Ossification is the process of bone formation. There are two types of ossification: intramembranous ossification and intracartilaginous (endochondral) ossification. In intramembranous ossification, bone is laid down directly in fibrous membranes by osteoblasts differentiating from mesenchymal cells. Bones formed this way include the skull and clavicle. In intracartilaginous ossification, a cartilaginous model is first formed and then replaced by bone. Long bones develop through this process, with osteoblasts eroding and replacing the cartilage matrix.

1. OSSIFICATION
D R . M D. M O H I U D D I N M A S U M
A s s i s t a n t p r o f e s s o r
A n w e r k h a n m o d e r n m e d i c a l c o l l e g e

2. Bone cells
Cells
Matrix
Osteoprogenitor
cell
Osteoblast
Osteocyte Osteoclast

3. Composition (contd.)

4. Ossification
It is a process of formation of bone and
it includes proliferation of collagen fibres
and ground substance
with subsequent deposition of calcium salt

5. Ossification (contd.)
Intramembranous ossification
Intracartilaginous ossification
or
Endochondral ossification
Types of ossification:

6. Ossification (contd.)
• Intramembranous ossification
in which osteoblasts differentiate directly from
mesenchyme and begin secreting osteoid
• Intracartilaginous ossification
in which the matrix of preexisting hyaline cartilage
is eroded and replaced by osteoblasts producing
osteoid.

7. Centre of Ossification
It is a particular point or points from which
ossification starts
It may be
1. Primary
2. Secondary

8. Centre of Ossification (contd.)

9. Intramembranous ossification
• In intramembranous ossification bone is laid down directly in a
fibrous membrane.
• The bones are called membrane bones.
eg. vault of skull, mandible and clavicle.

10. Intramembranous ossification: Process
• At the site where a membranous bone is to be formed-
mesenchymal cells closely packed (mesenchymal condensation
is formed)
• The region become highly vascularized.
• Some of the mesenchymal cells lay down collagen fibres in the
mesenchymal condensation and thus a membrane is formed.

11. Intramembranous ossification (contd.)
• An ossification center in the
fibrous membrane in formed
• Centrally located mesenchymal
cells cluster are differentiate into
osteoblasts, forming the
ossification center

12. Intramembranous ossification (contd.)
• These osteoblast come to lie along the
bundles of collagen fibers.
• These cells secrete a gelatinous matrix
in which the fibers get embeded. The
fibers also swollen up. This mass of
swollen fiber and matrix is called
osteoid.
• Under the influence of osteoblast,
calcium salts are deposited in osteoid
• Trapped osteoblasts become
osteocytes

13. Intramembranous ossification (contd.)
• As soon as this happens the layer of
osteoid becomes one lamellus of
bone.
• Over this lamellus another layer of
osteoid is laid down by osteoblasts.
In this way a number of lamellae lay
down one over another trabeculae.
• Accumulating osteoid forms a
network which encloses local blood
vessels
• Vascularized mesenchyme forms on
the external face of woven bone to
become periosteum.

14. Intramembranous ossification (contd.)
• Trabeculae just deep to the
periosteum thicken, forming a
woven collar which is later
replaced with mature lamellar
bone
• Spongy bone persists internally
and its vascular tissue becomes
red marrow

15. Intracartilaginous ossification
• In intra cartilaginous ossification → cartilaginous model is
formed which is closely resemble the bone to be formed.
• This cartilage is subsequently replaced by (not converted
into) bone.
eg. Almost all of the long bones.

16. Intracartilaginous ossification: Process
• At the site where bone is to be formed-mesenchymal cells
closely packed to form mesenchymal condensation.
• Some mesenchymal cells become chondroblast and lay down
hyaline cartilage.
• Mesenchymal cells form a membrane on the surface of
cartilage called the perichondrium.

17. Intracartilaginous ossification: Process (contd.)
• Perichondrium is vascular and
contain osteogenic cells. It
become vascularized and inner
layer is differentiated into
osteoblast
• Formation of a bone collar around
hyaline cartilage model under the
influence of an enzyme alkaline
phosphatase

18. Intracartilaginous ossification (contd.)
• Cartilage in the center of the
diaphysis calcifies
• Calcification of cartilage blocks
nutrients and chondrocytes die
leaving behind empty spaces
called primary areolae.

19. Intracartilaginous ossification (contd.)
• Some vessels of the perichondrium
invade the cartilage matrix.
• They are accompanied by
osteogenic cells. This mass of
vessels and cells (osteoblasts and
osteoclasts) is called the periosteal
bud.
• It eats away much of the calcified
matrix forming the walls of primary
areolae-creates large cavities called
secondary areolae.

20. Intracartilaginous ossification (contd.)
• The medullary cavity is formed as
ossification continues
• Secondary ossification centers form
in epiphyses
• Cartilage in epiphyses calcifies and
deteriorates opening cavities for
entry of periosteal bud

21. Intracartilaginous ossification (contd.)
• Ossification extend in the longitudinal direction by
successive processes of proliferation, maturation,
hypertrophy and death calcified cartilage

22. Intracartilaginous ossification (contd.)

23.  Epiphysis
 Epiphyseal cartilage
 Metaphysis
 Diaphysis

24. Structures of epiphyseal cartilage

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