Bone

Question 1

Specialized Connective Tissue with mineralized extracellular matrix

Answer 1

Roles:

• support body mass
• facilitate movement
• protect vital organs
• site of hematopoiesis
• calcium reservoir

Question 2

Bone Compared to cartilage

Answer 2

Similarities:
- Hard Tissues
- Contain living cells embedded in matrix (lacunae)
- Common mesenchymal progenitor cells
Differences:
- bone heavily vascularized/ cartilage avascular
- Bone access to blood vessels via canaliculi
- Cartilage less calcified, uses long range diffusion

Question 3

How mechanical stress affects bone structure

Answer 3

• compression and movement required for proper bone remodeling
• plasticity used by orthodontists to modify position of teeth in jaw
• loss of bone during space flight or immobilization
• Piezoelectric potential

Question 4

Piezoelectric potential

Answer 4

• since bone is crystalized, when you cause stress to it, it will generate an electric potential
• negative, bone deposition
• positive, bone reabsorbrion

Question 5

Non Cellular Bone Matrix: Organic Components

Answer 5

• 25% of total bone mass, mostly fibrous

Question 6

Fibers

Answer 6

• Non cellular, Organic component of bone
• Collagen type I
• provides elasticity, tensile strength

Question 7

Ground Substance (amorphous)

Answer 7

• Non cellular, organic component
• Glycosaminoglycans (chondroitin and Keratan sulfates)
• Glycoproteins
• Osteoid- newly secreted organic matrix ( not yet fully calcified)

Question 8

Osteonectin and osteopontin

Answer 8

• Glycoproteins

- anchors minerals to collagen, initiate mineralizaiton and promote crystal formation

Question 9

Osteocalcin and Bone Sialoprotein

Answer 9

• calcium binding proteins

Question 10

Non Cellular Inorganic Components of bone

Answer 10

• 50-70% of bone mass
• provide compressive strength
• Mainly salts of calcium phosphate in amorphous or crystalline form
• Crystals- Hydroxyapatite
• Water represents about 15% of bone mass
• Hydration shell around hydroxyapatite crystals facilitate calcium exchange with fluids

Question 11

Decalcification

Answer 11

• Flexible tissue

- Only the organic parts are left

Question 12

Grinding

Answer 12

• Grind down to the inorganic crystals

- translucent ground sections

Question 13

Mesenchymal Osteoprogenitor

Answer 13

• Osteogenic cell (stem cell)
• committed mesenchymal cell
• Commitment is controlled by Bone Morphogenic proteins
• Located in inner and outer linings of bone
• can self renew or differentiate
• Self renewal: PDGF, TGFB, IGF
• Differentiation: BMP, VitD3

Question 14

Osteoblast

Answer 14

• Bone forming cell
• non dividing cell
• located on bone matrix surface
• secrete bone matrix (OSTEOID)
• High secretory activity shown by abundant RER, Golgi
• Secretion activated by GH, sex steroids
• Deposition of osteoid between osteoblast cell layer and existing bone
• Secrete factors that promote osteoclast activity

Question 15

Osteocyte

Answer 15

• Terminally differentiated cell
• Osteoblast that becomes trapped in osteoid
• in spaces called lacunae
• extend filopodia (skinny microvilli) in canaliculi (canals connecting cells- gap junctions)

Question 16

Osteocytic Osteolysis (

Answer 16

• limited calcium release inside lacunae
• Parathyroid hormone (PTH) increases resorption (chief cells of parathyroid gland)
• Calcitonin decreases resorption (parafollicular cells of thyroid gland)

Question 17

Osteoclast

Answer 17

• Bone Resorbing Cell
• Macrophage/monocyte lineage
• multinucleated: fusion of monocytes promoted by Vitamin D
• large, non dividing, mobile
• on bone resoprtive and free surfaces in small cavitations called Howships Lacunae
• Has a ruffled border

Question 18

Ruffled Border

Answer 18

• Bone Resorption zone inside the Howship’s lacunae
• cytoplasmic processes
• surrounded by a ring shape sealing zone (clear zone)
• Increases surface area for resorption of bone

Question 19

Osteoclastic Osteolysis

Answer 19

= bone resoprtion
1)Focal decalcification by acidification (citric acid release, carbonic anhydrase)
2)Extracellular digestion by hydrolytic enzymes (collagenase, acid phosphatase, sulfatase) for proteolysis
- Required for bone remodeling and repair
Regulated by:
- PTH(via osteoblasts): increases resorption
- Calcitonin, estrogens: decreases resorption

Question 20

Periosteum

Answer 20

• Outer surfaces
• Tough connective tissue membrane
• Covers bone outer surface (except articular and tendon insertion surfaces)
• Fibrous periosteum (FP)- outer fibrous layer, highly vasularized
• Osteogenic periosteum (OP)- inner cellular layer (osteogenic cells, osteoblasts)
• Attached to bone by collagen fiber bundles- Sharpey’s fibers
• Point of origin of Volkmann’s canals containing blood vessels

Question 21

Endosteum

Answer 21

• inner surfaces
• Thin single cell layer (progenitor cells, osteoblasts, and osteoclasts)
• Lines bone internal surfaces (trabeculae, haversian canals)
• Important for bone nutrition and maintenance

Question 22

Role of Periosteum and endosteum in bone growth, repair and remodeling

Answer 22

• Both contain osteogenic cells
• cells that divide (mesenchymal)
• Cells that can give rise to either osteoblast or chondroblast in function of environment and vasularization
• commintment is dependent on the presence or absence of blood vessels in the area

Question 23

Bone Proper

Answer 23

Organization of collagen fibers in matrix determine the types of bone:
- Woven and Lamellar

Question 24

Woven

Answer 24

• Primary/ immature bone
• Random disposition of collagen fibrils, amorphous calcium phosphate
• in embryonic development and bone repair

Question 25

Lamellar

Answer 25

- Secondary/ mature bone - Organized disposition of collagen fibers (lamellae), crystalline calcium phosphate - Cancellous (spongy) - Compact (cortical) - comes from remodeling of woven bone - Adult bone

Question 26

Cancellous (Spongy) Bone

Answer 26

- Network of irregular lamellae: Trabeculae (spicules) - Farge spaces filled with bone marrow - Mostly converted to compact bone - Exceptions: flat bones, alveolar bone around teeth, short bones, epiphyses and diaphysis of long bones

Question 27

Anastomose

Answer 27

- fusion of trabeculae trapping blood and lymphatic vessels inside canals

Question 28

Compact (Cortical) Bone

Answer 28

``` - Forms diaphysis of long bones, found as thin layer around epiphyses, forms the tables of skull flat bones Highly organized bone lamellae: - Circumferential (outer and inner) - Harversian systems (osteons) - Interstitial (areas between osteons) ```

Question 29

Haversian system

Answer 29

- cylindrical columns of 4-15 concentric lamellae surrounding a canal called haversian canal containing blood and lymphatic vessels and nerves - osteons

Question 30

Volkmann's Canals

Answer 30

- Nutrient canals running at right anlge to the Haversian canals - contain blood vessels and nerves of bones

Question 31

Mechanism of Calcification

Answer 31

1) Osteoblast secretes the new organic matrix- osteoid 2) Osteoblast secrete factors allowing the subsequent mineralization of the matrix 3) osteoblasts become trapped in their own mineralized matrix: become osteocytes residing in lacunae 4) Osteocyte keeps a limited ability to secrete and reabsorb bone in its lacuna, allowing some bone turnover to occur

Question 32

Alkaline Phosphatase

Answer 32

- Secreted by osteoblasts - Promtes accumulation of inorganic phosphate in the osteoid, this is a point of saturation where calcium phosphate precipitates

Question 33

Calcium Binding Proteins

Answer 33

- secreted by osteoblasts in the matrix - High affinity - osteonectin, osteocalcin

Question 34

Matrix vesicles

Answer 34

- Secreted by osteoblasts in the matrix | - contain pyrophosphate and enzymes that inhibit calcification and allow phosphate release

Question 35

Balance between bone deposition and resorption

Answer 35

- bone deposition- ensure high vascularity - trapping of new vessels during bone deposition and anastamose of trabeculae - Bone resorption- allows formation of canals/ marrow cavity - required to maintain shape in growing bones - Allows bone response to mechanical stress - Osteons constantly remodeled - Help maintain calcium homeostasis

Question 36

Osteoporosis

Answer 36

- resorption--> deposition--> bone mass loss

Question 37

Osteopetrosis

Answer 37

- deposition--> resorption--> bone mass excess