Wednesday, August 1, 2007

Mechanical Properties of Tissue and Anatomy

As most of you know, I am here in MN and just wanted to say that I was not on the bridge that collapsed. I actually drive that way to class all the time, but I am off from classes until Sept. I won't speculated on what happened in though I took tons of classes in grad school on solid mechanics (the advanced study of how crap breaks) and was a teaching assistant for Statics (study of forces on not moving structures). Bridges are a specialty on to themselves and I will leave that to the Civil Engineers.
I know the Red Cross is in need of help, so anything you can do to help would be greatly appreciated. If you can help, please click here for a link

Tissue Properties
On to the blog for today which I wrote a few days ago actually.
I am probably the only one that thinks of this stuff, but I was doing some research and found some cool biomechanics studies on tissue properties. I will save the diatribe about stress strain curves, Young's Modulus etc for another time (I am sure you are all disappointed).

The interesting thing was that many of the studies were done on tissue harvested from embalmed cadavers. Embalming fluid is typically contains a mixture of formaldehyde, methanol, ethanol and other solvents. Formaldehyde fixes tissue or cells by irreversibly connecting a primary amine group in a protein molecule with a nearby nitrogen in a protein or DNA molecule through a -CH2- linkage called a Schiff base. In a nutshell, it is a protein fixative, so when you replace blood with it, it fixes all the proteins in space. Think of freeze tag that you played as a kid and once your touched (hopefully not by formaldehyde) you are frozen in time. Thus you are now able to poke and prod at it in Anatomy class.

When doing this poking and prodding, you will want to wear gloves and once you leave lab you will swear all your clothes smell funny and have a profound instinct to shower (well maybe that was just me). Nothing beats a good old fashion anatomy lab where you can pass around once living total knee replacements and other tissue! Seriously, it is super cool (another technical term).

Since all the proteins are fixed in space, this will effect the mechanical properties of tissue and is a big limitation on studies using cadaver tissue that is embalmed.

Fresh Tissue
So why don't they just use fresh tissue instead? Good question. The short answer is that it is a pain in the ass to do the experiment! If you have ever done any experiments with fresh or living tissue you know what I am talking about. If the tissue is fresh, where does it come from? How long as it been dead (not living, but fresh meaning it has not been embalmed). How long can it be ok? What does ok mean? How fast does it degrade? How do you measure that? What type bath or means are you going to use for testing to try to "keep it alive"? What parameters are you testing, etc ad nausem and you get the idea. The short answer is it's more work and complicates everything. The take away is to read the studies carefully and think for yourself!

Antatomy Class Rocks
Now that I am on my soapbox, if you ever get the chance to take an Anatomy class, by all means do not pass go and take it! I've been fortunate enough (read, colleges have made lots of money off me) to have taken a full cadaver Anatomy class as part of my undergrad and later even took a few more classes and used the med school Gross Anatomy lab here at the U of MN. I was even able to prosect (aka, I got to cut on my own cadaver--yippeee). That is quite the experience and it soon became apparent that anatomy is MESSY! There is no open spaces in the body (ok maybe a few open organs like the bladder, stomach, etc ) and you will be soon praying for a lean cadevar since fat, fascia, connective tissue in general, and more fat is EVERYWHERE! Nerves are crazy small and hard to find also--they should give you more lab bonus points for finding them. If there is more than one cadaver there, by sure to look around for various pathologies aka "things gone awry" like DCMs aka dilated cardiomyopathies (big ass hearts), AAA aka abdominal aortic aneurysms (think of a hose that is about burst-yep, you don't want this), and all sorts of other things (rectum--damm near killed em).

None of these comments are meant that the grad students are not eternally grateful to those that donate their bodies to science. All the bodies are treated with the utmost respect, and it is a true privilege to be able really see inside.

If you get the chance to work with fresh tissue, it is very different. There is a big difference between frozen and fresh tissue also. When you freeze something you form ice crystals and this tends to destroy/soften the tissue to a great degree (think slimy jello jigglers). Fresh tissue on the other hand has a much firmer feel to it. I only have experience with fresh and frozen cardiac and lung tissue in this regards so perhaps other tissue is different. Being able to dissect fresh tissue is an amazing experience and you will soon appreciate the anatomy and it becomes very real.

Visible Heart
For an even more real experience, check out the visible heart done here at the U of MN. So far they have done over a dozen human hearts that were not suitable for transplant. The heart is placed on a permanent bypass set up (switched between a Langendorf and 4 chamber working heart mode). In the 4 chamber mode, all the flow is physiologic (Langendorf is sort of backwards) and the fluid circulating is clear so that a tiny camera can be placed while the heart is actually beating. Amazing footage--click here

The why do you care part
The take aways are 1) anatomy is extremely complicated but exquisitely designed and worth checking into and 2) engage your brain when reading research and don't just take the researchers conclusions at face value. Look at the results and feel free to draw your own conclusions (it is a free county, now trying to publish your conclusions is a different matter). Also watch out for how big a "leap of faith" you take with the data. Embalmed tissue will have different mechanical properties than the live tissue that is your body right now so be aware. I will save the soap box speech about testing in isolation vs integration for a topic later perhaps (as you all wait with baited breath I know).

Rock on
Mike N