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I'm Stavros Manolagas,
Professor of Medicine and
the Director of the Division of
Endocrinology and Metabolism and
the Center for Osteoporosis and
Metabolic Bone Diseases at the University
of Arkansas for
Medical Sciences in Little Rock, AR.
The goal of the lecture is to
review current understanding of
the hormonal influences on bone
remodeling and their implications for
the pathophysiology and
treatment of osteoporosis.
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During development and growth,
the skeleton is sculpted to
achieve its shape and size by
the removal of bone from one site and
deposition at a different one,
this process is called modeling.
Once the skeleton has reached maturity,
however,
regeneration continues in the form of
a periodic replacement of all bone
with new at the same location,
this process is called remodeling.
Removal of bone,
otherwise known as bone resorption,
is the task of osteoclasts, formation
of new bone is the task of osteoblasts.
Bone resorption and
bone formation, however,
are not separate independent processes.
In the adult skeleton, all osteoclasts and
osteoblasts belong to a unique
temporary structure known as a basic
multicellular unit or BMU,
which is depicted in this cartoon.
The BMU is approximately 1 to 2
mm long and 0.2 to 0.4 mm wide,
it comprises a team of osteoclasts in the
front, a team of osteoblasts in the rear,
depicted here with a yellow monolayer of
cells, and a central vascular capillary.
Osteoclasts adhere to bone and
subsequently remove it by acidification
and proteolytic digestion.
As the BMU advances, in this cartoon
from the top of the image to the bottom,
osteoclasts leave the resorption site and
osteoblasts move in to
cover the excavated area and
begin the process of new bone formation
by secreting osteoid which is
eventually mineralized into new bone.
Importantly, some osteoblasts,
after they have finished their
bone-forming function are buried within
the mineralized matrix that they have
created and these are termed osteocytes.
Osteocytes are depicted as individual
cells in this cartoon with multiple
processes connected with each other
in the matrix surrounding the BMU.
A microscopic image of
an actual BMU in a section of
murine trabecular bone is shown here.
In this instance, the BMU travels from
the left to the right of the section.
Please note the group of
red osteoclasts stained for
tartrate-resistant acid phosphatase.
Trailing behind
the osteoclasts are teams of
teal-colored osteoblasts
bringing up the rear of the BMU.
Pale-yellow erythrocytes
are seen here in the vessel
that brought the osteoclast
precursors to the erosion cavity.
