📝 STANDARD DESCRIPTION:
Bone = specialised connective tissue; matrix mineralised with calcium hydroxyapatite + collagen fibres.
Properties:
• Hardest body tissue (after tooth enamel).
• Remarkable tensile + compressive strength.
• Living, remodelling, vascular tissue.
• Stores 99% body's Ca²⁺ + 85% PO₄.
• Site of haematopoiesis (red marrow).
Functions of Bone:
1. Support — body framework.
2. Protection — brain, spinal cord, heart, lungs.
3. Movement — levers for muscle action.
4. Mineral storage (Ca, P).
5. Haematopoiesis (red marrow).
6. Energy storage (yellow marrow = fat).
Composition: ~33% organic (mainly Type I collagen — 'ossein'), ~67% inorganic (calcium hydroxyapatite).
📚 DETAILED DESCRIPTION:
BONE — A LIVING COMPOSITE MATERIAL
Bone is a remarkable composite of collagen fibres + mineral crystals (hydroxyapatite). Collagen gives tensile strength (resists pulling); mineral gives compressive strength (resists crushing) — together they outperform any single material engineering can produce.
COMPOSITION:
• 33% organic — mainly Type I collagen ('ossein') + ground substance (proteoglycans).
• 67% inorganic — Calcium hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ crystals deposited along collagen fibres.
• Water — small but functional fraction.
BONE CELLS (4 types):
• Osteoblasts — bone-forming cells (build matrix).
• Osteocytes — mature osteoblasts trapped in their own matrix; live in lacunae, communicate via canaliculi.
• Osteoclasts — large multinucleate bone-resorbing cells (derived from monocytes).
• Bone-lining cells — quiescent cells on bone surfaces.
BONE TYPES (Histological):
• Compact (cortical) bone — dense outer shell; arranged in Haversian systems (osteons).
• Cancellous (spongy/trabecular) bone — internal latticework; aligned along stress lines.
BONE FORMATION (Ossification):
• Intramembranous — direct from mesenchyme (flat bones of skull, mandible).
• Endochondral — replaces a cartilage model (long bones of limbs).
STRUCTURE OF A LONG BONE:
• Diaphysis = shaft, with central medullary cavity.
• Epiphyses = ends, covered by articular hyaline cartilage.
• Metaphysis = junction zone with growth plate (epiphyseal plate / physis) in the young animal.
• Periosteum — fibrous outer covering; rich in osteoblasts and blood vessels.
• Endosteum — thin lining of medullary cavity.
• Nutrient foramen — passage for nutrient artery.
// COMPARATIVE ANALYSIS
| BIRD |
Pneumatic bones — many bones (humerus, sternum, vertebrae) are hollow and filled with air sacs to reduce weight for flight. |
| MAMMAL |
Bones contain marrow only (red or yellow) — solid, dense. |
| AQUATIC MAMMALS |
Pachyostotic bones in hippos/dugongs — extra-dense bones used as ballast for diving. |
CLINICAL RELEVANCE
Bone Marrow Aspiration: Sites — sternum or wing of ilium (cattle/horse), iliac crest or proximal humerus (dog). Used in haematology (anaemia, leukaemia diagnosis). Fracture healing: Depends on intact periosteum + nutrient artery + osteoblast activity.
