Anatomy and Functions of Bone

Functions of Bone

• Protects and supports soft tissues
• Provides attachment sites for muscles, enabling movement
• Stores minerals calcium and phosphate, crucial for mineral homeostasis
• Produces blood cells in red bone marrow (hemopoiesis)
• Stores energy in yellow bone marrow

Classification of Bone Shape

• Long Bones: Longer than wide, cylindrical shape (e.g., femur, phalanges)
• Short Bones: Nearly equal in width and length, cube-shaped (e.g., carpals, tarsals)
• Flat Bones: Thin and flat (e.g., skull bones, sternum, ribs, shoulder blades)
• Irregular Bones: Complex shape, don't fit other categories (e.g., vertebrae, hip bones)
• Sesamoid Bones: Develop within tendons under friction and stress (e.g., patella)
• Sutural Bones: Classified by location, tiny bones in sutures between cranial bones

Anatomy of a Long Bone

• Diaphysis: Shaft of the bone
• Epiphysis: Ends of the long bone
• Metaphyses: Areas between epiphysis and diaphysis, containing the epiphyseal plate in growing bones
• Articular Cartilage: Covers joint surfaces, reduces friction and acts as a shock absorber
• Medullary Cavity: Marrow cavity within the diaphysis
• Endosteum: Lining of the marrow cavity
• Periosteum: Tough membrane covering bone (excluding cartilage)

Histology of Bone Tissue

• Connective tissue with widely spaced cells
• Matrix composed of 25% water, 25% collagen fibers, and 50% mineral salts
  • Minerals: Primarily hydroxyapatite (85%), calcium carbonate (10%), and others like calcium fluoride, magnesium fluoride
• Four types of bone cells:
  • Osteogenic Cells: Divide and differentiate into osteoblasts
  • Osteoblasts: Bone-building cells
  • Osteocytes: Mature bone cells, maintain bone tissue
  • Osteoclasts: Derived from monocytes, break down bone tissue

Matrix of Bone

• Inorganic Mineral Salts: Provide bone's hardness (hydroxyapatite and calcium carbonate)
• Organic Collagen Fibers: Provide bone's flexibility, resist stretching and tearing
• Bone is not solid, contains spaces for vessels and red bone marrow
  • Spongy Bone: Many spaces
  • Compact Bone: Fewer spaces

Compact Bone

• Arranged in units called osteons
• Osteons contain:
  • Blood vessels, lymphatic vessels
  • Nerves, osteocytes
  • Calcified matrix
• Aligned along lines of stress, which can change over time
• Concentric rings (lamellae): Calcified matrix surrounding a central blood vessel
• Lacunae: Spaces containing osteocytes
• Canaliculi: Tiny canals filled with extracellular fluid, connecting osteocytes

Spongy Bone

• Lacks osteons
• Consists of trabeculae surrounding red marrow-filled spaces
• Forms most of the structure in short, flat, and irregular bones, and epiphyses of long bones
• Lightweight, supports and protects red bone marrow

Trabeculae of Spongy Bone

• Latticework of thin bone plates aligned along lines of stress
• Spaces between trabeculae filled with red marrow

Bone Formation (Osteogenesis or Ossification)

• Begins with mesenchymal cells forming template for ossification
• Two types:
  • Intramembranous Ossification: Bone formation directly from fibrous connective tissue membranes
  • Endochondral Ossification: Bone formation from hyaline cartilage models

Intramembranous Ossification

• Forms flat bones of skull, mandible, and clavicles
• Ossification Center: Mesenchymal cells convert to osteoblasts and lay down osteoid matrix
• Matrix calcifies as osteoblasts become osteocytes, forming bridges of trabeculae
• Peripheral mesenchyme forms periosteum

Endochondral Ossification

• Replaces cartilage with bone, forms most bones
• Development of Cartilage Model: Mesenchymal cells form cartilage model
• Growth of Cartilage Model:
  • Interstitial Growth: Chondrocyte cell division and matrix formation, lengthens cartilage
  • Appositional Growth: New matrix formation on periphery by chondroblasts, widens cartilage
  • Calcification: Cells in midregion burst, trigger calcification and chondrocyte death

Endochondral Ossification (Continued)

• Development of Primary Ossification Center:
  • Periosteal Bone Collar: Perichondrium lays down bone collar
  • Nutrient Artery: Penetrates cartilage model center
  • Periosteal Bud: Brings osteoblasts and osteoclasts, forming spongy bone trabeculae
  • Medullary Cavity: Formed by osteoclasts

Endochondral Ossification (Continued)

• Development of Secondary Ossification Center: Blood vessels enter epiphyses near birth, forming spongy bone without a medullary cavity
• Formation of Articular Cartilage: Cartilage on bone ends remains as articular cartilage

Growth in Length

• Epiphyseal plate (growth plate) is crucial for bone growth in length
• Zones of Epiphyseal Plate:
  • Zone of Resting Cartilage: Anchors plate to bone
  • Zone of Proliferating Cartilage: Rapid cell division
  • Zone of Hypertrophic Cartilage: Enlarged cells remain in columns
  • Zone of Calcified Cartilage: Thin zone, cells dead, matrix calcified, osteoclasts remove matrix, osteoblasts and capillaries form bone

Growth in Thickness (Appositional Growth)

• Bone grows in thickness by appositional growth at bone surface
• Process:
  • Periosteal cells differentiate into osteoblasts, forming matrix
  • Ridges fuse, periosteum becomes endosteum
  • New concentric lamellae form
  • Osteoblasts under periosteum form circumferential lamellae

Fractures

• Broken bones
• Typically heal on their own, but repositioning is necessary for proper healing.

This quiz covers the essential functions of bone, classifications of bone shapes, and the specific anatomy of long bones. Understand the protective, supportive, and mineral-storage roles of bones, along with details about different bone types and structures.