The bones
Bone is a strong and stiff connective tissue made up of minerals and collagen. Bones carry out many important functions in the body including:
- supporting the body’s tissues
- protecting important organs, such as the heart, lungs and brain
- working with muscles, tendons and ligaments to allow the body to move
- storing fat and minerals
- releasing minerals into the blood when the body needs them, such as calcium, phosphorus and magnesium
- making blood and immune cells in the bone marrow of some bones
At birth, the human body has about 270 bones. As we grow, some of these bones fuse together to form the adult skeleton. The adult human skeleton is made up of 206 bones.
Types of bone
Bones may be classified based on their shape.
Long bones are long and thin. The long bones include the humerus, ulna and radius in each arm, and the femur, tibia and fibula in each leg.
Short bones are often smaller and roughly cube-shaped. The bones in the wrists and ankles are short bones.
Flat bones are thin and often curved. The bones of the skull, breastbone and ribs are flat bones.
Irregular bones have complex shapes and canʼt be classified into the above categories. The bones of the spine (called vertebrae) and the pelvis are irregular bones.
Bone cells
There are 3 types of bone cells.
- Osteoblasts build new bone.
- Osteoclasts dissolve and remove old bone.
- Osteocytes maintain bone by carrying nutrients and removing waste.
Bone is very active tissue. Bone tissue is constantly being remodelled, meaning that it is being replaced by new bone tissue. Osteoblasts are building new bone while osteoclasts are removing old bone. When osteoblasts work faster than osteoclasts, the bone gets bigger and stronger. Bones can also be remodelled and heal after an injury. Along with bone cells, some hormones and minerals help bones remodel and heal.
Structure of a bone
Bone is made up of different layers of materials. Several terms are used to describe the different layers.
The medullary cavity is the space inside the bone that contains yellow bone marrow. The yellow bone marrow mainly stores fatty tissue.
Spongy bone (also called cancellous or trabecular bone) is very porous with lots of open space within it, sort of like a honeycomb. It surrounds the medullary cavity. The short bones are mostly made up of spongy bone.
Compact bone (also called cortical bone) is dense, hard and smooth. It forms the outer layer (or wall) of a bone. Compact bone surrounds spongy bone and yellow bone marrow in the shaft of the bone. Long bones are mostly made up of compact bone. Flat bones are made up of two layers of compact bone with a layer of spongy bone between them.
The periosteum is a thin layer of fibrous tissue that covers the outer surface of the bone, except at the ends.
Cartilage covers each end of a bone. It is a tough, fibrous connective tissue mixed with a gel-like substance. It is softer than bone, but firmer than most tissues. Cartilage stops the bones from rubbing together and acts like a cushion to absorb shock. It’s in the joints, spine, ears and nose.
There are 4 main parts of a long bone:
The diaphysis (shaft) makes up most of a bone’s length and is in the middle of the bone. It contains mostly compact bone.
The metaphysis connects the diaphysis to each end of a long bone.
The epiphysis is at each end of a long bone. It contains mostly compact and spongy bone.
The epiphyseal (growth) plate is between the metaphysis and the epiphysis. It is a layer of cartilage that allows the bones to grow longer. The epiphyseal plate gets smaller as children get older, and it disappears around the age of 20.
Bone growth
Bones grow throughout childhood. This growth is controlled by hormones, and supported by vitamins and minerals such as vitamin D and calcium. Bone growth ends by 18 to 25 years of age.
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