Fundamentals of Anatomy and Physiology - Bone

Function of Bones

• Support:
  • Framework that supports the body and cradles soft organs
• Protection:
  • Protective case for brain, spinal cord, vital organs
• Movement:
  • Provide levers for muscles
• Hematopoiesis:
  • Blood cell production occurs in the red marrow cavities
• Mineral Storage:
  • Reservoir for minerals, especially calcium and phosphorus
• Energy Storage:
  • Yellow bone marrow consists of adipose tissue

Classification of Bones

• Sutural Bones:
  • Small, irregular bones found between flat bones of the skull
• Irregular Bones:
  • Complex shapes (e.g., spinal vertebrae, pelvic bones)
• Short Bones:
  • Small and thick (e.g., ankle and wrist bones)
• Flat Bones:
  • Thin with parallel surfaces (e.g., skull, sternum, ribs, scapulae)
• Long Bones:
  • Long and thin (e.g., arms, legs, hands, feet, fingers, toes)
• Sesamoid Bones:
  • Small, round, flat bones (like sesame seeds), develop inside tendons near joints

Gross Anatomy of Bones

Bone Textures

• Compact Bone:
  • Dense outer layer, also known as cortical bone
• Spongy Bone:
  • Honeycomb structure filled with red or yellow bone marrow, also called cancellous or trabecular bone
  • Trabeculae develop along the bone’s lines of stress

Structure of Long Bones

• Diaphysis:
  • Tubular shaft forming the axis of long bones, composed of compact bone surrounding a medullary cavity
• Epiphyses:
  • Expanded ends with exterior compact bone and interior spongy bone; joint surface covered with articular cartilage
• Metaphyses:
  • Region where the diaphysis joins epiphyses; contains epiphyseal plate in growing bones

Surface Coverings of Bone

• Periosteum:
  • Double-layered outer membrane with dense fibrous outer layer and cellular inner layer containing osteogenic cells and osteoblasts
  • Contains nociceptors and blood vessels; does not cover articular surfaces
• Endosteum:
  • Delicate layer covering internal surfaces, contains osteogenic cells, osteoblasts, and osteoclasts

Composition of Osseous Tissue

• Extracellular Matrix:
  • Calcified matrix of calcium salts, collagen fibers, and specialized cells (osteogenic cells, osteoblasts, osteocytes, osteoclasts)
• Fibers:
  • Collagen fibers provide framework, allowing flexibility; combination of collagen and minerals gives compressive strength

Specialized Cells of Osseous Tissue

• Osteogenic Cells:
  • Divide and differentiate into osteoblasts, maintain osteoblast populations
• Osteoblasts:
  • Responsible for bone formation, secrete osteoid (unmineralized matrix), concentrate calcium phosphate
• Osteocytes:
  • Mature osteoblasts maintaining bone tissue, communicate through canaliculi
• Osteoclasts:
  • Multinucleated cells that break down bone matrix, regulate calcium and phosphate ion concentrations

Microscopic Structure of Bone

Compact Bone

• Osteon:
  • Basic functional unit, contains central canal, lamellae (layers of matrix), lacunae (cavities for osteocytes), and canaliculi (canals connecting lacunae)

Spongy Bone

• Contains trabeculae without central canals, gas and nutrient exchange occurs through canaliculi at trabecular surfaces

Bone Marrow

• Red Bone Marrow:
  • Found in spongy bone of skull, ribs, sternum, vertebrae, pelvis; involved in blood cell production
• Yellow Bone Marrow:
  • Found in medullary cavity of long bones; consists of adipose tissue, serves as an energy reserve

Bone Development

Ossification

• Endochondral Ossification:
  • Bone replaces hyaline cartilage; forms long bones
• Intramembranous Ossification:
  • Bone develops directly from fibrous tissue; forms flat bones of skull and clavicles

Zones of the Epiphyseal Plate

• Quiescent Zone:
  • Anchors the epiphyseal plate
• Proliferating Zone:
  • Chondrocytes divide and push epiphysis away from diaphysis
• Hypertrophic Zone:
  • Older cells enlarge and degrade, cartilage cells die
• Calcification Zone:
  • Bone formation occurs as cartilage is replaced

Bone Remodeling

• Active and continuous process involving bone resorption and deposition; maintains calcium and phosphate homeostasis; repairs microfractures
• Wolff’s Law:
  • Bone remodels in response to mechanical stresses placed upon it

Nutritional and Hormonal Effects on Bone Growth

• Essential minerals (e.g., calcium, phosphate) and vitamins (D3, C, A, K, B12) required for normal growth and remodeling
• Hormones (GH, T4, sex hormones) stimulate osteoblast activity; calcitonin and parathyroid hormone regulate calcium levels

Fracture Repair

1. Fracture Hematoma Formation:
   • Blood clot forms, stabilizes break
2. Callus Formation:
   • Internal and external calluses stabilize the break
3. Spongy Bone Formation:
   • Osteoblasts replace cartilage in callus with spongy bone
4. Compact Bone Formation:
   • Repair may result in thicker, stronger bone

Effects of Aging on the Skeletal System

• Bones become thinner and weaker (osteopenia, osteoporosis); sex hormones help maintain bone mass; aging affects epiphyses, vertebrae, and jaws, leading to increased risk of fractures and dental issues.