HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 7 BONE

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

A PARTS OF A BONE

See Chapter 8.B for terms, e.g., diaphysis, epiphysis, etc.

B CLASSIFICATIONS OF BONE

l Based on the size of the spaces within the bone, and its trabecular (lattice-like) or dense nature:
.. (a) Cancellous/spongy/trabecular
.. (b) Compact/dense

2 Based on the presence or absence of lamellae (layers) and osteons/Haversian systems:
.. (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.

C HAVERSIAN BONE

l An 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 centred 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, around which osteocytes are not especially ordered.

D MATURE HUMAN BONE

Studied from the outside working inwards has:
l Periosteum of dense CT divisible into:

2 Dense cortical bone. Where wide, e.g., femoral shaft, this layering is often present:

In practice, some areas of dense bone remain woven or primary and are not replaced by this classic lamellar architecture.

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.

E BONE CELLS

l Osteoblast
l 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: 3 Forms the collagen, glycoproteins, and proteoglycans of the matrix, and controls the deposition of mineral crystals on the fibrils.

2 Osteocyte
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.

3 Osteoclast
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.

F HISTOLOGICAL METHODS FOR BONE

Special techniques are needed because of the difficulty of cutting such hard material into sections thin enough for microscopy.
1 Ground sections with the mineral present are made by sawing out a slice of bone (or tooth) and grinding it thinner. They show osteons, lacunae and canaliculi, but these hold air or debris and no longer cells.
2 Decalcified sections are cut from bone imbedded in the usual way after removal of the mineral by dilute acids or chelating agents. Cells and the organic matrix remain. Eosin and selective collagen stains reveal the dense collagenous matrix, but individual fibrils and canaliculi are not seen unless special stains are used.
3 Mineral density can be studied by the magnified X-ray image of microradiography in ground sections or microtome-cut sections of plastic-imbedded undecalcified bone obtained by biopsy.
4 Electron microscopy of such plastic sections gives a comprehensive view of mineral, collagen, and cells, and their interactions. Scanning EM reveals bone's trabecular architecture, and how bone is formed and destroyed.
5 Vital labelling, with the fluorescing tetracyclines, alizarin red (in madder), or the radioactive isotopes, 45Ca or 31P, given at known times, permits the amount and sites of new bone formation, and its patterns of deposition and resorption to be identified, and related to bone diseases or experimental manipulations.

G JOINTS

1 Synarthroses (poorly movable)
1 Syndesmosis. Bones linked by dense fibrous CT, e.g., a skull suture, which may be replaced by bone with increasing age to become a synostosis.
2 Synchondrosis. Bones linked by cartilage, e.g., pubic symphysis.

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:
... superficial/tangential
... intermediate/transitional
... deep/radial
... 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.


William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159