  Oblique plane microscopy (OPM) enables high speed, volumetric fluorescence
imaging through a single-objective geometry. While these advantages have
positioned OPM as a valuable tool to probe biological questions in animal
models, its potential for in vivo human imaging is largely unexplored due to
its typical use with exogenous fluorescent dyes. Here we introduce a
scattering-contrast oblique plane microscope (sOPM) and demonstrate label-free
imaging of blood cells flowing through human capillaries in vivo. The sOPM
illuminates a capillary bed in the ventral tongue with an oblique light sheet,
and images side- and back- scattered signal from blood cells. By synchronizing
sOPM with a conventional capillaroscope, we acquire paired widefield and axial
images of blood cells flowing through a capillary loop. The widefield
capillaroscope image provides absorption contrast and confirms the presence of
red blood cells (RBCs), while the sOPM image may aid in determining whether
optical absorption gaps (OAGs) between RBCs have cellular or acellular
composition. Further, we demonstrate consequential differences between
fluorescence and scattering versions of OPM by imaging the same polystyrene
beads sequentially with each technique. Lastly, we substantiate in vivo
observations by imaging isolated red blood cells, white blood cells, and
platelets in vitro using 3D agar phantoms. These results demonstrate a
promising new avenue towards in vivo blood analysis.
