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Microscopic Structure of Bone

- Discussion:
    - there are three types of cells intimately associated with bone:  osteocytes, osteoblasts, and osteoclasts;
    - osteocytes dwell in small lacunae within the bone matrix;
          - they are oval in cross section, their longest diameter being roughly parallel to the lamellae of mature bone;

- Woven Bone: (primary bone)
    - forms embryonic skeleton, and is largely absent after age 4 yrs;
    - woven bone formation is seen in fracture callus in both children and adults;
    - woven bone does not contain lamellae, and rather has a relatively disorganized array of collagen and irregular mineralization pattern;

- Lamellar Bone (secondary bone):
    - layering of bone is the hallmark of mature bone;
    - it is in compact cortical bone of diaphysis that haversian systems are found;
          - nutrients pass thru haversian canals, which run in the direction of the long axis of the bone, to bone cells that lie buried in the thick cortex;
    - osteon:
          - refers to the bone surrounding the haversian system;
          - the haversian canals are surrounded by concentric rings or lamellae, each about 5 micrometers thick;
          - histologically, these layers of bone will contrast with one another under polarized light because of the different orientations of 
                collagen fibers in each layer;
          - cement lines surround the outer border of each osteon;
                - histologically they are seen as the demarcation between the point that bone resorption ceased, and bone formation began;
                       - collagen fibers and canaliculi do not cross cement lines;
                - these lines do not bind adjacent lamellae but rather than separate them;
    - canaliculi:
          - w/in concentric lamellae of mature bone are small lacunae containing osteocytes;
          - from lacunae extend fine canaliculi that contain cell processes of osteocytes;
          - these processes connect osteocytes to one another and to haversian canal, and are the pathways of nutrition from the canal to the 
                 bone cells;
          - canaliculi connect osteocytes within one haversian system only;
                 - they do not cross cement lines to other systems;
                 - since canaliculi are much too small to permit passage of RBCs, osteocytes must depend on diffusion & cell pumping mechanisms 
                        for their supply of oxygen & nutrients