Label-free photoacoustic microscopy of cytochromes

Photoacoustic microscopy (PAM) has achieved submicron lateral resolution in showing subcellular structures; however, relatively few endogenous subcellular contrasts have so far been imaged. Given that the hemeprotein, mostly cytochromes in general cells, is optically absorbing around the Soret peak (~420 nm), we implemented label-free PAM of cytochromes in cytoplasm for the first time. By measuring the photoacoustic spectra of the oxidized and reduced states of fibroblast lysate and fitting the difference spectrum with three types of cytochromes, we found that the three cytochromes account for more than half the optical absorption in the cell lysate at 420 nm wavelength. Fixed fibroblasts on slides were imaged by PAM at 422 and 250 nm wavelengths to reveal cytoplasms and nuclei, respectively, as confirmed by standard staining histology. PAM was also applied to label-free histology of mouse ear sections by showing cytoplasms and nuclei of various cells. PAM of cytochromes in cytoplasm is expected to be a high-throughput, label-free technique for studying live cell functions, which cannot be accomplished by conventional histology.