Bone is a composite material made up of cells and an extracellular matrix. At the micro level, bone contains osteoblasts that build bone tissue, osteoclasts that resorb bone, and osteocytes embedded in the matrix. The matrix is made of collagen fibers mineralized with hydroxyapatite crystals. Bones can be classified by their macro structure as long, short, flat, or irregular. Long bones have a diaphysis and epiphyses. Bone structure is adapted to its mechanical functions according to Wolff's law.

1. Structure and Composition of
Bone
Biomechanics aims to explain the
mechanics of life and living.
From molecules to organisms,
everything must obey the laws of
mechanics.
Y. C.
Fung

2. Learning Objectives
 Identify important structures of bone at
gross, macro and micro levels.
 Compare contributions of different
components of bone (such as cells) to
its functions
 Describe the effects of structure of
bone on its functions (Wolff’s Law)

3. Bone – a quick overview
 Composite of extracellular matrix proteins
◦ Mineralizes through elegant feedback and regulation
mechanisms
 Cellular components
◦ Maintain structural integrity
◦ Respond to metabolic and mechanical requirements
 Functions
◦ Protects organs
◦ Supports
◦ Allows movement through attachment for muscles
◦ Manufactures blood cells (bone marrow)
◦ Stores Ca2+ and PO4
3-

4. Bones can be classified on the macro
level several ways, such as by position...
 Axial skeleton – Bone forming axis of
body, e.g skull, rib, sternum and
vertebrae
 Appendicular skeleton – bones
forming appendages of body, e.g.
limbs, shoulder and hip.

5.  Long Bones - longer than
they are wide (e.g humerus)
 Short bones - cube shaped,
e.g. bones in wrist and ankle
 Flat bones - Thin, flattened
and a bit curved (e.g sternum
and most skull bones)
 Irregular bones - Complicated
shapes (e.g vertebrae,
maxilla and mandible)
… or by shape…

6. …or by structure.
 Compact (cortical) bone
 Spongy (cancellous, trabecular) bone

7. Structure of long bones
 Diaphysis
 Epiphysis
 Metaphysis
 Articular cartilage
 Periosteum
 Medullary canal
(marrow cavity)
 Endosteum

8. Structure of short, irregular and
flat bones
 Thin plates of
periosteum-covered
compact bone on the
outside
 Endosteum-covered
spongy bone on the
inside
 No diaphysis or
epiphysis
 Bone marrow between
trabeculae

9. Microstructure of adult bone:
Lamellar bone
 Adult bone made up of
layers (lamellae)
◦ Bundles of collagen in
parallel surrounded by
mineralized matrix
 Between layers are lacunae
(spaces)
 Lacunae contain
osteocytes (bone cells)
 Canaliculi (fine canals)
spread from lacunae

11. Interstitial lamellae
Concentric lamellae

12. Hierarchical structure of bone

13. Composition of bone
 ~1/3 organic
◦ Collagen fiber matrix (~88%) organized
around lamellae
◦ Non-collagen (~12%); Glycoproteins,
proteoglycans, lipids, etc)
 ~1/3 inorganic (mineral)
◦ Mostly dense calcium phosphate in the form
of highly insoluble crystals of hydroxyapatite
 ~1/3 water
 Cells – osteoblasts, osteoclasts,
osteocytes, bone lining cells

14. Bone cells: Osteoblasts
 Bone formers: produce organic matrix
 Found on surfaces where bone is formed
 Responsible for growth in thickness of long bones
 Can initiate bone resorption
 Synthesize osteoid, collagen
 Control mineralization
 Extend to adjacent osteoblasts through canaliculi
 Receptors – estrogen, parathyroid hormone,
Vitamin D, growth factors

15. Bone cells: Osteocytes
 Entrapped osteoblasts
 Most numerous bone cell
 Derived from osteoblast
that is no longer
synthesizing collagen
 Function: Maintenance
of bone

16. Bone cells: Osteoclasts
 Large multinucleated cell
 Plays active role in bone resorption during bone
remodelling
 Precursors: monocytes from marrow or blood
 Found in notches or indentations in bone (Howship’s
lacunae) – excavations
 Contain the enzyme acid phosphatase
 Receptors: calcitonin, estrogen (indirect)

17. Bone Lining Cells
 Cover inactive (nonremodeling) bone
surfaces
 Connect to other cells through
canaliculi
 May play important roles in
◦ Hematopoiesis
◦ Mechanotransduction – propagating
signals initiating bone resorption and
remodeling
◦ Maintenance of bone fluids and
movement of ions for mineral

18. Relationships between structure
and function
 Wolff’s law: change in
form follows change in
function
Bone structure changes
in response to
mechanical stimulation

19. Relationships between structure
and function
 Mechanotransduction:
◦ Mechanical signals  Biochemical
signals
◦ Response: cell activity
 Bone formation/resorption depends on
duration, magnitude and rate of
loading
 Cyclic loading induces bone formation
 Unloading induces bone resorption