Bones can be classified based on their position in the body as either the axial skeleton which forms the axis of the body or the appendicular skeleton which forms the appendages. Bones can also be classified by their shape as either long bones, short bones, flat bones, or irregular bones. The microstructure of adult bone consists of collagen fibers, hydroxyapatite crystals, and cells including osteoblasts, osteoclasts, osteocytes, and bone lining cells which are involved in bone formation and resorption.

1. Guided by - Dr. Rahimi sir
Presented by - Vikash Yadav

2. Bone – a quick overview
• Composite of extracellular matrix proteins
• Cellular components

3. Bones can be classified on the macro level by
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.

4. or by shapes...
• 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)

5. …or by structure
• Compact (cortical) bone
• Spongy (cancellous, trabecular)
bone

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

7. 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

8. Microstructure of adult bone

9. 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

10. 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

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

12. 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)

13. 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