title: The effect of weight loading and subsequent release from loading on the postnatal skeleton.
abstract: BACKGROUND
  The relationship between load and the structure and mechanical properties of mature bones has been thoroughly described. In contrast, this relationship has been studied much less in immature bones, which consist of bony tissue and cartilaginous growth plate, during the postnatal period. This paper describes the effect of an externally applied load on the bones of young fast-growing chicks; in particular, we examine the effect on the growth plate, which regulates longitudinal bone growth, and the consequences in terms of bone structural and mechanical properties.
  
  
  METHODS
  The tibial growth plates from chicks subjected to external load and control chicks, immediately after loading and following 5 days of load release, were studied by histological staining and quantitative PCR. The contralateral tibiae were mechanically tested by three-point bending and their structural features determined by micro-CT.
  
  
  RESULTS
  At the end of the external loading period, the tibias of the experimental group were shorter and their growth plate narrower than in controls. However, at this time point, effects were not yet apparent in the bones' structural or mechanical parameters. After a further 5 days of no external load, bones and growth plates of the experimental group demonstrated the phenomenon of 'catch-up': the thickness of the growth plate exceeded that of the control; however the relative expression of genes controlling chondrocyte differentiation (collagen II and X) did not change, while the expression of factors related to growth-plate ossification (osteopontin, alkaline phosphatase) and cartilage and bone calcification (matrix and bone Gla proteins) was upregulated as a result of the catch-up process. At this time, however, the tibiae of the experimental group showed inferior mechanical and structural properties relative to the control group.
  
  
  CONCLUSIONS
  External loading during bone elongation negatively affects the mechanical and structural properties of the skeleton. The effect is first noticeable in the growth plate, which regulates bone growth, and is exhibited in the bone phenotype after a lag period.
authors.affiliation: Institute of Biochemistry and Nutrition, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Israel. reich@agri.huji.ac.il
authors.forename: Adi
authors.initials: A
authors.lastname: Reich
authors.forename: Amnon
authors.initials: A
authors.lastname: Sharir
authors.forename: Elazar
authors.initials: E
authors.lastname: Zelzer
authors.forename: Lilach
authors.initials: L
authors.lastname: Hacker
authors.forename: Efrat
authors.initials: E
authors.lastname: Monsonego-Ornan
authors.forename: Ron
authors.initials: R
authors.lastname: Shahar
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000469:Alkaline Phosphatase / Q000235:genetics; D000818:Animals; D000831:Animals, Newborn; D001846:Bone Development / Q000502:physiology*; D001842:Bone and Bones / Q000166:cytology / Q000000981:diagnostic imaging / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000235:genetics; D002645:Chickens; D024043:Collagen Type II / Q000235:genetics; D024121:Collagen Type X / Q000235:genetics; D016326:Extracellular Matrix Proteins / Q000235:genetics; D020869:Gene Expression Profiling; D006132:Growth Plate / Q000166:cytology / Q000000981:diagnostic imaging / Q000378:metabolism*; D017403:In Situ Hybridization; D015675:Osteocalcin / Q000235:genetics; D053495:Osteopontin / Q000235:genetics; D011859:Radiography; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013977:Tibia / Q000166:cytology / Q000000981:diagnostic imaging / Q000378:metabolism; D016474:Weight-Bearing / Q000502:physiology*; D000097568:Matrix Gla Protein
chemical_list: D000469:Alkaline Phosphatase; D002135:Calcium-Binding Proteins; D024043:Collagen Type II; D024121:Collagen Type X; D016326:Extracellular Matrix Proteins; D015675:Osteocalcin; D053495:Osteopontin; D000097568:Matrix Gla Protein