Bone Flashcards
Specialized Connective Tissue with mineralized extracellular matrix
Roles:
- support body mass
- facilitate movement
- protect vital organs
- site of hematopoiesis
- calcium reservoir
Bone Compared to cartilage
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
How mechanical stress affects bone structure
- 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
Piezoelectric potential
- since bone is crystalized, when you cause stress to it, it will generate an electric potential
- negative, bone deposition
- positive, bone reabsorbrion
Non Cellular Bone Matrix: Organic Components
- 25% of total bone mass, mostly fibrous
Fibers
- Non cellular, Organic component of bone
- Collagen type I
- provides elasticity, tensile strength
Ground Substance (amorphous)
- Non cellular, organic component
- Glycosaminoglycans (chondroitin and Keratan sulfates)
- Glycoproteins
- Osteoid- newly secreted organic matrix ( not yet fully calcified)
Osteonectin and osteopontin
- Glycoproteins
- anchors minerals to collagen, initiate mineralizaiton and promote crystal formation
Osteocalcin and Bone Sialoprotein
- calcium binding proteins
Non Cellular Inorganic Components of bone
- 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
Decalcification
- Flexible tissue
- Only the organic parts are left
Grinding
- Grind down to the inorganic crystals
- translucent ground sections
Mesenchymal Osteoprogenitor
- 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
Osteoblast
- 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
Osteocyte
- Terminally differentiated cell
- Osteoblast that becomes trapped in osteoid
- in spaces called lacunae
- extend filopodia (skinny microvilli) in canaliculi (canals connecting cells- gap junctions)
Osteocytic Osteolysis (
- limited calcium release inside lacunae
- Parathyroid hormone (PTH) increases resorption (chief cells of parathyroid gland)
- Calcitonin decreases resorption (parafollicular cells of thyroid gland)
Osteoclast
- 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
Ruffled Border
- 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
Osteoclastic Osteolysis
= 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
Periosteum
- 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
Endosteum
- inner surfaces
- Thin single cell layer (progenitor cells, osteoblasts, and osteoclasts)
- Lines bone internal surfaces (trabeculae, haversian canals)
- Important for bone nutrition and maintenance
Role of Periosteum and endosteum in bone growth, repair and remodeling
- 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
Bone Proper
Organization of collagen fibers in matrix determine the types of bone:
- Woven and Lamellar
Woven
- Primary/ immature bone
- Random disposition of collagen fibrils, amorphous calcium phosphate
- in embryonic development and bone repair
Lamellar
- Secondary/ mature bone
- Organized disposition of collagen fibers (lamellae), crystalline calcium phosphate
- Cancellous (spongy)
- Compact (cortical)
- comes from remodeling of woven bone
- Adult bone
Cancellous (Spongy) Bone
- 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
Anastomose
- fusion of trabeculae trapping blood and lymphatic vessels inside canals
Compact (Cortical) Bone
- 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)
Haversian system
- cylindrical columns of 4-15 concentric lamellae surrounding a canal called haversian canal containing blood and lymphatic vessels and nerves
- osteons
Volkmann’s Canals
- Nutrient canals running at right anlge to the Haversian canals
- contain blood vessels and nerves of bones
Mechanism of Calcification
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
Alkaline Phosphatase
- Secreted by osteoblasts
- Promtes accumulation of inorganic phosphate in the osteoid, this is a point of saturation where calcium phosphate precipitates
Calcium Binding Proteins
- secreted by osteoblasts in the matrix
- High affinity
- osteonectin, osteocalcin
Matrix vesicles
- Secreted by osteoblasts in the matrix
- contain pyrophosphate and enzymes that inhibit calcification and allow phosphate release
Balance between bone deposition and resorption
- 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
Osteoporosis
- resorption–> deposition–> bone mass loss
Osteopetrosis
- deposition–> resorption–> bone mass excess
