1. Histology and Physiology of Bones
Chapter 6
Histology and Physiology of Bones
Osteon
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2. Functions of the Skeletal System
Skeletal system has four components
Bones
Cartilage
Tendons
Ligaments
Bones are organs composed of
Nerve tissue
Vascular tissue

3. Function of Bones
Support: form the framework that supports the body and cradles soft organs
Protection: provide a protective case for the brain, spinal cord, and vital organs
Movement: provide levers for muscles
Storage: reservoir for minerals, especially calcium and phosphorus
Blood cell production: hematopoiesis occurs within the marrow cavities of bones

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

5. Fig. 6.1

6. Bone Histology Bone Matrix
Approximately 35% organic and 65% inorganic material
Organic
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 one another through cell processes and surround themselves with bone matrix to become osteocytes
Osteocytes are located in lacunae and are connected to one another through canaliculi
Osteoclasts break down bone
Osteoblasts originate from osteochondral progenitor cells
Osteoclasts originate from stem cells in red bone marrow

9. Bone Histology Ossification (Osteogenesis)
Osteoblasts on a preexisting surface, such as cartilage or bone. The cell processes of different osteoblasts join together
Osteoblasts have produced bone matrix. The osteoblasts are now osteocytes
Fig. 6.3

10. Bone Histology
Bone tissue is classified as either woven or lamellar bone, according to the organization of collagen fibers
Woven bone
Has collagen fibers oriented in many different directions
It is remodeled to form lamellar bone
Lamellar bone
Mature bone
Arranged in thin layers called lamellae
Has collagen fibers oriented parallel to one another

11. Bone Histology
Bone can be classified according to the amount of bone matrix relative to the amount of space present within the bone
Cancellous bone has many spaces
Internal layer which 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
Trabeculae are oriented along lines of stress and provide structural strength
Fig. 6.4

13. Bone Histology Compact Bone Consists of organized lamellae
Circumferential lamellae form the 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 the exchange of gases, nutrients, and waste products
From the 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 are classified according to their shape
Long bones
Longer than they are wide
Most bones of the upper and lower limbs
Short bones
About as wide as they are long
Bones of the wrist (carpals) and ankle (tarsals)
Flat bones
Relatively thin, flattened shape and are usually curved
Certain bones of the skull, all the ribs, the breastbone (sternum), and the shoulder blades (scapulae)
Irregular bones
Do not fit into the other three categories
Vertebrae, pelvic girdle and facial bones

16. Bone Anatomy Structure of Long Bone
Long bones consist of a diaphysis and an epiphysis
Diaphysis
Tubular shaft that forms the axis of long bones
Composed of compact bone that surrounds the medullary cavity
Yellow bone marrow (fat) is contained in the medullary cavity
Not to the 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, and the interior is spongy bone
Joint surface is covered with articular (hyaline) cartilage
Epiphyseal line separates the diaphysis from the epiphyses
Epiphyseal plate is the site of bone growth in length

18. Bone Anatomy Bone Membranes
Periosteum: double layer of protective membrane covering the outer surface of bone
Outer fibrous layer is dense regular connective tissue, which 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 an interior framework of cancellous bone sandwiched between two layers of compact bone
Short and Irregular bones have a composition similar to the 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 the mandible, and the diaphyses of the clavicles
Endochondral ossification: bone forms by replacing hyaline cartilage
Bones of the base of the skull, part of the mandible, the epiphyses of the clavicles, and most of the remaining skeletal system

22. Bone Development Intramembranous Ossification
Within the membrane at centers of ossification, osteoblasts produce bone along the membrane fibers to form cancellous bone
Beneath the periosteum, osteoblasts lay down compact bone to form the outer surface of the 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. Bone Development Endochondral Ossification
Uses hyaline cartilage “bones” as models for bone construction
Requires breakdown of hyaline cartilage prior to ossification
The perichondrium covering the hyaline cartilage “bone” is infiltrated with blood vessels, converting it to a vascularized periosteum
The change in nutrition transforms the underlying osteochondral progenitor cells into osteoblasts
Formation of bone collar around the diaphysis of the hyaline cartilage model

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

26. Endochondral Ossification
Fig. 6.9a

27. Endochondral Ossification
Continued
Fig. 6.9b

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

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

30. Postnatal Bone Growth Growth in Long Bones Ossified bone
The epiphyseal plate is organized into four zones
Zone of resting cartilage
Cartilage attaches to the epiphysis
Zone of proliferation
New cartilage is produced on the epiphyseal side of the plate as the 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
The calcified cartilage on the diaphyseal side of the plate is replaced by bone

31. Fig. 6.10

32. Fig. 6.11

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

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

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

36. 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

37. Remodeling of a Long Bone
Fig. 6.13

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

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40. Bone Repair
Fig. 6.14

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

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

43. Bone Repair Callus formation cont. The external callus forms when:
Osteoblasts and fibroblasts migrate to the 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

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

45. Bone Repair 4. Bone remodeling
Excess material on the bone shaft exterior and in the medullary canal is removed
Compact bone is laid down to reconstruct shaft walls
The remodeling process may take more than a year to complete
Fig. 6.14

46. Bone Repair
Fig. 6.14

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

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

49. Osteoporosis
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