1. Cartilage and Bone
Kristine Krafts, M.D.

2. Cartilage and Bone Lecture Objectives
Describe the general functions of cartilage and bone.
Compare the function and composition of the three types of cartilage.
Describe the two methods of cartilage formation.
Describe the cells and extracellular matrix of bone.
Compare and contrast immature and mature bone.
Describe the two methods of bone formation.

3. Cartilage and Bone Lecture Outline
Function and characteristics of cartilage
Composition of cartilage
Three types of cartilage
Formation of cartilage
Bone
Function and characteristics of bone
Composition of bone
Two (okay, four) types of bone
Formation of bone

4. Cartilage and Bone Lecture Outline
Function and characteristics of cartilage

5. Functions and Characteristics of Cartilage
Cartilage is strong and sort of rigid but also flexible. It can withstand force but it can also bend.
It’s made of cells (chondroblasts and chondrocytes) and extracellular matrix.
Forms the supporting framework of some organs.
Lines the surface of articulating bones.
Forms the template for growth and development of long bones.

6. Metabolic facts that will make you feel sorry for cartilage
Bone is very metabolically active. Cartilage is not.
Bone has a good blood supply. Cartilage does not.
Bone usually heals well. Cartilage does not.

7. Cartilage and Bone Lecture Outline
Function and characteristics of cartilage
Composition of cartilage

8. Cartilage Cells
Cartilage is a specialized, supporting connective tissue composed of cells and extracellular matrix.
Chondroblasts Precursor cells that differentiate into chondrocytes.
Chondrocytes Mature cartilage cells that lie in little artifactual lacunae.

9. Cartilage Extracellular Matrix (ECM)
Cartilage ECM is composed of:
Collagen and/or elastic fibers
Lots of GAGs and proteoglycans
Nice choices! These substances make the ECM firm and resistant to mechanical forces.

10. Composition of ECM in Cartilage

11. Perichondrium
Perichondrium covers the surface of hyaline and elastic cartilage (but not fibrocartilage).
Dense connective tissue composed of fibroblasts and type I collagen fibers.
Contains blood vessels.

12. Cartilage is avascular. So what?
Cartilage itself has no blood vessels or nerves in its extracellular matrix (bone is a different story).
Nutrients must diffuse from perichrondrium through extracellular matrix to chondrocytes.
This means metabolic activity is low, the width of cartilage is limited, and cartilage heals poorly.

13. Cartilage and Bone Lecture Outline
Function and characteristics of cartilage
Composition of cartilage
Three types of cartilage

14. The fibers of the extracellular matrix determine the type of cartilage.
Hyaline cartilage
Type II collagen fibers
Elastic cartilage
Type II collagen fibers and elastic fibers
Fibrocartilage
Type I collagen fibers

15. Three Types of Cartilage
Hyaline cartilage
Most common.
Articular surfaces of joints, large respiratory passages (nose, larynx, trachea, bronchi), rib-sternum junction, and epiphyseal plate.
Functions: support soft tissue, line joints, growth of long bones.

16. Hyaline Cartilage: Histologic Features
Perichondrium
Chondroblasts
Extracellular matrix
Chondrocytes

17. Hyaline articular cartilage

18. Three Types of Cartilage
Hyaline cartilage
Elastic cartilage
Located in areas that are pliable and flexible
Pinna of ear, external auditory canal, auditory (eustachian) tube, epiglottis, larynx

19. Elastic cartilage: perichondrium

20. Elastic cartilage: chondrocytes and matrix

21. Three Types of Cartilage
Hyaline cartilage
Elastic cartilage
Fibrocartilage
Located in areas subjected to pulling forces
Intervertebral discs, pubic symphysis, and attachments of tendons and ligaments

22. Fibrocartilage: collagen fibers

23. Fibrocartilage: chondrocytes

24. Fibrocartilage: matrix

25. Cartilage and Bone Lecture Outline
Function and characteristics of cartilage
Composition of cartilage
Three types of cartilage
Formation of cartilage

26. Two Methods of Cartilage Formation
Interstitial growth: growth from within cartilage. Chondrocytes divide and secrete matrix. Appositional growth: growth along the outside of cartilage. Chondroblasts secrete matrix and differentiate into chondrocytes.

27. Cartilage and Bone Lecture Outline
Function and characteristics of cartilage
Composition of cartilage
Three types of cartilage
Formation of cartilage
Bone
Function and characteristics of bone

28. Functions of Bone Support and protection of soft tissues
Attachment of muscles for movement
Location of hematopoietic bone marrow where blood cells are produced
Stores and releases calcium, phosphate and other ions

29. Characteristics of Bone
Bone is highly vascularized and very metabolically active.
Bone remodels (turns over) constantly throughout life. Ideally, bone removal occurs at same rate as bone production.
Inhibition of bone turnover leads to poor quality, unhealthy bone.

30. Cartilage and Bone Lecture Outline
Function and characteristics of cartilage
Composition of cartilage
Three types of cartilage
Formation of cartilage
Bone
Function and characteristics of bone
Composition of bone

31. Composition of Bone Cells Osteoblasts: produce bone matrix
Osteocytes: lie in lacunae
Osteoclasts: resorb bone
Extracellular matrix
Organic matrix: osteoid, composed of type I collagen fibers and ground substance
Inorganic (mineralized) matrix: hydroxyapatite

32. Osteoblasts Found along edge of bone, with osteoid underneath.
Active osteoblasts are cuboidal-columnar with abundant RER, Golgi and secretory granules; inactive osteoblasts are flattened.

33. Osteoblast Functions Make bone!
Produce osteoid (organic bone matrix) with type I collagen fibers, proteoglycans and glycoproteins.
Deposit inorganic hydroxyapatite crystals: Ca10(PO4)6(OH)2
Bone formation promoted by testosterone, estrogen, growth hormone, weight-bearing exercise and muscle use.

34. Osteocytes
Osteocytes are involved in maintenance of bone matrix. Their death results in resorption of matrix.
Each osteocyte sits in its own lacuna.
Osteocytes have small, dark, flattened nuclei with minimal rough ER and Golgi.

35. Osteoclasts
Multinucleated giant cells ( μm). Lie in Howship’s lacunae.
Arise from fusion of blood monocytes.
Numerous lysosomes.
Actively resorbing osteoclasts adjacent to bone have cytoplasmic membrane branching (“ruffled border”).

36. How do osteoclasts resorb bone?
Clear zone: an area of cytoplasm that seals off an acidic micro-environment containing lysosomal enzymes. This is where bone resorption occurs.

37. How do osteoclasts resorb bone?
The osteoclast transfers collagenase and protons into this microenvironment, breaking down collagen fibers and calcium salts. Then, the osteoclast phagocytizes the remnants of bone resorption.

38. Bone Matrix Inorganic 50% of dry weight of bone Mostly calcium Organic
Type I collagen
Ground substance (proteoglycans and glycoproteins)

39. Serum Calcium and Bone Resorption
When serum calcium is low, parathyroid hormone is released, which tells osteoblasts to make osteoclast stimulating factor. Osteoclasts then resorb bone, which increases serum calcium. Cool! When serum calcium is high, calcitonin is released, which inhibits osteoclastic resorption of bone, leading to decreased serum calcium.

40. Cartilage and Bone Lecture Outline
Function and characteristics of cartilage
Composition of cartilage
Three types of cartilage
Formation of cartilage
Bone
Function and characteristics of bone
Composition of bone
Two (okay, four) types of bone

41. Two Gross Types of Bone
Compact bone Very dense, solid structure. Spongy bone Open, loose structure.

42. Compact Bone and Spongy Bone

43. Two Microscopic Types of Bone
Immature (primary, woven) bone The first bone laid down. Formed quickly. Later replaced by mature bone. Mature (secondary, lamellar) bone Arranged in osteons: layers (lamellae) of bone around little canals.

44. Collagen fiber arrangement
Immature vs. Mature Bone
Immature bone
Mature bone
Gross structure
Always spongy
Compact (outside) and spongy (inside)
Collagen fiber arrangement
Random
Parallel
Number of osteocytes
Lots
Relatively few

45. Two Microscopic Types of Bone
Immature (primary, woven) bone The first bone laid down. Formed quickly. Later replaced by mature bone.

46. Immature bone: super low-power view

47. Immature bone: high-power view

48. Two Microscopic Types of Bone
Immature (primary, woven) bone The first bone laid down. Formed quickly. Later replaced by mature bone. Mature (secondary, lamellar) bone Arranged in osteons: layers (lamellae) of bone around little canals.

50. Osteon (Haversian system)
Central canal
Osteocytes
Lamellae
Osteon (Haversian system)

51. Volkmann (perforating) canals connect osteons

52. Periosteum and Endosteum
Periosteum lines the outer surface of compact bone.
Two layers: outer dense connective tissue layer and inner layer of osteoprogenitor cells.
Sharpey’s fibers are collagen fibers which tightly attach the periosteum to the bone matrix.
Contains blood vessels for nutrition of bone.
Contains osteoblasts for bone growth, repair and remodeling.

53. Periosteum central canal osteon external lamellae perforating fibers
inner layer
outer layer
interstitial lamellae
trabeculae of spongy bone

54. Periosteum and Endosteum
Periosteum lines the outer surface of compact bone.
Endosteum lines the inner surface of compact bone.
Also lines the surface of bony trabeculae of spongy bone, and the Haversian canals.
Contains osteoblasts for bone growth, repair and remodeling.

55. Endosteum

56. Cartilage and Bone Lecture Outline
Function and characteristics of cartilage
Composition of cartilage
Three types of cartilage
Formation of cartilage
Bone
Function and characteristics of bone
Composition of bone
Two types of bone
Formation of bone

57. Bone Development (Osteogenesis)
Bone can be formed in two ways:
Intramembranous ossification Occurs in flat bones (most of skull, including maxilla and mandible). Endochondral ossification Occurs in long bones and irregular bones.

58. Eight Steps in Intramembranous Ossification
Capillaries grow into mesenchyme & release oxygen.
Surrounding mesenchymal cells round up, differentiate into osteoblasts and form osteoid.
Osteoid mineralizes. Osteoblasts become osteocytes.
Bone spicules form and enlarge to form trabeculae of immature bone.
A single plate (table) of bone is formed.
Immature bone is replaced by mature bone.
Remodeling continues, and 2 tables of bone are formed.
The two tables are separated by diploe:
Tables = compact bone
Diploe = spongy bone with marrow

59. Intramembranous Ossification

60. Intramembranous Ossification

61. Intramembranous Ossification

62. Intramembranous Ossification

63. Six Steps in Endochondral Ossification
Fetal hyaline cartilage develops.
Cartilage calcifies, and a periosteal bone collar forms around the diaphysis (long part of the bone).
A primary ossification center forms in the diaphysis.
Secondary ossification centers form in the epiphyses (ends of the bone).
Bone replaces cartilage (except the articular cartilage and epiphyseal plates).
Epiphyseal plates ossify.

64. Endochondral Ossification

65. Endochondral Ossification

66. Endochondral Ossification
Epiphysis: the end of a long bone. Consists primarily of spongy bone, with a layer of compact bone on the outside.
Diaphysis: the shaft of a long bone. Consists of compact bone on the outside and a marrow cavity inside.

67. Endochondral bone formation: super low-power view

68. Endochondral bone formation: zones of maturation

69. Endochondral bone formation: resting zone

70. Endochondral bone formation: proliferative zone

71. Endochondral bone formation: zone of hypertrophy

72. Endochondral bone formation: zone of calcification

73. Endochondral bone formation: zone of ossification

74. Epiphyseal plate in endochondral ossification

75. Synovial joint (diarthrosis)

76. Synovial Membrane

77. Articular cartilage
Synovial membrane
Joint cavity

78. Cartilage and Bone Lecture Outline
Function and characteristics of cartilage
Composition of cartilage
Three types of cartilage
Formation of cartilage
Bone
Function and characteristics of bone
Composition of bone
Two types of bone
Formation of bone