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A Close Look at Human Bones

Bones play an important role in the human body. They help us move around and give support and protection. Bones have special structures that allow them to carry out all of their important jobs.
 An illustration of the parts of a bone.
 

How Do Bones Form?

When you are born, you have about 300 bones in your body. By the time you become an adult, you will only have 206 bones! This is because when you grow, some bones fuse together, so two bones can become one. 
 

Many bones start off as cartilage, which is a rubbery material. Gradually the cells in the cartilage turn into solid bone. This is ossification. Some cartilage never turns into bone. You can feel cartilage in your ears and nose – it’s squishier than bone! 

An illustration fo the human ear, which is made of cartilage.

Your ear is made of cartilage, which is why it is squishy!

Over time, parts of your bones get old. Special cells inside of the bones absorb old bone tissue and help make new cells. Your bones are constantly changing, just like other parts of your body.
 

What are Bones Made Out Of? 

You might not realize it, but every bone in your body is a complicated structure with lots of layers! On the outside of every bone is a layer called the periosteum. The periosteum helps the bone grow, provides it with blood, and gives muscles a place to attach to the bone. 
 
Underneath the periosteum is a layer of compact bone. This layer gives the bone its strength. It is dense and mostly solid but has a few canals for nerves and blood to get through. 
 
Under the compact bone is the cancellous bone. It is also called spongy bone. This part of the bone is lightweight but still very strong. In some bones, red bone marrow fills the holes in spongy bone. Red bone marrow is important because it makes blood cells. At the very center of a bone is a hollow tube called the medullary cavity filled with yellow bone marrow. 
 

Bone Injury 

When a bone breaks we call it a fracture. Because bones are living organs they are able to heal themselves. A fracture damages blood vessels, which causes surrounding bone cells to die. Within a few hours, a blood clot forms and the whole area swells in response to the dead bone cells. New blood vessels grow in the area, bringing cells to clean up the damage.
 

Other cells form a callus of fibrous cartilage that eventually becomes new bone. The final stage is the remodeling of the new bone into its proper shape. The whole process can take months and isn’t much fun! If you break a bone you will have to go to a doctor who will take an X-ray and give you a cast. 

An image of small children playing football.

It’s important to protect your bones. Make sure to wear the right equipment when you play sports!

Bones and the Skeletal System

Bones are all part of an organ system called the skeletal system. There are different types of bones in the skeletal system. Each type has a specific job. Some bones help protect organs while others help you move. 
 
Visit the Skeletal System and Types of Bones pages to learn more! 

Other Great Resources

Keeping Your Bones Healthy: https://kidshealth.org/en/kids/bones.html

Ducksters on Bones: https://www.ducksters.com/science/bones.php

Skeleton and Bone Facts – Science Kidshttp://www.sciencekids.co.nz/sciencefacts/humanbody/skeletonbones.html

 

Written By: Leah Tolby