Bone Tissue Flashcards
1
Q
Bone tissue
A
Bone tissue
- Functions:
- provide solid support for the body
- protects vital organs
- encloses internal cavities containing bone marrow
- reservoir of Ca2+, phosphate and other ions
- system of levers
- Specialized CT composed of bone matrix
2
Q
Endosteum & Periosteum
A
Endosteum- lines the internal surface surrounding the marrow cavity
- covers small trabeculae of bony matrix that project into marrow cavities
- also contains: osteoprogenitor cells, osteoblasts (for bone growth), & bone lining cells
- sparse delicate matrix of collagen fibers
Periosteum- lines the external surface
- outer fibrous layer of dense connective tissue
- type 1 collagen, fibroblast, blood vessels
- Perforating or Sharpey’s fibers- periosteal collagen that penetrate the bone matrix and bind the periosteum to the bone
- Inner layer (more cellular): osteoblasts, bone lining cells, osteoprogenitor cells (mesenchymal for growth & repair)
3
Q
3 major cell types of bone tissue
A
3 major cell types of bone tissue:
-
Osteocytes
* found in lacunae between lamellae with cytoplasmic processes in canaliculi - Osteblast
- growing cells
- secrete osteoids (mineralized vesicles)
- Osteoclasts
- giant mulitnucleated cells
- involved in removing calcified bone matrix & remodeling bone tissue
4
Q
Osteoblasts
A
Osteoblasts
- originates from mesenchymal stem cells
- produce organic compounds of bone matrix including Type 1 collagen fibers, proteoglycans, matricellular glycoproteins (osteonectin)
- active osteoblasts
- located exclusively at surface of bone matrix
- bound by integrins
- joined by adherent & gap junctions
- differentiate as osteocytes
- Prominent among the noncollagen proteins secreted by osteoblast is osteocalcin (Vit K-dependent)
- binds Ca2+ & concentrate mineral locally
- release membrane-enclosed matrix vesicles rich in ALP
- raise PO43
- Hydroxyapatite [Ca10(PO4)6(OH)2]- 1st step in calcification
- Osteosarcoma- bone cancer arising in osteoprogenitor cells
- Skeleton is often site of secondary metastatic tumors (cancer via small blood or lymph vessels from breast, lung, prostate, thyroid)
5
Q
Osteocytes
A
Osteocytes
- communicate with one another & ultimately with nearby osteoblasts & bone lining cells via gap junctions at the end of their processes
- normally the most abundant
- almond-shaped
- exhibit less RER, smaller Golgi, & more condensed nuclear chromatin
- maintain calcified matrix
- death is followed by rapid matrix resorption
- lacunar-canalicular network serve as sensitive detectors of stress or fatigue-induced microdamage in bone
- Mechanostat
- network of dendritic processes that signal cells to adjust ion levels & maintain adjacent bone matrix
- lack of exercise (or weightlessness by astronauts) decrease bone density d/t lack of mechanical stimulation of osteocytes
6
Q
Osteoclasts
A
Osteoclasts
- large, motile cells with multiple nuclei (d/t origin from fusion of bone marrow derived monocytes)
- essential for matrix resoprtion during bone growth & remodeling
- development requires 2 polypeptides produced by osteoblasts:
- macrophage-colony stimulating factor (M-CSF)
- receptor activator of nuclear factor-kB ligand (RANKL)
- lie in matirx known as resorption lacunae (Howhsip lacunae)
- Active osteoclast:
- sealing zone (bind cells to bone matrix)
- ruffled border (surface projections)
- pump proton (H+) to acidify & dissolve hydroxyapatite
- release matrix metalloproteinases & other hydrolytic enzymes for localized digestion of matrix proteins
-
Osteopetrosis
- heavy bones (marble bones)- obliterates marrow activities- leading to anemia
- osteoclasts lack ruffle borders & bone resorption is defective
- mutations in gene for cells’ proton ATPase pumps on Cl channels
-
Osteoporosis
- bone resorption exceeds bone formation
- found in immobilized patients & post-menopausal women
- Ca2+ loss from bones reduced bone mineral density (BMD)
- Dual-energy X-ray Absorptiometry- routine scan for BMD
7
Q
Bone matrix
A
Bone matrix
- inorganic (no carbon)
- Calcium hydroxyapatite [Ca10(PO4)6(OH)2] is most abundant
- bicarbonate, citrate, Mg2+, K+, Na+ are also found
- organic (with carbon)
- 90% Type 1 collagen, small proteglycans, multiadhesive glycoproteins (osteonectin)
- Osteocalcin & Phosphatases promote calcification
- decalcified bone matrix is acidophilic d/t increase collagen content
8
Q
Types of Bone
A
Types of Bone:
- Compact (cortical bone)
- 80% of all lamellar bone
- parallel lamellae or densely packed osteons with interstitial lamellae
- located at thick, outer region (beneath periosteum) of bones
- Cancellous (spongy bone, trabecular bone, medullary bone)
- 20% of all lamellar bone
- interconnected thin spicules or trabeculae covered by endostem
- found in inner region of bones, adjacent to marrow cavities
9
Q
Short bones and Flat bones
A
Short bones
- wrist & ankle
- cores of cancellous bone
- surrounded by compact bone
Flat bones
- calvaria (skull cap)
- Plates- 2 layers of compact bone
- Diploe- separates plates with cancellous bone
10
Q
2 types of bone organization
A
2 types of bone organization:
- Lamellar bone/Mature/Secondary bone
- characterized by multiple layers of lamellae of calcified matrix
- Heavily calcified
- Not all lamellar bones have osteons
- All normal regions of adult bones
-
Type 1 collagen fibers are aligned in each lamellae
- oriented orthogonally in successive lamellae
- orientation adds strength of lamellar bone
-
Osteon (Haversian system)
- complex of concentric lamellae (5-20)
- surrounds central canal (contains blood vessels, nerves, & endosteum
-
Lacunae
- found between successive lamellae
- each one with one osteocyte all interconnected by the canaliculi containing the cells’ dendritic processes.
- processes are in contact with gap junctions
- Cement line- outer boundary of each osteon
- Perforating canals (Volkmann canals)- transvers canals to communicate different canals
-
Interstitial lamellae
- scattered among the intact osteons
- lamellae remaining from osteons destroyed by osteoclasts
- External circumferential lamellae- beneath the periosteum
- Internal circumferential lamellae- around the marrow cavity which encloses & strengthens the middle region containing osteons
- Woven bone/Immature/Primary bone
- nonlamellar
- random distribution of Type 1 collagen fibers
- 1st bone to appear in embryonic development & fracture repair
- temporary & replaced in adults by lamellar bone except in sutures of calvaria & insertions of tendons
- lower mineral content
- higher proportion of osteocytes
11
Q
Bone remodeling
A
Bone remodeling
- occurs througout life
- resorbs part of old osteons & produce new ones in compact bone
- process:
- osteoclasts remove old bone
- small tunnel-like cavities are formed
- tunnels are quickly invaded by osteoprogenitor cells
- osteoblasts develop, line wall of tunnels, secrete osteoid
- forms new osteon (concentric lamellae and trapped osteocytes)
- 5-10% of the bone turns over annually (200X faster in active young children)
12
Q
Osteomalacia & Osteitis fibrosa cystica
A
Diagnosis on the conditions below can be aided with antibiotic tetracycline in which fluorescent lamellae can be seen on bone formed
Osteomalacia - condition in which bone mineralization is impaired
Osteitis fibrosa cystica- increased osteoclast activity resulting in removal of bone matrix & fibrous degeneration
13
Q
2 process of Osteogenesis
A
2 process of Osteogenesis
- Intramembranous ossification (fetus before 8 weeks)
- Osteoblast differentiate directly from mesenchyme & begin secreting osteoid
- which most flat bones (skull, jaws, scapula, clavicle) begin to form
- Ossification centers:
- Osteoprogenitor cells arise
- proliferare
- form incomplete layers of osteoblasts around a network of developing capillaries
- Osteoid secreted calcifies forming woven bone with osteocytes in lacunae & cannaliculi
- continued matrix secretion & calcification enlarges areas
- fusion of neighboring ossification centers
- woven bone is replaced by compact bone (encloses cancellous bone)
- Mesenchymal regions that do not undergo ossifcation give rise to endosteum & periosteum
- Endochondral ossification (8 weeks or 2 months)
- preexisting matrix of hyaline cartilage is eroded & invaded by osteoblasts (begins osteoid production)
- forms most bones of the body
- Appositional growth
- Primary ossification center forms in diaphysis (1st trimester):
- begins with hyaline cartilage as a mold
- Bone collar produced by osteoblasts differentiate within the perichondrium (transitioning to periosteum) around the cartilage model’s diaphysis
- the collar impedes diffusion of O2 & nutrients causing local chondrocytes to swell up (hypertrophy) releasing osteocalcin & ALP
- the hypertrophic chondrocytes eventually die creating empty spaces within the calcified matrix
- Blood vessel from the perichondrium (now periosteum) penetrate the bone collar bringing osteoprogenitor cells to the porous central region
- Osteoblasts move into available spaces & produce woven bone
- Secondary ossification centers form in epiphyses (birth):
- Bone replaces cartilage except the articular cartilage & epiphyseal plates
- Epiphyseal plate separates Primary & Secondary ossification centers for continued bone elongation (until 20 yrs old)
- Epiphyseal plates ossify & form epiphyseal lines (epihyseal closure
- Articular cartilage remain
14
Q
Osteogenesis imperfecta
A
Osteogenesis imperfecta
- brittle bone disease
- osteoblasts produce deficient amounts of Tyoe 1 collagen or defective Type 1 collagen mutation
- fragility of bones
15
Q
Distinct zones of Epiphyseal growth plate
A
Distinct zones of Epiphyseal growth plate:
-
Zone of Reserve (or Resting)
* composed of hyaline cartilage - Zone of Proliferation
- cartilage cells divide repeatedly
- cells enlarge & secrete more Type II collagen & proteoglycan
- cells are organized into columns
- Zone of Hypertrophy
- swollen chondrocytes
- compressed matrix into aligned spicules
- stiffens matrix by secretion of type X collagen which limits diffusion in the matrix and with growth factors promotes vascularization from the adjacent primary ossification center
- Zone of Calcified Cartilage
- chondrocytes about to undergo apoptosis release matrix vesicles & osteocalcin to begin matrix calcification by the formation of hydroxyapatite crystals
- chondrocytes have disappeared, hence the white blank color
- Zone of Ossification
- bone tissue appears first
- capillaries & osteoprogenitor cells invade
- Osteoblasts secrete osteoid which becomes woven bone & remoldeled to lamellar bone
18
Q
Bone Repair
A
Bone Repair
- (with periosteum in place) Clotted blood is removed by macrophages
- a fibrocartilaginous (soft) callus forms
- invaded by regenerating blood vessels & proliferating osteoblasts
- fibrocartilage is replaced with a hard (bony) callus
- Bone is remodeled
