These cells look like fairly typical bone cells. They appear to be connected to each other by thin branch-like projections and are embedded in a white matrix of fibres. At their centres are dark red spots that are probably their nuclei. But it's not their appearance that singles out these extraordinary cells - it's their source. You're looking at the bone cells of a dinosaur.
They come from an animal called Brachylophosaurus, a duck-billed dinosaur that lived over 80 million years ago. By looking at one of its thigh bones, Mary Schweitzer from North Carolina State University has managed to recover not just bone cells, but possible blood vessels and collagen protein too. Their presence in the modern day is incredible. Time usually isn't kind to such tissues, which decay and degrade long before harder structures like bones, teeth and armour are fossilised.
This is the second time that Schweitzer's team have recovered ancient protein from dinosaur bones. Two years ago, they pulled off a similar trick with collagen protein from the bones of Tyrannosaurus rex. That discovery was a controversial one, and many scientists were justly sceptical. Last year, one group reinterpreted the so-called soft tissues as nothing more than bacterial biofilms, "cities" of bacteria not unlike the plaque on your teeth or slime on moist rocks.
Now, Schweitzer has returned with another volley in the debate, and one which considerably strengthens the case for preserved Cretaceous proteins. From the bone of Brachylophosaurus, she has uncovered tissues that bind to antibodies designed to target collagen and other proteins not found in bacteria, including haemoglobin and elastin. And her experiments were duplicated by independent researchers from five different laboratories. It seems that her Tyrannnosaurus discovery was far from a one-hit wonder.
Schweitzer attributed the staying power of the Tyrannosaurus tissues to the fact that the animal was deeply buried in sandstone. So this time, when she discovered the skeleton of a Brachylophosaurus in the same condition, she was ready to ensure that its thigh bone made it from dig site to test tube as quickly as possible. The bone itself was never exposed to the elements. It was sealed in a jacket of the same sediment it was preserved in. Back in the lab, it was uncovered, quickly wrapped in foil using sterile tools, and placed in sealed jars with dehydrating crystals until it could be analysed.
The team stripped the minerals away from some fragments to leave behind microscopic structures that strongly resembled blood vessels and cells, preserved in a matrix of parallel fibres. For comparison, they subjected the bones of a modern ostrich to the same treatment.
The fibrous matrix was indistinguishable from those in ostrich bone and glowed under a mercury lamp just as modern bone collagen does. The cells had the same texture, size and features of modern bone cells, including potential nuclei at their centre, and thin projections called filopodia that connected them to each other and the surrounding matrix. The vessels were transparent, hollow and flexible and contained a red substance that looked like dried blood; again, their texture was very similar to that of ostrich blood vessels.
According to Schweitzer, biofilms couldn't produce structures this diverse. "Biofilms are essentially slime and bugs," she says. "As such, they're rather amorphous and their chemical profile is based on gradients." The dinosaur tissues, however, are tremendously varied in texture, shape and colour at a very small scale.
Schweitzer found that glowing antibodies designed to target modern proteins also stuck to the tissues recovered from the dinosaur bone. These included collagen, osteocalcin (a bone protein), elastin (a protein unique to back-boned animals) laminin and haemoglobin (the oxygen-carrying protein in red blood cells). If the dinosaur tissues were first mixed with enzymes that break down these proteins, such as collagenase and elastinase, the antibodies no longer stuck to them (see below). And none of the sediment from the dig site yielded any traces of these proteins, proving that they weren't coming from any other source.
These results strongly support Schweitzer's assertion that the white fibres she recovered from dinosaur bones were indeed collagen, the cells were bone cells and the vessels were blood vessels. She even had her experiments successfully repeated by two other independent laboratories headed by Raghu Kalluri and Lewis Cantley.
Schweitzer even managed to analyse some of the amino acids that made up the dinosaur collagen, using a technique called mass spectrometry. The technique revealed the sequences of eight stretches of amino acids that hailed from two versions of collagen. Schweitzer managed to sequence more than twice as many amino acids as she did from the tyrannosaur bones. And among these was hydroxyproline, a chemically modified version of the amino acid proline that bacteria can't produce - another major blow to the biofilm theory. Again, these protein sequences were confirmed independently by the laboratory of William Lane.
Schweitzer says that the scepticism that greeted her tyrannosaur discovery was "appropriate" but her new results strongly support the idea that soft tissues and their original proteins can be preserved at least since the late Cretaceous period. How is that even possible?
"Well that's the real interesting question, isn't it?", agrees Schweitzer. "According to all of our models and all bench-top experiments exposing proteins to artificial conditions, it [shouldn't be possible]. That is an area of active research in my lab. We're in the process of testing that very question through actualistic experiments and other means. We have some ideas about the chemistry that might allow such preservation, but we're still in the process of working it out. Stay tuned!"
"I see this research expanding exponentially," she says. "The biggest hurdle is showing and convincing people that it can be done. Next is convincing them it is worth the effort." Certainly, some scientists branded her previous work as trendy but niche science. Rich of Evolgen described it as "a sexy method that will probably be difficult to implement elsewhere."
But others see wider potential. One of her co-authors is Raghu Kalluri, a cancer researcher who works on the biochemistry of collagen, and who sees great implications for his seemingly unrelated field. "This study further proves that collagen I is very stable and thus offers new insights into its role in organ fibrosis and tumor growth," he says.
As tumours progress, they produce huge amounts of collagen I, which helps them to grow. The same build-ups happen as organs age, which leads to organ fibrosis. The exceptionally stable protein doesn't go away easily and slowly compromises the function of organs as it accumulates. Kalluri is interested in finding ways of melting away collagen from tumours and fibrotic organs without altering it in places like bones where it's needed. "We hope to use the knowledge gained by this dinosaur study to help come up with new therapies," he says. "The collagen sequences found in this study might also offer new insight into the most stable regions of this protein and help in tissue engineering efforts."
For Schweitzer, the search for dinosaur soft tissues is just beginning. "We are working at the very threshold of the most sensitive technologies that we have. I think that as technologies increase in sensitivity and resolution, this will become a viable avenue of investigation, despite the risky nature of it."
Images: All microscope images by Mary Schweitzer. Photo of Mary by Kelly Gorham. Dinosaur painting by Julius Csotonyi
Reference: Schweitzer, M., Zheng, W., Organ, C., Avci, R., Suo, Z., Freimark, L., Lebleu, V., Duncan, M., Vander Heiden, M., Neveu, J., Lane, W., Cottrell, J., Horner, J., Cantley, L., Kalluri, R., & Asara, J. (2009). Biomolecular Characterization and Protein Sequences of the Campanian Hadrosaur B. canadensis Science, 324 (5927), 626-631 DOI: 10.1126/science.1165069
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*mouth agape*
This is turning out to be a very good year for paleontology!
and some bits of DNA in their nuclei?
and some bits of DNA in their nuclei?
This is excellent, I can't wait to read the paper in detail.
I was involved in some of the discussions concerning the accuaracy of the predictions made in their T.rex and Mastadon paper when it came out, and noticed that the hydroxyglycine residues they calimed were in the sequence were unlikely...something my structural biologist colleagues later ran with. It looks as if their analysis programs have got it right this time, at leastI hope that's the case.
I want this incorporated into a gene tree.
Can these proteins be sequenced like DNA, to be compared against (e.g.) birds' corresponding proteins? (I understand that nothing akin to PCR amplification is possible, which would make it hard, but I have not kept up on advances in this area, and would welcome being surprised.)
Ed: The remarks about treating tumors seem like extreme reaching. Am I missing something? Is he suggesting that the points at which this collagen broke would be good points to attack in tumor collagen?
I really really REALLY want to see replication of this from a different lab.
Nathan - they did that. It was the only bit of the paper I left out because (a) the piece was getting very long and (b) it's probably the least exciting bit about it. What you get is a phylogenetic tree that doesn't tell us anything new beyond what bones have done in the past. The last time they did this (for the T.rex and mammoth sequences), they were slightly ridiculed for confirming hypotheses that had long been confirmed. Also the tree wasn't great, with some species being greatly misplaced.
The new one's better - it groups Brachylopohosaurus and Tyrannosaurus together, and places them closest to an ostrich and a chicken. The only real "mistake" is perhaps that the rex ought to group with the chicken, but I think the point is that these are only very partial sequences. Even with twice the reported amino acids of the previous paper, the Brachylophosaurus collagen sequences only represent less than 10% of the whole protein. I think that's what Kalluri's interested in - which areas of the protein degrade before the others?
I can definitely understand why that could be seen as reaching, although I think it's fair to report why someone is excited about something if they genuinely are. I've railed before against focusing on "practical implications" over and above the actual science, but this is a case where people have actively criticised Schweitzer's work for having no practical implications. That being so, it's interesting then to see what her co-workers from different fields make of it.
RBH - the paper does make it clear that the key parts of the experiments (antibody-binding, collagen-sequencing) were all independently replicated in different labs, using separates bone samples. Are you looking for the entire process, from start to finish, to be completed elsewhere without Schweitzer's involvement?
This is amazing, whether it turns out there are practical applications or not. Retrieving 80 million year old cells is just amazing.
I'm stunned that they could recover anything after millions of years. Wow! First time I heard about it I thought there'd be an error. But the results have been duplicated, and done so in a more rigorous fashion. Wow again!
Ed, if you come across any explanations as to how it is possible that these tissues could last this long, please post it. I didn't think tissues could last a few millenia let alone a few million years.
Sometimes what's interesting in a story isn't the main topic. The importance of collagens in tumor growth is new to me. Likewise, I had no idea that protein sequencing had been automated. It must make finding genes responsible for a protein much easier, though I doubt actually as easy as anyone hoped.
I would like to see that soft tissue dated by every available means. There is no way it could have survived 80,000,000 years.
I remember reading the first couple of papers on this matter and being particularly impressed with the antibody staining results rather than the mass spec data (it was the mass spec data that got all the initial attention - both overly favorably in the beginning and then overly negatively when the contamination/bacterial film hypothesis was suggested). They have really strengthened the antibody side of the work in this new paper. As for the question of how proteins can survive this length of time - (nucleic acids almost certainly cannot - they are much more suscepible to biological and chemical degradation and frequently have radiactive phosphorus present that causes further degradation) its certainly an interesting point. WHat I've heard about this matter is that certain conditions (high levels of stable protein like collagen in close proximity to mineralized bone matter) can provide remarkably stable conditions that result in the partial preservation of these proteins under the right circumstances.