Histology of the Bone PDF

Histology of the Bone Dr.Abdulhussain Sahib  OBJECTIVES a. Important features. b. Composition. c. Its types of bone and covering. d. Its cells and their function.  Bone is a specialized connective tissue , has a rich vascular supply composed...

Histology of the Bone Dr.Abdulhussain Sahib  OBJECTIVES a. Important features. b. Composition. c. Its types of bone and covering. d. Its cells and their function.  Bone is a specialized connective tissue , has a rich vascular supply composed of calcified extracellular material, the bone matrix, and following three major cell types : 1.Osteocytes 2.Osteoblasts 3.Osteoclasts Function of the bone: Support the body. Protect many internal organs. Act as the body’s Ca2+ reservoir.  Osteoblasts Osteoblasts Originating from mesenchymal stem cells. Osteoblasts produce the organic components of bone matrix, including: Type I collagen fibers. Proteoglycans. Glycoproteins such as osteonectin. Deposition of the inorganic components of bone also depends on osteoblast activity.  Active osteoblasts are located exclusively at the surfaces of bone matrix, to which they are bound by integrins, typically forming a single layer of cuboidal cells joined by adherent and gap junctions. When their synthetic activity is completed, some osteoblasts differentiate as osteocytes entrapped in matrix-bound lacunae, some flatten and cover the matrix surface as bone lining cells, and the majority undergo apoptosis.  Matrix components are secreted at the cell surface in contact with existing bone matrix, producing a layer of unique collagen-rich material called osteoid between the osteoblast layer and the preexisting bone surface. This process of bone appositional growth is completed by subsequent deposition of calcium salts into the newly formed matrix.  Important components of osteoid include type I collagen, the protein osteocalcin, which binds Ca2+ and matrix vesicles with enzymes generating PO4-. High concentrations of Ca2+ and PO4- ions cause formation of hydroxyapatite crystals, whose growth gradually calcifies the entire matrix.  Osteocytes These are the cells of mature bone. They lie in the lacunae of bone, and represent osteoblasts that have become ‘imprisoned’ in the matrix during bone formation. During the transition from osteoblasts to osteocytes, the cells extend many long dendritic processes, which also become surrounded by calcifying matrix. The processes thus come to occupy the many canaliculi, 250-300 nm in diameter, radiating from each lacuna.  In contrast to osteoblasts, osteocytes have eosinophilic or lightly basophilic cytoplasm. This is to be correlated with: The fact that these cells have negligible secretory activity The presence of only a small amount of endoplasmic reticulum in the cytoplasm. Osteocytes are present in greatest numbers in young bone, the number gradually decreasing with age.  Diffusion of metabolites between osteocytes and blood vessels occurs through the small amount of interstitial fluid in the canaliculi between the bone matrix and the osteocytes and their processes. Osteocytes also communicate with one another and ultimately with nearby osteoblasts and bone lining cells via gap junctions at the ends of their processes.  Functions: 1. They probably maintain the integrity of the lacunae and canaliculi, and thus keep open the channels for diffusion of nutrition through bone. 2. They play a role in removal or deposition of matrix and of calcium when required.  Osteoclasts These are bone removing cells. They are found in relation to surfaces where bone removal is taking place (Bone removal is essential for maintaining the proper shape of growing bone). At such locations the cells occupy pits called resorption bays or lacunae of Howship. Osteoclasts are very large cells (20 to 100 μm or even more in diameter). They have numerous nuclei: up to 20 or more. The cytoplasm shows numerous mitochondria and lysosomes containing acid phosphatase.  At sites of bone resorption the surface of an osteoclast shows many folds that are described as a ruffled membrane. Removal of bone by osteoclasts involves demineralization and removal of matrix. Bone removal can be stimulated by factors secreted by : osteoblasts, by macrophages,  or by lymphocytes. It is also stimulated by the parathyroid hormone.  bone matrix 1.Inorganic salts ,about 65% of the dry weight of bone. 2.Organic ground substance and collagen fibres(35%). The ground substance (or matrix) of bone consists of an organic matrix in which mineral salts are deposited.  THE ORGANIC MATRIX This consists of a ground substance in which collagen fibres are embedded. The ground substance consists of : Glycosaminoglycans. Proteoglycans Water. Two special glycoproteins osteonectin and osteocalcin are present in large quantity. They bind readily to calcium ions and, therefore, play a role in mineralization of bone. Various other substances including chondroitin sulphates, phospholipids and phosphoproteins are also present.  The collagen fibres (Type I collagen) (They are sometimes referred to as osteoid collagen). The fibres are usually arranged in layers, running parallel to one another. Collagen fibres of bone are synthesized by osteoblasts. The matrix of bone shows greater density than elsewhere immediately around the lacunae, forming capsules around them, similar to those around chondrocytes in cartilage. The term osteoid is applied to the mixture of organic ground substance and collagen fibres (before it is mineralized).  THE INORGANIC IONS The ions present are predominantly calcium and phosphorus (or phosphate). Others also present in significant amounts, include: Magnesium. Carbonate. Hydroxyl. Chloride. Fluoride. Citrate. Sodium. Potassium.  Periosteum & Endosteum External and internal surfaces of all bones are covered by connective tissue of the periosteum and endosteum, respectively. The periosteum is an outer fibrous layer of dense connective tissue, containing mostly bundled type I collagen, but also fibroblasts and blood vessels. Bundles of periosteal collagen, called perforating (or Sharpey) fibers, penetrate the bone matrix and bind the periosteum to the bone.  Periosteal blood vessels branch and penetrate the bone, carrying metabolites to and from bone cells. The periosteum’s inner layer is more cellular and includes osteoblasts, bone lining cells, and mesenchymal stem cells referred to as osteoprogenitor cells. With the potential to proliferate extensively and produce many new osteoblasts, osteoprogenitor cells play a prominent role in bone growth and repair.  Internally the very thin endosteum covers small trabeculae of bony matrix that project into the marrow cavities. The endosteum also contains osteoprogenitor cells, osteoblasts, and bone lining cells, but within a sparse, delicate matrix of collagen fibers. TYPES OF BONE  Woven bone Woven bone is nonlamellar and characterized by random disposition of type I collagen fibers and is the first bone tissue to appear in embryonic development and in fracture repair. Woven bone is usually temporary and is replaced in adults by lamellar bone, except in a very few places in the body, for example, near the sutures of the calvaria and in the insertions of some tendons. In addition to the irregular, interwoven array of collagen fibers, woven bone typically has a lower mineral content (it is more easily penetrated by x-rays) and a higher proportion of osteocytes than mature lamellar bone. These features reflect the facts that immature woven bone forms more quickly but has less strength than lamellar bone.  Lamellar bone Most bone in adults, compact or cancellous, is organized as lamellar bone, characterized by multiple layers or lamellae of calcified matrix, each 3-7 μm thick. The lamellae are organized as parallel sheets or concentrically around a central canal. In each lamella, type I collagen fibers.  An osteon (or Haversian system) refers to the complex of concentric lamellae, typically 100- 250 μm in diameter, surrounding a central canal that contains small blood vessels, nerves, and endosteum. Between successive lamellae are lacunae, each with one osteocyte, all interconnected by the canaliculi containing the cells’ dendritic processes  Processes of adjacent cells are in contact via gap junctions, and all cells of an osteon receive nutrients and oxygen from vessels in the central canal. The outer boundary of each osteon is a layer called the cement line that includes many more noncollagen proteins in addition to mineral and collagen.  Each osteon is a long, sometimes bifurcated, cylinder generally parallel to the long axis of the diaphysis. Each has 5-20 concentric lamellae around the central canal that communicates with the marrow cavity and the periosteum. Canals also communicate with one another through transverse perforating canals (or Volkmann canals) that have few, if any, concentric lamellae.  All central osteonic canals and perforating canals form when matrix is laid down around areas with preexisting blood vessels. Scattered among the intact osteons are numerous irregularly shaped groups of parallel lamellae called interstitial lamellae. These structures are lamellae remaining from osteons partially destroyed by osteoclasts during growth and remodeling of bone.  Compact bone Compact bone (eg, in the diaphysis of long bones) also includes parallel lamellae organized as multiple external circumferential lamellae immediately beneath the periosteum and fewer inner circumferential lamellae around the marrow cavity. The lamellae of these outer and innermost areas of compact bone enclose and strengthen the middle region containing vascularized osteons. Transverse section and longitudinal section through compact bone to show Haversian canals and the Volkmann's canal  Cancellousbone (spongy bone) Cancellous bone is made up of a meshwork of bony plates or rods called trabeculae. Each trabeculus is made up of a number of lamellae between which there are lacunae containing osteocytes. Canaliculi, containing the processes of osteocytes, radiate from the lacunae. The trabeculae enclose wide spaces that are filled in by bone marrow. They receive nutrition from blood vessels in the bone marrow. The trabeculae are covered externally by vascular end steum containing osteoblasts, osteoclasts and osteoprogenitor cells.