The adult human skeleton (or 'skellington') is made up 206 bones. The foetal skeleton starts as flexible cartilage, but within a few weeks it begins the process of ossification, as deposits of calcium phosphate and stretchy collagen are laid down.
A young human has zones called growth plates. These consist of columns of multiplying cartilage cells that gradually transform into bone. It takes about 20 years for this process to be completed.
Bone contains three types of cells:
Osteoblasts manufacture new bone and repair damage
Osteocytes transport nutrients and waste products
Osteoclasts sculpt and shape
Bones are composed of minerals including calcium, phosphorus and sodium, as well as a protein called collagen. Calcium is the stuff that makes your bones hard. Bones also act as a calcium store for the rest of the body.
In the centre of your larger bones there are groups of stem cells (bone marrow), which produce the body's red blood cells and platelets. Red blood cells carry oxygen to the body's tissues, and platelets contribute to blood clotting.
Bones are made up of two types of material
Compact bone is the hard, structural part of your skeleton. It is permeated by a complex matrix of tunnels, containing blood vessels and nerves.
Cancellous bone comprises the inner parts of bone. It is a spongy, mesh-like network, filled with red marrow, found mainly at the ends of bones, and yellow marrow, which is mostly fat.
Ligaments are fibrous straps that join your bones together.
Cartilage is a flexible, rubbery substance that absorbs shock and reduces friction where bones meet.
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