The human skeleton
gives the body its shape
and provides physical support
for the systems
contained within.
It also forms part
of the musculoskeletal system
that enables us to move.
The structure of bone
is optimized
so that it is strong
but relatively lightweight.
The interior of bone
is composed of bone marrow.
It is surrounded by two
major types of bone tissue:
cortical bone, or the hard,
outer shell of bone,
and trabecular bone,
the spongey-looking center.
The amount of each type
of tissue in bone
is dependent on
the function of that bone.
The basic unit of cortical
or compact bone is the osteon.
It is composed of successive,
concentric lamellae.
This structure contributes
to bone strength
by resisting bending.
Cells called osteocytes
are distributed
within the concentric lamellae.
Osteocytes form
a complex network
that is thought to be important
in maintaining
the viability and structural
integrity of bone.
At the center of the osteon
is the Haversian canal.
These canals contain
blood vessels and nerves.
The blood vessels
within bone facilitate
the exchange between osteocytes
and the blood.
Trabecular bone is present
in the interior of some bones
and resists compression.
Osteocytes are also contained
within its structure,
and again play an important role
in sensing local changes
in strain.
Trabecula are covered in a layer
of flat and lining cells
that are thought to be
involved in the dynamic process
by which bone is formed
and broken down.
Bone marrow is found
within the interior of bones.
The surrounding trabecula
and vascular network provides
structural support, nutrition,
and a waste removal system
that a heterogenous group of
cells found within this space.
Bone marrow is a site
for hematopoiesis,
the process by which the cellular
components to blood are formed.
