  Ultrathin lensless fibre endoscopes offer minimally invasive investigation,
but they mostly operate as a rigid type due to the need for prior calibration
of a fibre probe. Furthermore, most implementations work in fluorescence mode
rather than label-free imaging mode, making them unsuitable for medicine and
industry. Herein, we report a fully flexible ultrathin fibre endoscope taking
3D holographic images of unstained tissues with 0.87-{\mu}m spatial resolution.
Using a bare fibre bundle as thin as 200-{\mu}m diameter, we design a lensless
Fourier holographic imaging configuration to selectively detect weak
reflections from biological tissues, a critical step for stain-free reflectance
imaging. A unique algorithm is developed for calibration-free holographic image
reconstruction, allowing us to image through a narrow and curved passage
regardless of fibre bending. We demonstrate endoscopic reflectance imaging of
unstained rat intestine tissues that are completely invisible to conventional
endoscopes. The proposed endoscope will expedite more accurate and earlier
diagnosis than before with minimal complications.
