Bones develop in two different ways. Most of the long bones develop from a softer cartilage which is gradually replaced by bone in a process called ossification. Flat bones develop by minerals such as calcium and phosphorus being deposited on membranes, without going through a cartilaginous stage. In the unborn baby the first rudiments of bones appear as soft cartilage. At just eight weeks after fertilization the first signs of true bone appear. Groups of osteocytes form at the centre of the stem of cartilage, called the primary ossification centre. Enzymes in the body fluids of the baby cause theseto lay down calcium phosphate in the first stage of ossification.
This process then continues, spreading outwards from the centre of the shaft to the ends. The shaft of the new bone thickens by other bonein the periosteum laying down minerals on their inside. Once a baby is born the ossification process starts at new sites – secondary ossification centres – at the ends of the long bones, the epiphyses. Even in adults who have stopped growing, bone is continually being remodelled and reshaped, with old bone being removed (resorption) and new bone being deposited (accretion). This is normally well balanced, with an equal amount of bone being made and destroyed. As we get older, resorption occurs faster than accretion; if we are inactive for long periods bone resorption also speeds up.
Exercise, on the other hand, stimulates the bone to grow, strengthening it along the lines of greatest. It is thought that activity encourages bone formation by stimulating electric currents to travel through the body. The rate of bone formation is also affected by levels of and certain vitamins in the body. Parathyroid hormone, secreted from glands in the thyroid, stimulates the breakdown of bone, usually to raise low levels of calcium in the stream. If the level of calcium in the is high, parathyroid hormone is not released. Vitamin D, obtained both from food and sunlight, also influences bone formation. This vitamin is needed for the body to absorb calcium from food as it passes through the gut. If the body is lacking in vitamin D, calcium is not absorbed and the bones start to atrophy. This is the cause of the condition called rickets in children and osteomalacia in adults. In rickets the bones fail to harden as the child grows and the soft bones often become deformed. Growth hormone, released by the pituitary gland, also stimulates the body to make more bone. It is naturally important in children as they grow, but also plays a role in adulthood by influencing the pancreas’ release of sugar into the stream.
Breaking and mending
Bones are remarkable in their ability to repair themselves if damaged. When a bone breaks (fractures), blood from vessels in the bone clots, and cells from the ends of the broken bones move into the break and start to form a fibrous network. The join is completed by osteocytes laying down hard bone. The area of new bone made around the break, called the callus, is at first large and unwieldly and the bone may be misshapen and lumpy. But, over several months the bone becomes remodelled into its former shape. In young children, with their softer and more flexible bone tissue, bones tend to split rather than snap (this is called a ‘greenstick’ fracture). Older people, whose bones are more brittle, also take longer to recover from fractures.