skeleton make up a strong, fl exible
framework that protects our vital
organs and allows our bodies to
move, as well as being a mineral store
and stem-cell reserve.
Bone is a composite material,
constructed from three basic ingredients:
collagen strands, a sugary protein glue
and inorganic calcium salts. The collagen
fi bres are arranged in alternating layers,
crossing over one another, providing a
fl exible scaffold, and calcium salts are
glued in between for strength and rigidity.
The outside of each bone is composed
of plates, or hollow tubes, of dense
cortical bone, supported on the inside by a
honeycomb network of spongy trabecular
bone. This network is slightly fl exible and
helps to distribute the load, curving the
tensile and compressive forces across the
ends of the bone, while providing
maximum strength.
Spongy bone is also home to the bone
marrow, and which houses stem cells
capable of producing most of the cells of
the blood and immune system. They are
constantly active, and millions upon
millions of new red and white blood cells
are produced every minute.
Embedded within the
bone matrix are cells
known as osteocytes. They do not move, but are
capable of detecting stresses inside the
bone itself, and can trigger the formation of
new bone in a process known as
remodelling. The old bone is broken down
by large cells known as osteoclasts, and
new collagen and minerals are deposited
by smaller osteoblasts.
Together, the two cell types are able to
release and store calcium and phosphorous
in the skeleton for use elsewhere in the
body. They are under the infl uence of
hormones released by glands in the brain,
and when levels of minerals run low in the
body, the signals encourage the osteoclasts
to begin wearing away at the surface of the
bone, releasing minerals into the
bloodstream. When mineral
levels are high, osteoblasts
lay down new bone,
replenishing the store.
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