Page 265 - Atlas of Histology with Functional Correlations

remodeling (renewal or restructuring) and are often located on the resorbed

surfaces or in shallow depressions in the bone matrix called Howship
lacunae. Lysosomal enzymes released by osteoclasts erode these
depressions and demineralize the bony surface. During bone development,
bone deposition by osteoblasts is closely coordinated with bone remodeling
by the osteoclasts to maintain proper bone development and the bone mass.

Osteoclast activities are influenced by several hormones, including
parathyroid hormone (PTH) from the parathyroid gland and calcitonin from
the thyroid gland. Osteoblasts, activated by PTH, stimulate the development
of osteoclasts by producing a molecule called RANKL (receptor for

activation of nuclear factor-kappa B ligand). This molecule in turn binds
to RANK receptors on osteoclasts and stimulates the formation and activity
of osteoclasts. Presence of membrane-bound proteins RANKL and M-CSF
(monocyte colony-stimulating factor), produced by surrounding stromal

cells and osteoblasts, is needed for osteoclast formation. Activated
osteoclasts increase the resorption of bone matrix and increase the levels of
calcium in the blood.

BONE MATRIX

The bone matrix consists of inorganic (minerals), organic (collagen fibers)

components, living cells, and extracellular material. Due to calcification or
mineralization, the bone matrix is harder than cartilage, and no diffusion takes
place. As a result, the bone matrix is highly vascularized. Blood vessels from the
periosteum penetrate and enter the bone matrix via the perforating (Volkmann)
canals. These canals run perpendicular to and join the vessels in the central

canals of the osteon, which then supply all of the cellular components of the
bone matrix.

The organic components enable bones to resist tension, whereas the mineral
components resist compression. The major organic components of bone matrix

are the coarse type I collagen fibers, the predominant proteins. The other
organic components are sulfated glycosaminoglycans and hyaluronic acid that
form larger proteoglycan aggregates. The glycoproteins osteocalcin and
osteopontin bind tightly to calcium crystals and promote mineralization and

calcification of the bone matrix. Another matrix protein, sialoprotein, binds
osteoblasts to the extracellular matrix through the integrins of the plasma

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