Pictures worth thousands of words and dollars

ResearchBlogging.orgLooking into subdural empyema, which is a meningeal infection you don’t want, I stumbled upon a study from the roaring 1970s – the glorious Nixon-Ford-Carter years – using computerized axial tomography (hence, CAT scan) to visualize lesions within the skull (Claveria et al. 1976). Nowadays people refer to various similar scanning techniques simply as “CT” (for computed tomography, though this is not exactly the same as magnetic resonance imaging, MRI).

It’s pretty amazing how medical imaging has advanced in the 35 years since this study. For example, to the right is a CAT scan from Claveria et al. (1976, Fig. 4). These are transverse images (“slices”) through the brain case, the top of the images corresponding to the front of the face. You can discern the low-density (darker) brain from the higher density (lighter) bone – the sphenoid lesser wings and dorsum sellae, and petrous pyramids of the temporal bones are especially prominent in the top left image. In the bottom two images you can see a large, round abscess in the middle cranial fossa. Whoa.

What makes this medical imaging technique so great is that it allows a view inside of things without having to dissect into them. Of course, the downside is that it relies on radiation, so ethically you can’t be so cavalier as to CT scan just any living thing. If I’d been alive in 1976, CAT scanning would’ve blown my mind. Still, the image quality isn’t super great here, there’s not good resolution between materials of different densities, hence the grainy images.

But since then, some really smart people have been hard at work to come up with new ways to get better resolution from computerized tomography scans, and the results are pretty amazing. To the left is a slice from a synchrotron CT scan of the MH1 Australopithecus sediba skull (Carlson et al. 2011, Supporting on line material, Fig. S10). You’re basically seeing the fossil face-to-face … if someone had cut of the first few centimeters of the fossil’s face. Just like the movie Face Off.

Quite a difference from the image above. Here, we can distinguish fossilized bone from the rocky matrix filling in the orbit, brain case and sinuses. Synchrotron even distinguishes molar tooth enamel from the underlying dentin (see the square). The post-mortem distortion to the (camera right) orbit is clear. It also looks as though the hard palate is thick and filled with trabecular bone, as is characteristic of robust Australopithecus (McCollum 1999). Interesting…

Even more remarkable, the actual histological structure of bone can be imaged with synchrotron imaging. Mature cortical bone is comprised of these small osteons (or Haversian systems), that house bone cells and transmit blood vessels to help keep bone alive and healthy. Osteons are very tiny, submillimetric. To the right is a 3D reconstruction of an osteon and blood vessels, from synchrotron images (Cooper et al. 2011). The scale bar in the bottom right is 250 micrometers. MICROmeters! Note the scan can distinguish the Haversian canal (red part in B-C) from vessels (white part in B). Insane!

Not only has image quality improved over the past few decades, but CT scanning is being applied outside the field of medicine for which it was developed; it’s becoming quite popular in anthropology. What I’d like to do, personally, with such imaging is see if it can be used to study bone morphogenesis – if it can be used to distinguish bone deposition vs. resorption, and to see how these growth fields are distributed across a bone during ontogeny. This could allow the study the proximate, cellular causes of skeletal form, how this form arises through growth and development. If it could be applied to fossils, then we could potentially even see how these growth fields are altered over the course of evolution: how form evolves.

Carlson KJ, Stout D, Jashashvili T, de Ruiter DJ, Tafforeau P, Carlson K, & Berger LR (2011). The endocast of MH1, Australopithecus sediba. Science (New York, N.Y.), 333 (6048), 1402-7 PMID: 21903804

Claveria, L., Boulay, G., & Moseley, I. (1976). Intracranial infections: Investigation by computerized axial tomography Neuroradiology, 12 (2), 59-71 DOI: 10.1007/BF00333121

Cooper, D., Erickson, B., Peele, A., Hannah, K., Thomas, C., & Clement, J. (2011). Visualization of 3D osteon morphology by synchrotron radiation micro-CT Journal of Anatomy, 219 (4), 481-489 DOI: 10.1111/j.1469-7580.2011.01398.x

McCollum, M. (1999). The Robust Australopithecine Face: A Morphogenetic Perspective Science, 284 (5412), 301-305 DOI: 10.1126/science.284.5412.301

Tooth formation rates – what do species comparisons really mean?

A paper just came out in PNAS, by Tanya Smith and others, in which they estimate tooth-crown formation times in a large sample of modern humans (n=>300 individuals), a modest sample of Neandertals (n=8), and a poor sample of ‘fossil Homo sapiens‘ (n=3). Teeth form by the periodic deposition of enamel (the hard, white part visible in teeth in the mouth) and dentin (forms the tooth root and internal part of the crown). These periodicities are fairly regular (though variable), thus allowing researchers to estimate how long it took for teeth to develop. As previous studies have shown, Smith and colleagues find that Neandertals formed most of their teeth faster than modern humans.

Growth and development are part of an organism’s life history strategy, and so the observation that Neandertals (and other fossil human species/lineages) form their teeth faster than modern people suggests that perhaps they ‘lived faster’ and died younger than us. It has also been used as evidence that Neandertals are a different species from modern humans.
But I don’t know how well the latter taxonomic argument works. Along these lines, I wish the authors had discussed the meaning of the estimated crown formation times for their fossil ‘modern’ humans (Qafzeh 10 & 15 from Israel ~100 thousand years ago, and Irhoud 3 from Morocco ~160 thousand years ago). The boxplot summaries of crown extension rates (above) show that Neandertals are, indeed, generally fast relative to the large modern sample. However the fossil-modern humans (asterisks, which I’ve circled in red) show a bizarre, not easily interpretable pattern. For the central upper incisors (I1), fossil-moderns are either within the Neandertal range or an outlier at the high end of the human sample. For the lower second incisor (I2) the two fossil-moderns are either waaaaaay above the human range, or a little below it -either way it’s outside the human range. In addition, the sole fossil-modern lower first molar has a lower rate than the modern sample – suggesting an even slower development time. Only the fossil-modern canine formation time fits comfortably within the range of modern humans. Given this wide range of variation in tooth crown formation times in the very small sample of fossil-modern humans, I don’t think we can use this information to make taxonomic arguments.
I think these dental histology studies are very interesting, but I don’t know how much stock we can put in any taxonomic interpretations of them. That Neandertal teeth form faster than modern humans’ is old news, and the discussion focused solely on the neandertal-modern human comparison. It’s too bad that the really interesting part of the paper – the variation in formation time displayed by the fossil-moderns – got no discussion.
The paper
Smith TM et al. 2010. Dental evidence for ontogenetic differences between modern humans and Neandertals. Proceedings of the National Academy of Sciences, in press.