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ossification or osteogenesis: process by which bone forms

INITIAL BONE FORMATION IN EMBRYO AND FETUS

-cartilage formation and ossification occurs in 6th week of embryo development
-intramembrous ossification: bone forms directly within mesenchyme arrange in sheetlike layers
-endochondrial ossification: bone forms within hyaline cartilage that develops from mesenchyme

INTRAMEMBRANOUS OSSIFICATION

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development of ossification center

-chemical messages cause mesenchymal cells to form together from osteogenic cells to osteoblasts
-osteoblasts secrete organic extracellular matrix until its surrounded by it

calcification

-secretion stops
-osteocytes lie in lucanae and extend through canaliculi
-calcium and other salts are deposited in extracellular matrix and hardens

formation of trabeculae

-form spongy bone
-blood vessel differentiate into red bone marrow

development of periosteum

much of newly formed bone is destroyed and renewed into adult size bone

bones formed by intramembranous ossification

-skull and mandible

ENDOCHONDRAL OSSIFICATION

-replacement of cartilage to bone

development of cartilage model

-chemical messages cause mesenchymal cells to crowd together where bone will form and develop chondroblasts

growth of cartilage model

-chondroblasts turn into chondrocytes
-interstitial growth: cartilage grows by continual cell division of chondrocytes and further secretion of cartilage extracellular matrix; increase in length
-appositional growth: growth on outer surface and thickness

development of primary ossification center

-nutrient artery penetrates periochondrium and calcifying cartilage model through nutrient foramen and change cells from chondroblasts into osteoblasts
-periochondrium turns into periosteum to form bone
-forms spongy bone
-primary ossification center: region where bone tissue replace most cartilage

development of medullary cavity

-as primary ossification center grow towards end of bones, osteoclasts break down spongy bone and is replaced with compact bone

development of secondary ossification center

-develops when epiphyseal artery enters epipyses

formation of articular cartilage and epiphyseal plate

-hyaline cartilage becomes articular cartilage

BONE GROWTH DURING INFANCY, ADOLESCENCE, ADULTHOOD

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growth in length

two stages:
-interstitial growth of cartilage in epiphyseal side
-replacement of cartilage on diaphyseal side of epiphyseal plate with bone by endochondral ossification
-epiphyseal plate: layer of hyaline cartilage in metaphysis of growing bone that has four zones:

zone of resting cartilage

-nearest epiphysis
-consist small scattered chondrocytes
-anchor epiphyseal plate to epiphysis of bone

zone of proliferating cartilage

-larger chondrocytes stacked like coins
-interstitial growth as they divide and secrete extracellular matrix

zone of hypertrophic cartilage

-large maturing chondrocytes in columns

zone of calcified cartilage

-dead chondrocytes because extracellular matrix around them has calcified
-becomes new diaphysis
-as bone grows, new chondrocytes are formed on epiphyseal side of plate, while old chondrocytes on diaphyseal side of [plate are turned into bone

when bone stops growing

-epiphyseal cartilage stops dividing
-bone replace all cartilage
-epiphyseal plate turns into epiphyseal line

GROWTH IN THICKNESS

-periosteum form osteoblasts which secrete extracellular matrix
-osteoblast turn into osteocytes
-osteoclasts in endosteum destroy bone tissue lining of the medullary cavity, therefore enlarging bone thickness

REMODELING OF BONE

-ongoing replacement of old bone tissue by new bone tissue
-bone resorption: removal of minerals and collagen fibers from bone by osteoclasts
-bone deposition: addition of minerals and collagen fibers to bone by osteoblasts
-remodeling removes injured bones, make it stronger than before

bone resorption

-osteoclasts attach to endosteum or periosteum and release lysosomal enzyme and acids to digest collagen fibers and other organic substances while acid dissolve bone mienrals
-products of bone resorption diffuse into blood capillaries

Pagets disease

abnormal acceleration of remodeling process
-newly formed bone especially in pelvis, limb, skull, vertebrae becomes hard and brittle and fractures easily

FACTORS AFFECTING BONE GROWTH AND REMODELING

-minerals
-vitamins; vitamin A for osteoblasts
-hormones; insulin like growth factors stimulate osteoblasts
-during puberty, sex hormones increase osteoblast activity and responsible for growth spurt

FRACTURE AND REPAIR OF A BONE

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TYPES OF FRACTURES

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open (compound)

-broken bone protrude skin
-close simple fracture doesnt

comminuted

bone is crushed or broken to pieces and smaller bone fragments lie between two main fragments; most difficult to treat

greenstick

-one side of bone is broken and other side bends
-occurs in children whose bones are not fully ossified yet

impacted

-one end of bone is driven into interior of the other

pott's

fracture of distal end of lateral leg bone (fibula) with serious injury of distal tibial articulation

colle's

-fracture of distal end of lateral radius in arm and distral fragment is displaced

stress fracture

-fissures in bone that has no evidence of injury to other tissues
-caused by strenous activity like running and jumping

REPAIR OF BONE FROM FRACTURE

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formation of fracture hematoma

-fracture hematoma: mass of clotted blood formed around site of fracture where circulation of blood stops
-osteoclasts and phagocytes remove dead bone cells

fibrocartilaginous callus formation

fibrocartilaginous callus: mass of repair tissue consisting of collagen fibers and cartilage that bridges broken ends of bone
-developed by fibroblasts and periosteum

bony callus formation

-osteogenic cells develop into osteoblasts which begin to produce spongy bone trabeculae
-bony callus: cartilage is converted to spongy bone

bone remodeling

-dead portions of original fragments of broken bone are resorbed by osteclasts
-compact bone replace spongy bone

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