Page 264 - Atlas of Histology with Functional Correlations

the mineralization of osteoid by releasing matrix vesicles that contain

alkaline phosphatase, which increases phosphate ions that then combine
with calcium ions. Increased concentrations of phosphate and calcium ions
combine to form hydroxyapatite crystals and the initial centers of
calcification. Further calcification surrounds these centers and embeds the
collagen fibers and the glycoproteins.

Osteocytes are the mature osteoblasts that become surrounded by the
mineralized bone matrix. They are also smaller than osteoblasts and are the
principal cells of the bone. Like the chondrocytes in cartilage, osteocytes are
trapped by the surrounding bone matrix in their lacunae. Osteocytes in the

lacunae are very close to blood vessels. In contrast to cartilage, only one
osteocyte is found per bony lacuna. Also, because mineralized bone matrix is
harder than cartilage, nutrients and metabolites cannot diffuse through it to
the osteocytes. Consequently, bone is highly vascular and possesses a unique

system of channels or tiny canals called canaliculi, which open into the
osteons.

Osteocytes exhibit numerous cytoplasmic extensions that enter the
canaliculi and then radiate in all directions from each lacuna, and contact

neighboring osteocytes via gap junctions, thus allowing the passage of ions
and small molecules from cell to cell. The canaliculi contain extracellular
fluid, and the gap junctions allow individual osteocytes to communicate with
adjacent osteocytes and with materials in the blood vessels of the central
canal. In this manner, the canaliculi form complex connections around the

blood vessels in the osteons and constitute an efficient exchange mechanism:
nutrients are brought to the osteocytes, gaseous exchange takes place
between the blood and cells, and metabolic wastes are removed from the

osteocytes. The canaliculi system keeps the osteocytes alive, and the
osteocytes, in turn, maintain the homeostasis of the surrounding bone matrix
and blood concentrations of calcium and phosphates. When an osteocyte
dies, the surrounding bone matrix is reabsorbed by osteoclasts.

Osteoclasts are large, multinucleated cells found along bone surfaces

where resorption (removal of bone), remodeling, and repair of bone take
place. Although osteoblasts and osteocytes arise from mesenchyme
osteoprogenitor cell line, the osteoclasts are multinucleated cells that
originate by fusion of blood or hematopoietic progenitor cells of the

mononuclear macrophage–monocyte cell line of the red bone marrow.
Osteoclasts are phagocytic cells that function in bone resorption during bone

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