  The vast majority of the microscopy methods currently available to study
biological samples require staining prior to imaging. Nevertheless, the ability
to reveal specific cell structures or organelles in a label-free manner remains
desirable in different contexts. Polarization microscopy has long been
considered as an interesting alternative to fluorescence-based methods in order
to gain specificity on the imaged biological samples. In this work, we show how
an original polarization imaging technique, implementing micro-wave photonics
and referred to as orthogonality-breaking (OB) microscopy, can provide
informative polarization images from a single scan of the cell sample in a fast
and sensitive way. For OB imaging, the sample is probed with a laser setup
simultaneously generating two orthogonal polarizations shifted in frequency by
a few tens of MHz. If the imaged samples display some polarimetric properties,
the orthogonality between the two polarizations is broken, leading to a
beatnote interference signal that can be detected with a fast detector. The
comparison of the images of various cell lines at different cell-cycle stages
obtained by OB polarization microscopy and fluorescence confocal images shows
that an endogenous polarimetric contrast arizes on compacted chromosomes during
cell division. This technique paves the way to label-free real-time
polarization imaging of mitotic chromosomes with further potential applications
in histology and cancer diagnosis.
