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.
The diploic channels are placed between the vault cortical layers (external and internal). The calvarial diploe contains large and valveless diploic veins interconnected through a complex network of microscopic channels. We have now published a procedure for segmentation of diploic channels and localization of the main vessel pathways by reducing the noise of the cancellous bone. We also provide quantitative description of the diploic vessel variation in modern humans and three Neanderthals. One modern human was reformatted at three different resolutions namely high, medium and low, to estimate the effect of the pixel resolution on the final anatomical rendering. The use of computed tomography at high resolution can hamper semi-automatic segmentation of the diploic channels. Optimal resolution should be sufficient to reveal the channels without increasing noise associated with the trabecular structure. We have found that modern humans present a remarkable variation of diploic channels in their morphological patterns, being the parietal area the most vascularized. There is a correlation in the degree of vascularization of the frontal, parietal, and occipital bone, and no asymmetries can be apparently detected. The three Neanderthals analyzed in the study also display a parietal vascular network, but less developed than modern human, suggesting these vessels may be involved in evolutionary changes. The diploic network is commonly connected with the meningeal artery at the temporal fossa, with the emissary veins at the occipital bone, and with the venous sinuses at the parieto-occipital areas. The brain and braincase of our specie are characterized by larger parietal areas, and changes in the vascular organization can be associated with thermoregulation and heat management. In this sense future research may help us to understand the possible involvement of the diploic veins in brain thermoregulation. The study of diploic channels may be relevant in anthropology, medicine, paleontology, and forensic sciences.
Gizéh Rangel de Lázaro
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