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Bone

Bone formation and growth


Reviewers: Dariusz Borys, M.D., (see Reviewers page)
Revised: 5 February 2012, last major update January 2012
Copyright: (c) 2003-2012, PathologyOutlines.com, Inc.

Bone formation-general
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● Bone tissue is formed by intramembranous ossification or by endochondral ossification
● The original or model tissue is gradually destroyed and replaced with bone tissue
● Woven bone is primarily formed and later converted to lamellar bone by subsequent remodeling

Intramembranous ossification
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● Source of flat and less commonly short bones
● Occurs through condensation of mesenchymal tissue
● Process begins when multiple groups of cells differentiate into osteoblasts in a primary ossification center
● Osteoid is synthesized, then mineralizes surrounding the osteoblasts, which mature to osteocytes
● When ossification centers fuse, loose trabecular structures known as primary spongiosa are formed
● Then blood vessels grow into the connective tissue between trabeculae

           
Intramembranous bone formation

Endochondral ossification
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● Responsible for formation of long and short bones
● Hyaline cartilage model, which provides template of shape of the bone
● May be divided into 2 phases:
● 1st phase: chondrocytes are hypertrophic and degenerated, then calcified
● 2nd phase: osteoprogenitor cells and blood capillaries invade the spaces left by degenerating cartilage; osteoblasts arise from osteoprogenitor cells and lay down a layer of rapidly mineralized osteoid on the surface of calcified cartilage, called primary spongiosa, which later is remodeled to lamellar bone (secondary spongiosa); calcified cartilage is resorbed by chondroblasts and replaced by bone and marrow cavities

       
Endochondral ossification

Histology of bone growth
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● Epiphyseal cartilage of long bone is located between epiphysis and metaphysis, is responsible for longitudinal growth; has 5 zones:
(a) Resting zone - small chondrocytes
(b) Proliferative zone rapidly dividing chondrocytes in columns, parallel to the long axis of bone
(c) Hypertrophic zone large chondrocytes with clear cytoplasmic glycogen
(d) Calcified cartilage zone (zone of provisional calcification) chondrocyte graveyard, followed by blood vessel invasion and bone deposition
(e) Ossification zone formation of primary spongiosa by rapidly mineralized osteoid

       
Epiphyseal plate

Bone growth
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● Bone grows by either endochondral or intramembranous ossification
● Endochondreal ossification of the epiphyseal plate is responsible for longitudinal growth of long bones
● Periosteal deposition is responsible for length and thickness of long bones
● Endosteal bone deposition is responsible for growth of trabecular bone and endosteal cortex, including the haversian system

Modeling and remodeling
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● Bone formation is an ongoing process that alters the size and shape of bone by partial resorption of preformed bone tissue and simultaneous deposition of new bone (modeling and remodeling)
● Modeling is a process in which bone achieve its proper shape
● Modeling is responsible for the circumferential growth of bone and expansion of marrow cavity
● Remodeling is a continuous process, in which damaged bone is repaired, ion homeostasis is maintained, and bone is reinforced for increased stress; entire remodeling cycle requires ~ 6 months
● In healthy adults, remodeling rate varies by type of bone: 25% per year in trabecular bone versus 3% in cortical bone
● Resorption and deposition are normally balanced, and bone density is maintained
● Resorptive activity exceeding deposition activity represents a pathologic state, may cause lytic lesions
● The cement line (reversal line) is evidence of previous remodeling activity; is formed by filling of new bone in a previously resorbed cavity; is strongly basophilic due to high content of inorganic matrix and is normally found in the haversian and interstitial systems of adult bone
● Cement line from normal remodeling is relatively long and straight; indented or mosaic pattern indicates a pathologically accelerated remodeling process

End of Bone > Bone growth


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