1. Histology and Physiology of Bones
Chapter 6
Histology and Physiology of Bones
Osteon
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Skeletal System 4 components Bones are organs composed of Bones
Cartilage
Tendons
Ligaments
Bones are organs composed of
Nerve tissue
Vascular tissue

3. Functions of Bones
Support: framework that supports body and cradles soft organs
Protection: protective case for brain, spinal cord, vital organs
Movement: provide levers for muscles
Storage: reservoir for minerals, especially Ca & P
Blood cell production: hematopoiesis within marrow cavities of bones

4. Cartilage Chondroblasts produce cartilage and become chondrocytes
Chondrocytes located in lacunae surrounded by matrix
matrix contains collagen fibers (for strength) and proteoglycans (trap water)
perichondrium surrounds cartilage
outer layer contains fibroblasts
inner layer contains chondroblasts

5. Cartilage grows by appositional and interstitial growth
Fig. 6.1

6. Bone Histology Bone Matrix ~35% organic and 65% inorganic material
Collagen provides flexible strength
Proteoglycans
Inorganic
Hydroxyapatite (calcium phosphate crystal) provides weight-bearing strength

7. Effects of Changing the Bone Matrix!!!
Fig. 6.2

8. Bone Histology Bone Cells
Osteoblasts produce bone matrix and become osteocytes
Osteoblasts connect to 1 another through cell processes
surround themselves with bone matrix to become osteocytes
Osteocytes located in lacunae, connected to 1 another through canaliculi
Osteoblasts originate from osteochondral progenitor cells
Osteoclasts break down bone
originate from stem cells in red bone marrow

9. Bone Histology Ossification (Osteogenesis)
Osteoblasts on a preexisting surface, such as cartilage or bone. Cell processes of different osteoblasts join together
Osteoblasts produce bone matrix. Osteoblasts then become osteocytes.
Fig. 6.3

10. Bone Histology
Bone tissue is woven or lamellar bone, according to the organization of collagen fibers
Woven bone
collagen fibers in many diff directions
remodeled to form lamellar bone
Lamellar bone
Mature bone
collagen fibers parallel to 1 another
Arranged in thin layers called lamellae

11. Bone Histology
classified according to amount of bone matrix relative to amount of space present within bone
Cancellous bone has many spaces
Internal layer is a honeycomb of trabeculae filled with red or yellow bone marrow
Compact bone is dense with few spaces
External layer
Fig. 6.4

12. Bone Histology Cancellous Lamellae combine to form trabeculae
Beams of bone that interconnect to form a lattice-like structure with spaces
filled w/bone marrow and blood vessels
Trabeculae oriented along lines of stress: provide structural strength
Fig. 6.4

13. Compact Bone Histology
organized lamellae
Circumferential lamellae form outer surface of compact bones
Concentric lamellae surround central canals, forming osteons
Interstitial lamellae are remnants of lamellae left after bone remodeling
Canals within compact bone provide a means for exchange of gases, nutrients, and waste products
From periosteum (endosteum), perforating canals carry blood vessels to central canals
Canaliculi connect central canals to osteocytes

14. Fig. 6.5

15. Bone Anatomy Individual bones classified according to shape Long bones
Longer than wide
Most bones of upper and lower limbs
Short bones
About as wide as long
Bones of wrist (carpals) and ankle (tarsals)
Flat bones
Relatively thin, flattened shape, usually curved
Certain bones of skull, all ribs, breastbone (sternum), shoulder blades (scapulae)
Irregular bones
Do not fit into other 3 categories
Vertebrae, pelvic girdle, facial bones

16. Bone Anatomy Structure of Long Bone: diaphysis and epiphysis Diaphysis
Tubular shaft, forms axis of long bones
compact bone, surrounds medullary cavity
Yellow bone marrow (fat) in medullary cavity
Not to same extent, but certain bones also contain red marrow

17. Bone Anatomy Structure of Long Bone Epiphyses
Expanded ends of long bones
Exterior is compact bone, interior is spongy bone
Joint surface is covered with articular (hyaline) cartilage
Epiphyseal line separates diaphysis from epiphyses
Epiphyseal plate is site of bone growth in length
Epiphyseal plate becomes epiphyseal line when all its cartilage is replaced with bone

18. Bone Anatomy Bone Membranes
Periosteum: double layer of protective membrane covering outer surface of bone
Outer fibrous layer is dense regular conn tissue, contains blood vessels and nerves
Inner osteogenic layer contains osteoblasts, osteoclasts, and osteochondral progenitor cells
Endosteum: delicate membrane covering internal surfaces of bone
Contains osteoblasts, osteoclasts, and osteochondral progenitor cells

19. Fig. 6.6

20. Bone Anatomy Structure of Flat, Short, and Irregular Bones
Flat bones contain interior framework of cancellous bone sandwiched between 2 layers of compact bone
Short and Irregular bones have a composition similar to ends of long bones

21. Bone Development Begins at week 8 of embryo development
Intramembranous ossification: bone develops from a fibrous membrane
Some skull bones, part of mandible, diaphyses of clavicles
Endochondral ossification: bone forms by replacing hyaline cartilage
Bones of base of skull, part of mandible, epiphyses of clavicles, most of remaining skeletal system

22. Intramembranous Ossification
Within membrane at centers of ossification, osteoblasts produce bone along membrane fibers to form cancellous bone
Beneath periosteum, osteoblasts lay down compact bone to form outer surface of bone
Fontanels are areas of membrane that are not ossified at birth
Bones formed by intramembranous ossification are yellow and bones formed by endochondral ossification are blue
Fig. 6.7

23. Fig. 6.7

24. Fig. 6.8

25. Endochondral Ossification
Uses hyaline cartilage “bones” as models for bone construction
Requires breakdown of hyaline cartilage prior to ossification
perichondrium covering hyaline cartilage “bone” infiltrated with blood vessels, converting it to vascularized periosteum
change in nutrition transforms underlying osteochondral progenitor cells into osteoblasts
Formation of bone collar around diaphysis of hyaline cartilage model

26. Endochondral Ossification cont.
Blood vessels grow into calcified cartilage, bringing osteoblasts and osteoclasts from periosteum
primary ossification center forms as osteoblasts lay down bone matrix
Medullary cavity forms
Appearance of secondary ossification centers in epiphyses
Ossification of epiphyses, with hyaline cartilage remaining only in epiphyseal plates

27. Endochondral Ossification
Fig. 6.9a

28. Endochondral Ossification
Continued
Fig. 6.9b

29. Bone Growth Bones increase in size only by appositional growth
Adding of new bone on surface of older bone or cartilage
Trabeculae grow by appositional growth

30. Bone Growth Growth in Bone Length
Bone length increases because of growth at epiphyseal plate
Epiphyseal plate growth involves
Interstitial growth of cartilage
Followed by appositional bone growth on cartilage
Epiphyseal plate growth results in an increase in length of diaphysis and bony processes
Bone growth in length ceases when epiphyseal plate becomes ossified and forms epiphyseal line

31. Postnatal Bone Growth Growth in Long Bones Ossified bone
epiphyseal plate
Zone of resting cartilage
Cartilage attaches to epiphysis
Zone of proliferation
New cartilage produced on epiphyseal side of plate as chondrocytes divide and form stacks of cells
Zone of hypertrophy
Chondrocytes mature and enlarge
Zone of calcification
Matrix is calcified, and chondrocytes die
Ossified bone
calcified cartilage on diaphyseal side of plate is replaced by bone

32. Fig. 6.10

33. Fig. 6.11

34. Bone Growth Growth at Articular Cartilage
interstitial growth of cartilage followed by appositional bone growth on cartilage
Results in larger epiphyses and increase in size of bones that do not have epiphyseal plates

35. Bone Growth Growth in Bone Width
Appositional bone growth beneath periosteum increases diameter of long bones and size of other bones
Osteoblasts from periosteum form ridges with grooves between them
ridges grow together, converting grooves into tunnels filled with concentric lamellae to form osteons
Osteoblasts from periosteum lay down circumferential lamellae, which can be remodeled
Fig. 6.12

36. http://highered. mcgraw-hill

37. Bone Growth Factors Affecting Bone Growth
Genetic factors determine bone shape and size
expression of genetic factors can be modified
Factors that alter mineralization process or production of organic matrix
Deficiencies in vitamin D
Hormones
Growth hormone, estrogen, testosterone stimulate bone growth
Estrogen and testosterone cause closure of epiphyseal plate

38. Bone Remodeling
Remodeling converts woven bone to lamellar bone and allows bone to
Change shape
Adjust to stress
Repair itself
Regulate body calcium levels
Basic multicellular units (BMUs) make tunnels in bone, which are filled with concentric lamellae to form osteons
BMUs are temporary assemblies of osteoclasts and osteoblasts
BMU activity renews the entire skeleton every 10 years
Interstitial lamellae are remnants of bone not removed by BMUs

39. Remodeling of a Long Bone
Fig. 6.13

40. Bone Fractures (Breaks)
Bone fractures classified by:
position of bone ends after fracture
completeness of break
orientation of bone to long axis
Whether or not bone ends penetrate skin

41. Page 137

42. Bone Repair
Fig. 6.14

43. Bone Repair Hematoma formation Torn blood vessels hemorrhage
mass of clotted blood (hematoma) forms at fracture site
Site becomes swollen, painful, and inflamed
Fig. 6.14

44. Bone Repair Callus formation
Granulation tissue (soft callus) forms a few days after fracture
Capillaries grow into tissue and phagocytic cells begin cleaning debris
Fig. 6.14

45. Bone Repair Callus formation cont. external callus forms when:
Osteoblasts and fibroblasts migrate to fracture and begin reconstructing the bone
Fibroblasts secrete collagen fibers that connect broken bone ends
Osteoblasts begin forming woven bone
Osteoblasts furthest from capillaries secrete an externally bulging cartilaginous matrix that later calcifies

46. Bone Repair Callus ossification
fibers and cartilage of internal and external calluses are ossified to produce woven, cancellous bone
Cancellous bone formation in callus usually complete 4-6 weeks after injury
Fig. 6.14

47. Bone Repair 4. Bone remodeling
Excess material on bone shaft exterior and in medullary canal is removed
Compact bone laid down to reconstruct shaft walls
remodeling process may take >1 year to complete
Fig. 6.14

48. Bone Repair
Fig. 6.14

49. Calcium Homeostasis Bone is major storage site for calcium (Ca2+)
2 hormones regulate Ca2+ levels in blood: parathyroid hormone (PTH) and calcitonin
PTH is major regulator of blood Ca2+
Falling blood Ca2+ levels signal parathyroid glands to release PTH
PTH signals
Osteoclasts to degrade bone matrix and release Ca2+ into blood
Ca2+ absorption from small intestines
Reabsorption of Ca2+ from urine
Calcitonin
Rising blood Ca2+ levels trigger thyroid to release calcitonin
Calcitonin stimulates calcium salt deposition in bone by decreasing osteoclast activity

50. Fig. 6.15

51. Effects of Aging on the Skeletal System
With aging, bone matrix is lost and matrix becomes more brittle
Cancellous bone loss results from a thinning and loss of trabeculae
Compact bone loss mainly occurs from inner surface of bones and involves less osteon formation
Loss of bone
Increases risk for fractures
Causes deformity
Loss of height
Pain
Stiffness
Loss of teeth

52. Osteoporosis
Page 138

53. http://highered. mcgraw-hill

54. Page 142