96
UNIT 2
Basic Tissues
Figure 5-14A.
Bone remodeling, nasal.
H&E,
3
136;
inset
3
363
Bone remodeling
is necessary during bone formation in
order to mold the bone into a proper shape to carry out its
function. Remodeling usually occurs on the surface of the
bone where
osteoblasts
and
osteoclasts
play different roles.
In order to achieve a certain shape, bone matrix is continu-
ally being deposited by
osteoblasts
in one region, and, at
the same time, bone matrix is being absorbed by osteoclasts
in another area.
Osteoclasts
are large, multinucleated cells,
which originate from monocytes and act as phagocytes. They
often sit in the
Howship lacunae
(eroded grooves
produced
by ongoing reabsorption) on the bone surface. Osteoclasts
are under the infl uence of the hormone
calcitonin
, which is
synthesized by the
thyroid gland
,
and
parathyroid hormone
produced by the
parathyroid gland
.
Calcitonin
directly
inhibits osteoclast activity and reduces bone reabsorption.
Parathyroid
hormone
indirectly increases osteoclast activity
and increases bone reabsorption.
Bone matrix
Bone matrix
Bone matrix
Howship
Howship
lacuna
lacuna
Howship
lacuna
Osteoclasts
Osteoclasts
Osteoclasts
Osteoclast
Osteoclast
Osteoclast
A
Figure 5-14B.
Osteoclast.
EM,
3
14,000
Osteoclasts
are large, multinucleated cells derived from cells seen in circulating blood as monocytes, which are derived, in turn, from
progenitor cells in the bone marrow. Key features in identifying an osteoclast are multiple nuclei, abundance of
mitochondria
in the
cytoplasm, and intimate attachment to the surface of bone matrix. The mitochondria provide the energy for pumping protons into
the space adjacent to the
bone matrix
. The cytoplasm near the matrix contains
lysosomes
, the acid hydrolases of which are secreted
into the space adjacent to the bone matrix. This area of the cytoplasm also contains numerous electron lucent
vacuoles
that prob-
ably refl ect
endocytosis
of degraded matrix components. In an active osteoclast, the
plasmalemma
in the central part of the interface
between the cell and the matrix is highly folded into a ruffl ed border, a structure that is not discernible in this electron micrograph.
B
Euchromatin
Lysosomes
Vacuoles
Howship lacuna
Endothelial cell
Lumen of venule
Bone matrix
Bone matrix
Bone matrix
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