Bone is composed of cells and an extracellular matrix. There are four main bone cell types: osteoblasts which form bone, osteocytes embedded in the bone matrix, osteogenic cells which differentiate into osteoblasts, and osteoclasts which resorb bone. Bone has two types - compact bone which forms the dense outer shell and cancellous or spongy bone found at the ends of bones and in flat bones. Compact bone contains concentric lamellae that form haversian systems around central haversian canals containing blood vessels. Cancellous bone contains trabeculae that form a network with bone marrow in between.

1. HISTOLOGY OF
BONE

2. Objectives:
❖ Components of bone
1. Bone cells [Osteogenic cells, Osteoblasts, Osteocytes & Osteoclasts]
2. Bone matrix [Extracellular matrix]
3. Periosteum & endosteum
❖ Types of bone
Maturity : Primary & secondary bone
Arrangement and regularity: Compact & cancellous bone

3. Bone is a weight-bearing type of connective tissue characterized by its
stony hard, solid matrix
- Bone is formed of cells and extracellular matrix
- Bone is vascular (Unlike cartilage which is non-vascular)
❖ Components of the bone
- Bone cells: Osteogenic cells, Osteoblasts, Osteocytes & Osteoclasts
- Extracellular matrix (bone matrix)
- Periosteum & Endosteum
❖ Types of bone
❖ Bone development, growth & remodeling

4. Components of the bone
Periosteum
Endosteum
Bone cells: Osteogenic cells, Osteoblasts, Osteocytes & Osteoclasts
Extracellular matrix (bone matrix)

5. 1) Periosteum:
The membrane that covers the outer surface of bone
It is formed of 2 layers:
a) Outer fibrous layer: composed of dense irregular C.T. . which is
vascular
b) Inner osteogenic layer: consists
of one or two rows of cells.
In the resting periosteum (inactive),
it is formed of osteogenic cells.
In the active periosteum, (during
growth or healing of fracture) it
changes to osteoblasts
Functions of the periosteum:
1) The outer fibrous layer provides
attachment for the muscles
Periosteum
I. Components of Bone

8. 2) The inner osteogenic layer is responsible for:
a) Appositional growth of bone: osteogenic cells differentiate
into osteoblasts which lay down bone matrix under the
periosteum causing bone growth in width
b) Healing of bone after fracture
Periosteum
Endosteum
2) Endosteum
The membrane that lines bone
cavities (bone marrow cavity,
Haversian canals and Volkman’s
canals)
Structure and function:
It is formed of one layer of
osteogenic cells
Its function is similar to the inner
osteogenic layer of the periosteum
Periosteum

9. 3) Bone Cells
1. Osteogenic cell (Osteoprogenitor cell):
- Arise from pericytes around blood
capillaries
- Osteogenic cells are capable of
mitotic division and differentiation into
osteoblasts (bone forming cells)
- Function:They are responsible for
a) Appositional growth of bone
b) Bone repair after fracture
- Present in the inner osteogenic layer of the periosteum and in the
endosteum
- Flat cells with pale basophilic cytoplasm and oval pale (open-face)
nucleus

10. 2. Osteoblast (Bone forming cell):
- Present in the inner osteogenic layer of
the periosteum and in the endosteum
when this membranes are active
- They arise from osteogenic cells.
- Osteoblasts can't divide (non-dividing
cells)
- Osteoblasts are small oval deeply
basophilic cells showing –ve Golgi image.
Their nucleus is eccentric. They are rich in
alkaline phosphatase enzyme which
deposits calcium phosphate & calcium
carbonate in the matrix
Osteoblast
Microscopic structure of
osteoblast
Bone Cells

11. Osteoblast
Microscopic structure of
osteoblast
Function of the osteoblast:
They are responsible for bone formation.
a) They synthesize and secrete the
organic components of bone matrix
(collagen, glycosaminoglycans &
glycoproteins)
b) They cause calcification (deposition of
calcium) in bone matrix through their
content of alkaline phosphatase
When osteoblasts surround themselves by
bone matrix, they become imprisoned
inside lacunae, thus change to
osteocytes
Bone Cells

12. Bone Cells
Mineralization in bone matrix

13. Bone Cells

14. 3. Osteocytes (Mature bone cells):
- Osteocytes derived from osteoblasts that
become imprisoned in lacunae within the
bone matrix
- Osteocytes are non-dividing cells.They are
branched oval cells with basophilic
cytoplasm and oval nucleus.
- The cellular processes of osteocytes pass
through bony canaliculi that connect the
lacunae together.The processes of osteocytes
communicate through gap junctions
-Tissue fluid pass through the lacunae and
canaliculi to conduct oxygen and nutrients to
osteocytes & remove waste products
Function of the osteocyte:
- Maintain bone matrix by continue producing
small amount of matrix throughout life
Osteocyte
Bone Cells

15. 4. Osteoclasts (Bone resorping cells):
- Osteoclast is responsible for bone
resorption
- Osteoclast doesn’t belong to the osteogenic
cell lineage.The precursor of osteoclast is
blood monocyte.
- Osteoclast plays a role in bone remodeling.
This process involves removal of bone matrix
at several sites by osteoclasts followed by
replacement with new bone by osteoblasts
- Osteoclast is a large multinucleated cell, has
foamy acidophilic cytoplasm and a striated
(brush, ruffled) border facing the bone
surface. Osteoclast resides in a shallow cavity
on the bone surface called “Howship’s lacuna”
[subosteoclastic compartment]
Howship’s
lacuna
Osteoclast
Bone Cells

16. Osteoclast
Function of the osteoclast:
Bone resorption which involves 2 steps:
1) Dissolution of the inorganic
component of bone matrix (bone
demineralization)
- This is achieved as the subosteoclastic
compartment becomes acidic
- Within the osteoclast:
CO2 + H2O H2CO3
H2CO3 H+ + HCO3-
- H+ ions are transported into the
subosteoclastic compartment, so the pH
is reduced (acidic environment)
2) Enzymatic degradation of the organic
matrix by lysosomal enzymes
Bone Cells

17. Bone Cells
Osteoclast has 4 zones:
1. Ruffled border 3. Vesicular zone
2. Clear zone (Sealing zone) 4. Basal zone

18. 4) Bone matrix (Extracellular matrix)
Bone matrix
Inorganic component
(65%)
Organic component
(35%)
Mainly Calcium
phosphate, carbonate
and some other
minerals
a) Collagen type I (90%)
b) Glycosaminoglycan
c) Glycoproteins
(Osteocalcin,
Osteopontin,
osteonectin,
sialoprotein)

19. II. Types of Bone
Bone is classified in two different ways:
I) According to its maturity
II) According to its arrangement and regularity
I. Classification of bone according to its maturity:
A. Primary bone
(Immature bone)
B. Secondary bone
(Mature bone)
A) Primary bone (immature bone- woven bone)
Primary bone is the 1st bone to form during fetal development and
during bone healing after fracture.Then it is replaced by 2ry bone.
Primary bone is characterized by:
Irregular collagen bundles – Abundant osteocytes randomly
arranged - More water content - Less mineral content

20. B) Secondary bone (mature bone- lamellar bone): is characterized
by:
Collagen fibers are regularly arranged - The matrix is more
calcified - Less water content
Secondary bone is stronger than primary bone
II. Classification of bone according to its arrangement or regularity:
A) Cancellous bone
Spongy bone
B) Compact bone
Ivory bone
Types of Bone

21. A) Cancellous bone:
Site:
- All flat bones: iliac bone, scapula, diploae of the vault of the skull
- All irregular bones: bodies of vertebrae
- Epiphysis of long bones
Types of Bone

22. Microscopic structure of cancellous bone:
- It is composed of branching and anastomosing bone trabeculae with
bone marrow spaces inbetween these trabeculae
- Bone trabeculae contain osteocytes in lacunae
- The surface of trabeculae (endosteum) is lined by osteogenic cells,
osteoblasts and osteoclasts
A section in
cancellous bone
Types of Bone

23. Magnification of
the section in
cancellous bone
Types of Bone

24. B) Compact bone:
Site:
- Shaft (diaphysis) of long bone
- Covering any cancellous bone
Types of Bone

25. Microscopic structure of compact bone:
Bone lamellae are arranged in regular pattern
a) Outer circumferential lamellae:
Bone lamellae present under the periosteum and parallel to the outer
surface of the bone
b) Concentric lamellae (Haversian systems, Osteons)
- Regular concentric bone lamellae surrounding Haversian canal
- Haversian canal: is a canal that lies longitudinally parallel to the
long axis of the bone . It is lined with endosteum and contains blood
vessels
- Haversian canal and the surrounding bone lamellae constitutes
Haversian system (Osteon)
- Volkman’s canal: transverse canals that connect the Haversian
canals together and also with the periosteum and endosteum. It is
lined with endosteum and contains blood vessels
c) Interstitial lamellae: Present inbetween the Haversian systems
d) Inner circumferential lamellae:
Bone lamellae present under the endosteum and parallel to the
medullary bone cavity
Types of Bone

26. 1. Outer circumferential
lamellae
3. Interstitial lamellae
4. Inner circumferential
lamellae
2. Haversian system
(Osteon)
Types of Bone

27. Haversian system
(Osteon)
Haversian
canal
Concentric
bone
lamellae
Concentric bone lamellae contain
osteocytes inside lacunae
Types of Bone

28. Haversian system
(Osteon)
Haversian
canal
Volkman’s
canal
Volkman’s
canal
Haversian
system (Osteon)
is formed of regular
concentric bone
lamellae
surrounding
longitudinal
Haversian canal
that contains blood
vessels
Volkman’s canal
are transverse
canals connect the
Haversian canals
together and
contains blood
vessels
Blood vessel
Concentric bone
lamellae
Types of Bone

29. A section in the
compact bone
Higher magnification
(H&E stain)
Higher magnification
(H&E stain)
Types of Bone

30. A section in the compact bone Higher magnification
Types of Bone

31. Types of Bone