Obscure vertebral anatomy term of the day: bouton

Long-time readers will recall that I’m fascinated by neurocentral joints, and not merely that they exist (although they are pretty cool), but that in some vertebrae they migrate dorsally or ventrally from their typical position (see this and this).

A few years ago I learned that there is a term for the expanded bit of neural arch pedicle that contributes to the centrum in vertebrae with ventrally-migrated neurocentral joints: the bouton, which is French for ‘button’. Here’s an example in the unfused C7 of a subadult sheep. Somebody gifted me a handful of these things a few years ago, and I’ve been meaning to blog about them forever. Many thanks, mysterious benefactor. (I mean, only mysterious to me, because my memory is crap; I’m sure you know who you are, and if you ever read this, feel free to remind me. And thanks for the dead animal parts!)

Guess what? You have these things, too! Or at least you did; if you’re old enough to be reading this, your boutons fused with the rest of the separate bits of your vertebrae a long time ago, between the ages of 2 and 5 (according to Bagnall et al. 1977). Here’s a diagram from Schaefer et al. (2009: p.99) showing the separate centrum and neural arch elements in a thoracic vertebra of a human toddler. So, hey, cool, we all had boutons, just like sheep. And just like some sauropods. (You didn’t think I was going to do a whole OVATOD post without sauropods, did you?)

Here’s our old friend BIBE 45885, an unfused caudal neural arch (or perhaps neural ring) of Alamosaurus, which I’ve been freaking out over for five years now. Those fat bits of neural arch that very nearly close off the neural canal ventrally? Boutons, baby! Big, beautiful boutons. In this photo it looks like the paired boutons meet on the midline, but in fact they merely overlap from this point of view — there is a narrow (<1mm) squiggly gap between them. Given how narrow that gap is, I suspect the two boutons probably would have fused to each other before either of them fused to the centrum, if this particular animal hadn’t died first.

Here’s an unfused dorsal centrum of Giraffatitan, MB.R. 3823, which I yapped about in this post. This vertebra is the spiritual opposite of the Alamosaurus caudal shown above: instead of the neural canal being nearly enclosed by bits of the neural arch wrapping around ventrally, the neural canal is nearly enclosed dorsally by bits of the centrum sticking up on either side and wrapping around dorsally. As with the boutons of the Alamosaurus caudal, the two expanded bits of centrum stuff in this Giraffatitan dorsal approach each other very closely but don’t quite meet; you can fit a piece of paper between them, but not a heck of a lot more. In essence, those “two expanded bits of centrum stuff” are centrum boutons that project up into what I suppose we’ll keep calling a ‘neural arch’ even though it’s neither very neural nor an arch. Or perhaps anti-boutons? With apologies to Gould and Vrba (1982), here we have another missing term in the science of form.

Why do we, and sheep, and prolly lots of other mammals, and some sauropods, have boutons? Presumably to strengthen the neurocentral joints by expanding the joint surface area. I don’t know if anyone has ever tested that — if you do, please let me know in the comments.

Many thanks to Thierra Nalley, who may be the only person I know besides Mike who spends more time thinking about vertebrae than I do, for introducing me to the term ’bouton’ a few years ago. If for some reason you want to corrupt your sensibilities reading about primate vertebrae, you could do a lot worse than checking out Thierra’s papers.

I don’t expect we’ll have a ton of OVATOD posts, in part because there aren’t a heck of a lot of vertebra parts that we haven’t already blogged about. But who knows, maybe Mike will write about prepostepipophyses or something. Stay tuned!

References

• Bagnall, K.M., Harris, P.F., and Jones, P.R.M. 1977. A radiographic study of the human fetal spine. 2. The sequence of development of ossification centers in the vertebral column. Journal of Anatomy 124(3): 791–802.
• Gould, S.J. and Vrba, E.S. 1982. Exaptation—a missing term in the science of form. Paleobiology 8(1): 4-15.
• Schaefer, M., Black, S., and Scheuer, L. 2009. Juvenile Osteology: A Laboratory and Field Manual. Academic Press, Burlington, MA, 369pp.

P.S. Can we all pitch in and make ’bouton’ the new ‘aglet‘? Please? Please?