Bone
Matrix and Bone Cells
Bone Matrix
Bone matrix consists of collagen fibres (about 90%
of the organic substance) and ground substance.
Collagen type I is the dominant collagen form in bone. The hardness of the
matrix is due to its content of inorganic salts (hydroxyapatite; about 75% of
the dry weight of bone), which become deposited between collagen fibres.
Calcification begins a few days after the
deposition of organic bone substance (or osteoid)
by the osteoblasts. Osteoblasts are capable of producing high local
concentration of calcium phosphate in the extracellular space, which
precipitates on the collagen molecules. About 75% of the hydroxyapatite is
deposited in the first few days of the process, but complete
calcification may take several months.
Bone Cells
Osteoprogenitor
cells (or stem cells of bone)
are located in the periosteum
and endosteum. They are very difficult to distinguish from the surrounding
connective tissue cells. They differentiate into
Osteoblasts
(or bone forming cells).
Osteoblasts may form a low
columnar "epitheloid layer" at sites of bone deposition. They contain
plenty of rough endoplasmatic reticulum (collagen synthesis) and a large Golgi
apparatus. As they become trapped in the forming bone they differentiate into
Osteocytes.
Osteocytes contain less
endoplasmatic reticulum and are somewhat smaller than osteoblasts.
Osteoclasts
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are very large (up to 100 µm),
multi-nucleated (about 5-10 visible in a histological section, but up to 50
in the actual cell) bone-resorbing cells. They arise by the fusion of
monocytes (macrophage precursors in the blood) or macrophages. Osteoclasts
attach themselves to the bone matrix and form a tight seal at the rim of the
attachment site. The cell membrane opposite the matrix has deep invaginations
forming a ruffled border. Osteoclasts empty the
contents of lysosomes into the extracellular space between the ruffled border
and the bone matrix. The released enzymes break down the collagen fibres of
the matrix. Osteoclasts are stimulated by parathyroid
hormone (produced by the parathyroid gland) and inhibited by calcitonin (produced by specialised cells of the
thyroid gland). Osteoclasts are often seen within the indentations of the
bone matrix that are formed by their activity (resorption
bays or Howship's lacunae).
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Title:
Bone Matrix and Bone Cells
by:
om
at
2013-02-15T03:02:00+07:00
Rating: 4.8
of 5 Reviews
Bone Matrix and Bone Cells