Bones can be separated into five different bone types, which are explained below:
- Long bones - these are the bones connected with large movement. They are long and cylindrincal with growth heads - epiphyses (singular epiphysis - pronounced epi-physis) at either end. The epiphysis is covered by articular cartilage. The outer layer of the bone is hard, and is called "compact bone". The inside of the bone is spongy, called "cancellous bone". Examples of long bones include the femur (thigh bone), the humerus (upper bone in the arm) and the phalanges (fingers and toes).
- Short bones - these bones are almost cube shaped and associated with smaller, more complex movements. Examples of complex bones include the carpals (small bones in the base of the hand) and tarsals (in the feet).
- Flat bones - these bones protect the internal organs and include the skull (cranium), ribs, scapula (shoulder blade), sternum (breast bone) and the pelvic girdle.
- Irregular bones - these bones are irregular in shape and include the vertebrae and some facial bones.
- Sesamoid bones - these are small bones held within tendons and include the patella (knee cap). Cartilage separates the femur and the patella, and acts as a shock absorber.
Diagram of a Long Bone (Left Femur)
When the foetus in the womb initially starts to develop it has no bone, only cartilage. At 6-7 weeks, the ossification process starts. When the baby is born, it has over 300 bones, but as the baby grows up, many bones fuse together and a fully grown adult has just 206 bones. When ossification occurs, cartilage is replaced with bone by laying down calcium. This process is known as calcification.
During the growth phase, the bone grows from the growth plates (epiphyseal plates) which are situated at the end of the bone, just before the epiphysis (see the x-ray of a child's epiphyseal plates to the right). At the end of growth, between the ages of about 16 - 21, these epiphyseal plates turn to bone.
Two types of cell involved in bone growth are osteoblasts and osteoclasts. Osteoblasts lay down new bone, whilst osteoclasts clear away the old bone. Growth occurs when the cells in the cartilage divide and push the older cartilage cells down towards the bone. The diaphysis ossifies first, followed by the epiphyses.
What affects bone growth?
- Sufficient calcium
- Sufficient phosphorus
- Vitamins, especially vitamin D which is involved in the absorption of calcium
- The correct hormone balance, specifically:
- Growth hormone from the pituitary gland
- Calcitonin from the thyroid gland. Calcitonin metabolises calcium and phosphorus.
- Parathormone - from the parathyroid gland - almost works in opposition to calcitonin to balance it out.
- The sex hormones, testosterone and oestrogen.
- The aging process. Women start to lose calcium from their bones at around the age of 40, and in men at around the age of 60, perhaps eventually leading to brittle bone disease - osteoporosis. Other causes of osteoporosis include prolonged treatment with cortisone steroids, anorexia nervosa and an inadequate diet, especially during pregnancy and breast feeding. However, it is possible to increase bone density by performing weight bearing exercise, taking calcium supplements (where the diet does not contain sufficient calcium) and, in women, oestrogen replacement therapy.
Protein formation in bone decreases with age, which can make bones more liable to fracture.