Chapter 6:

Osseous Tissue and Bone Structure

1

The Skeletal System

• Skeletal system includes:– bones of the skeleton– cartilages, ligaments, and other

connective tissues that stabilize the bones

2

Skeletal System

Functions:1. Support: framework & structure of body2. Storage of minerals and lipids

Minerals: calcium and phosphate - for osmotic regulation, enzyme

function, nerve impulses

Yellow marrow: triglycerides3. Blood cell production: all formed elements - red marrow: stem cells hematopiesis4. Protection: surround soft tissues

5. Leverage for movement:- levers upon which skeletal muscles act3

Classification of Bones

• Bone are identified by:– shape– internal tissues– bone markings

SHAPE:1. Long bones2. Flat bones3. Sutural bones4. Irregular bones5. Short bones6. Sesamoid bones

4

Shape of Bones

1. Long Bones:- Longer than wide, consist of

shaft and 2 ends - e.g. bones of appendages

2. Short Bones:- Approx. equal in all dimensions- e.g. carpals, tarsals

3. Flat Bones:- Thin, 2 parallel surfaces - e.g. skull, sternum, ribs,

scapula

Figure 6–1a5

Shape of Bones

4. Irregular Bones:- Complex shapes- E.g. vertebrae, os coxa

5. Sesamoid Bones:- Seed shaped, form in tendon- E.g. patella, total number can

vary

6. Sutural Bones:- Extra bones in sutures of skull

6

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 tissue1. Compact bone:

- solid, dense bone, makes up surfaces and shafts

2. Spongy Bone/Cancellous bone:- meshy, makes up interior of bones, houses

red marrow in spaces7

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 8

Bone Markings

Table 6–1 (2 of 2)9

Long Bones Structure

1. Diaphysis:- Hollow shaft of compact bone

2. Medullary (marrow) cavity:- Center of diaphysis, contains

yellow marrow- Triglycerides for energy reserve

3. Epiphysis:- Expanded end of bone, surface

of compact bone- Center filled with spongy bone

with red marrow in spaces- Produces blood cells

Figure 6–2a10

Long Bones Structure

4. Epiphyseal line or plate:- Cartilage that marks connection of

diaphysis with epiphysis- Line: adults, narrow (aka metaphysis)- Plate: thick, allows growth during

childhood

5. Periosteum:- 2 layer covering around outside of bone:

- Outer Fibrous Layer- Inner Cellular Layer

6. Endosteum:- Cellular layers, covers all inside surfaces

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12

7. Articular Cartilage:- 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

13

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

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Characteristics of Bone Tissue

• Periosteum: – covers outer surfaces of bones – consist of outer fibrous and inner

cellular layers

• Endosteum:– Inner, cellular layer of periosteum

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Bone Histology

• Bone = osseous tissue, supporting CT• Consists of specialized cells in a

matrix of fibers and ground substance• Characteristics of bone:

1. Dense matrix packed with calcium salts2. Osteocytes in lacunae3. Canaliculi for exchange of nutrients and

waste4. Two layer periosteum, covers bone except

at articular surfaces 16

Bone Histology

• Matrix = 98% of bone tissue– 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 bone1. Osteocytes2. Osteoblasts3. Osteoprogenitor cells4. Osteoclasts

17

Cells located in Bones1. 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 matrix2. Can also participate in bone repair:

-become stem cell like when broken free of lacuna

18

LM X 362

Canaliculi

Osteocytesin lacunae

Matrix

Bloodvessels

Central canal

PERIOSTEUM

Fibrouslayer

Cellularlayer

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Cells located in Bones

2. 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 20

21

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

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23

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

remodeling24

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Cells located in Bones

Osteocyte: Mature bonecell that maintains thebone 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 MatrixMatrix

Marrow cavity

Osteoprogenitorcell OsteoclastCanaliculiOsteocyte

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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

27

How would the strength of a bone be affected if the ratio of

collagen to hydroxyapatite increased?

1. Strength increases, flexibility increases.

2. Strength increases, flexibility decreases.

3. Strength decreases, flexibility. decreases.

4. Strength decreases, flexibility increases. 28

If the activity of osteoclasts exceeds the activity of osteoblasts in a bone,

how will the mass of the bone be affected?

1. stable mass, but re-positioned matrix

2. mass will not be affected3. more mass4. less mass

29

The difference between compact bone

and spongy bone.

30

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

31

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 32

Compact Bone

Figure 6–533

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 34

Bone Marrow

• Red 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

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Structure of Spongy Bone

36

Periosteum and Endosteum

• Compact bone is covered with membrane:– periosteum on the outside – endosteum on the inside

37

Periosteum

1. Fibrous outer layer:- Dense irregular CT

2. Cellular Inner layer:- Osteoprogenitor cells

Functions:1. Isolate bone from surrounding tissues2. Site for attachment for tendons and

ligaments3. Route for nerves and blood vessels to enter

bone4. Participates in bone growth and repair 38

39

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

40

Endosteum

Figure 6–8b41

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:

1. Intramembranous: forms flat bones2. Endochondrial: forms long bones 42

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

43

The difference between intramembranous ossification and

endochondral ossification.

44

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

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Intramembranous Ossification: Step 1

• Ossification center appears in the fibrous CT membrane– Mesenchymal cells aggregate– Differentiate into osteoblasts– Begin ossification at the ossification center

46

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

47

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

48

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

49

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

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Endochondral Ossification

• Growth and ossification of long bones occurs in 6 steps

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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)52

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)53

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)54

Endochondral Ossification: Step 4

Figure 6–9 (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

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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)56

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)57

• Appositional growth:– compact bone thickens

and strengthens long bone with layers of circumferential lamellae

Figure 6–9 (Step 2)

Endochondral Ossification

58

During intramembranous ossification, which type(s) of

tissue is/are replaced by bone?

1. hyaline cartilage2. fibrous connective tissue3. mesenchymal connective

tissue4. osteoid tissue

59

In endochondral ossification, what is the original source of osteoblasts?

1. de novo synthesis2. cells brought with via the

nutrient artery3. cells of the inner layer of

the perichondrium4. chondrocytes from the

original model60

The characteristics of adult bones.

61

Epiphyseal Lines

Figure 6–1062

Epiphyseal Lines

• When long bone stops growing, after puberty:– epiphyseal cartilage disappears – is visible on X-rays as an epiphyseal

line

63

A child who enters puberty several years later than the average age

is generally taller than average as an adult. Why?

1. Epiphyseal plates fuse during puberty.

2. Bone growth continues throughout childhood.

3. Growth spurts usually occur at the onset of puberty.

4. All of the above.64

The skeletal system remodels and maintains

homeostasis.

The effects of nutrition, hormones, exercise, and

aging on bone.65

Bone Remodeling

• Bones are not static: constantly recycled and renewed

• 5-7% of skeleton is recycled/week• Osteoclasts secrete:

1. Lysosomal enzymes: digest osteoid2. Hydrochloric acid: solubilize calcium salts

• Osteoblasts secrete:1. Osteoid (organic matrix)2. Alkaline phosphatase: induces

mineralization of osteoid- Complete mineralization takes ~1 week 66

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 67

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, death68

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

69

Skeleton as a Calcium Reserve

• Calcium homeostasis depends on:1. Storage in the Bones2. Absorption in the GI3. Excretion at the Kidneys

** These factors are controlled by hormones to regulate blood calcium levels

70

If blood calcium levels Low:

• Parathyroid hormone (from parathyroid gland) triggers:

1. Increase osteoclast activity - decrease storage

2. Enhanced calcitriol action - increase absorption

3. Decreased calcium excretion at the kidneys 71

If Blood Calcium levels High

• Calcitonin (from thyroid gland) triggers:

1. Inhibition of osteoclast activity

2. Increased calcium excretion at the kidneys

72

Nutritional and Hormone Effects on Bone

• Many nutrients and hormones are required for normal bone growth and maintenance:

1. Calcium and phosphate salts2. Calcitriol3. Vitamin C4. Vitamin A5. Vitamin K and B126. Growth Hormones7. Thyroxin8. Estrogens and Androgens9. Calcitonin10. Parathyroid Hormone 73

Nutritional and Hormone Effects on Bone

1. Calcium and phosphate salts- From food, for mineralization of matrix

2. Calcitriol- From kidneys, for absorption of calcium and

phosphate

3. Vitamin C- From food, for collagen synthesis and osteoblast

differentiation

4. Vitamin A- From carotene in food, for normal bone growth in

children

5. Vitamin K and B12- From food, for synthesis of osteoid proteins

74

Nutritional and Hormone Effects on Bone

6. Growth Hormones- From pituitary gland, for protein synthesis and cell

growth7. Thyroxin

- From thyroid gland, for cell metabolism and osteoblast activity

8. Estrogens and Androgens- From gonads, for epiphyseal closure

9. Calcitonin- From thyroid gland AND

10. Parathyroid Hormone- From parathyroid gland, to regulate calcium and

phosphate levels in body fluids- Affects bone composition 75

Hormones for Bone Growth and Maintenance

Table 6–276

Abnormalities

• Genetic/Physiological Abnormalities1. 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

77

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 deformity78

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?

1. The individual will be taller.2. The individual will be

shorter.3. Growth of the individual will

be erratic and slow.4. Excessive growth will be

limited to axial skeleton.79

Why does a child who has rickets have difficulty walking?

1. Joints become fused, preventing movement.

2. Bones are brittle and break under body weight.

3. Bones are flexible and bend under body weight.

4. Motor skills are impaired.80

What effect would increased PTH secretion have on blood calcium

levels?

1. higher level of calcium2. lower level of calcium3. uncontrolled level of

calcium4. no effect on blood calcium,

PTH effects calcium in the bones

81

How does calcitonin help lower the calcium ion concentration of

blood?

1. by inhibiting osteoclast activity

2. by increasing the rate of calcium excretion at the kidneys

3. by increasing the rate of calcium uptake by intestinal cells

4. 1 and 2

82

Types of fractures and how do they heal.

83

Fractures

• Fractures:– 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

84

Fracture Repair: Step 1

• Bleeding:– produces a clot (fracture

hematoma)– Seals off dead

osteocytes and broken blood vessels

Figure 6–15 (Step 1)85

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)86

Fracture Repair: Step 3

• Osteoblasts:– replace cartilage with

spongy bone

• Fracture gap is now filled with all spongy bone

Figure 6–15 (Step 3)87

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)88

The effects of aging on the skeletal system.

89

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

90

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

91

(a) Normal spongy bone SEM X 25

(b) Spongy bone in osteoporosis SEM X 21

92

Effects of Bone Loss

• The epiphyses, vertebrae, and jaws are most affected:– resulting in fragile limbs– reduction in height– tooth loss

93

Hormones and Bone Loss

• Estrogens and androgens help maintain bone mass

• Bone loss in women accelerates after menopause

94

Why is osteoporosis more common in older women than in

older men?

1. Testosterone levels decline in post-menopausal women.

2. Older women tend to be more sedentary than older men.

3. Declining estrogen levels lead to decreased calcium deposition.

4. In males, androgens increase with age.

95

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

96

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

97