The bones
Your body has about 270 bones at birth. Some eventually grow together (fuse) to form the 206 bones in the adult skeleton. Together, groups of bones and cartilage make up the skeleton, which gives shape and form to the body. Bones also protect important organs, such as the heart, lungs and brain.
Bones help us to move when they are attached to muscle. The bone marrow inside bone has other important functions, such as making blood cells and storing fat and minerals.
Types of bone
Bones may be described by their shape.
Long bones act as levers and help us move. The long bones are the humerus, ulna and radius in each arm and the femur, tibia and fibula in each leg.
Short bones provide support and help with movement. The bones in the wrists and ankles are short bones.
Flat bones shield and protect important organs. They are the bones of the skull, the breast bone (also called the sternum) and the ribs.
Irregular bones don’t fit into the other categories. The bones of the spine (called vertebrae) and the pelvis are irregular bones.
Cellular makeup of a bone
Bones are made up of bone matrix and different types of cells.
Bone matrix
Bone matrix (osteoid) is the tissue that surrounds bone cells. It contains minerals such as calcium and strong fibres called collagen. Together the minerals and collagen help make bones strong and stiff.
Bone cells
There are 3 main types of bone cells.
Osteocytes are mature bone cells. They help control the amount of protein and minerals in the bone matrix.
Osteoblasts are immature bone cells. They make bone matrix, which can harden into bone. Osteoblasts are found on the outer and inner surfaces of bone. When an osteoblast is surrounded by bone matrix, it matures into an osteocyte.
Osteoclasts are bone cells that break down and remodel bones as they grow or if there is stress on the skeleton. Osteoclasts also help control blood calcium levels because they release calcium into the blood as they break down bone.
When osteoclasts work faster than osteoblasts, bone becomes weak. When osteoblasts work faster than osteoclasts, bone gets bigger and stronger.
Structure of a bone
Bone is made up of the following layers.
The periosteum is a layer of fibrous tissue that covers the bone.
Compact bone is the dense, hard, smooth outer part of a bone. It surrounds the medullary cavity and contains blood vessels and nerves.
The medullary cavity is the hollow part of bone that contains bone marrow. The bone marrow makes blood cells and stores fat.
Spongy bone (also called cancellous bone) is made up of small, needle-like pieces of bone arranged like a honeycomb. Spongy bone helps to lessen the weight of a bone while still providing strength.
Cartilage covers the end of bones. It is a tough, fibrous connective tissue that stops the bones from rubbing together and acts like a cushion to absorb shock.
Long bones are divided into the following regions.
The diaphysis (shaft) of a long bone makes up most of a bone’s length. It contains mostly compact bone.
The epiphysis is at each end of a long bone. It contains mostly spongy bone.
The metaphysis is the part of a long bone between the diaphysis and the growth plate.
The growth plate (epiphyseal plate) is a thin disc of cartilage between the epiphysis and metaphysis. It allows a bone to get longer and disappears by about 20 years of age.
Bone growth
The growth of bone is controlled by
Long bones in the arms and legs, as well as the pelvis, develop from cartilage. These bones have growth plates that allow them to grow longer. The growth plates are made up of multiplying cartilage cells. Osteoblasts gradually replace cartilage with bone. Because girls mature at an earlier age than boys, their growth plates change into bone at an earlier age.
Some bones, including bones in the skull, the lower jaw (called the mandible) and the collarbone (called the clavicle), develop differently than the pelvis and the long bones in the arms and legs. These bones develop from a type of cell found in developing embryos, called a mesenchymal cell. Mesenchymal cells can change into the cells that make bone (called osteoblasts).