does not support a thermoregulatory role for the meningeal vascular network, at least in adult normal blood flow conditions. On the other hand, biomechanical protective functions (hydraulic skeleton for shock adsorption) cannot be ruled out.

The spatial relationships between brain and braincase represent a major topic in surgery and evolutionary neuroanatomy. In paleoneurology, neurocranial landmarks are often used as references for brain areas. In this study, we analyze the variation and covariation of midsagittal brain and skull coordinates in a sample of adult modern humans in order to demonstrate spatial associations between hard and soft tissues. The correlation between parietal lobe size and parietal bone size is very low, and there is a marked individual variation. The distances between lobes and bones are partially influenced by the dimensions of the parietal lobes. The main pattern of morphological variability among individuals, associated with the size of the precuneus, apparently does not influence the position of the neurocranial sutures. Therefore, variations in precuneal size modify the distance between the paracentral lobule and bregma, and between the parietal lobe and lambda. Hence, the relative position of the cranial and cerebral landmarks can change as a function of the parietal dimensions. The slight correlation and covariation among these elements suggests a limited degree of spatial integration between soft and hard tissues. Therefore, although the brain influences the cranial size and shape during morphogenesis, the specific position of the cerebral components is sensitive to multiple effects and local factors, without a strict correspondence with the bone landmarks. This absence of correspondent change between brain and skull boundaries suggests caution when making inferences about the brain areas from the position of the cranial sutures. The fact that spatial relationships between cranial and brain areas may vary according to brain proportions must be considered in paleoneurology, when brain anatomy is inferred from cranial evidence.

Modern humans display a large variation in their morphological patterns, being the parietal area the most vascularized. The pattern of the diploic channels may be relevant in anthropology, medicine, and paleontology, taking into account their possible involvement in thermoregulation.