The ability to survive intracellular freezing in nematodes is related to the pattern and distribution of ice formed

A few species of nematodes can survive extensive intracellular freezing throughout all their tissues, an event that is usually thought to be fatal to cells. How are they able to survive in this remarkable way? The pattern and distribution of ice formed, after freezing at -10°C, can be observed using freeze substitution and transmission electron microscopy, which preserves the former position of ice as white spaces. We compared the pattern and distribution of ice formed in a nematode that survives intracellular freezing well (Panagrolaimus sp. DAW1), one that survives poorly (Panagrellus redivivus) and one with intermediate levels of survival (Plectus murrayi). We also examined Panagrolaimus sp. in which the survival of freezing had been compromised by starvation. Levels of survival were as expected and the use of vital dyes indicated cellular damage in those that survived poorly (starved Panagrolaimus sp. and P. murrayi). In fed Panagrolaimus sp. the intracellular ice spaces were small and uniform, whereas in P. redivivus and starved Panagrolaimus sp. there were some large spaces that may be causing cellular damage. The pattern and distribution of ice formed was different in P. murrayi, with a greater number of individuals having no ice or only small intracellular ice spaces. Control of the size of the ice formed is thus important for the survival of intracellular freezing in nematodes.