2. Bone Is a specialized connective tissue and is one of the hardest tissues of the body Second only to cartilage in its ability to withstand stress
3. Functions of bone Provides mechanical support Provides levers for the action of muscles/help in locomotion Serves as site for storage of calcium in the body Protects vital organs Encloses the blood forming elements of the bone marrow
4. Structural Elements of Bone (composition) Bone cells Osteoprogenitor cells – stem cells Osteoblasts – forms bone Osteocytes – maintains bone Osteoclasts – resorbs bone Bone matrix Sharpey’s fibers Blood vessels, nerves, lymph vessels (Haversian canals)
6. Osteoprogenitor cells Stem cells derived from the mesenchyme Possess mitotic potential and the ability to differentiate into mature bone cells Resemble mesenchymal cells and are spindle-shaped, with pale-staining elongated nuclei and sparse cytoplasm (small amount of rough ER and poorly developed golgi complex) Mostly found in the inner portion of the periosteum, in the endosteum, and within vascular canals of compact bone
7. Types of Osteoprogenitor cells Preosteoblasts Possess some endoplasmic reticulum and gives rise to the osteoblasts Preosteoclasts Has more mitochondria and free ribosomes and give rise to the osteoclasts
8. Osteoblasts Associated with bone formation Found where osseous matrix is being deposited Vary in shape, some being cuboidal and others pyramidal, and are frequently in continuous layer suggestive of an epithelial arrangement
9. Osteoblasts Have numerous finger-like cytoplasmic processes that extend into the developing bone matrix to contact cell processes from neighboring osteoblasts
10. Functions of the osteoblasts Contain ribose nucleoprotein concerned with the synthesis of the organic components of bone matrix, i.e. collagen and glycoproteins Contain the enzyme alkaline phosphatase concerned not only with the elaboration of matrix but also with its calcification
11. Osteocytes Principal bone cells Are osteoblasts which have become imprisoned within the bone matrix and are lodged inside cavities called LACUNA Have faintly basophilic cytoplasm which can be shown to contain fat droplets, some glycogen and fine granules similar to those of or are present within osteoblasts
12. Osteocytes Has fine cytoplasmic processes which extend for some distance and are called radiating canaliculi. These form an extensive continuous communication system among osteocytes
13. Functions of the radiating canaliculi Passageway of nutrients and metabolites (intracellular transport) Intercommunication system – interconnection to other osteocytes For storage of calcium
14. Osteoclasts Bone destroyers Are multinucleated giant cells (5-50 nuclei), varying in size and shape with serrated borders and vacuolated cytoplasm Found to lie within enzymatically etched depressions in the matrix known as HOWSHIP’S LACUNA Under the EM, the surface of the osteoclasts facing the matrix has numerous cytoplasmic projections and microvilli – this is described as a RUFFLED BORDER
15. Osteoclasts Derived from the mononuclear cells (monocytes) of hemopoietic bone marrow Secrete acid, collagenase, and other proteolytic enzymes that attack the bone matrix and liberate the calcified ground substance FUNCTIONS: Responsible for bone resorption activities Engaged in elimination of debris formed during bone resorption
18. Periosteum The outermost covering of bones Made up of a dense fibrous CT that covers all portions of the compact bone except those covered b articular cartilages Important for growth and regeneration Consists of two layers: the outer and inner layer (osteogenetic layer or cambium)
19. Layers of the Periosteum OUTER LAYER – consists of collagen fibers and fibroblasts. Made up of loose/areolar CT, blood vessels and nerves INNER LAYER – OSTEOGENETIC LAYER OR CAMBIUM. It is where regeneration occurs. It is more cellular, composed of flattened cells or osteoprogenitor cells with potential to divide by mitosis and differentiate into osteoblasts. Presence of bundles of Sharpey’s fibers – penetrate the bone matrix and serving to bind the periosteum to the bone
20. Endosteum Lines all internal surfaces of cavities within the bone. Composed of a single layer or flattened osteoprogenitor cells and a very small amount of CT. Considerably thinner than the periosteum
21. Principal Functions of the Periosteum and Endosteum For nutrition of osseous tissue Provision of a continuous supply of new osteblasts for repair or growth of bone
22. Bone Matrix Composed of concentrically arranged bony lamellae Types of layering (lamella) Circumferential lamella – external, internal Concentric/Haversian lamella Interstitial or Intermediate lamella
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24. External Circumferential lamella or Periosteal lamella Continuous uninterrupted layer found immediately beneath the periosteum around the circumference of the bone
25. Internal Circumferential Lamella or Endosteal Lamella Continuous uninterrupted lamellae immediately surrounding the central medullary cavity
26. Concentric or Haversian Lamella Layers of bone surrounding the Haversian canal Varies from 5 – 20 layers
27. Interstitial or Intermediate lamella Found in between haversian systems Triangular or irregularly shaped groups of parallel lamella May represent lamella left by former haversian systems destroyed during growth and remodelling of bone
28. Haversian system Composed of small central canal (HAVERSIAN CANAL) which is in constant communication with the blood vessels Surrounding the canal are concentric bony lamella called haversian lamella
29. Haversian system A bright thin line called CEMENTING LINE separates one haversian system from another Haversian systems or OSTEONES (OSTEON) are found underneath the periosteal lamella
30. Volkmann’s canal Canals that are not surrounded by bony lamella Traverse the bone in a direction perpendicular or oblique to the lamella Larger than haversian canals
33. Spongy or Cancellous bone Composed of anastomosing bone trabeculae or spicules Has a simple and less organized architecture Has a lattice-work pattern with numerous small cavities Found internal to compact bone Has no haversian system
34. Compact bone Composed of dense and concentrically arranged bony trabeculae or lamella More solid with fewer cavities Found external to spongy bone Presence of haversian system
35. Microscopic Approach Immature bone /Fibrous bone: These have more cells & fibers in them. In humans they are found only in fetus, sockets of alveolar bone, sutures of the skull Mature bone /Lamellar bone: The type of bone which are composed of thin plates (lamellae) of bony tissue. Most mature human bones are lamellar bones.
36. Types of Immature bone Bundle bone Have thick bundles of collagen fibers which lie parallel to each other with osteocytes between them Woven bone or coarsely bundled bone Bundles of collagen fibers of its matrix run in various directions
37. According to the Manner of Development Intramembranous or Membrane bone Endochondral or Substitution bone
38. Intramembranous/Membrane bone Develop directly from the mesenchymewithout the intervention of the cartilage Examples are: flat bones of the skull and face (part of the maxilla and mandible)
39. Endochondral/Substitution bone Develop within fetal hyaline cartilages Pass cartilage development first then calcifies to bones Examples are: all bones except the flat bones of the skull and face
