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MONITORING MICROMETER SCALE COLLAGEN ORGANIZATION IN TENDON UPON

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MONITORING MICROMETER SCALE COLLAGEN ORGANIZATION IN TENDON UPON Powered By Docstoc
					      MONITORING MICROMETER-SCALE COLLAGEN ORGANIZATION IN
      TENDON UPON MECHANICAL STRAIN BY USE OF SHG MICROSCOPY.

        I. Gusachenko1, Y. Goulam Houssen1, G. Latour1, V. Tran2, J.-M. Allain2,
                                M.-C. Schanne-Klein1

(1) Lab. d'Optique et Biosciences, Ecole Polytechnique – CNRS - INSERM U696, 91128
                                   Palaiseau, France
(2) Lab. Mécanique des Solides, Ecole Polytechnique – CNRS, 91128 Palaiseau, France
KEY WORDS: Biomechanics, SHG microscopy, polarimetry, collagen, 3D imaging,
multiphoton microscopy, tendon
     Collagen is the major component of the extracellular matrix and is responsible for the
architecture of tissues. It is characterized by triple helical domains and shows a highly
structured macromolecular organization that is specific for every tissue. Tendons are mainly
composed of type I collagen that forms 200 nm diameter fibrils aligned within fibers and
fascicles with a crimped pattern. This hierarchical organization is responsible for the
biomechanical properties of this tissue. However, the relationship between the macroscopic
mechanical behavior and the microscopic collagen structure is not fully understood.
     We therefore combined nonlinear optical microscopy and strain-stress experiments to
visualize the micrometer-scale structure of rat-tail tendon during mechanical loading cycles
[1]. Indeed, collagen has been shown to exhibit large second order optical nonlinearities at the
molecular scale [2] and at the macromolecular scale in tissues [3, 4]. We also implemented
polarization-resolved second harmonic generation (SHG) imaging [5] and performed
ratiometric measurements of the two main tensorial components of collagen nonlinear
response since this parameter is sensitive to molecular order at the sub-micrometer scale.
     We performed load cycles with
                                                             4
increasing maximum strain and recorded                                   0%           4%           6%
simultaneously SHG images and stress
using a sensitive 10 N sensor. Crimped                       3

pattern were observed at 0 % strain, then
                                              Stress (MPa)




they disappeared when increasing the
                                                             2
strain, and they were again observed with
increasing spatial frequency when
relaxing the strain (see fig.1).                             1
Polarimetric measurements were also
observed to vary with the strain and
                                                             0
presumably indicate variation of the                             0   1        2   3   4    5   6        7   8   9   10
molecular distribution within the focal                                               Strain (%)
volume. Finally, stress measurements
evidenced a hysteresis behavior at >4%                       Fig. 1: SHG imaging and Strain-Stress
strain that may be related to the                            measurements of a rat-tail tendon during
aforementioned modifications of the                          load cycles. Scale bar: 100 µm.
microscopic structure.
[1]    Goulam Houssen et al, in preparation (2011)
[2]    Deniset-Besseau et al, J. Phys. Chem. B 113, 13437 (2009)
[3]    Strupler et al, J. Biomed. Optics 13, 054041 (2008)
[4]    Pena et al, J. Biomed. Opt. 15, 056018 (2010)
[5]    Gusachenko et al, Opt. Express 18, 19339 (2010)