Bone - Bone formation and growth

Home Chapter Home Jobs Conferences Fellowships Books

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
=========================================================================

● 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
=========================================================================

● 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
=========================================================================

● 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
=========================================================================

● 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
=========================================================================

● 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
=========================================================================

● 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

This information is intended for physicians and related personnel, who understand that medical information is often imperfect, and must be interpreted in the context of a patient's clinical data using reasonable medical judgment. This website should not be used as a substitute for the advice of a licensed physician.

All information on this website is protected by copyright of PathologyOutlines.com, Inc. Information from third parties may also be protected by copyright. Please contact us at copyrightPathOut@gmail.com with any questions (click here for other contact information).