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
- Fracture Hematoma Formation:
- Blood clot forms, stabilizes break
- Callus Formation:
- Internal and external calluses stabilize the break
- Spongy Bone Formation:
- Osteoblasts replace cartilage in callus with spongy bone
- 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.