Pursuing praxis

So a couple weeks ago my ear got hurt during a Krav class. We were learning how to break out of headlocks. It’s good to practice near or on that line between go-easy-for-practice and real-world-hard. And, frankly, ears prevent your head from being essentially ball-like and hard to stabilize between flexed biceps and a ribcage. They are flexible but robust speedbumps that slow your exit from a scene you’re trying to remove yourself from.

Poor ears. It’s been a good 3 weeks and I still don’t like to sleep on my left side and mash my ear cartilage. I was wondering why it was taking so long to heal, which took me down the mental path of ear anatomy. And it’s kinda interesting.

As you probably know, there are no bones in the external ear on the outside of your head (called the pinna, plural pinnae). Like your nose, it’s held up and given shape by cartilage. Unlike your nose, though, ears have elastic cartilage, which is a bit different from hyaline cartilage, which is in your nose and is the most common kind in your body. For example, hyaline cartilage covers the ends of all the bones in your limbs, and the top and bottom of all the vertebrae in your back. There is a third kind of cartilage - fibrocartilage - that makes up the intervertebral discs in the spine, and the free-floating cartilage pads in your knee joints. It’s fibrous and tough, as you might have guessed.

Anyway, back to ears. Although the cartilage of your nose is softer than bone, it doesn’t like to bend very much. Contrast this with your ears. Fold that sucker in half. Fold it in half in the other direction. Bend it inside out. Let it spring back. This is elastic cartilage. Throw on a network of blood vessels, and some skin more or less closely attached, and you’ve got a pinna. There are some cool modifications to the hair follicles inside your ears, and I could discuss muscle attachment points and the ability to wiggle your ears, but let’s stay on cartilage.

Why so long to heal? I think it is explained by the nature of cartilage as a tissue. Unlike most tissue, cartilage has relatively few cells in it. The cartilage cells (literally Latin-ized into chondrocytes) are comparatively large and widely scattered. They sit in a matrix of cartilage, so they don’t get out much, so to speak. They are responsible for building, maintaining and (presumably) assisting in repair of damaged cartilage. They most usually (I won’t say always, because I don’t know) are produced by chondroblasts (cells whose job is to produce more cartilage cells, which mature into chondrocytes). Perhaps chondroblasts don’t produce chondrocytes as quickly as in growing tissue, or they are relatively rare in mature tissue.

In any case, the non-cellular part of cartilage is mostly composed of collagen fibers, glycoproteins (sugar-coated proteins that attract and bind to a lot of water), water, various ions, and elastic fibers. All of these are outside cells, so if they get damaged, they just sit there until cells come by and fix things up. Hmm… bone is about equally low on cells and high on matrix (although bone is quite a bit different than cartilage), and you know how long it takes a bone to heal, even just a little fracture. Detection of damage, mobilization of repair processes, repair, and return to equilibrium are all pretty slow processes in bone as compared to, say, muscle or skin (very cellular tissues).

Pictures:
Here’s a picture of the tissue of the ear. The cartilage is the darker band in the middle with big cells. They are wierdly big and bubbley in appearance; very typical of chondrocytes. The darkness of the cartilage comes from the elastic fibers; they stain more darkly than other types of tissue with this kind of staining technique (but they are too small to see individually at this magnification). (Here’s a closeup of the elastic cartilage).

In the first picture, you can see the circular blood vessels in the lower half of the slide. The big pink one in the lower middle is an artery; the big pink band around it is the smooth muscle of the arterial wall. Arteries have more muscles than veins, which are weak, wimpy and passive by comparison. The two smaller vessels to the left of the artery are veins. At this magnification, I can’t tell whether the other vessels in the picture are arterioles (small arteries) or venules (small veins); they get a bit harder to tell apart as they get smaller. At the top you can see the numerous, compact layers of cells forming the outer-most layer of skin: the epidermis. Below that is a lot of loose, less-organized connective tissue that helps connect the skin to the cartilage and provide a bit of padding. I don’t know what that dark horizontal streak is in the top half of the slide.

If you’re still with me … contrast this with hyaline cartilage (good side-by-side comparisons at this site). More space between the chondrocytes, no dark elastin fibers. And finally, fibrocartilage is characterized by a rather wavy appearance, due to all the collagen fibers in the matrix. They are usually oriented relative to the plane of strain.

More complicated, but more beautiful, pictures of cartilage:
Hyaline cartilage from the trachea (those bumps in your throat are C-shaped rings of cartilage that keep your windpipe propped open).
Fibrocartilage from - I think - the pubic symphysis. The stacks of bubbley chondrocytes are probably hyaline cartilage that lines the pubic bones (I don’t know for sure though); the mature fibrocartilage cells are very dispersed in the wavy fibrocartilage matrix. But it’s supercool how that matrix appears to extend, by interlaced filaments, to the actual bone. Makes sense. The pubic symphysis cartilage isn’t a distinct object like the discs in your back or in your knees. The job of the symphysis is to bind together the two pubic bones so you have an intact, bowl-like pelvis that doesn’t wiggle around when you move. You wouldn’t be able to run if your pelvis flopped about (it’s got 6 bones and a sacrum that make it up, afterall).

Interestingly, the symphysis changes during child labor (that is, the end of pregnancy, not kids working in coal mines). The hormone oxytocin acts to degrade the symphysis so that the pubic bones can pull apart a bit more and make room for the baby passing through. I can’t say that sounds like fun. But hey, I’m here, and I came into the world by the normal route, so I know it works.

These are essentially 2D images. The somewhat 3D appearance of the fibrocartilage slides is a very cool optical illusion. 

Other random, cool slide photo: the pyramidal cells (neurons) of the cortex of the brain. They are specially dyed to show up, to the exclusion of the myriad supportive cells surrounding them. I also read that for some unknown reason, only about 1 in 10 pyramidal cells actually stains. There are about 100 billion neurons in the adult brain, and about one trillion support cells. I’ve read that a normal, healthy adult loses anywhere between 9,000 and 80,000 brain cells a day. Just estimates. Not my area of research, either. Still: take care of your brain! Encourage it to grow (yes, physically, by exercising it mentally), and you’ll have scads more brains (literally) by the age of 85 than if you neglect or abuse your brain!