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Ch.6 Skeletal System (6.3 Ossification and appositional growth are…

- - - - Step 1: Small fracture many blood vessels broke and extended bleeding occurs- large blood clot called fracture hematoma forms and closes off injured blood vessels- resulting lack of blood supply kills osteocytes- dead bone extends in either direction from break
      - Step 2: Cells of periosteum and endosteum undergo mitosis and daughter cells migrate to fracture zone- form localized thickenings- an external callus and internal callus- center of external callus cells differentiate into chondrocytes and produce hyaline cartilage.
      - Step 3: Osteoblasts replacenew central cartilage of external callus with spongy bone- external and internal calluses form continuous brace of spongy bone at fracture site- ends of fracture help firmly now in place and can withstand normal stresses from muscle contractions
      - Step 4: Remodeling of spongy bone at fracture site continues for 4 months-year. when complete fragment of dead bone and spongy bone of calluses are gone and only living compact bone will remain and will be good as new with slightly thicker than normal fracture site
- - - - Basic functional unit of compact bone is osteon
      - osteocytes are arranged in concetric layers around a central canal that contain one or more blood vessels.
      - Perforating canals provide passageways for linking the blood vessels of the central canals with those of the periosteum and the narrow cavity.
      - Spongy bone has different lamellar arrangement and no osteons instead the lamellae form rods or plates called trabeculae
      - Layer of compact bone covers bone surfaces everywhere except inside joint capsules where articular cartilages protect oposing surfaces. Spongy bone is found where bones are not heavily stressed or where stresses arrice from many directions. Spongy bone is also much lighter than compact bone
  - - - mature bone cells, maintain normal none structure by recycling the calcium salts in bony matrix around themselves and by assisting in repairs
    - - Giant cells with 50 or more nuclei
    - - Cells responsible for production of new bone, produce new bone matrix and promote deposition of calcium salts in the organic matrix