1. Chapter 6: Osseous Tissue and Bone Structure

2. The Skeletal System Skeletal system includes: bones of the skeleton
cartilages, ligaments, and other connective tissues that stabilize the bones

3. Skeletal System Functions: 1. Support: framework & structure of body
2. Storage of minerals and lipids
Minerals: calcium and phosphate
- for osmotic regulation, enzyme function,
nerve impulses
Yellow marrow: triglycerides
3. Blood cell production: all formed elements
- red marrow: stem cells  hematopiesis
4. Protection: surround soft tissues
5. Leverage for movement:
- levers upon which skeletal muscles act

4. Classification of Bones
Bone are identified by:
shape
internal tissues
bone markings
SHAPE:
Long bones
Flat bones
Sutural bones
Irregular bones
Short bones
Sesamoid bones

5. Shape of Bones Long Bones: Short Bones: Flat Bones:
Longer than wide, consist of shaft and 2 ends
e.g. bones of appendages
Short Bones:
Approx. equal in all dimensions
e.g. carpals, tarsals
Flat Bones:
Thin, 2 parallel surfaces
e.g. skull, sternum, ribs, scapula
Figure 6–1a

6. Shape of Bones Irregular Bones: Sesamoid Bones: Sutural Bones:
Complex shapes
E.g. vertebrae, os coxa
Sesamoid Bones:
Seed shaped, form in tendon
E.g. patella, total number can vary
Sutural Bones:
- Extra bones in sutures of skull

7. Bone Structure A bone is an organ consisting of many tissue types:
Osseous, nervous, cartilage, fibrous CT, blood, etc.
All bones consist of 2 types of bone tissue
Compact bone:
- solid, dense bone, makes up surfaces and shafts
Spongy Bone/Cancellous bone:
- meshy, makes up interior of bones, houses red marrow in spaces

8. Bone Markings Bones are not flat on the surface:
Have projections, depressions, and holes for muscle attachment, blood & nerve supply
Depressions or grooves:
along bone surface
Projections:
where tendons and ligaments attach
at articulations with other bones
Tunnels:
where blood and nerves enter bone

9. Bone Markings
Table 6–1 (2 of 2)

10. Long Bones Structure Diaphysis: Medullary (marrow) cavity: Epiphysis:
- Hollow shaft of compact bone
Medullary (marrow) cavity:
Center of diaphysis, contains yellow marrow
Triglycerides for energy reserve
Epiphysis:
Expanded end of bone, surface of compact bone
Center filled with spongy bone with red marrow in spaces
Produces blood cells
Figure 6–2a

11. Long Bones Structure Epiphyseal line or plate: Periosteum: Endosteum:
Cartilage that marks connection of diaphysis with epiphysis
Line: adults, narrow (aka metaphysis)
Plate: thick, allows growth during childhood
Periosteum:
2 layer covering around outside of bone:
Outer Fibrous Layer
Inner Cellular Layer
Endosteum:
Cellular layers, covers all inside surfaces

13. Articular Cartilage: Joint/Articulation:
Hyaline cartilage on end where bone contacts another, no periosteum or perichondrium
Joint/Articulation:
- connection between two bones, surrounded by CT capsule, lined with synovial membrane
Joint cavity filled with synovial fluid to reduce friction on articular cartilage

14. Flat Bone Structure
Thin layer of spongy bone with red marrow between two layers of compact bone
Covered by periosteum and endosteum
Site of most hematopoiesis
Production of blood cells and cell fragments that are suspended in plasma (RBC, WBC, and platelets

15. Characteristics of Bone Tissue
Periosteum:
covers outer surfaces of bones
consist of outer fibrous and inner cellular layers
Endosteum:
Inner, cellular layer of periosteum

16. Bone Histology Bone = osseous tissue, supporting CT
Consists of specialized cells in a matrix of fibers and ground substance
Characteristics of bone:
Dense matrix packed with calcium salts
Osteocytes in lacunae
Canaliculi for exchange of nutrients and waste
Two layer periosteum, covers bone except at articular surfaces

17. Bone Histology Matrix = 98% of bone tissue Cells = only 2% of bone
1/3 = osteoid; organic part:
Collagen fibers + ground substance
Tough and flexible
2/3 = densely packed crystals of hydroxyapatite (calcium salts, mostly calcium phosphate)
Hard but brittle
Cells = only 2% of bone
Osteocytes
Osteoblasts
Osteoprogenitor cells
Osteoclasts

18. Cells located in Bones Osteocytes = mature bone cells
-no cell division
-located in lacunae between layers of matrix called lamellae
-canaliculi link lacunae to each other and blood supply
-osteocytes linked to each other via gap junctions on cell projections in canaliculi:
- allow exchange of nutrients and wastes
-Function
1. To maintain protein and mineral content of matrix
2. Can also participate in bone repair:
-become stem cell like when broken free of lacuna

19. Osteocytes in lacunae Blood vessels
LM X 362
Canaliculi
Osteocytes
in lacunae
Matrix
Blood
vessels
Central canal
PERIOSTEUM
Fibrous
layer
Cellular

20. Cells located in Bones Osteoblasts - Immature bone cells
Perform osteogenesis:
Formation of new bone matrix
Produce osteoid
Organic components of matrix that is not yet calcified to form bone
Promote deposit of calcium salts which spontaneously form hydroxyapatite
Once enclosed in lacuna by matrix, osteoblast differentiates into osteocyte and no longer produces new matrix

22. Cells located in Bones 3. Osteoprogenitor Cells – mesenchymal cells
- bone stem cell that produces daughters
- daughters become osteoblasts for repair and growth
- located in endosteum and inner periosteum

24. Cells located in Bones 4. Osteoclasts - large, multinuclear
- derived from monocytes (macrophages)
- perform osteolysis =
- digest and dissolve bone matrix
- release minerals:
1. For use in blood or
2. Recycling during bone remodeling

26. Cells located in Bones Osteocyte: Mature bone cell that maintains the
bone matrix
Osteoblast: Immature
bone cell that secretes
organic components
of matrix
Osteoclast: Multinucleate
cell that secretes acids and
enzymes to dissolve bone matrix
Osteoprogenitor cell:
Stem cell whose divisions
produce osteoblasts
Osteoid
Osteoblast
Matrix
Marrow
cavity
Osteoprogenitor
cell
Osteoclast
Canaliculi
Osteocyte

27. Homeostasis
Bone building (by osteocytes) and bone recycling (by osteoclasts) must balance:
more breakdown than building, bones become weak
exercise causes osteocytes to build bone

28. How would the strength of a bone be affected if the ratio of collagen to hydroxyapatite increased?
Strength increases, flexibility increases.
Strength increases, flexibility decreases.
Strength decreases, flexibility. decreases.
Strength decreases, flexibility increases.

29. stable mass, but re-positioned matrix mass will not be affected
If the activity of osteoclasts exceeds the activity of osteoblasts in a bone, how will the mass of the bone be affected?
stable mass, but re-positioned matrix
mass will not be affected
more mass
less mass

30. The difference between compact bone and spongy bone.

31. Structure of Compact Bone
Consists of osteons:
Parallel to surface
Each osteon is around a central canal:
Contains blood vessels and nerves
Perforating canals perpendicular to osteons act to connect the osteons
Osteon is built of layers of matrix secreted by osteoblasts
Each layer = concentric lamella
Osteocytes are located in lacunae between lamellae
Ostocytes are connected to neighboring cells and central canal via canaliculi

32. Structure of Compact Bone
Interstitial lamellae fill spaces between osteons
Circumferiential lamellae run perimeter inside and out in contact with:
endosteum and periosteum
Compact bone is designed to receive stress from one direction
Very strong parallel to osteons
Weak perpendicular to osteons

33. Compact Bone
Figure 6–5

34. Structure of Spongy Bone
Lamellae = meshwork called trabeculae (no osteons)
Red marrow fills spaces around trabeculae
Osteocytes in lacunae are linked by canaliculi
No direct blood supply (no central canals)
Nutrients diffuse into canaliculi in trabeculae from red marrow
Spongy bone make up:
low stress bones
Areas of bone where stress comes from multiple directions
Provide light weigh strength

35. Bone Marrow Red Marrow: Yellow Marrow:
Located in space between trabeculae
Has blood vessels
Forms red blood cells
Supplies nutrients to osteocytes
Yellow Marrow:
In some bones, spongy bone holds yellow bone marrow:
is yellow because it stores fat

36. Structure of Spongy Bone

37. Periosteum and Endosteum
Compact bone is covered with membrane:
periosteum on the outside
endosteum on the inside

38. Periosteum Fibrous outer layer: Cellular Inner layer: Functions:
- Dense irregular CT
Cellular Inner layer:
Osteoprogenitor cells
Functions:
Isolate bone from surrounding tissues
Site for attachment for tendons and ligaments
Route for nerves and blood vessels to enter bone
Participates in bone growth and repair

40. Endosteum Thin cellular layer
Lines medullary cavity, central canals, and covers trabeculae
Consists of:
osteoblasts, osteoprogenitor cells, and osteoclasts
Cells become active during bone growth and repair

41. Endosteum
Figure 6–8b

42. Bone Growth Begins 6-8 weeks post fertilization
Continues through puberty (18-25 y)
Osteogenesis = ossification = formation of bone
Not calcification
Hardening of matrix or cytoplasm with calcium
Can happen to many tissues
Two types of Ossification:
Intramembranous: forms flat bones
Endochondrial: forms long bones

43. Bone Development Human bones grow until about age 25 Osteogenesis:
bone formation
Ossification: Deposition of calcium salts
the process of replacing other tissues with bone

44. The difference between intramembranous ossification and endochondral ossification.

45. Intramembranous Ossification
Bone develops from mesenchyme or fibrous CT in deep layers of dermis
Also called dermal ossification:
because it occurs in the dermis
produces dermal bones such as mandible and clavicle
Produces skull bones
There are 4 main steps in intramembranous ossification

46. Intramembranous Ossification: Step 1
Ossification center appears in the fibrous CT membrane
Mesenchymal cells aggregate
Differentiate into osteoblasts
Begin ossification at the ossification center

47. Intramembranous Ossification: Step 2
Bone matrix (osteoid) is secreted within the fibrous membrane
Osteoblasts begin to secrete osteoid, which is mineralized within a few days
Trapped osteoblasts become osteocytes

48. Intramembranous Ossification: Step 3
Woven bone and periosteum form
Accumulating osteoid is laid down between embryonic blood vessels, which form a random network
Vascularized mesenchyme condenses on the external face of the woven bone and becomes periosteum around spongy bone

49. Intramembranous Ossification: Step 4
Bone collar of compact bone forms and red marrow appears
Trabeculae just deep to the periosteum thickens, forming a woven bone collar that is later replaced with mature lamellar bone
Spongy bone, consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow

50. Endochondral Ossification
Ossifies bones that originate as hyaline cartilage
Most bones originate as hyaline cartilage
Cartilage grows by interstitial and appositional growth
Cartilage is slowly replaced from the inside out

51. Endochondral Ossification
Growth and ossification of long bones occurs in 6 steps

52. Endochondral Ossification: Step 1
Primary ossification center begins to form:
Chondrocytes in the center of hyaline cartilage:
Enlarge in diaphysis
Surrounding matrix calcifies killing the enclosed chondrocytes
die, leaving cavities in cartilage
Figure 6–9 (Step 1)

53. Endochondral Ossification: Step 2
Blood vessels grow around the edges of the cartilage
Cells in the perichondrium change to osteoblasts:
Secrete osteoid
Osteiod is mineralized and produces a layer of superficial bone around the shaft which will continue to grow around the diaphysis and become compact bone (appositional growth)
Figure 6–9 (Step 2)

54. Endochondral Ossification: Step 3
Capillaries and fibroblast migrate into the primary ossification center:
Blood vessels enter the cartilage
Bringing fibroblasts that become osteoblasts and secrete osteoid
Mineralized into rebeculae
Spongy bone develops at the primary ossification center and continues to growth toward the epiphysis
Figure 6–9 (Step 3)

55. Endochondral Ossification: Step 4
Remodeling creates a marrow cavity:
Osteoclasts degrade trabeculae in the center to create the marrow cavity
Bone increases in length by interstital growth of the epiphyseal plate followed by replacement of plate cartilage by spongy bone
Cartilage continues to grow on epiphyseal side and is replaced by bone on diaphysis side
Bone increases in diameter by appositional growth from cellular layers of peristeum
Figure 6–9 (Step 4)

56. Endochondral Ossification: Step 5
Secondary ossification centers form in epiphyses:
Capillaries and osteoblasts enter the epiphyses:
creating secondary ossification centers
Figure 6–9 (Step 5)

57. Endochondral Ossification: Step 6
Epiphyses become ossified with spongy bone
Hyaline cartilage remains on articular surfaces (not calcified or ossified)
Ossification continues at both 1°and 2° ossification centers until all epiphyseal cartilage has been replaced with bone  epiphyseal closure
Adult bone retains the epiphyseal line
Figure 6–9 (Step 6)

58. Endochondral Ossification
Appositional growth:
compact bone thickens and strengthens long bone with layers of circumferential lamellae
Figure 6–9 (Step 2)

59. During intramembranous ossification, which type(s) of tissue is/are replaced by bone?
hyaline cartilage
fibrous connective tissue
mesenchymal connective tissue
osteoid tissue

60. In endochondral ossification, what is the original source of osteoblasts?
de novo synthesis
cells brought with via the nutrient artery
cells of the inner layer of the perichondrium
chondrocytes from the original model

61. The characteristics of adult bones.

62. Epiphyseal Lines
Figure 6–10

63. Epiphyseal Lines When long bone stops growing, after puberty:
epiphyseal cartilage disappears
is visible on X-rays as an epiphyseal line

64. A child who enters puberty several years later than the average age is generally taller than average as an adult. Why?
Epiphyseal plates fuse during puberty.
Bone growth continues throughout childhood.
Growth spurts usually occur at the onset of puberty.
All of the above.

65. The skeletal system remodels and maintains homeostasis
The skeletal system remodels and maintains homeostasis. The effects of nutrition, hormones, exercise, and aging on bone.

66. Bone Remodeling Bones are not static: constantly recycled and renewed
5-7% of skeleton is recycled/week
Osteoclasts secrete:
Lysosomal enzymes: digest osteoid
Hydrochloric acid: solubilize calcium salts
Osteoblasts secrete:
Osteoid (organic matrix)
Alkaline phosphatase: induces mineralization of osteoid
- Complete mineralization takes ~1 week

67. Bone Remodeling Bones Adapt:
Stressed bones grow thicker
Bumps and ridges for muscle attachment enlarge when muscles are used heavily
Bones weaken with inactivity: up to 1/3 or mass is lost with few weeks of inactivity
Heavy metals can get incorporated
Condition of bones depends on interplay between osteoclast and osteoblast activity

68. Skeleton as a Calcium Reserve
Calcium is important for normal function of neurons and muscle
Blood calcium: 9-11 mg/100ml
If blood levels are too high:
Nerve and muscle cells are non responsive
If blood levels are too low:
Nerve and muscle cells are hyper-excitable  convulsions, death

69. The Skeleton as Calcium Reserve
Bones store calcium and other minerals
Calcium is the most abundant mineral in the body
Calcium ions are vital to:
membranes
neurons
muscle cells, especially heart cells

70. Skeleton as a Calcium Reserve
Calcium homeostasis depends on:
Storage in the Bones
Absorption in the GI
Excretion at the Kidneys
** These factors are controlled by hormones to regulate blood calcium levels

71. If blood calcium levels Low:
Parathyroid hormone (from parathyroid gland) triggers:
Increase osteoclast activity
- decrease storage
Enhanced calcitriol action
- increase absorption
Decreased calcium excretion at the kidneys

72. If Blood Calcium levels High
Calcitonin (from thyroid gland) triggers:
Inhibition of osteoclast activity
Increased calcium excretion at the kidneys

73. Nutritional and Hormone Effects on Bone
Many nutrients and hormones are required for normal bone growth and maintenance:
Calcium and phosphate salts
Calcitriol
Vitamin C
Vitamin A
Vitamin K and B12
Growth Hormones
Thyroxin
Estrogens and Androgens
Calcitonin
Parathyroid Hormone

74. Nutritional and Hormone Effects on Bone
Calcium and phosphate salts
- From food, for mineralization of matrix
Calcitriol
- From kidneys, for absorption of calcium and phosphate
Vitamin C
- From food, for collagen synthesis and osteoblast differentiation
Vitamin A
- From carotene in food, for normal bone growth in children
Vitamin K and B12
- From food, for synthesis of osteoid proteins

75. Nutritional and Hormone Effects on Bone
Growth Hormones
- From pituitary gland, for protein synthesis and cell growth
Thyroxin
- From thyroid gland, for cell metabolism and osteoblast activity
Estrogens and Androgens
- From gonads, for epiphyseal closure
Calcitonin
- From thyroid gland AND
Parathyroid Hormone
From parathyroid gland, to regulate calcium and phosphate levels in body fluids
Affects bone composition

76. Hormones for Bone Growth and Maintenance
Table 6–2

77. Abnormalities Genetic/Physiological Abnormalities 1. Giantism:
too much Growth hormone prior to epiphyseal closure, bones grow excessively large
2. Acromegaly:
- too much GH after closure, bones don’t
grow but all cartilage does
- ribs, nose, ears, articular cartilage
3. Pituitary Dwarfism:
- not enough GH, bones fail to elongate

78. Abnormalities Diet Related Abnormalities: 1. Scurvy: - lack of Vit. C
- causes low collagen content, reduced bone
mass, bones brittle
2. Osteomalacia:
- lack calcitriol, osteoid produced but
not mineralized, bones flexible
-Called Rickets in children and leads to
permanent deformity

79. The individual will be taller. The individual will be shorter.
A seven-year-old child has a pituitary tumor involving the cells that secrete growth hormone (GH), resulting in increased levels of GH. How will this condition affect the child’s growth?
The individual will be taller.
The individual will be shorter.
Growth of the individual will be erratic and slow.
Excessive growth will be limited to axial skeleton.

80. Why does a child who has rickets have difficulty walking?
Joints become fused, preventing movement.
Bones are brittle and break under body weight.
Bones are flexible and bend under body weight.
Motor skills are impaired.

81. What effect would increased PTH secretion have on blood calcium levels?
higher level of calcium
lower level of calcium
uncontrolled level of calcium
no effect on blood calcium, PTH effects calcium in the bones

82. How does calcitonin help lower the calcium ion concentration of blood?
by inhibiting osteoclast activity
by increasing the rate of calcium excretion at the kidneys
by increasing the rate of calcium uptake by intestinal cells
1 and 2

83. Types of fractures and how do they heal.

84. Fractures Fractures: Bones break in response to excessive stress
cracks or breaks in bones
caused by physical stress
Bones break in response to excessive stress
Bones are designed to heal
Fractures are repaired in 4 steps

85. Fracture Repair: Step 1 Bleeding: produces a clot (fracture hematoma)
Seals off dead osteocytes and broken blood vessels
Figure 6–15 (Step 1)

86. Fracture Repair: Step 2 Cells of the endosteum and periosteum:
Divide and migrate into fracture zone
Cells of Periosteum:
create external callus of fibrocartilage
Cells of Endosteum:
create internal callus of spongy bone
Calluses stabilize the break:
external callus of cartilage and bone surrounds break
internal callus develops in marrow cavity
Figure 6–15 (Step 2)

87. Fracture Repair: Step 3 Osteoblasts:
replace cartilage with spongy bone
Fracture gap is now filled with all spongy bone
Figure 6–15 (Step 3)

88. Fracture Repair: Step 4
A bulge from the callus marks the fracture point
Osteoblasts and osteocytes remodel the fracture for up to a year:
Spongy bone is replaced with compact bone and excess callus material is removed
Figure 6–15 (Step 4)

89. The effects of aging on the skeletal system.

90. Effects of Aging Bones become thinner and weaker with age
1. Osteopenia = reduction in bone mass
All adults suffer in some degree
Osteoclasts out-work osteoblast
sex hormones in youth inhibit osteoclasts
Women: 8%/decade after 40
Men: 3%/decade after 40

91. Effects of Aging
2. Osteoporosis = reduction in bone mass that compromises function
More common in women:
Over age 45, occurs in:
29% of women
18% of men
Thinner bones to start
Greater rate of osteopenia

92. (b) Spongy bone in osteoporosis
(a) Normal spongy bone
SEM X 25
(b) Spongy bone in osteoporosis
SEM X 21

93. Effects of Bone Loss
The epiphyses, vertebrae, and jaws are most affected:
resulting in fragile limbs
reduction in height
tooth loss

94. Hormones and Bone Loss Estrogens and androgens help maintain bone mass
Bone loss in women accelerates after menopause

95. Why is osteoporosis more common in older women than in older men?
Testosterone levels decline in post-menopausal women.
Older women tend to be more sedentary than older men.
Declining estrogen levels lead to decreased calcium deposition.
In males, androgens increase with age.

96. SUMMARY (1 of 2) Bone shapes, markings, and structure
The matrix of osseous tissue
Types of bone cells
The structures of compact bone
The structures of spongy bone
The periosteum and endosteum
Ossification and calcification
Intramembranous ossification
Endochondrial ossification

97. SUMMARY (2 of 2) Blood and nerve supplies
Bone minerals, recycling, and remodeling
The effects of exercise
Hormones and nutrition
Calcium storage
Fracture repair
The effects of aging