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Introduction:
The Osteon
The principal
organizing feature of compact bone is the osteon.
A synonym for osteon is Haversian system.
Haversian is derived from the name Clopton Havers, a 17th
century English physician. We will use the term osteon.
At its very simplest, the osteon has been characterized
as a long narrow cylinder that is 0.2 mm (200 µm) wide
and 10 mm (10,000 µm) long. However, the osteon is not
that simple as will be discussed below and in the
analysis section.
You need to know
how bone sections are made for microscopy. One method is
the dry bone section. A piece of dead bone is broken or
sawed from the main bone. The piece is ground and
polished to where it is very thin (about 15 to 45 µm
thick). That polished piece is placed on a microscope
slide and viewed directly. The bone cells are quite
missing from the dead, polished bone piece. In a humorous
sense you are looking at a skeleton of the skeleton.
A second common
method for preparing bone for histology is to soak a
piece of bone in an acid solution for a period of time.
The acid treatment dissolves the bone salts from the
tissue in a process called demineralization. With this
method, the cells stay behind and can be stained before
observation in the microscope. The acid treatment can
distort the tissue somewhat. Much of this exercise is
based on looking at bone tissue prepared by the dry bone
process.
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There are five key
terms that you should know when describing the
structure of an osteon: Haversian canal,
lamellae, lacunae, canaliculi, and cement lines.
The photograph of a bone model at the left
identifies these components. The Haversian
canal is from 20 to 150 µm in diameter. The
variation in diameter can be due to the age of
the osteon and its relative position within the
bone. Literature reports for typical canal
average diameter range from 50 to 100 µm. One or
two small blood vessels occupy the canal along
with a nerve and possibly a lymphatic vessel.
The outer
perimeter of the osteon has a special coating
which is known as a cement line.
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| When compact bone
osteons are being formed, collagen fibers are
laid down first. The collagen patterns are
reflected in the structure known as a lamella.
Osteons have from between four to about twenty
lamellae with each measuring between 3 to 7 µm
in width. It takes special microscope lighting in
order to see all the lamellae within an osteon. Lying between or
within the lamellae are special holes known as lacunae.
Each lacuna provides enough space for an
individual bone cell to reside. The osteocyte
inside the lacuna is responsible for secreting
the bone salts surrounding it.
Osteocytes
within their cave-like lacunae communicate with
each other through unique passages called canaliculi.
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The human
osteocyte under normal conditions is said to live
for 25 years or so. This information would
suggest that in the lifetime of a person there
would be about four generations of osteocytes, or
four population cycles of these critical cells.
As has been mentioned periosteum covers the
outside of the bone and endosteum covers the
marrow vault walls. The total surface area
covered by these two layers of bone membrane is
eight meters squared. If one where to measure the
surface area provided by the walls of the
Haversian canals, lacunae, and canaliculi, a
really significant value is obtained: 1,500 to
5,000 m2. The surface area of the
skeleton exceeds by more than ten times the total
surface area of the human lungs which are built
for surface area.
The focus
of this exercise is upon the lacunae. How are the
lacunae distributed within the osteon? We trust
you will find this an interesting question.
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Figure two was
created to indicate what a perfect osteon might
look like in cross section and in longitudinal
section. Please understand that osteons do not
look like this in real life but that the diagram
is an approximation of "the perfect
osteon." One can learn a great deal by
building models and then discovering how the real
structure varies from the modeled structure. Figure three is an
image from a real cross-sectioned osteon. You
might consider how the model disagrees with the
"real" structure. An obvious difference
is that the real osteon is not perfectly round
with perfect concentric lamellae within. However,
the model does capture the "essence" of
the real structure; in other words, it is a good
approximation.
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| The Haversian canal,
the lacunae, and the canaliculi within an osteon
are all connected to each other; and by lateral
passages called Volkmann’s canals between
Haversian canals, adjacent osteons are in
communication with each other. Thus the
osteocytes with the lacunae are in communication
with each other and with a supply of nutrients. |
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