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The progression of AS can be highlighted by:
The initial inflammation of the joint where the
spine meets the pelvis.
The eventual fusion of the bones of that joint and subsequent
fusion of the vertebras on up the spinal column.
The cause of this inflammation and fusion in unknown,
though research continues.
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Typical bone growth, ossification, occurs as cartilage cells that are determined
to become bone, called chondracytes, develop into cells called
osteoblasts, that in turn develop into bone tissue. This bone tissue is of two types, cancellous bone, the inner largely porous
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bone where blood cells are stored, and cortical bone,
which is the strong denser outer bone.
The AS condition provides for abnormal ossification in
which the cartilage-to-bone process is bypassed and the osteoblasts
that are intended for repair instead form new bone structures. The
osteblasts are deposited in the body's attempt to repair the
damage to the bone caused at the point of damage, the enthesopathy. 
As the enthesopathy's inflammation cells are infused with
blood vessels, osteoblasts assemble at the site of the enthesopathy,
weaving into the existing bone's cellular matrix, producing
small points
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of cortical bone called syndamorphytes. These syndamorphytes
comprise the bony protusions that ultimately replace the anulus
fibrosus' fibrous tissue, thereby fusing the vertebrae as
they grow across the intervertebral space.
AS is a chronic, systemic inflammatory disorder of the
axial skeleton. In simpler terms, AS is a slowly developing
disease that takes years to manifest itself on the spine
and associated joints. Prior to advanced stages of AS, the
course is highly variable and can be characterized by spontaneous
remissions and exacerbation, that is, that there are episodes
of moderate to high levels of pain and immobility between
periods of time when no symptoms are present. The cause of
the ascending spinal inflammation which characterizes AS is
unknown. Though research continues, a number of studies seems
to reveal potential enteric (intestinal) bacteria involvement.
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