Histology of bone Classification of bone Ossification of bone

1. HISTOLOGY OF BONE
I
DR. FIZZA SAHER
ASSISTANT PROFESSOR
ORAL BIOLOGY

2. WHAT IS BONE?
�specialized form of connective tissue and is the main element of the skeletal
tissues.
�composed of cells and an extracellular matrix in which fibers are embedded.
�unlike other connective tissues in which the extracellular matrix becomes
calcified.

3. FUNCTIONS OF BONE
1. Support: provides framework that supports and anchors all
soft organs.
1. Protection: skull and vertebrae surround soft tissue of the
nervous system, and the rib cage protects vital thoracic
organs.
1. Movement: skeletal muscles use the bones as levers to
move the body.
1. Storage: fat stored in the interior of the bones. Bone matrix
serves as a storehouse for various minerals.
1. Blood Cell Formation: hematopoiesis occurs within the
marrow cavities of the bones.

4. CLASSIFICATION OF BONE
Bone may be classified in several ways:
Developmentall
y
Macroscopic
structure
Microscopic
structure
By shape
Endochondral
bone
Compact
(Cortical) Bone
Embryonic or
woven bone
Long bones
Intramembranou
s bone
Spongy
(Cancellous)
Bone
lamellar bone Short bones
Flat bones
Irregular bones

5. ACC. TO DEVELOPMENT
A. Endochondral bone
� Where bone is preceded by a cartilaginous model that is eventually replaced by bone
In a process termed endochondral ossification.

6. Enlarging
chondrocytes
within
calcifying matrix
Chondrocytes at
the center of the
growing
cartilage model
enlarge and then
die as the matrix
calcifies.
Newly derived
osteoblasts
cover the shaft
of the cartilage
in a thin layer of
bone.
Blood vessels
penetrate the
cartilage. New
osteoblasts form a
primary ossification
center.
The bone of the
shaft thickens,
and the cartilage
near each
epiphysis is
replaced by
shafts of bone.
Blood vessels invade
the epiphyses and
osteo-blasts form
secondary centers of
ossification.
Cartilage
model
Epiphysis
Diaphysis Marrow
cavity
Primary
ossification
center
Blood
vessel
Marrow
cavity
Blood
vessel
Secondary
ossification
center
Epiphyseal
cartilage
Articular
cartilage

7. B. Intramembranous bones:
Intramembranous ossification/direct bone formation

8. ACC. TO SHAPE
Limbs Carpals
Tarsals
Vertebrae Skull bones, Ribs
Sternum, Scapula

9. CHEMICAL PROPERTIES OF
BONE

10. 67 % Inorganic material
�is formed from carbonated
hydroxyapatite
33 % Organic material
�mainly composed of Type I collagen
�The organic part is also composed of
various growth factors:
�Glycosaminoglycans,
osteocalcin, osteonectin,
Bonesialoprotein, osteopontin
67%
33%
Composition of bones
Inorganic Substances
Organic Substances

11. ORGANIC MATRIX OF THE
BONE
Bone is formed by the hardening of
the matrix, entrapping the cells.
When these cells become
entrapped from osteoblasts they
become osteocytes.
The organic matrix of bone is
about:
�90% collagen
�10% non-collagenous proteins.
90%
10%
Organic Components of
the Bone Matrix
Collagen
Non-collagenous proteins

12. NON-COLLAGENOUS
PROTEINS OF BONE
�heterogeneous group which vary from entrapped serum protein to glycoproteins
�plays a role in mineralization.
�The main non-collagenous proteins comprise the:
�Proteoglycans
�Glycoproteins
�Bone Gla-containing proteins
�Serum proteins

13. COLLAGEN
�Contributes towards the important biomechanical properties of tissue in terms of
resisting loads and providing necessary resilience that prevents fractures.
�The dominant collagen in bone is Type I.
�Intrinsic collagen
�collagen as secreted by osteoblasts.
�Extrinsic collagen
�Collagen formed by adjacent fibroblasts.

14. GROSS HISTOLOGY OF
BONES
Periosteum
Is an outer fibrous
sheath of dense regular connective
tissue covering of the bone except
articular surface.
Two layers
�outer fibrous layer
�an inner cellular (osteoprogenic) layer
�is well defined If active bone formation is in progress

15. PERIOSTEUM
The relatively few periosteal cells are
capable of undergoing division and
becoming osteoblasts under appropriate
stimulus.
Sharpey’s fibers
�Collagen fibers from ligaments and tendons
extend directly into the bone tissue, where they
are continuous with the collagen fibers of the
extracellular matrix of the bone tissue.

16. ENDOSTEUM
Is often only one cell thick a layer of
connective tissue cells lining Bone
cavities
consists of osteoprogenitor cells that
can differentiate into osteoblasts, and
bone-lining cells

17. COMPACT BONE
Also called as Cortical bone
As its name implies. . .
�“cortical” bone forms the cortex or outer shell of most bones.
�“compact” bone is much denser than cancellous bone. Furthermore, it is harder,
stronger and stiffer than cancellous bone.
The primary anatomical and functional unit of cortical bone is
the osteon.

18. composed of cylindrical units
called osteons or Haversian
systems
osteons consist of concentric
lamellae of bone matrix
surrounding a central canal,
the Haversian canal, which
contains the vascular and
nerve supply of the osteon.
OSTEON

19. OSTEOCYTE
Within the bone matrix are spaces called
lacunae, each containing osteocyte.
The osteocyte extends numerous processes
into small tunnels called canaliculi.
�communicate by gap junctions with other osteocytes
canaliculi generally arranged in a radial
pattern with respect to the Haversian canal
�serves for the passage of
substances between the osteocytes and blood
vessels

22. 3 DISTINCT LAYERING OF
COMPACT BONE
Circumferential Lamellae
�enclose the entire adult bone, forming its outer perimeter
Concentric Lamellae
�make up the bulk of compact bone and form the basic metabolic unit of bone, the
osteon
Interstitial Lamellae
�interspersed between adjacent concentric lamellae and fill the spaces between them

23. Between the osteons are
remnants of previous concentric
lamellae called interstitial
lamellae
Circumferential lamellae follow
the entire inner and outer
circumferences of the shaft of a
long bone
Volkmann’s canals:
�blood vessels and nerves travel from the
periosteal and endosteal surfaces to
reach the haversian canal; they also
connect haversian canals to one another
�not surrounded by concentric lamellae

26. IMMATURE / WOVEN BONE
Seen in skeleton of a developing fetus
Also known as bundle bone or woven bone because of the
interlacing arrangement of the collagen fibers.
Comparison with mature bone
�does not display an organized lamellated appearance.
�contains randomly arranged, relatively
more cells per unit area.
�more ground substance, not heavily mineralized
Areas of immature bone are present in adults, especially
where bone is being remodeled. E.g. in the alveolar sockets
and where tendons insert into bones.

27. SPONGY BONE
Also called Cancellous bone or
Trabecular bone
Compared to compact bone,
cancellous bone has a higher
surface area but is less dense,
softer, weaker, and less stiff.
Cancellous bone is highly
vascular and frequently contains
red bone marrow where
hematopoeisis occurs.
The primary anatomical and
functional unit of cancellous bone
is the trabecula.

28. CELLS OF BONE TISSUE
1. osteoprogenitor cells,
2. osteoblasts,
3. osteocytes,
4. bone-lining cells
5. Osteoclasts
bone tissue Cells are surrounded by matrix.
- 25% water
- 25% protein
- 50% mineral salts
Differentiated
form of the same
basic cell type

29. CELL TYPES

30. OSTEOPROGENITOR CELLS
derived from mesenchymal stem cells.
It is a resting cell that can differentiate into an osteoblast
and secrete bone matrix.
found on the external and internal surfaces of bones

31. OSTEOBLAST
the differentiated bone-forming cell that secretes bone matrix
i.e. type I collagen and bone matrix proteins (BMPs)
active osteoblasts are cuboidal or polygonal in shape and
aggregate into a single layer of cells lying in apposition to the
forming bone.
inactive osteoblasts are flat or attenuated cells that cover the
bone surface.

33. FUNCTIONS
▪ New bone formation
▪ Controls bone mineralization.
▪ Regulation of bone remodeling and mineral metabolism.

34. OSTEOBLASTS
▪ Osteoblasts secrete type I collagen, small amount of type V collagen,
osteonectin, osteopontin, RANKL,
Proteoglycans, latent proteases and growth factors including bone
morphogenic proteins.
▪Osteoblasts exhibit high levels of alkaline phosphatase -cytochemical
marker.

35. OSTEOCYTES
When completely surrounded by osteoid or bone matrix, the
osteoblast is referred to as an osteocyte and the space occupied is
lacuna
Osteocytes processes communicate through the canaliculi with
other Osteocytes and bone-lining cells by gap junctions.
They are responsible for maintaining
the bone matrix.
�synthesize new matrix, as well as
participate in matrix degradation →
maintain calcium homeostasis
arranged with their long axes in the same direction

36. BONE-LINING CELLS
Bone-lining cells are derived from osteoblasts
and cover bone that is not remodeling.
layer of flat cells with attenuated cytoplasm
�on external bone surfaces = periosteal cells
�on internal bone surfaces = endosteal cells
Function
�maintenance and nutritional support of the
osteocytes and regulate the movement of calcium
and phosphate into and out of the bone.

37. OSTEOCLASTS
are phagocytotic cells derived from
fusion of hemopoietic progenitor cells
of neutrophilic granulocyte and
monocyte lineages. → multinucleated
cells
are bone-resorbing cells present on
bone surfaces where bone is being
removed or remodeled
�a shallow bay called a resorption bay
(Howship’s lacuna) can be observed in the
bone directly under the osteoclast.

40. A
B
C
A= Osteoclast
B= Osteoblast
C= Osteocyte

41. A
B
C
A= cementum/ root
B= Sharpeys fibers
C= Alveolar bone/ compact bone

42. A
C
D
E
B
A= Circumferential lamella
B= Concentric lamella
C= Interstitial lamella
D= haversian canal
E= Osteocyte

43. THANK YOU