Second-harmonic microscopy of biological tissue

We present the first reported optical second-harmonic microscope images of a biological sample-rattail tendonand discuss, also for the first time, the need to distinguish between coherent and incoherent second-harmonic imaging. Our data show that the currently unexplained macroscopic polar order of this classic representative of connective tissue is due both to a coherent network containing a large number of fine, polar, filamentlike structures that permeate the entire tendon volume and to a small number of intensely polar surface patches.