Bone Histology

Bone tissue also called (osseous tissue) is a type of specialized dense connective tissue.


Histology

Junqueira’s Basic Histology Text and Atlas, 15th Ed

1. Bone tissue "Mohmmadrjab" S. Seder Faculty of Medicine Histology

2. Table of contents Introduction Bone Cells Bone Matrix Periosteum and Endoesteum Types of Bone Osteogenesis Bone Remodeling and Repair Metabolic Role of Bone Joints

3. Introduction Bone tissue also called (osseous tissue) is a type of specialized dense connective tissue. Bone tissue is made up of different types of bone cells.

4. Bone tissue has a honeycomb-like matrix internally, which helps to give the bone rigidity.

5. As the main constituent of the adult skeleton, Bone tissues have many Functions: Movement . Provides solid support for the body. Protects vital organs such as those in the cranial and thoracic cavities. Encloses internal (medullary) cavities containing bone marrow where blood cells are formed. Serves as a reservoir of calcium, phosphate, and other ions. Endocrine regulation. Storage of minerals and fat, and hematopoiesis.

6. Normal bone is composed of 67 % mineral , 33 % organic matrix , 5 to 10% water , and <3% lipids . Calcium and phosphorus are chief minerals found in the bone along with small amount of carbonate, magnesium.

7. Classification of Bones by shapes

8. Bone is a specialized connective tissue composed of calcified extracellular material: The bone matrix Three major cell types: Osteocytes (Oc) Osteoblasts (Ob) Osteoclasts (Ocl) M: mesenchymal regions

9. Osteocytes, which are found in cavities (lacunae) between bone matrix layers (lamellae) , with cytoplasmic processes in small canaliculi (L. canalis , canal) that extend into the matrix. Osteoblasts, growing cells which synthesize and secrete the organic components of the matrix. Osteoclasts, which are giant, multinucleated cells involved in removing calcified bone matrix and remodeling bone tissue

10. Osteogenic cells  are undifferentiated stem cells. They are the only bone cells that can divide. When they do, they differentiate and develop into osteoblasts.

11. Bone lacuna is a small cavity within the bone matrix, containing an osteocyte.

12. Osteons are cylindrical structures that contain a mineral matrix and living osteocytes connected by canaliculi , which transport blood. They are aligned parallel to the long axis of the bone. Each osteon consists of lamellae , which are layers of compact matrix that surround a central canal called the Haversian canal.

13. Number 1 indicates a recently formed osteon. Number 2 indicates a somewhat older osteon. Number 3 indicates interstitial lamellae, representing the remnants of still older osteons.

14. All bones are lined on their internal and external surfaces by layers of connective tissue containing osteogenic cells; endosteum on the internal surface surrounding the marrow cavity and periosteum on the external surface.

15. Because of its hardness, bone cannot be sectioned routinely. Bone matrix is usually softened by immersion in a decalcifying solution before paraffin embedding, or embedded in plastic after fixation and sectioned with a specialized microtome.

16. Bone marrow  is a (spongy) soft connective tissue that is found inside a cavity, called the marrow cavity. There are two types of marrow in adults, yellow bone marrow , and red bone marrow. Yellow bone marrow consists mostly of fat . while r ed bone marrow contains hematopoietic stem cells that give rise to red blood cells, white blood cells, and platelets in the process of hematopoiesis.

17. All marrow is red in newborns, but by adulthood, much of the red marrow has changed to yellow marrow. In adults, red marrow is found mainly in the femur, ribs, vertebrae, and pelvic bones.

18. Bone cells Osteoblast: a mononucleate cell from which bone develops. Osteocyte: a mature bone cell involved with the maintenance of bone. Osteoclast: a large multinuclear cell associated with the resorption of bone.

19. Osteoblasts Osteoblast : a cell which secretes the substance of bone. Originating from mesenchymal stem cells . Osteoblasts produce the organic components of bone matrix, including type I collagen fibers, proteoglycans, and matricellular glycoproteins such as osteonectin. Active osteoblasts are located exclusively at the surfaces of bone matrix.

20. 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.

21. Osteoid is the unmineralized, organic portion of the bone matrix that forms prior to the maturation of bone tissue. Osteoblasts begin the process of forming bone tissue by secreting the osteoid as several specific proteins.

22. Bone mineralization is a well regulated process in which crystals of calcium phosphate are produced by bone-forming cells [osteoblasts] and are laid down in precise amounts within the fibrous matrix. The process of matrix mineralization is not completely understood, but basic aspects of the process are shown in the Figure in the next slide.

23. O steocytes Some osteoblasts become surrounded by the material they secrete and then differentiate as osteocytes enclosed singly within the lacunae spaced throughout the mineralized matrix. An osteocyte

24. C: Canaliculi Osteocytes in lacunae

25. Osteoclasts Osteoclasts are very large , motile cells with multiple nuclei that are essential for matrix resorption during bone growth and remodeling . Osteoclasts are multinucleated due to their origin from the fusion of bone marrow-derived monocytes.

26. Osteoclast

27. Bone Matrix The bone matrix (also known as osteoid ) is that part of the bone tissue and forms most of the mass of the bone. The bone matrix consists of about 33% organic matter (mostly Type I collagen) and 67% inorganic matter (calcium phosphate, mostly hydroxyapatite crystals).

28. The organic matter embedded in the calcified matrix is 90% type I collagen , but also includes mostly small proteoglycans and multiadhesive glycoproteins such as osteonectin.

29. Periosteum and Endoesteum External and internal surfaces of all bones are covered by CT of the periosteum and endosteum.

30. Periosteum Is a layer of dense CT on the outer surface of bone, bound to bone matrix by bundles of type I collagen called perforating (or Sharpey ) fibers.

31. Periosteum

32. 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.

33. Endosteum Endosteum is a thin layer of active and inactive osteoblasts, which lines all the internal surfaces within bone; Endosteum covers small trabeculae of bony matrix that project into the marrow cavities.

34. Endosteum

35. Characteristic ENDOSTEUM PERIOSTEUM Location Medullary canal, spongy bone, Volkmann’s and Haversian canals of all bones Outer bone surfaces, except articular surfaces; not in sesamoid bones (patella) Structure A single cellular layer, loose connective tissue Two layers: fibrous and cellular layer (cambium), dense irregular connective tissue Thickness ~10 micrometers (0.01 mm) 13 0.1-0-5 mm Function Bone growth, remodeling, repair Bone growth, remodeling, repair, bone sensitivity, nourishment Difference Between Endosteum and Periosteum

36. Types of Bone Gross observation of a bone shows 2 types of bone: Compact (cortical / hard) bone: a dense area near the surface, which represents 80% of the total bone mass. Cancellous (spongy / trabecular) bone: deeper areas with numerous interconnecting cavities, constituting about 20% of total bone mass.

37. In long bones , the bulbous ends—called epiphyses are composed of cancellous bone covered by a thin layer of compact cortical bone. In long bones ,t he cylindrical part—the diaphysis is almost totally dense compact bone, with a thin region of cancellous bone on the inner surface around the central marrow cavity .

38. Short bones such as those of the wrist and ankle usually have cores of cancellous bone surrounded completely by compact bone.

39. The flat bones that form the calvaria (skullcap) have two layers of compact bone called plates , separated by a thicker layer of cancellous bone called the diploë.

40. At the microscopic level both compact and cancellous bones typically show two types of organization: Mature lamellar bone , with matrix existing as discrete sheets. Woven bone , newly formed with randomly arranged components.

41. Lamellar bone Woven bone

42. Summary of bone types and their organization

43. Lamellar Bone Lamellar bone represents the main type of bone in a mature skeleton. Lamellar bone is characterized by the organized arrangement of collagen fibers into layers or lamellae . This arrangement gives lamellar bone greater stiffness when compared to the disorganized nature of woven bone.

44. 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.

45. Each osteon is a long, sometimes bifurcated, cylinder generally parallel to the long axis of the diaphysis. Each osteon 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.

46. L: Lamellae C: Canaliculi CC: Central canal I: Interstitial lamellae O: Osteocytes An osteon

47. P: Perforating (Volkmann) canals

48. Two photographs of the same area of an unstained section of compact bone, showing osteons with concentric lamellae around central canals. Lamellae are seen only faintly by brightfield microscopy (a) , but they appear as alternating bright and dark bands under the polarizing light microscope (b) . Bright bands are due to birefringence from the highly ordered collagen fibers in a lamella. Alternating bright and dark bands indicate that fibers in successive lamellae have different orientations, an organization that makes lamellar bone very strong.

49. 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.

50. Compact bone (e.g, 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.

51. Woven Bone Woven bone is nonlamellar and characterized by bone tissue with a disorganized collagen I fibers arrangement. It primarily develops embryonically and is gradually replaced between three and four years of age by lamellar bone. Woven bone is not frequently found in the adult skeleton, except in pathological conditions (such as Paget’s disease and osteosarcoma, ...)

52. 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. Immature woven bone forms more quickly but has less strength than lamellar bone.

53. Osteogenesis Bone ossification , or osteogenesis , is the process of bone formation. This process begins between the 6 th and 7 th weeks of embryonic development and continues until about age 25. Osteogenesis occurs by one of two processes: Intramembranous ossification Endochondral ossification

54. Each of these processes( Intramembranous , Endochondral ossification ) begins with a mesenchymal tissue precursor, but how it transforms into bone differs. The names refer to the mechanisms by which the bone forms initially; in both processes woven bone is produced first and is soon replaced by stronger lamellar bone. During growth of all bones, areas of woven bone, areas of bone resorption, and areas of lamellar bone all exist contiguous to one another.

55. Intramembranous vs. Endochondral ossification Intramembranous ossification directly converts the mesenchymal tissue to bone and forms the flat bones of the skull, clavicle, and most of the cranial bones. Endochondral ossification begins with mesenchymal tissue transforming into a cartilage intermediate, which is later replaced by bone and forms the remainder of the axial skeleton and the long bones.

56. Intramembranous ossification Intramembranous ossification is the process of bone development from fibrous membranes. It is involved in the formation of the flat bones of the skull, the mandible, and the clavicles. Ossification begins as mesenchymal cells form a template of the future bone.

57. (a) Mesenchymal cells group into clusters, and ossification centers form. (b) Secreted osteoid traps osteoblasts, which then become osteocytes. (c) Trabecular matrix and periosteum form. (d) Compact bone develops superficial to the trabecular bone, and crowded blood vessels condense into red marrow. Intramembranous ossification steps

58. Intramembranous ossification

59. Five steps can summarize intramembranous ossification: Mesenchymal cells differentiate into osteoblasts and group into ossification centers. Osteoblasts become entrapped by the osteoid they secrete, transforming them to osteocytes. Trabecular bone and periosteum form. Cortical bone forms superficially to the trabecular bone. Blood vessels form the red marrow.

60. Endochondral ossification Endochondral ossification takes place within hyaline cartilage shaped as a small version, or model, of the bone to be formed. This process involves the replacement of hyaline cartilage with bone. Forms most bones of the body.

61. Five steps can summarize endochondral ossification: Mesenchymal cells differentiate into chondrocytes and form the cartilage model for bone. Chondrocytes near the center of the cartilage model undergo hypertrophy and alter the contents of the matrix they secrete, enabling mineralization. Chondrocytes undergo apoptosis due to decreased nutrient availability; blood vessels invade and bring osteogenic cells. Primary ossification center forms in the diaphyseal region of the periosteum called the periosteal collar. Secondary ossification centers develop in the epiphyseal region after birth.

62. The epiphyseal (growth) plate

63. The epiphyseal (growth) plate

64. The physeal growth plate is separated into various sections based on pathologic characteristics: Reserve Zone Proliferative Zone Hypertrophic Zone Primary Spongiosa (zone of calcified cartilage) Secondary Spongiosa (zone of ossification)

65. Reserve Zone Storage site for lipids, glycogen, proteoglycan Proliferative Zone Proliferating chondrocytes leading to longitudinal growth Hypertrophic Zone Site of chondrocyte maturation Within the hypertrophic zone, the chondrocytes go through a transformation process. The chondrocyte mature and prepare a matrix for calcification; then they degenerate which allows calcium release for calcification of the matrix

66. Primary Spongiosa (zone of calcified cartilage) Site for mineralization to form woven bone Vascular invasion occurs Secondary Spongiosa (zone of ossification) Internal modeling with the replacement of fiber bone with lamellar bone External modeling with funnelization

67. Epiphyseal growth plate: Locations and zones of activity.

68. Growth disorders

69. Appositional growth Appositional growth is the increase in the diameter of bones by the addition of bony tissue at the surface of bones. Osteoblasts at the bone surface secrete bone matrix, and osteoclasts on the inner surface break down bone.

70. Bone Remodeling and Repair Is a lifelong process where mature bone tissue is removed from the skeleton ( bone resorption ) and new bone tissue is formed ( ossification or new bone formation). These processes also control the reshaping or replacement of bone following injuries like fractures.

71. Main features of bone fracture repair

72. Metabolic Role of Bone Calcium ions are required for the activity of many enzymes and many proteins mediating cell adhesion, cytoskeletal movements, exocytosis, membrane permeability, and other cellular functions.

73. The skeleton serves 99% of the body’s total calcium.

74. The conc. of calcium in the blood (9-10 mg/dL) The principal mechanism for raising blood calcium levels is the mobilization of ions from hydroxyapatite to interstitial fluid, primarily in cancellous bone. Ca +2 mobilization is regulated mainly by paracrine interactions among bone cells. Two polypeptide hormones target bone cells to influence calcium homeostasis: parathyroid hormone (PTH) , calcitonin.

75. Hypocalcemia Hypocalcemia, commonly known as calcium deficiency disease , occurs when calcium levels in the blood are low. A long-term deficiency can lead to dental changes, cataracts, alterations in the brain, and osteoporosis , which causes the bones to become brittle.

76. Osteoporosis A medical condition in which the bones become brittle and fragile from loss of tissue, typically as a result of hormonal changes, or deficiency of calcium or vitamin D.

77. Calcium-Rich Foods Oranges Milk Cheese Fish Kiwifruit Yoghurt Kale ....

78. Joints Joints are regions where bones meet, capped and held together firmly by other connective tissues , allowing at least the potential for bending or movement in that portion of the skeleton. The type of joint determines the degree of movement between the bones.

79. Anatomy of Joints A joint is an articulation between two bones in the body and are broadly classified by the tissue which connects the bones. The three main types of joints are: Synovial Cartilaginous Fibrous Fibrous joints have fibrous tissue joining the bone and these joints are typically very strong.

80. Classifications of Joints 2 Methods of Classification: Functional Classification *Focuses on the amount of movement allowed. Structural Classification *Focuses on the material that binds joints together

81. Classifications of Joints Fibrous Fixed (Synarthrosis) A. Synostosis / Sutures B. Gomphosis C. Syndesmoses D. Symphyses Cartilaginous Slightly movable (Amphiarthrosis) A. Pri. Cart. Joints Synchondrosis B. Sec. Cart. Joints Symphysis Synovial freely movable (Diarthrosis) 1. Plane 2. Hinge 3. Pivot 4. Bicondylar 5. Ellipsoid 6. Saddle 7. Ball and socket

82. Types of Joints

83. Synarthrosis (Sutures, Gomphosis, Symphyses and Syndesmoses ) Synarthrosis (Fibrous joints) formed by Dense connective tissue

84. Amphiarthrosis ( Cartilaginous) Are united by Hyaline cartilage or fibrocartilage Synchondrosis ( Primary Cartilaginous Joints ) Temporary s ynchondrosis Permanent synchondrosis Symphysis ( Secondary Cartilaginous Joints) Intervartebral symphysis Manubriosternal symphysis Pubic symphysis

85. Diarthroses (synovial joints)

86. Synovial membrane

87. Articular cartilage

88. THANK YOU THE END