  Cryopreservation is the only fully established procedure to extend the
lifespan of living cells and tissues, a key to activities spanning from
fundamental biology to clinical practice. Despite its prevalence and impact,
central aspects of cryopreservation, such as the cell's physico-chemical
environment during freezing, remain elusive. Here we address that question by
coupling in situ microscopic directional freezing to visualize cells and their
surroundings during freezing with the freezing medium phase diagram. We extract
the freezing medium spatial distribution in cryopreservation, providing a tool
to describe the cell vicinity at any point during freezing. We show that two
major events define the cells' local environment over time: the interaction
with the moving ice front and with the vitreous moving front - a term we
introduce here. Our correlative strategy may be applied to cells relevant in
clinical research and practice, and help designing new cryoprotective media
based on local physico-chemical cues.
