HISTOLOG Y OF BONE BY DR. MUDASSAR ALI ROOMI (MBBS, M. Phil.)
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HISTOLOGY OF
BONE
BY
DR. MUDASSAR ALI ROOMI (MBBS, M. Phil.)
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FUNCTIONS OF BONE
Definition of bone : Bone isa specialized supportingconnective tissuecomposed of calcifiedintercellular material,
the bone matrix, andthree cell types.
1. Support & Protectionfor soft body parts
2. Points of attachmentsfor muscles (Makesmovement possible)
3. Red & Yellow marrow ->Blood cell production
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Bone cells
1. Osteocytes (Gr. osteon, bone+ cytos, cell), which arefound in cavities (lacunae)between layers (lamellae) of bone matrix
2. Osteoblasts (osteon + Gr.blastos, germ), whichsynthesize the organiccomponents of the matrix
3. Osteoclasts (osteon + Gr.clastos, broken), which aremulti-nucleated giant cellsinvolved in the resorptionand remodeling of bonetissue.
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OSTEOBLASTS (Bone forming cells)
Synthesize and secrete osteoid
Osteoid= organic components of bone
matrix, consisting of type I collagen fibers,
proteoglycans, and several glycoproteins
including osteonectin.
Deposition of the inorganic components
of bone also depends on them via
secretion of ALP enzyme.
located at the surfaces of bone matrix, in a
layer somewhat resembling a simple
epithelium.
When active, they have a cuboidal to
columnar shape and basophilic cytoplasm
(RER).
When their activity declines, they flatten
and cytoplasmic basophilia is reduced.
stimulated by PTH
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osteocytes (mature bone cells)
Individual osteoblasts are graduallysurrounded by their own secretion and
become osteocytes
enclosed singly within spaces called lacunae.
In the transition from osteoblasts to
osteocytes the cells extend many long
cytoplasmic processes, which also becomesurrounded by calcifying matrix.
An osteocyte and its processes occupy each
lacuna and the canaliculi radiating from it .
maintain matrix, if they die matrix is
resorbed
Processes of adjacent cells make contact viagap junctions.
they are flat and almond-shaped cells
Cytoplasm: reduced RER and Golgi apparatus
Nucleus: more condensed nuclear chromatin
.
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Osteoclasts (bone resorbing cells)
very large (dia=150 um), motile cells with
multiple nuclei.
Origin: from the fusion of monocytes.
osteoclasts lie within depressions in the bone
matrix known as resorption bays (Howship
lacunae).
In active osteoclasts, the surface against the
bone matrix is folded into irregular projections,
which form a ruffled border.
Surrounding the ruffled border is a clear
cytoplasmic zone rich in actin filaments which is
the site of adhesion to the bone matrix.
This circumferential adhesion zone creates a
microenvironment between the osteoclast andthe matrix in which bone resorption occurs .
Into this subcellular pocket the osteoclast
secretes collagenase and other enzymes and
pumps protons, forming an acidic environment
locally for dissolving hydroxyapatite and
promoting the localized digestion of collagen.
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Osteoclasts (bone resorbing cells)
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Functions of osteoclasts
1. Remodeling of the bone during bone repair
and bone growth.
2. Calcium homeostasis:
maintenance of constant calcium ions in blood.