We all know what bones are, right? The hard bits that your skeleton is made up of come in loads of different shapes, are made of calcium and are not really alive.
Wrong, as it turns out.
Yes, bone is hard, but there are also soft bits and it is constantly, if slowly, being destroyed and reformed. How else do we heal from fractures?
But there’s more to it than that.
Starting off with the obvious, the hard bit does contain calcium in the mineral form calcium phosphate which is what provides the traditional hardness to the bone, and is the reason that bones can snap under enough pressure.
The reason that it takes ‘enough pressure’ to snap a bone is because the calcium phosphate crystals are found within and surrounding fibres of the soft tissue component, the vast majority of which is made up from collagen – the same protein that can apparently reduce skin wrinkles. This soft collagen scaffold allows the bone to bend, squash or stretch slightly when pressure is applied i.e. it makes the bone less brittle. It’s possible to dissolve the calcium phosphate by soaking a bone in vinegar for a few days/weeks, leaving just the bendy collagen scaffold behind, and there are plenty of YouTube videos that show the results.
(None really worth linking to here but if you don’t believe me please feel free to have a look for yourself).
Something has to make all this in the first place, which must mean that bones are somehow alive. The cells responsible for producing the collagen are called osteoblasts (from the Greek for ‘bone’ [osteo] and ‘germinate’ [blast]; Wikipedia). The osteoblasts produce the collagen inside themselves then secrete it, where it self-assembles into long fibres that interlink to form the scaffold. This scaffold then mineralises with calcium phosphate to provide more rigidity.
As it grows, bone is subjected to forces produced by the muscles which encourage it to arrange the collagen scaffold in a way that resists the dominant direction of force. In a chimpanzee, the arm bones have to resist stretching forces which could just pull the bone apart as the chimp hangs from a tree. However, chimpanzees also use their arms for walking on so the bones need to be able to resist bending in the middle when weight is put on them. So forces can often act in more than one direction on the same bone, meaning that the collagen needs to be arranged in different directions within the same bone.
As a result, bone grows in distinct layers around tiny canals which house things like blood vessels and nerves:
In each layer, the collagen fibres can be orientated in different directions to deal with various directions of force, resulting in something which can look like this:
Normally as collagen is deposited and mineralised the osteoblasts ride up on the surface of the bone, but occasionally they become trapped within the bone and surrond themselves to create a cavity which show up in fig. 1 as the dark black blobs. These cells are now known as osteocytes (‘cyte’ meaning ‘container’ in Greek; Wiktionary) although it is used in science to refer to a cell. So, literally: osteocyte = bone cell. Fig. 1 also shows tiny connections between the osteocytes, and it is thought that these could be used for communication to co-ordinate the deposition of collagen in different directions in each layer.
I’m not happy with the word death but lets go with it anyway. A third type of cell is responsible for the destruction of bone – the osteoclast (from the Greek ‘klastos’ meaning ‘broken’; Wiktionary). To break down the bone, osteoclasts seal themselves very tightly to the surface and secrete acid onto the surface to dissolve the calcium phosphate, then break down the collagen using enzymes. Osteoclasts leave a scalloped outline as they move across the surface of a bone (figure 3).
This can pose a problem in later life as resorption (fig. 3) exceeds growth, leading to osteoporosis where bones become weaker and more delicate. However, resorption is also a part of normal turnover of the cells. Figure 4 shows how osteons (fig. 1) can been destroyed and eaten into by newer ones during the normal life of bones. Our entire skeleton is completely renewed every 10 years or so.
Two Types of Bone.
So what we saw above is the actual fabric of bone itself, but how that’s arranged on a slightly larger scale can produce 2 different types of bone.
This is dense bone of the sort described above and is pretty much solid. It is found around the outside of bones where it provides greatest strength (see figure 5). It is also relatively heavy.
So called because it looks like a sponge, not because it is actually squashy, although it’s not quite as strong as cortical bone. It is a lot lighter however, because as figure 5 shows it has lots of air spaces within it.
The most important part to realise here is that the struts you can see here are not individual osteons as in figure 1, but that thousands of osteons and blood vessels and nerves are inside each strand of bone visible in spongy bone.
All About Bones
And that’s it! Bones are constantly changing and are slightly soft, and we can tell what they were designed for by looking at the proportion of cortical to spongy bone, and by looking at them through a powerful microscope to see the arrangement of the collagen scaffold. This has important implications for working out how our ancient ancestors moved around 4 or 5 million years ago when they were coming down from the trees. I think this is very possible, since the collagen component has recently been isolated from dinosaur bones between 140 and 65 million years old.
Until next week,
As soon as I find references I shall update this post. I do not claim to own any of these images.
Bone Research society: http://www.brsoc.org.uk/
Dinosaur Bones: Schweitzer M H, Wittmeyer J L, John R Horner J R (2007). Soft tissue and cellular preservation in vertebrate skeletal elements from the Cretaceous to the present. Proceedings of the Royal Society Biological Sciences. Vol. 274 pp. 182-197. Freely available.
Giraud-Guille, 1988. Twisted plywood architecture of collagen fibrils in human compact bone osteons. Calcified Tissue International. Vol. 42; pp. 167-180.
Gray, 1918. Anatomy of the Human Body. Available from: www.bartelby.com. Accessed: 01/11/13, 11.46 am.