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Abstract

The present work was aimed to study in vivo a rat animal model under a constant static compressive loading precisely control, to quantify the mechanical bone growth modulation and to determine the role of the extracellular matrix (synthesis and/or degradation) in the modulation process. In order to fulfill this goal, a loading device was fixed on the sixth (Cd6) and the eight (Cd8) caudal vertebrae of male rats 28 days old to mechanically load the seventh caudal (Cd7) vertebra with a stress (force/area) of 0.2 MPa for 15 days. Three experimental groups were studied: a baseline control group, a sham group (loading device installed but no load applied), and a loaded group. The Cd5 and Cd9 vertebrae, external to the external fixator, were used as internal controls. In the three groups, the rats were divided in two subgroups for the different biological analysis. A first subgroup was used to measure the growth rates on the Cd5, Cd7 and Cd9 vertebrae, using two calcein—an antibiotic which binds specifically to actively mineralizing matrix—injections made before sacrifice. This subgroup was also used to perform different histological assays, including immunohistochemistries to characterize the type II and X collagens distribution and safranin-O coloration to study the proteoglycans distribution, in the growth plate extracellular matrix of the Cd5, Cd7 and Cd9 vertebrae. Histomorphometric measures were also performed on the Cd7 vertebra. The second subgroup was used for qRT-PCR assays to quantify the three main extracellular matrix components (type II and X collagens and aggrecan) and four proteolytic enzymes degrading the above mentioned components (MMP-3 and -13 and ADAMTS-4 and -5).

The experimentations showed that the compressive loading reduced the Cd7 growth of the loaded group by 29% (p<0.05) and 15% (p=0.07) as compared to the control group and the sham group, respectively. In the growth plate matrix, the type X and II collagens were less expressed in the loaded group than in the other group in 50% and 83% of the rats, respectively. No significant change was observed neither in the mRNA production of the two collagens nor in MMP-13, one of the proteolytic enzyme studied able to degrade the collagens. A significant over-expression was observed in the loaded group for MMP-3, the second collagens degradative enzyme studied, as compared to the control group (p<0.05), yet the difference was not significant as compared to the sham group (p=0.72).

Then this over-expression in the loaded group could not exclusively be attributed to the load applied, because it was very similar to the expression measured in the sham group. The aggrecan, the third main component studied, did not demonstrate any change in the protein distribution, in the mRNA production or in the mRNA production of the proteolytic degraders studied, ADAMTS-4 and -5. In addition, the histomorphometric studies showed that the loaded Cd7 vertebra had a cortex thicker and porosity more important than the same vertebra in the control group.

This study showed that an in vivo static compressive loading induces a remodeling of the growth plate extracellular matrix, which could lead to modifications in the proliferative and hypertrophic cellular activities, and in consequence explain the growth rate reduction observed. The nature of the reorganization in the matrix proteins though, was not determined. Following studies could investigate the possibility of a mechanical denaturation of the collagens, as it has already been observed in articular cartilage under mechanical loading. Other loading levels could also been investigated to confirm the results obtained here with 0.2 MPa. The increase understanding in the growth plate mechanobiology that will come from these studies will provide new insights in the mechanical growth modulation mechanism and a more scientific basis for the medical treatments of progressive skeletal deformities. (Abstract shortened by UMI.)

Details

Title
Étude mécanobiologique in vivo: Effets de la modulation mécanique de croissance dans la matrice extracellulaire des plaques de croissance de vertèbres caudales d'un modèle animal de rat
Author
Cancel, Mathilde
Publication year
2008
Publisher
ProQuest Dissertations & Theses
ISBN
978-0-494-46040-5
Source type
Dissertation or Thesis
Language of publication
French
ProQuest document ID
304800112
Copyright
Database copyright ProQuest LLC; ProQuest does not claim copyright in the individual underlying works.