Bones grow
When you were first growing inside your mother, your skeleton was not made of bone, it was made of cartilage. Over the years since then your skeleton of cartilage has been slowly changing to bone and the process will not be complete until you are over twenty years old.

You may think that you would not survive if your skeleton were made of cartilage but some animals have a skeleton of cartilage all their lives. These animals are a group of fish simply called the cartilaginous or gristly fish. The fish in this group are sharks, rays, skates and dogfish. They all have a skeleton made of cartilage to support them and help them move through the oceans of the world.

How does a bone form in a skeleton of cartilage? It might help to think of a leg bone. This bone is long. The middle part between the two ends is called the shaft and it is here that the cartilage begins to change. Bone forms in the sides of the shaft while the rest of the shaft and the ends remain made of cartilage. Changes begin, inside the shaft, which destroy the cartilage and make a hollow space, which fills with marrow. While this space is forming, the rest of the shaft wall turns to bone.

At the ends of the bones changes are taking place too. Bone is starting to form deep in the cartilage and grows up towards the surface and down towards the shaft. Bone never reaches the surface of the end. The end always remains covered in cartilage so that a joint can be made where the bones move freely. Lower down, where the end of the bone meets the shaft, more bone is produced to make the shaft grow longer. This place can produce new bone for many years. As new bone is added to the shaft your height increases. Eventually bone production stops and the ends of the bone join or fuse with the shaft to make the bone complete.

The time it takes a bone finally to stop growing varies. The bones of the foot may be fully formed by the age of twelve or as late as twenty two. The long bones of the leg may be complete by the age of fifteen to twenty five while the skull is fully formed at sixteen and the fingers by twenty-one years of age. Other bones have different rates of growth but by about thirty-five years of age your skeleton will have stopped increasing in size.

Bones not only change their size they can also change their shape. This is due to the activities of two types of cells in the bone. You can think of a cell as a microscopic box filled with living material. The body is made from billions of them. The two kinds of cell in bones do different things. One kind builds up the bone and the other breaks it down. This may seem a bit strange but the two kinds of cells work together to shape the bone as it grows. For example, imagine that you had to wear shoes that were too tight for you. The cells in your bones would reshape your toe bones to fit in the smaller space. This would result in the bones becoming deformed. That is why it is important to wear shoes that fit properly. They give your feet space to grow freely and form the shape that is best for supporting the rest of your body.

The two kinds of cells also work together well if you break a bone. At first the bone building cells get to work and repair the damage but make a thicker ring of bone than was present before the break. The bone destroying cells get to work on this ring and reduce its size so it is almost impossible to tell that a break has occurred.

Next time you measure your height think of the cells in your bones busily building up the bones and shaping them. Think of the places just below the ends of the long bones in our legs, which are gradually increasing in size and pushing you higher into the air.

How can sharks survive with a skeleton of cartilage?
The water helps to support the body of the shark. This takes the strain off the backbone and allows it to be made of cartilage instead of stronger bone.

Could animals with a skeleton of cartilage live on land?
Probably not. The skeleton would be too weak to support all the parts of the body such as the lungs, heart and intestines and they would press on each other and stop working. If the animal had legs they would bend so much that the animal would not be able to walk.

Why do we have a skeleton of cartilage first?
Most scientists believe that each kind of animal has developed from some other animal in the past. For example, they believe that animals such as dogs developed from wolves. They think this happened because dogs have many features in common with wolves. In a similar way, scientists think that animals with a skeleton of bone developed from animals which had a skeleton of cartilage. This helps to explain why we go through the cartilage skeleton stage as we grow before we develop our skeleton of bones.

Does that mean that we have developed from sharks?
No. It means that we, and all other animals with a bony skeleton, developed from a simple fish-like animal which had a skeleton of cartilage. These changes took place over hundreds of millions of years. During that time sharks also developed from the simple fish-like animal too.

Why is cartilage in the joint?
Cartilage is tough and has a smooth, slippery surface. The toughness prevents the cartilage wearing away and allowing the bones to come together. This would make movement painful as bones have nerves in them. The slippery surface also makes the ends of the bones move easily over each other.

Doctors look at X-ray photographs for broken bones. How can they see the bones?
Energy can move through air and space as waves. We are sensitive to two kinds of waves that move in this way: light and heat. Light is stopped by an opaque object but can pass through a transparent one. Heat waves strike the surface of any object and warm it up. X-rays are also waves of energy but they have a special property. They can go through flesh but cannot go through bone. When X-rays are shone onto a broken limb they pass through the skin and muscles and make a black mark on a piece of film. The X-rays that reach the bones are stopped and so a white mark is made on the film. The white mark makes the shape of the bone on the film.

What happens to bones when you grow old?
They go smaller again. That is why old people become shorter. The bones also become weaker and break more easily. That is why old people need to take more care when they are moving around and other people should give them extra consideration.

Should everyone's bones grow at the same rate?
No. People vary a great deal in the times they grow, the speed at which they grow and how much they grow. All these variations are normal.