1 Bone Structure and Composition

Overview of the structure and composition of bone, its hierarchical structure and 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 Ca 2+ and PO 4 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

10. Interstitial lamellae Concentric lamellae

11. Hierarchical structure of bone

12. 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

13. 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 C ontrol mineralization Extend to adjacent osteoblasts through canaliculi Receptors – estrogen, parathyroid hormone, Vitamin D, growth factors

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

15. 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)

16. 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 homeostasis

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

18. Relationships between structure and function Mechanotransduction : M echanical 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