There are two types of bone tissue: compact and spongy.
The names imply that the two types of differ in density, or
how tightly the tissue is packed together. There are three
types of cells that contribute to bone homeostasis. Osteoblasts
are bone-forming cell, osteoclasts resorb or break down bone,
and osteocytes are mature bone cells. An equilibrium between
osteoblasts and osteoclasts maintains bone tissue.
Compact Bone
Compact bone consists of closely packed osteons or haversian
systems. The osteon consists of a central canal called the
osteonic (haversian) canal, which is surrounded by concentric
rings (lamellae) of matrix. Between the rings of matrix, the
bone cells (osteocytes) are located in spaces called lacunae.
Small channels (canaliculi) radiate from the lacunae to the
osteonic (haversian) canal to provide passageways through
the hard matrix. In compact bone, the haversian systems are
packed tightly together to form what appears to be a solid
mass. The osteonic canals contain blood vessels that are parallel
to the long axis of the bone. These blood vessels interconnect,
by way of perforating canals, with vessels on the surface
of the bone.
Spongy (Cancellous) Bone
Spongy (cancellous) bone is lighter and less dense than compact
bone. Spongy bone consists of plates (trabeculae) and bars
of bone adjacent to small, irregular cavities that contain
red bone marrow. The canaliculi connect to the adjacent cavities,
instead of a central haversian canal, to receive their blood
supply. It may appear that the trabeculae are arranged in
a haphazard manner, but they are organized to provide maximum
strength similar to braces that are used to support a building.
The trabeculae of spongy bone follow the lines of stress and
can realign if the direction of stress changes.
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