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Osteoblasts are the cells that
build bone by secreting bone matrix around themselves. Once surrounded
by the matrix, they become osteocytes – the mature bone cells
that occupy lacunae in the solid matrix and have cytoplasmic extensions
that extend through canaliculi to reach neighboring lacunae containing
osteocytes. Osteocytes communicate with each other via gap junctions.
They also maintain bone matrix thus playing an important role in
homeostasis.
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Osteoblast: building bone |
Osteoclast: resorbing bone |
Osteoclasts are bone resorption cells. They originate from
pluripotent cells within the bone marrow. These pluripotent cells
also give rise to macrophage and monocytes. Osteoclasts are multinucleated
giant cells, with multiple vacuoles and lysosomes, and a highly
acidophylic cytoplasm. The surface of the osteoclast in contact
with the bone being resorbed forms a ruffled border due to extensive
infolding. The ruffled border is indicative of an "activated"
osteoclast; osteoclasts not currently involved in bone resorption
do not display this border. This border increases the surface area
of the portion of the osteoclast's plasma membrane in contact with
the adjacent bone surface. The osteoclast binds to a portion of
bone matrix and induces a drop in pH through the release of hydrogen
ions, produced by carbonic anhydrase within the cell. The increased
acidity causes the hydroxyapatite crystals of the bone matrix to
dissolve. The organic portion of the matrix is subsequently dissolved
by proteolysis. This rocess forms pits on the surface of bones called
Howship's lacunae.
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