Bone lecture

1- Is the highest differentiation of CT
2-Form the skeleton
3-sites for attachment of the muscles and
tendons essential for locomotion.
4-Supports and protects vital organs
5- Harbors bone marrow, fat store
6- Acts as a reservoir of Calcium & Phosphate
BoneBone

Gross anatomy of bones
 Compact bone
 Spongy
(trabecular)
bone
 Blood vessels
 Medullary
cavity
 Membranes
Periosteum
Endosteum

Cartilage Bone
Water content: ~70%
Collagen II: ~40% of organic content.
Grows interstitially and by apposition.
Avascular
Water content: 25%
Collagen I: 90% of organic
content.
Other Ground Substance
Osteonectin: anchor
collagen to bone mineral.
Osteocalcin: Calcium binding
protein involved in bone
calcification.
Osteopontin: Binding of
osteoblasts and osteoclasts to
bone.
Grows only by apposition.
Highly vascular

Types of BoneTypes of Bone
(Flat, long, short and irregular bones)(Flat, long, short and irregular bones)

Flat bones
 Spongy bone :
Compact + spongy
 Have bone marrow

Long bones
 Diaphysis= shaft
 Epiphyses at the ends: covered with “articular”
(=joint) cartilage
 Epiphyseal line in adults
 Kids: epiphyseal growth plate (disc of hyaline
cartilage that grows to lengthen the bone)
 Blood vessels
 Nutrient arteries and veins through nutrient foramen

Cells Fibers
Matrix
1. Osteogenic cells
2. Osteoblasts
3. Osteocytes
4. Osteoclasts
Type 1 Collagen (90%)
Structure of bone
Organic
1.GAGs: Hyaluronan,
Chondroitin & Keratan Sulfate
2.Proteoglycans
Inorganic
Hydroxyapatite crystals
[Ca10(PO4)6(OH)2]
Calcium phosphate

1. Osteogenic or osteoprogenitor
cells:
- stem cells derived from mesenchyme.
- They are spindle-shaped with elongated nuclei.
- present in:
• inner layer of periosteum
• endosteum
• lining the vascular canals of compact bone.
Function: differentiation into osteoblast cells.
Cells of Bone

2. Osteoblasts:
- young bone cells, located at the surfaces .
- known as bone-forming cells.
- cuboidal cells with short, slender processes.
- has a large nucleus + prominent nucleolus.
- The cytoplasm: basophilic, well-developed Golgi
apparatus, mitochondria.
- can not divide.
Functions: synthesis of the organic component and
deposition of inorganic components of bone matrix.

3. Osteocytes
Once the osteoblast cells surrounded by matrix:
1. The cytoplasmic processes become more evident.
2. The cells become flattened.
3. lacunae and canaliculi appear
osteoblasts are referred to as Osteocytes.
- Processes make contact via gap junctions (?????)
- can not divide.
- faint basophilic cytoplasm.
- Dark nucleus.
- Few RER & Golgi.
Function:
maintenance of the bony matrix.

4. Osteoclasts:
- Bone-eating cells.
- found in depressions known as Howship’s lacunae.
- derived from the fusion of blood monocytes.
- very large cells (20 – 100 µm).
- multinucleated (5 –50).
- acidophilic cytoplasm.
-The surface facing the bone matrix is irregular.
- contain a well-developed Golgi + great number of
lysosomes.
Function:
secrete acid collagenase, and proteolytic enzymes that
attack the bone matrix and liberate the calcified
ground substance.

Bone cells
Osteoblasts
Active
Inactive
Osteocytes
Osteoclasts
Howship’s lacuna
(resorption bay)

Osteocytes, Lacunae and Canaliculi
Source Undetermined

Types of bone
Compact Bone
Cancellous bone

Compact (dense) and Spongy (cancellous)
Bone
cancellous bone
Compact
bone
• found in diaphysis (shaft) of long
bones.
• dense, hard, with no cavities.
• found in epiphyses (bulbous ends) of long
bones, flat (Skull) & irregular (vertebrae) bone
• with cavities.

.
1.Compact bone
Compact bone is a solid mass composed of:
1. Haversian system.
2. Circumferential lamellae.
3. Interstitial lamellae (O&I).
4. Periostem
5. Endosteum

1. Haversian system (osteon):
the structural unit of compact bone, composed of:
1. Haversian canal: it runs parallel to the long axis of bone. It
contains loose connective tissue rich in blood vessels and nerves
and lined with osteogenic cells.
2. Bone lamellae: they are calcified osteoid tissue arranged in
concentric layers (4-20 layers).
3. Osteocytes: they are present in their lacunae between the bone
lamellae.
2. Circumferential lamellae:
bone lamellae under the periosteum (outer circumferential lamellae)
or adjacent to the endosteum (inner circumferential lamellae).
3. Interstitial lamellae: lamellae between Haversian system
Volkmann`s canal: transverse or oblique canals that connect
haversian canals together.

Haversian system (osteon), Haversian canal (HC)
and Volkmann’s canal (VC)
Transverse section Longitudinal section
Osteon
VC
HC
VC
HC

•Nutrients diffuse from vessels in central canal
•Alternating direction of collagen fibers
increases resistance to twisting forces
Isolated osteon:

• Canaliculi
•Tiny canals
•Radiate from the
central canal to
lacunae
•Form a transport
system
Figure 5.3

Dark spots are called ‘lacunae’ and
would contain osteocytes in living bone
Central
canal
containing
an artery,
vein,
lymph
vessel and
nerves.

Haversian system (osteon) /Harversian canal
HC
HC

Periosteum:
 Connective tissue membrane
 Covers entire outer surface of bone except at epiphyses
 Two sublayers
 1. Outer fibrous layer
 2. Inner (deep) cellular osteogenic layer (Osteoblasts: bone
depositing cells, also osteoclasts: bone destroying cells)
 Secured to bone by perforating fibers (Sharpey’s fibers)
Endosteum:
 Lines the internal surfaces of bones - one cell thick
 Is also osteogenic
• Important roles of the periosteum and endosteum:
1. nutrition
2. histogenesis and repair.

Periosteum and Endosteum
Endosteum
Source Undetermined (Both images)

Cancellous
Bone
1- NO Haversian system.
2- Irregularly arranged bone.
3- Irregular marrow spaces.
4- Periostem and endosteum.

Compact and Cancellous Bone
Michigan Medical School Histology Slide Collection

Bone lecture

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