ABSTRACT
SHG has several advantageous features that make it an ideal approach in the microscopy of l iving organisms. Si nce SHG sig nals a rise f rom a n i nduced p olarization r ather t han f rom a bsorption, t his leads to a sub stantially re duced ph otobleaching a nd ph ototoxicity rel ative to uorescence methods (including m ultiphoton). A dditionally, b ecause t he e xcitation t ypically u ses t he s ame ne ar-infrared wavelengths (800-1000 nm) produced by titanium-sapphire lasers, which are also used for two-photon excited uorescence (2PEF), this method is well suited for studying intact tissue samples. Moreover, excellent depths of penetration can be obtained. Furthermore, detailed information about the organization of protein matrices at the molecular level can be extracted from SHG imaging data. is is because the SHG signals have well-de ned polarizations with respect to the laser polarization and the specimen orientation, which can be used to de termine t he absolute orientation of t he protein molecules in t he array, as well as the degree of organization of proteins in tissues.
