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!
OSSIFICATION/OSTEOGENESIS/
HISTOGENESIS OF BONE
Bone

specialized connective tissue whose
extracellular matrix is calcified,
imprisoning the cells that secreted it.



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


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.
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.

Contain a central cavity, the marrow
cavity, which houses the bone marrow, a
hemopoietic organ.
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.


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
Osteons
Bone Matrix
Inorganic and organic constituents
Inorganic ComponentCrystals of calcium hydroxyapatite
[Ca10(PO4)6(OH)2], composed mostly of calcium
and phosphorus.
About 65% of dry weight

Organic Component


Predominantly type I collagen.
About 35% of dry weight
Collagen fibres
Cells of Bone




Osteoprogenitor cells
Osteoblasts
Osteocytes
Osteoclasts.
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.
Embryonic mesenchyme.


Under certain conditions of low oxygen tension,
these cells may differentiate into chondrogenic
[cartilage forming]cells
Most active during the period of intense bone
growth
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


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


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 (uncalcified bone matrix).
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.


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.
OSTEOCYTES
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.
PRIMARY BONE
newly formed, immature bone, rich in osteocytes, with randomly arranged
bundles of calcified collagen. Osteoclasts and osteoblasts are numerous in
the surrounding endosteum.
SECONDARY BONE
organized as lamellae, seen faintly here as concentric
lines surrounding osteonic canals
An osteon.
Lamellar bone: Perforating canals
Hyaline cartilage
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.



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

As layers of calcifying matrix are added to these
early trabeculae, the osteoblasts enclosed by
matrix come to lie within primitive lacunae.
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
ENDOCHONDRAL
OSSIFICATION


Requires the presence of a cartilage template
Most of the long and short bones of the body
develop by endochondral ossification
Phases
1.
2.
3.
Formation of a miniature hyaline cartilage
model
Continued growth of the model, which serves
as a structural scaffold for bone development
Eventual resorption and replacement by bone
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).
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.


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.


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

This is followed by resorption of the calcified
cartilage/calcified bone complex.
Bone remodelling
Fracture healing