Diploic growth

After our first survey on diploic channels in humans, here a second one on their growth and development. We have analyzed the ontogenetic variation in their length, lumen size and volume, in the frontal, parietal and occipital bones, and their correlation with skull size and bone thickness. Interestingly, there is a gradual increase of the vascular complexity, but a noticeable and outstanding spurt only in the adult stage. The development of these vessels is probably constrained by the thickness of the cancellous bone, and that’s why only in the adult stage we can observe a marked increase of their network complexity, as well as a marked increase of the individual diversity. If these vessels are involved in the thermal regulation of the endocranial cavity, their role is not patent until adulthood, at least if we consider their largest branches. Studies on the smaller ones are in course. It is worth noting that a large and complex diploic network is only observed in Homo sapiens, and not in living apes or extinct hominids. This may suggest some recent evolutionary adaptations. Here another post with more details.

Here a couple of recent reviews on craniovascular traits and anthropology and on craniovascular traits and human evolution. One paper specifically on the parietal bone, and a large survey of the prevalence of these features in modern populations.

Bones and vessels

The vascular traces left on the bones are remnants of physiological processes associated with blood flow and functions. Craniovascular traits can be used in archaeology, paleontology, and forensic science to deal with normal and pathological variations of the circulatory system, bridging interests between evolutionary and medical fields. Current information on these characters is, at best, scarce. After our recent work on diploic channels, this week we publish another morphometric study on the vascular traces, and specifically on their relationships with parietal bone size and thickness. We provide a quantitative description of the lumen size in adult modern humans for the middle meningeal and diploic vessels, as calculated from cranial anatomy after computed tomography, for different orders of branches. Vessel size and cranial thickness can be proportional if sharing growth factors, or inversely proportional if competing through structural constraints. However, we do not find any clear relationship between vascular size, cranial size, and cranial thickness. This result suggests that bone and vessel morphogenesis are probably influenced by independent factors, at least when dealing with differences among adult individuals.

Diploic veins

Diploic channels run within the vault bones, and are therefore protected from external agents. This condition makes them an interesting topic in paleontology, archaeology, and forensics. At the same time, such secluded position has hampered detailed studies on their morphology, variations, and functions. In 1999 Hershkovitz and colleagues published a first pioneering survey on these “elusive” anatomical elements. This week we publish a segmentation procedure to visualize these channels after computed tomography, applying this method to modern humans and Neandertals. The diploic network displays a marked individual variability. It is frequently connected with the meningeal system at the pteric area, and with the emissary and venous systems at the occipital area. As for the meningeal arteries, also the diploic vasculature is apparently more complex in modern humans than in other hominids, mostly at the parietal area. Taking into account the large size of the parietal lobes and bones of our species, it is likely that such vascular development can be associated with metabolic and thermal functions. Beyond the large diploic channels, this vascular system counts with a widespread network of microvessels, which should be carefully investigated in the future.