We created man from gentle extraction of clay;

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We created man from gentle extraction of
clay Then We placed him as (a drop of) sperm in
a place of rest firmly fixed Then We made the
drop into an Alaqah (leech like) then We changed
the leech like structure into a Mudghah (chewed
substance) then We made out of the Mudghah bones
and clothed the bones with flesh then We
developed out of it another creature so blessed
be Allah the Best creator!
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Ossification/osteogenesis/histogenesis of bone
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Bone

• specialized connective tissue whose extracellular
  matrix is calcified, imprisoning the cells that
  secreted it.

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• one of the hardest substances of the body
• dynamic tissue that constantly changes shape in
  relation to the stresses placed on it
• pressures applied to bone lead to its resorption,
  whereas tension applied to it results in
  development of new bone

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• Applying these facts, the orthodontist is able to
  remodel the bone of the dental arches by moving
  and straightening the teeth to correct
  malocclusion
• This provides the patient with a more natural and
  pleasant smile.

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Functions of bones

• Primary structural framework for support and
  protection of the organs of the body
• Serve as levers for the muscles attached to them,
  thereby multiplying the force of the muscles to
  attain movement
• Reservoir -store about 99 of the body's calcium.

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• Contain a central cavity, the marrow
  cavity, which houses the bone marrow, a
  hemopoietic organ.

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Composition

• Cells lying in an extracellular matrix that has
  become calcified.
• The calcified matrix is composed of fibers and
  ground substance.
• The fibers constituting bone are primarily type I
  collagen.

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• The ground substance is rich in proteoglycans
  with chondroitin sulfate and keratan sulfate side
  chains.
• In addition, glycoproteins such as osteonectin,
  osteocalcin, osteopontin, and bone sialoprotein
  are present

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Osteons
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Bone Matrix

• Inorganic and organic constituents
• Inorganic Component-
• Crystals of calcium hydroxyapatite
  Ca10(PO4)6(OH)2, composed mostly of calcium and
  phosphorus.
• About 65 of dry weight

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Organic Component

• Predominantly type I collagen.
• About 35 of dry weight

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Collagen fibres
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Cells of Bone

• Osteoprogenitor cells
• Osteoblasts
• Osteocytes
• Osteoclasts.

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Osteoprogenitor Cells

• Derived from embryonic mesenchymal cells and
  retain their ability to undergo mitosis
• Located in the inner cellular layer of the
  periosteum, lining haversian canals, and in the
  endosteum
• Have the potential to differentiate into
  osteoblasts.

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Embryonic mesenchyme.
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• Under certain conditions of low oxygen tension,
  these cells may differentiate into chondrogenic
  cartilage formingcells
• Most active during the period of intense bone
  growth

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OSTEOBLASTS

• synthesize the organic matrix of bone
• possess receptors for parathyroid hormone.
• derived from osteoprogenitor cells
• located on the surface of the bone in a
  sheet-like arrangement of cuboidal to columnar
  cells

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• osteoblasts exocytose their secretory products,
  each cell surrounds itself with the bone matrix
  it has just produced
• when this occurs, the imprisoned cell is referred
  to as an osteocyte, and the space it occupies is
  known as a lacuna

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• Most of the bone matrix becomes calcified
• Osteoblasts as well as osteocytes are always
  separated from the calcified substance by a thin,
  noncalcified layer known as the osteoid (uncalcifi
  ed bone matrix).

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Osteocytes

•  Mature bone cells derived from osteoblasts that
  became trapped in their lacunae
• Radiating out in all directions from the lacunaa
  are narrow, tunnel-like spaces (canaliculi) that
  contain cytoplasmic processes of the osteocyte.
• Processes make contact with similar processes of
  neighboring osteocytes, forming gap
  junctions through which ions and small molecules
  can move between the cells.

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• Processes make contact with similar processes of
  neighboring osteocytes, forming gap
  junctions through which ions and small molecules
  can move between the cells
• Canaliculi also contain extracellular fluid
  carrying nutrients and metabolites that nourish
  the osteocytes.

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OSTEOCYTES
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OSTEOCLASTS

• Multinucleated cells originating from
  granulocyte-macrophage progenitors
• Play a role in bone resorption
• Occupy shallow depressions, called Howship's
  lacunae, that identify regions of bone resorption.

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PRIMARY BONEnewly formed, immature bone, rich in
osteocytes, with randomly arranged bundles of
calcified collagen. Osteoclasts and osteoblasts
are numerous in the surrounding endosteum.
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SECONDARY BONEorganized as lamellae, seen
faintly here as concentric lines surrounding
osteonic canals
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An osteon.
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Lamellar bone Perforating canals
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Hyaline cartilage
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INTRAMEMBRANOUS OSSIFICATION

• Direct formation of bone (membrane bone) within
  highly vascular sheets or membranes of
  condensed primitive mesenchyme
• Mesenchymal stem cells differentiate into
  osteoprogenitor cells which proliferate around
  the branches of a capillary network, forming
  incomplete layers of osteoblasts in contact with
  the primitive bone matrix.

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• Osteoblasts secrete a fine mesh of collagen
  fibres and ground substance, osteoid, from the
  surface which faces away from the blood vessels
• Earliest crystals appear in association with
  extracellular matrix vesicles produced by the
  osteoblasts
• Crystal formation subsequently extends into
  collagen fibrils in the surrounding matrix,
  producing woven bone

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• As layers of calcifying matrix are added to these
  early trabeculae, the osteoblasts enclosed by
  matrix come to lie within primitive lacunae.

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Intramembranous ossification forming the nasal
bones of a 7-month human fetus. Islands of bone
(solid pink matrix M, enclosing osteocytes)
enlarge through the deposition of new matrix by
osteoblasts (arrows). They subsequently fuse and
are remodelled by osteoclasts to form mature
lamellar bone
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ENDOCHONDRAL OSSIFICATION

• Requires the presence of a cartilage template
• Most of the long and short bones of the body
  develop by endochondral ossification

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Phases

1. Formation of a miniature hyaline cartilage model
2. Continued growth of the model, which serves as a
   structural scaffold for bone development
3. Eventual resorption and replacement by bone

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Cells and matrices of a primary ossification
center.A small region of a primary ossification
center showing key features of endochondral
ossification. Compressed remnants of calcified
cartilage matrix (dark purple), now devoid of
chondrocytes, are enclosed by more lightly
stained osteoid or bone matrix. This newly formed
bone is surrounded by a layer of large, active
osteoblasts. Some osteoblasts that were captured
by the matrix have become smaller osteocytes
(arrowheads).
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Zones of epiphyseal plate

• Zone of reserve cartilage Chondrocytes randomly
  distributed throughout the matrix are mitotically
  active.
• Zone of proliferation Chondrocytes, rapidly
  proliferating, form rows of isogenous cells that
  parallel the direction of bone growth.

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• Zone of maturation and hypertrophy Chondrocytes
  mature, hypertrophy, and accumulate glycogen in
  their cytoplasm
• The matrix between their lacunae narrows with a
  corresponding growth of lacunae.

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• Zone of calcification Lacunae become confluent,
  hypertrophied chondrocytes die, and cartilage
  matrix becomes calcified.
• Zone of ossification Osteoprogenitor cells
  invade the area and differentiate into
  osteoblasts, which secrete matrix that becomes
  calcified on the surface of calcified cartilage

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• This is followed by resorption of the calcified
  cartilage/calcified bone complex.

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Bone remodelling
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Fracture healing