Bone
General
Bone formation and growth

Author: Dariusz Borys, M.D. (see Authors page)

Revised: 14 September 2016, last major update January 2012

Copyright: (c) 2003-2016, PathologyOutlines.com, Inc.

PubMed Search: Bone formation [title] growth
Table of Contents
Definition / General
Cite this page: Bone formation and growth. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/bonebonegrowth.html. Accessed December 6th, 2016.
Definition / 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

    Images hosted on other servers:
    Missing Image

    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

    Images hosted on other servers:
    Missing Image

    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

    Images hosted on other servers:
    Missing Image

    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