Bones start out as flexible cartilage templates, but before
birth this cartilage is mostly replaced by bone, which consists
of fine collagen fibres (similar to those present in skin
/ leather) that are hardened by the addition of large amounts
of relatively insoluble calcium and phosphorus salts. This
composite structure makes the bone stiff enough to support
the body but resistant to fracture. These two key components
of bone are illustrated here.
Most bones have a more or less solid outer layer, with a honeycomb-like
structure in the centre which also contains bone marrow.
In the central part of the bones of the skull, spine, shoulder,
ribs and pelvis there are special cells that form a reservoir
from which replacement blood cells develop. The marrow also
contains a reserve of cells that can develop into osteoclasts
Calcium is an essential part of the mineral component of
bone. It occurs in milk and other dairy products but also
in tinned fish (the bones are soft enough eat and should not
be discarded), nuts, dried fruit and green leafy vegetables
such as broccoli.
We need around 800 milligrammes of calcium per day –
roughly one pint of milk or the equivalent. Women over the
age of 50 and those diagnosed with osteoporosis may need to
increase their calcium intake by taking supplements.
Essential for the absorption of calcium from the intestine.
Vitamin D is made in the skin by the action of sunlight and
this is the easiest way to ensure the body has sufficient
supplies. In sunlight, mild exposure of the skin can make
up to 100 micrograms per day. In temperate climates this production
ceases in winter and if insufficient Vitamin D isalready stored
in the body supplementation from food may become important.
Vitamin D occurs in oily fish, eggs and cod liver oil. The
body needs between 10 and 20 micrograms per day.
Osteoporosis sufferers will usually be prescribed Vitamin
Coffee and tea adversely affect the uptake of Vitamin D from
Two types of cells are important in keeping the bone structure
intact to withstand the changing stresses on the skeleton.
bone (by excavating pits) and osteoblasts
lay down new bone. If the activity of these cells gets out
of balance either bone becomes too thin and fragile (osteoporosis)
or too thick and heavy (osteopetrosis).
An osteoclast absorbing bone Osteoblasts
When bones are fractured the osteoclasts
(and other cells) clear up the damaged fragments and osteoblasts
then lay down new bone in order to effect the repair. Bone
has a much greater capacity to repair itself than cartilage.
This is because living bone has a good blood supply (providing
nutrients and oxygen), whereas there are no blood vessels