Bone is a hard CT with cells, osteocytes, in much matrix, and
serves for support, attachment, leverage, protection and mineral storage.
l 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. 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 (Chapter 5.C.8).
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. Powerpoint
2 Based on the presence or absence of lamellae (layers) and
.. (a) Woven/primitive
.. (b) Lamellar/Haversian
Woven bone's matrix has disorderly fibrils, whereas in lamellar bone the fibrils of a lamella share a predominant orientation. Note that a particular bone will have areas of woven and lamellar bone, depending on how far remodelling has involved all regions.
2 Dense cortical bone. Where wide, e.g., femoral shaft, this layering is often present:
3 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.
4 Marrow cavities lie between trabeculae, inside the tubular shaft, or in the diploic spaces of flat skull bones.
l 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. SEM evidence puts into doubt the proposal that osteocytes can resorb bone by osteolysis. Lacunae empty of osteocytes indicate dead bone.
l Large, multinucleated cell, with a pale acidophilic cytoplasm.
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 Bone cell dynamics
l 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 partipate indirectly as macrophages in the bone resorption.
2 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.
2 Diarthrosis (movable)
1 Articular cartilage, usually hyaline, covers the moving bone ends, and is nourished and lubricated by synovial fluid.
2 Joint capsule of dense irregular fibrous CT, continuous with the periostea, encloses a joint space for synovial fluid.
3 Nervous joint receptors for proprioception are in the capsule.
4 Synovial membrane: lines the capsule; a cellular layer, with macrophage (A/M) and fibroblastic (B/F) cells, lies on a loose vascular CT, sometimes thrown up into folds, synovial villi. The cells make lubricating hyaluronic acid and glycoproteins, and determine the nature of the cartilage-sustaining synovial fluid.
5 Articular cartilage layers: although the cartilage is not thick, variation in the amounts and arrangements of proteoglycans and collagen with depth distinguishes these layers:
... calcified/mineralized, attached to the 'subchondral' bone.
A lamina splendens is at the free surface of the superficial layer: below it the collagen fibrils are better organized, in a packed series of 'leaves' that curve up from the radial layer, run parallel with the surface superficially, then descend to the radial layer.
6 In arthritis, inflamed synovium threatens articular cartilage. Synovial cytokines stimulate chondrocytes to emphasize cartilage breakdown over renewal.