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Bone is a hard CT with cells, osteocvtes, in much matrix, and serves for support, attachment, leverage, protection and mineral storage.
  1. To obtain great strength and rigidity with some elasticity, the matrix is composed of densely packed collagen fibrils infiltrated with bone mineral as fine crystals of calcium salts resembling hydroxyapatite crystals (HACs). Mineral constitutes about 65 per cent of the dry weight of bone. The densely packed collagen fibrils are primarily type I. There are small amounts of distinctive non-collagenous proteins, e.g., calcium-binding osteocalcin and bone sialoproteins
  2. Matrix is strong but dense, thus nutritive fluids cannot diffuse freely through it. Osteocytes therefore have to differ from chondrocytes in having many long processes extending through canaliculi (narrow passages; and making contact with one another and, indirectly, with blood vessels. The cell body lies in a cavity, a lacuna, in the matrix.
  3. Throughout life, for mineral homeostasis, and for its special problems of growth, bone is subject to an unending turnover, with selective destruction and replacement - the remodelling process.

Classifications Of Bone
  1. Based on the size of the spaces within the bone, and its trabecular (lattice-like) or dense nature:
    1. Cancellous/spongy/trabecular
    2. Compact/dense
  2. Based on the presence or absence of lamellae (layers) and osteons/Haversian systems:
    1. Woven/primitive
    2. Lamellar/Haversian
Woven bone’s matrix has disorderly fibrils, whereas in lamellar bone the fibrils of a lamella share a predominant orientation. A particular bone will have areas of woven and lamellar bone, depending on how far remodelling has involved all regions.

Haversian Bone
  1. A Haversian system is roughly cylindrical and arranged around one or two small vessels in a central Haversian canal.
  2. Osteocytes and bone lamellae making up the system are disposed in 4-20 concentric rings centered on the canal.
  3. A lamella is the territory formed and maintained by the osteocytes lying in a ring when seen in a cross-section. From the orderliness of the fibrils, lamellae can be distinguished in polarized light, but it is only in a smaller unit, the domain, that SEM reveals the fibrils to be aligned in the same direction.
  4. Haversian canals branch and join up with others. Their vessels originally entered the bone from the periosteum or marrow via Volkmann’s canals (these run in transverse directions connecting Haversian canals), around which osteocytes are not especially ordered.
Mature Human Bone
Studied from the outside working inwards has:
  1. Periosteum of dense CT divisible into:
    1. An external fibrous layer of collagen and elastic fibres, fibroblasts, other cells, vessels and nerves; and
    2. An inner cambial layer of bone cells, mostly resting osteoblasts.
  2. Dense cortical bone. Where wide, e.g., femoral shaft, this layering is often present:
    1. External circumferential/basic lamellae lie outside;
    2. The main thickness with many osteons of various generations (primary, secondary, etc); interstitial lamellae fill the chinks between osteons and are lamellae of earlier osteons that have been spared total erosion;
    3. endosteal/intemal circumferential lamellae lie to the inside, with their osteocyte bodies lying parallel with the inner surface.
  3. In practice, some areas of dense bone remain woven or primary and are not replaced by this classic lamellar architecture.
  4. Cancellous medullary bone whose trabeculae are lined by a thin cellular endosteum and have some lamellae, but can be sustained by marrow blood vessels without the need for Haversian canals.
  5. Marrow cavities lie between trabeculae, inside the tubular shaft, or in the diploic spaces of flat skull bones.
Bone Cells
  1. Osteoblast
    1. Lies on the surfaces of bone, in a one-cell thick layer, as most of the endosteum and inner periosteum.
    2. May be in two states:
      1. Active, forming bone matrix, with a large Golgi complex and much GER in a plump cell, appearing in LM to have a pale juxtanuclear vacuole (Golgi) in the basophilic cytoplasm;
      2. Resting or bone-maintaining; small cell with a dark nucleus and flattened against the bone.
    3. Forms the collagen, glycoproteins, and proteoglycans of the matrix, and controls the deposition of mineral crystals on the fibrils.
  2. Osteocyte
    1. Osteoblast becomes an osteocyte by forming matrix around itself and becoming buried or immured.
    2. Young osteocyte thus resembles an active Osteoblast; older ones have smaller, flattened bodies.
    3. Processes extending from the body down the canaliculi are not visible by LM; but EM shows that osteocytes, like osteoblasts, remain connected by gap junctions.
    4. The mature osteocyte is involved in maintaining the matrix of its territory. Lacunae empty of osteocytes indicate dead bone.
  3. Osteoclast
    1. Large, multinucleated cell, with a pale acidophilic cytoplasm (derived from the monocytes).
    2. Lies on the surface of bone, often in an eaten-out hollow - Howship’s lacuna.
    3. Cell surface is attached to the bone by podosomes to create a sealed compartment against the bone, in which the moving long cell processes of the ruffled border can agitate the resorbing - bone-destroying - materials.
    4. Cytoplasm has vacuoles and lysosomes, since the mechanism of bone resorption is partly an enzymatic digestion, by cathepsins and collagenase, and also from acid made by an osteoclastic proton pump.
    5. In dense bone, many osteoclasts act together to erode resorption tunnels, which are later partially filled in with lamellar bone to become osteons.
  4. Osteoprogenitor cells: The osteoprogenitor cell is a small, organelle-poor cell on the surface or lying just behind the osteoblasts. It might be just an inactive osteoblast: that it is more of a stem cell is shown by its occasionally becoming chondroblastic, e.g., in tumours and fracture repair.
Bone cell dynamics : Skeletal growth, changes of shape, and the physiological responses of bone need changes in the populations of ‘blasts’ and ‘clasts’, These rely on a proliferation of osteoblasts or a precursor, while osteoclasts come from the fusion of blood-derived monocytes, which also participate indirectly as macrophages in the bone resorption.

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