François Légaré

Imaging and modeling collagen architecture from the nano to micro scale

The collagen meshwork plays a central role in the functioning of a range of tissues including cartilage, tendon, arteries, skin, bone and ligament. Because of its importance in function, it is of considerable interest for studying development, disease and regeneration processes. Here, we have used second harmonic generation (SHG) to image human tissues on the hundreds of micron scale, and developed a numerical model to quantitatively interpret the images in terms of the underlying collagen structure on the tens to hundreds of nanometer scale. Focusing on osteoarthritic changes in cartilage, we have demonstrated that this combination of polarized SHG imaging and numerical modeling can estimate fibril diameter, filling fraction, orientation and bundling. This extends SHG microscopy from a qualitative to quantitative imaging technique, providing a label-free and non-destructive platform for characterizing the extracellular matrix that can expand our understanding of the structural mechanisms in disease.

Femoral epiphyseal cartilage matrix changes at predilection sites of equine osteochondrosis: Quantitative MRI, second‐harmonic microscopy and histological findings

Osteochondrosis is an ischemic chondronecrosis of epiphyseal growth cartilage that results in focal failure of endochondral ossification and osteochondritis dissecans at specific sites in the epiphyses of humans and animals, including horses. The upstream events leading to the focal ischemia remain unknown. The epiphyseal growth cartilage matrix is composed of proteoglycan and collagen macromolecules and encases its vascular tree in canals. The matrix undergoes major dynamic changes in early life that could weaken it biomechanically and predispose it to focal trauma and vascular failure. Subregions in neonatal foal femoral epiphyses (nu2009=u200910 osteochondrosis predisposed; nu2009=u20096 control) were assessed for proteoglycan and collagen structure/content employing 3T quantitative MRI (3T qMRI: T1ρ and T2 maps). Site‐matched validations were made with histology, immunohistochemistry, and second‐harmonic microscopy. Growth cartilage T1ρ and T2 relaxation times were significantly increased (pu2009<u20090.002) within the proximal third of the trochlea, a site predisposed to osteochondrosis, when compared with other regions. However, this was observed in both control and osteochondrosis predisposed specimens. Microscopic evaluation of this region revealed an expansive area with low proteoglycan content and a hypertrophic‐like appearance on second‐harmonic microscopy. We speculate that this matrix structure and composition, though physiological, may weaken the epiphyseal growth cartilage biomechanically in focal regions and could enhance the risk of vascular failure with trauma leading to osteochondrosis. However, additional investigations are now required to confirm this. 3T qMRI will be useful for future non‐invasive longitudinal studies to track the osteochondrosis disease trajectory in animals and humans.

Interferometric Second Harmonic Generation microscopy for tissue imaging

The results obtained with interferometric second harmonic generation (I-SHG) microscopy show that this technique is complementary to standard SHG microscopy since it provides additional information about the structural organization of tissues containing large groups of noncentrosymmetric proteins.

Efficient Wavelength Conversion and Net Parametric Gain via Fwm in a High Index Doped Silica Waveguide

We demonstrate C-band subpicosecond wavelength conversion over > 100nm, exploiting four wave mixing in a high index doped silica waveguide spiral of 45cm, showing a +16.5dB net gain for a 40W peak pump power.