Cartilage And Bone

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Cartilage

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resilient, flexible support
avascular
smooth gliding surface for articulatoin
temporary template skeleton (hyaline cartilage is the first layer down and then is eventually replaced by bone)
found in places where you need protection but always if flexiblr

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Components of Cartilage

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cells
EMC
Perichondrium

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Components of Cartilage: Cells

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chondroblasts, chondrocytes

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Components of Cartilage: ECM

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type II collagen, hyaluronic acid and GAGs rich in keratin sulfate and chondroitin sulfate, high water content

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Components of Cartilage: perichondrium

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dense CT on outer surface of hyaline or elastic cartilage, out fibrous layer and inner cellular layer consisting of progenitor cells (contains some stem cells that will give rise to more cartiledge so you can have some repair)

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development of cartiledge

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mesenchymal cells to chondroblasts to secrete matrix constituents, become "trapped" in spaces (lacunae) in matrix and now called chondrocytes

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Growth of Cartilage

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1. Interstitial = growth from within, chondroblasts multiply within lacunae forming isogenous groups and secrete matrix around lacunae

2. appositional = growth on top (on the surface), progenitor cells from perichondrium differentiate into chondroblasts which lay down cartilage matrix on top of exisiting cartilage

early growth is interstital and appositional but later growth and rapair is mainly appositional

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cartilage types

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hyaline
elastic
fibrocartilage

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hyaline

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respiratory passages, joints, ventral ribs, early template skeleton, glistening cartilage, nice smooth gliding surface

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elastic

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epiglottis, larynx, ear, eustachian tube, have collagen type II along with elastic fibers

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fibrocartilage

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IV discs, pubic symphysis, cross between hyaline cartilage and dense regular cartilage

all cartilage contains collagen II fibrils, elastic cartilage also contains elastic fibers, fibrocartilage also contains type I collagen

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Fibrocartilage

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cross between dense CT and hyaline cartilage, contains chondrocytes in lacunae

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bone

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supports fleshy structures
protects vital organs
provides a system of levers which multiplies the forces of muscle contraction
resouvior of minerals
bone marrow - blood cell formation
bone is vascular

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compact bone

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lamellar bone

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spongy bone

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trabecular bone
cancellous

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long bones

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diaphysis and epiphysis

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bone marrow

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red (active) or yellow

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periosteum

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dense CT on outer surface of bones

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endosteum

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CT on the inner surfaces of bone marrow and bone

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osteoblasts

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function in bone deposition, secrete organic matrix components
secrete non-collagenous proteins that regulate osteoclast differentiation and resorption: (indirectly determine bone resorption)
macrophage-stimulating colony, osteoprotegin, RANKL,
ostoponin

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osteocytes

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function in bone maitenance, gap junctions b/w cells

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osteoclasts

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function in bone resorption
derived from monocyte/phagocyte cell lineage. monocytes enter the bone via blood and differentiate into multinucleated osteoclasts
occupy acidic depressions in bone called howship's lacunae

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steps in bone resorption

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osteoclasts recognize and attach to bone,they produce and release acidic vesicles and enzymes that demineralize bone and degrade organic matrix,

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fibers

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collagen type I

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matrix

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glycoproteins and minerals, mainly hydroxyapatite crystals

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specimen preparation
decalcified section
dry ground section

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organic components
inorganic mineral "shell"

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compact bone

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haversian system: haversian canal surrounded by concentric layers of (lamallae) of compact bone
lacunae: spaces in the lamellae, osteocytes sit in them and extend their cytoplasmic processes into the caniculi,
caniculi: look like centipede legs in hte lamellae, face towards where the blood is coming from,
volkmans canal: interconnect haversian's canals
inner and outter circumfrential lamellae

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bone step formation

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1. osteoid deposition - osteoblast secrete organic matrix constituents, the first bone laid during development and fracture repair is woven bone (irregular arrangment of collagen fibers). it is later replaced by mature bone
2. mineralization - matrix vesicles secreted by osteoblasts contain enzymes and proteins that attract calcium and phosphate ions and mediate the formation of hydroxyapatite crystals
3. bone deposition and resorption allows for reshaping or remodeling of bone

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all bone formation is appositional (deposited on the surface) but their are two mechanisms of development:

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intramembraneous - bone develops directly from the mesenchyme
endochondral bone development - long bones, cartilage forms first and is replaced by bone, allows for growth in length

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intramembranous bone development

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mesenchyme to osteblasts to synthesize matrix to mineralization and formation of spicules and other osteoblasts deposit bone on spicules and osteoclasts resorb bone for reshaping

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endochondral bone formation

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in the fetus a temporary skeleton of hyaline cartilage forms first, it is surrounded by dense CT perichondrium
as the hyaline cartilage skeleton develops and grows, bone formation begins in specific areas of cartiledge (ossification centers) the first primary ossification center is the center of the diaphysis, secondary ossification centers develop in the epiphysis
cartilage breaks down and is replaced by bone

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chondrocyte proliferation, cartilage growth

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longitudinal growth at ends of diaphysis, proliferating chondrocytes secrete Indian Hedgehog Protein (Ihp) which regulates and determines bone length and regulates formatoin of the periosteal collar, lack of Ihp results in dwarfism

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chondrocyte hypertrophy, apoptosis and matrix calcification

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chondrocytes hypertrophy and mediate matrix calcification and degradation as well as perichondrium angiogenesis
growing vessels from the perichondrium invade the calcified matrix, carrying osteoprogenitor cells and hemopoietic cells
mediate calcification in the cartilage matrix
chrondrocytes undergo apoptosis

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ossification

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osteoprogenitor cells differentiate into osteoblasts and deposit on bone matrix on calcified cartilage
monocytes differentiate into osteoclasts/chondroclasts that resorb calcified cartilage matrix and remodel newly deposited bone
hemopoietic cells develop in marrow spaces within developing spongy bone

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ossification

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osteoprogenitor cells differentiate into osteoblasts and deposit on bone matrix on calcified cartilage
monocytes differentiate into osteoclasts/chondroclasts that resorb calcified cartilage matrix and remodel newly deposited bone
hemopoietic cells develop in marrow spaces within developing spongy bone

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Concurrent Periosteal Collar formation

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osteoprogenitor cells from perichondrium differentiate into osetoblasts and deposit a thin collar of bone around diaphsis. Periosteal collar is initially formed by intramembranous bone formation and additional layers added by appositional growth
as the periosteal collar bone is deposited, it surrounds vessels from the periosteum laying the framework for Haversian systems and Volksmans canal
bone collar starts out as just a little ring around the surface and then spreads like a tube longitudinally
inner and outter circumfrential lamellae are laid down

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secondary ossification centers and epiphyseal plate

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endochondral bone formation occurs in secondary centers of the epiphysis in a similar fashion except hyaline cartiledge remains on the articular surface
bone growth in length is dependent on cartilage growth at either end of the diaphysis
the cartilagenous zone between the diaphysis and the epiphysis is called the epiphyseal plate

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epiphyseal plate

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very thick in growing bones as cartilage grows on either end of the diaphysis and then degenerates and is replaced by bone, when bones have reached their adult length, the epiphyseal closes

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additional factors that regulate bone formation and remodeling

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hypertrophic chondrocytes: secrete VEGF which stimulates perichondrium angiogenesis and vascular invasion of the cartilage matrix, mediates calcification of the cartilage matrix, stimulate periosteal collar formaiton
osteoblasts - secrete factors that stimulate osteoclastgenosis, deficiencies can result in osteoporosis = high density bone resulting from abnormal or absent osteoclast activity
calcium and phosphate are needed for mineralization
PTH, calcitonin, vit D: regulate osteoclast/osteoblast formation

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rickets and osteomalacia

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defects in mineralization of bone. can be caused by calcium deficieny but more often caused b deficient vitamin D metabolism or adsorption

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osteoporosis

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loss of bone mass leading to bone fragility and susecptibility to fracture, major cause is estrogen deficiency causing increased osteoclast acivity without osteoblast compensation

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paget's disease

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unknown cause, osteoblasts compensate for increased osteoclast activity by rapidly laying down woven bone, results in abnormally large bones prone to fracture

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tumors

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osteoma, chondroma (benign)
osteosarcoma, chondrosarcoma (malignant)
bone is frequent site of metastatic spread of malignant epithelal tumors

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synarthroses joint

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immovable - sutures of the skull

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amphiarthroses joint

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slightly moveable - pubic symphasis

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diarthroses joint

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freely movable - synovial joints

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articular cartilage

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hyaline cartilage with unique overlapping collagen fibers, at either end of the bone

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joint capsule

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outer dense CT (fibrous), inner synovial membrane,

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synovial membrane

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inner surface, consists of two types of synovial cells that secrete fluid rich in hyaluronic acid and glycoproteins

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synovial fluid

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combo of synovial cell secretion, capillary ultrafiltrate and WBC's

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osteoarthritis

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degenerative disorder of articluar cartilage due to excessive wear and tear. inflammation is secondary, cartilage erosion exposes bone surface, bone thickens with irregular growth at margins of surface, then done degenerates

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rheumatoid arthritis

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systemic disorder, chronic relapsing inflammation of joints, synovial membrane thickens and is thrown into folds, infiltration of inflammatory cells, excess synoival fluid with precipitation of fibrin. Articular cartilage destruction with replacement by fibrovascular tissue, ankylosis (fusion) of joints may occur

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gout (hyperuricemia)

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precipitation of urate crystals in joints cause acute inflammatory reaction