Bones Are Not Stone-Like Inanimate Objects. They are fully alive, and Continually Tearing down and renewing.

Most people’s knowledge of bones comes from the remains of a meal, or from those that amuse their dogs. They are seen as hard, rock-like, hollow, and inert. They look like they will last hundreds of years. These are dead bones. If they have been outside for a while, all that’s left is the calcium deposits that make it so hard. This is deceiving. A bone is quite complex, and is often considered an organ. Besides providing structural support, it also acts as a mineral depot, and houses the cellular factory that manufactures red and white blood cells. It has an elaborate blood supply and is continually tearing itself down and rebuilding itself.

A live bone is a lot stronger than Rover’s plaything. A bone is constructed rather like a steel reinforced concrete pillar. Concrete is immensely strong under compression, but not strong at all if pulled apart: a tension type stress. For this reason, reinforcing steel bars, called rebar, are embedded in the concrete pillars to provide the needed tension strength. Civil engineering copied this from bones. The hard mineral part of a bone is quite strong for compressive sorts of forces. It’s built around a cellular generated matrix made of collagen, which has enormous tension strength, just like the rebar. Ligaments and tendons are also made of collagen. Collagen is made of fibrous strands, and for its weight, is actually stronger than steel.

This structure is maintained by a busy construction crew of bone cells. The three types of interest are osteoblasts, osteocytes, and osteoclasts. Generally speaking, in cell-world, -blasts make things, and -clasts tear it down.

The first two of these three bone cells are differentiated from mesenchyme stem cells, which are plentiful both inside bones and in the surrounding sheath. We recently posted on this type of cell here.

To build new bone, the osteoblasts tightly align themselves in a rectangular formation. They then secrete collagen, forming a very strong scaffolding like matrix. To this they attach mineral crystals made mainly from calcium. This is the bone material. As they build layer upon layer of this, they effectively wall themselves in. At this point they are renamed osteocytes. However, they do not die. They live on, and still have several functions. Thus bone is living tissue, even the “bony” part.

How Bone is Made

How we get bone in the first place is quite complicated. A baby has mainly cartilage. This is replaced by bone via an elaborate process that we won’t go into, assuming here that you are already grown-up, and have a full set of bones. Let’s consider the part we normally think of as bone, the hard stuff. It is a bunch of layers of osteocytes, which are alive, but imprisoned by the hard calcium wall they themselves built. Surrounding this is a tough layer called the periosteum. It is loaded with mesenchyme stem cells, and will, if conditions are right, lay down another layer of osteoblasts, which will form a fresh collagen matrix, already strong, and then coat that with the mineral bone material, making new bone. The periosteum also has a very strong outer sheath that will tend to hold the bone in place in case of a fracture. It basically acts as a built-in cast.

New bone tends to be made where it is needed. A weight lifter will build new bone in his or her back and legs. If a jogger develops new bone it will be in his or her feet. (Sadly, they usually lose it elsewhere as ‘the body always makes sense’ and does not need to be carrying around unneeded weight just to jog, as the body interprets this as survival behavior.) These are easily seen on X-rays as striations of calcium.

The body “knows” where to build bone because of an amazing property of collagen. It is piezoelectric. Remember piezoelectricity from high school physics? If you put pressure on a piezoelectric crystal, an electric field results. Bone is built over a scaffolding of piezoelectric collagen. Where bone is weak, the collagen scaffold gets pushed and twisted and creates an electric field that attracts new calcium and other minerals.

The bones also play a major role in calcium regulation. Very precise regulation of calcium is important, and the body uses the bones as a storage depot. To regulate calcium, the body will secrete various hormones that either cause the bones to release calcium, or to take it up.

Bone is Also Torn Down

Bone is torn down for two reasons. First is repair. Although fractures are rare, micro-fractures are common and inevitable. They are just what they sound like. Second, if the body needs calcium, it will tear down bone to get it.

The cells that tear down bone, called osteoclasts, are odd indeed. They have multiple nuclei (five of them typically) and are huge: about five times wider than a normal cell. They look a bit like leeches. It appears these form by fusing multiple white cells together, meaning that they are effectively part of the immune system. However, instead of gobbling bacteria or other invaders, they go after bone. But which bone? The huge cell-leeches are looking for micro-fractures. Micro-fractures occur wherever a rigid structure is used over and over. Remember the walled in osteocytes? If the micro-fracture involves the bone they made, they secrete a chemical that attracts the osteoclasts. The osteoclasts arrive, attach, and dissolve the bone and collagen matrix. The grand plan is that osteoblasts will come along after this and build new bone.

Bone Renewal is Regulated

About 10% of bone is renewed per year. Bone density peaks at about age 21 at around 200 mg/cc. These are strong bones. At that point, the body starts tearing down more bone than it is building, losing bone at a rate of about 2% per year. At first this has little effect, but eventually, bone mineral density is down to around half, at 105 mg/cc. This is the fracture threshold. The bones can no longer support the weight required of them, and start to crush in on themselves. This is osteoporosis and can happen to both men and women. It can be stopped and reversed by putting more demand on the body’s large bones through exercise. The Quantitative Medicine protocols describe how. This is covered here and here.

Take care of your bones.