  A structural understanding of whole cells in three dimensions at high spatial
resolution remains a significant challenge and, in the case of X-rays, has been
limited by radiation damage. By alleviating this limitation, cryogenic coherent
diffraction imaging (cryo-CDI) could bridge the important resolution gap
between optical and electron microscopy in bio-imaging. Here, we report for the
first time 3D cryo-CDI of a whole, frozen-hydrated cell - in this case a
Neospora caninum tachyzoite - using 8 keV X-rays. Our 3D reconstruction reveals
the surface and internal morphology of the cell, including its complex,
polarized sub-cellular architecture with a 3D resolution of ~75-100 nm, which
is presently limited by the coherent X-ray flux and detector size. Given the
imminent improvement in the coherent X-ray flux at the facilities worldwide,
our work forecasts the possibility of routine 3D imaging of frozen-hydrated
cells with spatial resolutions in the tens of nanometres.
