S

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.