Here's the low-down on bones and some of the diseases that can affect them. Bones are formed in three ways:
At birth: Cartilage turns into bone.
At post-birth: You're born. You're a baby. You're surrounded by all these adults saying, 'Eat you're greens, or you'll never get big and strong!' You're probably ignorant of the fact that bones mostly require calcium, phosphorus, and Vitamin D to form, all of which are not in your greens. But you eat your greens anyway, you get big and strong - so strong you don't have to share your toys with the other children or wash behind your ears. In this time, your infant bones are gone and you have adult bones. This is the process of 'modelling'.
Remodeling: This is maintenance. Picture yourself on a highway in Nevada, surrounded by desert, and you're on your Harley-Davidson. All of a sudden, your front tyre hits a pot hole and you crash. While you're in hospital, recovering from your wounds, you scream at the highway department to repair the highway. Being a bit lazy, the workers do not replace the entire highway but rather dig up a bit of the pot hole, refill it with new pavement, and go down to the roadhouse for lots of beer and to listen to Bruce Springsteen music. Much of the same is going on in your own body as you recover from the motorcycle accident. The bones are fractured and they recruit cells (workers) to fix the problem. The first set are the osteoclasts. Think of them as demolition men, cells that degrade bone. The second set are the osteoblasts. Osteoblasts reform bone. The final set are the osteocytes. Osteocytes are retired osteoblasts. The osteocytes help recruit osteoclasts and osteoblasts to do useful work. The process of osteocytes recruiting cells, osteoclasts removing damaged bone, and osteoblasts filling in new bone is much like pavement remodelling.
But what's all this got to do with bone disease? Well, many bone diseases have to do with the remodeling process going wrong. This can happen in many ways. With loads and loads of regulatory molecules governing remodeling, it isn't surprising that just one going wrong could have disastrous results. Here's a very quick synopsis of just three different disease types:
Osteoporosis: This is what your grandmother might have. What happens is that she has stopped making oestrogen, a sex hormone that regulates many activities, including bone remodeling. Think of oestrogen as the important guy in the yellow hard hat who tells the demolition men that although blowing up an entire building might be fun, they are only supposed to dig up a sidewalk. When oestrogen, the guy in the yellow hard hat, is gone, the osteoclasts go berserk, digging up bone left, right and centre. The osteoblasts, who are supposed to repair the bones the osteoclasts dig up, can't keep up. The net result is decreased bone density. So, when your grandma falls down, she is going to break her hip.
Osteopetrosis : You know how Superman has 'Bizzaro Superman', who is like Superman only his opposite? Ostopetrosis is bizzaro osteoporosis. In this case, the osteoclasts are on strike and don't degrade any bone. The osteoblasts, who are bored, decide to build new bone anyway. This results in increased bone density. In this case, more is definitely not better and several problems may result, including deformity and joint problems.
Paget's disease: Now our worker cells are going mad, working at an accelerated rate, so fast that they get sloppy. Instead of blowing up an ancient building and replacing it with a nice looking hotel, they blow up ancient buildings and replace them with cardboard boxes. In more technical terms, remodeled bones are no longer highly structured. Instead, they are weak and brittle, and this is a bad thing.