BONE

Department Of General Histology

INTRODUCTION As the main constituent of the adult skeleton, bone tissue

supports fleshy structures, protects vital organs such as those in the cranial and thoracic cavities, and harbors the bone marrow, where blood cells are formed. Bone also serves as a reservoir of calcium, phosphate, and other ions that can be released or stored in a controlled fashion to maintain constant concentrations of these important ions in body fluids.

In addition, bones form a system of levers that multiply the forces generated during skeletal muscle contraction and transform them into bodily movements. This mineralized tissue therefore confers mechanical and metabolic functions to the skeleton.

Bone is a specialized connective tissue composed of calcified intercellular material, the bone matrix, and three cell types: Osteocytes (Gr. osteon, bone + kytos, cell), which are

found in cavities (lacunae) between layers (lamellae) of bone matrix (Figure 8–1)         

Osteoblasts (osteon + Gr. blastos, germ), which synthesize the organic components of the matrix         

Osteoclasts (osteon + Gr. klastos, broken), which are multi-nucleated giant cells involved in the resorption and remodeling of bone tissue.

ENDOOSTEUM AND CAVITY

OSTEOBLASTS

The photomicrograph of developing bone shows the location and morphological differences between osteoblasts (OB) and osteocytes (O). Rounded osteoblasts, derived from the mesenchymal cells nearby, appear as a simple row of cells adjacent to a very thin layer of lightly stained matrix covering the more heavily stained matrix. The lightly stained matrix is osteoid. Osteocytes are less rounded and located within lacunae. In thin spicules of bone like those seen here, canaliculi are usually not present.

OSTEOBLASTS AND OSTEOCYTES

Schematic diagram shows the relationship of osteoblasts to osteoid, bone matrix, and osteocytes.

MINERALIZATION IN BONE MATRIX

OSTEOCYTES IN LACUNAE

TEM section of bone showing an osteocyte with its cytoplasmic processes surrounded by matrix. Such processes are extended as osteoid is being secreted and this material calcifies around the processes giving rise to canaliculi in the bony matrix. The ultrastructure of the cell nucleus and cytoplasm is that of a cell no longer active in matrix synthesis.

Photomicrograph of bone, not decalcified and sectioned, but dried and ground very thin for demonstration of lacunae and canaliculi, but not cells. The lacunae and canaliculi appear dark and show the communication between these structures through which nutrients derived from blood vessels diffuse and are passed from cell to cell in living bone. X400. Ground bone.

OSTEOCLASTS AND THEIR ACTIVITY

Microscopic section showing two osteoclasts (arrows) digesting or resorbing bone matrix in resorption bays on the matrix surface. X400. H&E.

Diagram showing each osteoclast has a circumferential zone where integrins tightly bind the matrix and surround a ruffled border of cytoplasmic projections close to this matrix. The sealed space between the cell and the matrix is acidified by a proton pump localized in the osteoclast membrane and receives hydrolytic enzymes secreted by the cell. It is the place of decalcification and matrix digestion and can be compared to a giant extracellular lysosome. Acidification of this confined space facilitates the dissolution of CaPO4 from bone and creates the optimal pH for activity of the lysosomal hydrolases. Bone matrix is thus resorbed and ions and products of matrix digestion are released for re-use.

showing an active osteoclast cultured on a flat substrate of bone. A trench is formed on the bone surface as the osteoclast crawls along.

PERIOSTEUM AND ENDOSTEUM

Section through a thin portion of the wall of a long-bone diaphysis showing both periosteum (P) and endosteum (E). The periosteum covers bone and provides a supply of osteoprogenitor cells which become osteoblasts for new bone formation.

COMPACT AND CANCELLOUS (SPONGY OR TRABECULAR) BONE.

Close gross examination of a thick section of dried bone illustrating the cortical compact bone and the lattice of trabeculae in cancellous bone at the bone's interior. In living tissue the compact bone is covered externally with periosteum and all surfaces of cancellous bone are covered with endosteum.

PRIMARY (WOVEN) BONE AND SECONDARY (LAMELLAR) BONE

Micrograph of a fractured bone undergoing repair. Primary bone is newly formed, immature bone, rich in osteocytes, with randomly arranged bundles of calcified collagen. Osteoclasts and osteoblasts are numerous in the surrounding endosteum.

Secondary or mature bone shows matrix organized as lamellae, seen faintly here as concentric lines surrounding osteonic canals.

AN OSTEONIn preparations of dried, ground bone osteons can be seen with lacunae (L) situated between concentric lamellae and interconnected by fine canaliculi (C). Although it is not apparent by light microscopy, each lamella consists of multiple parallel arrays of collagen fibers. In adjacent lamellae, the collagen fibers are oriented in different directions. The presence of large numbers of lamellae with differing fiber orientations provides the bone with great strength, despite its light weight. Only remnants of the osteocytes (O) in some lacunae and of the osteonic canal's contents are seen in ground bone. In living tissue osteocytic processes connected via gap junctions are present in successive canaliculi, making cells in all the lamellae in communication with the blood vessels in the central canal.

LAMELLAR BONE: PERFORATING CANALS AND INTERSTITIAL LAMELLAE

Transverse perforating canals (P) connecting adjacent osteons are shown at the left side of the micrograph. Such canals "perforate" lamellae and provide another source of microvasculature for the central canals of osteons. Among the intact osteons are also found remnants of eroded osteons, seen as irregular interstitial or intermediate lamellae (I).

Schematic diagram shows remodeling of compact lamellar bone showing three generations of osteonic haversian systems and their successive contributions to the formation of interstitial lamellae. Remodeling is a continuous process that involves the coordinated activity of osteoblasts and osteoclasts, and is responsible for adaptation of bone to changes in stress, especially during the body's growth.

DEVELOPMENT OF OSTEONSDuring remodeling of compact bone, a group of osteoclasts acts as a boring cone to make a tunnel into bone matrix. Behind these cells a population of osteoblasts enters the tunnel and lines its walls. As the osteoblasts secrete osteoid in a cyclic manner, they produce layers of new matrix with cells trapped in lacunae. The cells in lacunae are now osteocytes. The tunnel becomes constricted with multiple concentric layers of new matrix and its lumen finally exists as only a narrow central canal with small blood vessels.

VESSEL INSIDE THE OSTEON

VESSEL INSIDE THE CENTRAL CANAL OF OSTEON

INTRAMEMBRANOUS OSSIFICATION

Groups of mesenchymal cells in a "membrane" or sheet of this embryonic tissue, round up and differentiate as osteoblasts producing osteoid.

Cells trapped in the calcifying matrix differentiate as osteocytes.

Woven bone is produced in this manner, with vascularized internal spaces that will form the marrow cavity and surrounded on both sides by developing periosteum.

Remodeling of the woven bone produces the two layers of compact lamellar bone with cancellous bone in between, which is characteristic of these flat bones.

INTRAMEMBRANOUS OSSIFICATION

Areas of typical mesenchyme (M), condensed mesenchyme (CM) adjacent to aggregates of new osteoblasts (O). Some osteoblasts have secreted matrices of bone (B) which remain covered by osteoblasts. Between these trabeculae of newly formed primary bone are vascularized areas (V) that will form marrow cavities. X40. H&E.

Higher magnification shows the developing periosteum (P) that covers masses primary bone that will soon merge to form a continuous plate of bone. The larger mesenchyme-filled region at the top is the developing marrow cavity.

OSTEOGENESIS OF LONG BONES BY ENDOCHONDRAL OSSIFICATION

CELLS AND MATRICES OF A PRIMARY OSSIFICATION CENTER

A small region of a primary ossification center showing key features of endochondral ossification. Compressed remnants of calcified cartilage matrix (dark purple), now devoid of chondrocytes, are enclosed by more lightly stained osteoid or bone matrix. This newly formed bone is surrounded by a layer of large, active osteoblasts. Some osteoblasts that were captured by the matrix have become smaller osteocytes (arrowheads).

EPIPHYSEAL GROWTH PLATE: LOCATIONS AND ZONES OF ACTIVITY

CELLS AND MATRICES OF THE EPIPHYSEAL GROWTH PLATE

At the top of the micrograph the growth plate (GP) shows its zones of hyaline cartilage with cells undergoing rest (R), proliferation (P), and hypertrophy (H). As the chondrocytes swell and degenerate they release phosphatase, activities which compress the matrix and cause an initial deposition of CaPO4. This produces calcified spicules (C) in the former cartilage matrix. The tunnel-like lacunae in which the chondrocytes have undergone apoptosis are invaded from the diaphysis by large, thin-walled blood vessels which begin to convert these spaces into marrow (M) cavities. Endosteum with osteoblasts also moves in from the diaphyseal primary ossification center and these cells cover the spicules of calcified cartilage and lay down layers of osteoid, forming a supportive matrix that is now largely primary woven bone (B).

Higher magnification shows more detail of the cells and matrix spicules in the zones undergoing hypertrophy (H) and ossification. Staining properties of the matrix clearly change in this process: first when it is compressed and begins to calcify (C), and then when osteoid and bone (B) are laid down. The large spaces between the ossifying matrix spicules become the marrow cavity (M), in which sinuses of eosinophilic red blood cells and aggregates of basophilic white blood cell precursors can be distinguished. The marrow is the major site of blood cell formation in adults.

MAIN FEATURES OF BONE FRACTURE REPAIR

METABOLIC ROLE OF BONE Calcium ions are required for the activity of many enzymes and other proteins

mediating cell adhesion, cytoskeletal movements, exocytosis, membrane permeability, and other functions in cells throughout the body. The skeleton serves as the calcium reservoir and contains 99% of the body's total calcium in crystals of hydroxyapatite. The concentration of calcium in the blood and tissues is generally quite stable because of a continuous interchange between blood calcium and bone calcium.

The principal mechanism for raising blood calcium levels is the mobilization of ions from hydroxyapatite crystals to interstitial fluid. This takes place mainly in cancellous bone. The younger, more lightly calcified lamellae that exist even in adult bone (because of continuous remodeling) receive and lose calcium more readily. These lamellae are more important for the maintenance of calcium concentration in the blood than are the older, more densely calcified lamellae, whose role is mainly that of support and protection.

The action of two key hormones on cells in bone regulates the process of calcium mobilization from hydroxyapatite. Parathyroid hormone (PTH) from the parathyroid glands raises low blood calcium levels. The principal target cells of this polypeptide are osteoblasts, which stop production of osteoid and matrix vesicles and instead secrete a paracrine protein, osteoclast stimulating factor. This factor promotes osteoclastic resorption of the bone matrix, liberating calcium. Osteoclast activity is inhibited by another hormone, calcitonin, which is synthesized by the parafollicular cells of the thyroid gland. This slows matrix resorption and thereby gradually lowers blood calcium levels.

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