Microscopic Bone Structure

Overview

• Bones consist of organic components including collagen fibers and different types of cells.
• One-third of bone is organic, containing collagen and cells.

Types of Bone Cells

Osteoprogenitor Cells

• Definition: Stem cells that can differentiate into various bone cells.
  • "Osteo" means bone; "progenitor" implies they are precursors or "bone babies."
  • Can undergo mitosis and become osteoblasts or remain as stem cells.
• Location: Found in the endosteum and inner layer of the periosteum.
• Function: Source of new bone cells.

Osteoblasts

• Definition: Cells that build bone by secreting the matrix.
  • "Blast" means to build.
• Function: Create the bone matrix by secreting collagen ropes and attaching calcium phosphate.
• Cannot undergo mitosis; only form from osteoprogenitor cells.

Osteocytes

• Definition: Mature bone cells that are no longer building bone.
• Location: Live inside lacunae (small spaces within bone matrix).
• Function: Maintain bone tissue.
• Structure: Linked by caniculi to central canal for nutrients and waste.

Osteoclasts

• Definition: Multinucleated cells that break down bone tissue.
• Function: Resorb bone to release minerals like calcium and phosphorus into the blood.
• Origin: Derived from white blood cells, not osteoprogenitor cells.
• Activity: Crucial for bone repair and remodeling.

Bone Composition

• Collagen (Osteoid): Provides flexibility and strength.
• Calcium Phosphate (Hydroxyapatite): Provides hardness and rigidity, constituting about 65% of bone.

Bone Structure

• Osteon (Haversian System): Functional unit of compact bone.
  • Central canal (Haversian canal) surrounded by concentric rings (lamellae).
  • Osteocytes reside in lacunae connected by caniculi.
• Perforating (Volkmann's) Canals: Connect central canals of different osteons.

Bone Development and Growth

Hematopoietic Tissue

• Newborns: Red bone marrow in nearly all bones due to rapid growth.
• Adults: Red marrow primarily in flat and irregular bones like sternum and coxal bones.

Bone Formation

• Osteogenesis: Process of bone formation.
• Ossification: Hardening of bones into mature structure.

Ossification Processes

• Intramembraneous Ossification: Formation of flat bones like skull bones from fibrous connective tissue.
• Endochondral Ossification: Conversion of hyaline cartilage models into bone, common for most bones.
  • Involves anti-angiogenesis factor which initially prevents blood supply.
  • Blood supply invasion allows osteoblasts to replace cartilage with bone.

Bone Growth

• Longitudinal Growth: Lengthening of bones via growth plates until about age 25.
• Appositional Growth: Thickening of bones throughout life due to osteoblast and osteoclast activity.

Important Structures

• Articular Cartilage: Covers ends of bones at joints to reduce friction.
• Fontanelles: Soft spots on newborn skulls that ossify by age two.

Bone Repair and Remodeling

• Bones are constantly being remodeled through the coordinated actions of osteoblasts and osteoclasts.
• Factors like diet, exercise, and hormonal balance influence bone strength and integrity.

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These notes cover the key concepts of bone structure, types of bone cells, the process of ossification, and the continuous remodeling and repair that bones undergo throughout life. They aim to provide a detailed yet concise overview of the lecture's main points.