MBChB MSC PhD DCH UK MRCPCH Part 1 Bone Special type of connective tissue Has a hard Mineralized extracellular matrix It is Vascular tissue Although it is one of the hardest substances of the body ID: 1010609
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1. BoneDr. Mohammed HusseinM.B.Ch.B, MSC, PhD, DCH (UK), MRCPCHPart 1
2. BoneSpecial type of connective tissueHas a hard, Mineralized, extracellular matrixIt is Vascular tissue
3. Although it is one of the hardest substances of the bodyIt is a dynamic tissue that constantly changes shape in relation to the stresses placed on it.Bone
4. Medical ApplicationThe orthodontist is able to remodel the bone of the dental arches by moving and straightening the teeth to correct malocclusion, thus providing the patient with a more natural and pleasant smile.
5. FunctionsBone is the primary structural framework for support and protection of the organs of the body, including the brain and spinal cord and the structures within the thoracic cavity, namely the lungs and heart. The bones also serve as levers for the muscles attached to them, thereby multiplying the force of the muscles to attain movement. Bone is a reservoir for several minerals of the body; for example, it stores about 99% of the body’s calcium. Bone contains a central cavity, the marrow cavity, which houses the bone marrow, a hemopoietic organ.
6. HISTOLOGY
7. Bone Sections Because bone is such a hard tissue, two methods are employed to prepare it for study. Decalcified sections can be prepared by decalcifying the bone in an acid solution to remove the calcium. The tissue can then be embedded, sectioned, and routinely stained for study. Ground sections are prepared by sawing the bone into thin slices, followed by grinding the sections with abrasives between glass plates. When the section is sufficiently thin for study with light microscope, it is mounted for study.
8. Decalcified sections Ground sections Bone Sections Because bone is such a hard tissue, two methods are employed to prepare it for study.
9. ConnectiveTissue CellsExtracellular Matrix(ECM)FibersGroundSubstancesCartilageChondrogenicChondroblastChondrocytesCollagen type I and IIElasticGAGsBoneOsteoprogenitor OsteoblastOsteocytesOsteoclastsInorganic Matrix(calcium & phosphorous)Organic MatrixCellsCollagen type IChondroitin sulfate and Keratan sulfate
10. Bone MatrixInorganic Component65% of its dry weightcalcium and phosphorus. hydroxyapatite crystals.Organic Component35% of the dry weight almost exclusively type I collagenchondroitin sulfate and keratan sulfate.
11. Cells of BoneOsteoprogenitor Osteoblasts Osteocytes Osteoclasts
12. Bone Structure
13. Bones are classified according to their shape:Long bones display a shaft located between two heads (e.g. ).Short bones have more or less the same width and length (e.g., ).Flat bones are flat, plate-like (e.g., ).Irregular bones have an irregular shape (e.g., ).Sesamoid bones develop within tendons (e.g., )..Long bone Short boneFlat boneIrregular boneSesamoid bone
14. Gross Observation of Bone
15. Compact Bone: the very dense bone on the outside surface Cancellous or Spongy bone: the porous portion lining the marrow cavity isGross observations of the femur (a long bone) cut in longitudinal section reveal two different types of bone structure.
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17. Bone Types Based on Microscopic Observations
18. Microscopic observations reveal two types of bone: Primary bone (immature or woven bone) is immature in that it is the first bone to form during fetal development and during bone repair. It has abundant osteocytes and irregular bundles of collagen.Secondary bone (mature or lamellar bone) is mature bone composed of parallel or concentric bony lamellae. Because the matrix of secondary bone is more calcified, it is stronger than primary bone.
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21. The Periosteum
22. The periosteum is a noncalcified, dense, irregular, collagenous connective tissue covering the bone on its external surface and inserting into it via Sharpey’s fibers. Periosteum is composed of two layers: The outer fibrous layer helps distribute vascular and nerve supply to boneThe inner cellular layer possesses osteoprogenitor cells and osteoblasts
23. PeriosteumSharpey’s fibers
24. The Endosteum
25. Specialized thin, connective tissue Lining the central cavity of a bone Composed of a monolayer of osteoprogenitor cells and osteoblasts
26. Lamellar Systems of Compact BoneCompact bone is composed of wafer-like thin layers of bone, lamellae, that are arranged in lamellar systems that are especially evident in the diaphyses of long bones. These lamellar systems are:Outer circumferential lamellaeInner circumferential lamellaeInterstitial lamellaeOsteons (Haversian canal systems) surrounded by concentric lamellae
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28. Haversian Canal Systems (Osteons)
29. The bulk of compact bone is composed of an abundance of haversian canal systems (osteons)
30. Each system is composed of cylinders of lamellae, concentrically arranged around a vascular space known as the Haversian canal
31. Each osteon is bounded by a thin cementing line, composed mostly of calcified ground substance with a scant amount of collagen fibers.
32. Each haversian canal, lined by a layer of osteoblasts and osteoprogenitor cells, houses a neurovascular bundle with its associated connective tissue.
33. Cement lineHaversian canalOsteoprogenitor cellOsteoblast
34. Haversian canals of adjacent osteons are connected to each other by Volkmann’s canals. These vascular spaces are oriented oblique to or perpendicular to haversian canals.
35. As bone is being remodeled, osteoclasts resorb osteons and osteoblasts replace them. Remnants of osteons remain as irregular arcs of lamellar fragments, known as interstitial lamellae, surrounded by osteons.
36. Cancellous bone
37. Cancellous bone is also called spongy bone. It has a lower density than compact bone and consists of bony trabeculae, or spicules, within a marrow-filled cavity.Bone marrow fills the space between the bony trabeculae. Osteoblasts line the surface of the bony trabeculae. Most osteocytes in the matrix are arranged in an irregular pattern rather than in circular rings
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39. Cells of BoneOsteoprogenitor Osteoblasts Osteocytes Osteoclasts
40. Osteoprogenitor CellsOsteoprogenitor cells are located in the inner cellular layer of the periosteum, lining haversian canals, and in the endosteum . These cells, derived from embryonic mesenchyme, remain in place throughout postnatal life Can undergo mitotic division and have the potential to differentiate into osteoblasts.
41. Osteoblasts are derived from osteoprogenitor cells Responsible for the synthesis of the organic protein components of the bone matrixOsteoblasts
42. Medical ApplicationOsteoblast are rich in the enzyme alkaline phosphatase (ALP). During active bone formation, these cells secrete high levels of ALP, elevating the levels of this enzyme in the blood. Thus, we can monitor bone formation by measuring the blood ALP level.
43. Osteocytes are mature bone cells, derived from osteoblasts, that are housed in lacunae within the calcified bony matrix. Radiating out in all directions from the lacunaa are narrow, tunnel-like spaces (canaliculi) that house cytoplasmic processes of the osteocyte. These processes make contact with similar processes of neighboring osteocytes, forming gap junctions through which ions and small molecules can move between the cells. The canaliculi also contain extracellular fluid carrying nutrients and metabolites that nourish the osteocytes. Osteocytes
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45. OsteoclastsOsteoclasts are large, motile, multinucleated cells; they contain up to 50 nuclei and have an acidophilic cytoplasm. Osteoclasts occupy shallow depressions, called Howship’s lacunae, that identify regions of bone resorption. Osteoclasts are responsible for resorbing bone, and after they finish doing so, these cells probably undergo apoptosis. The precursor of the osteoclast originates in the bone marrow. Osteoclasts were once thought to be derived from the fusion of many blood-derived monocytes, but the newest evidence shows that they have a bone marrow precursor in common with monocytes termed the mononuclearphagocyte system.
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48. Medical ApplicationOsteopetrosis, not to be confused with osteoporosis Is a genetic disorder where osteoclasts cannot resorb bone and persons with osteopetrosis display increased bone density. Individuals suffering from this disease may exhibit anemia resulting from decreased marrow space, as well as blindness, deafness, and cranial nerve involvement because of impingement of the nerves due to narrowing of the foramina.
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