Monday, March 1, 2010

Which tissues are anisotropic?

Today in my biomechanics class we moved out of molecular mechanics and into tissue mechanics. This will involve a lot of stereotypical mech-E stuff like stress and strain and elasticity that all us biologists have never heard of before -- deterministic, bulk properties of continuous solids (or gooey things), rather than stochastic models of single molecules or long chains of molecules being buffeted around by thermal motion.

In this field, as in every other, you have to make simplifying assumptions. Two of the key assumptions we often make about bulk materials are that they are homogeneous and isotropic. That is, they're uniform throughout, and they're the same in every direction. Tissue like muscle (with fibers) or substances like string cheese (also with fibers) are anisotropic (there's your five dollar word for the day). If you're stretching them and you want to find out how they deform, it matters which direction you're pulling. After we learned what all these fancy words meant, we categorized a few tissues:

  • Liver

  • Fat

  • Blood

  • Cartilage

  • Muscle

  • Bone

  • Skin

  • Ligaments

As I was writing these down, I noticed a pattern. All the isotropic tissues are what I'd call "biochemical tissues". Their main role is to store chemicals or make reactions happen. In contrast, all the isotropic tissues are "mechanical tissues", that make stuff move or stick together. Huh! Insight! But then it occurred to me that this makes total sense. After all, if you're doing a chemical reaction in a test tube, and you pour it into a bowl, the reaction will still happen. Whereas if you suddenly make all the fibers in a muscle run in a different direction, the muscle will do something completely different. If a tissue's job is to apply a force, it needs to apply that force in the right direction. Biochemical tissues are OK being isotropic, because their job is isotropic, and it seems like it ought to be harder for a developing organism to grow an anisotropic tissue, so why bother.

(Cartilage goes under "isotropic tissues", not because it does much in the way of chemical reactions, but because it's basically just a cushiony substance.)

Neat, huh?

Addendum: what about neurons and nerve tissue?

Well, we can throw out "neurons" straight out, because neurons are single cells, and tissues can only be treated as bulk materials if you've got a lot more than single cells. Trying to treat tissues on the tens-of-micrometers scale as bulk materials is like trying to calculate the viscosity of minestrone. It just doesn't apply. The viscosity of chicken broth is way different from the viscosity of beans. It only works if you zoom way, way out so that the effects of all the little bits and bobs become uniform over the entire blob of whatever you're looking at.

But we can totally look at nerve tissue this way. Your spinal cord, I would imagine, is a little bit like string cheese or rope. It's a whole bunch of long cellular fibers in parallel. If you pulled on it lengthwise, I would expect it to stretch; if you pulled on it widthwise, I would expect it to fray apart. It's anisotropic.

Gray matter in your cortex, on the other hand, I would call isotropic, or at least closer to isotropic (the cortex does have layers after all). Gray matter is mostly made up of cell bodies arranged more or less randomly, not a bunch of fibers all aligned with each other. Having dissected the odd brain or two, I think the best comparison for gray matter might be a firm jelly. (Exactly how firm it is depends on whether it's been preserved and how.)

So, does nerve tissue fit into the "biochemical" vs. "mechanical" tissue dichotomy? Not very cleanly. Then again, it's an atypical tissue. Its job is to send signals from point A to point B. For spinal cords and nerve bundles, point A and point B are far enough apart that it becomes important, on a macro scale, for the cells to go in the right direction. Gray matter contains mostly cell bodies rather than axons, so it's like a pile of Point As (or Points A, for the Captains Pedantic), whose job is to turn chemical signals into electrical activity.

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