  Super-resolution microscopy normally requiring complex and cumbersome optics
is not applicable for in situ imaging through a narrow channel. Here, we
demonstrate single hair-thin multimode fiber (MMF) endoscope (less than 250
${\mu}m$) for in vivo light-field nano-imaging, which is called
spatial-frequency tracking adaptive beacon light-field encoded nano-endoscopy
(STABLE nano-endoscopy) that enables three-dimensional (3D) subcellular-scale
imaging. Spatial-frequency tracking provides up to $10^3$ Hz disorder tracking
that ensures stable imaging in long-haul MMFs (up to 200 m) under various
conditions. Full-vector modulation and fluorescence emission difference are
combined to enhance the imaging signal-to-noise ratio two times and to improve
the resolution to sub-diffraction-limited 250 nm (${\lambda}/3NA$). STABLE
nano-endoscopy and white-light endoscopy (WLE) are integrated to achieve
cross-scale in vivo imaging inside the lumen. This high-resolution and robust
observation in a minimally invasive manner paves the way to gain a deeper
understanding of the disease mechanisms and to bridge clinical and biological
sciences.
