Ice can penetrate invertebrate tissues via paracellular pathways

Ice-cell interactions especially the question whether and how ice crystals invade tissues and organs during freezing were examined in small transparent organs (salivary glands) in which many structural details remained visible despite of freeze-induced cell darkening. In most glands no invasion of ice into the lumen was observed since ice dendrites stopped growing after touching the gland. Here I report that in rare cases a so far unknown type of ice crystals developed which aggressively pushed against cell membranes before invading the gland via paracellular pathways (septate junctions). Aggressive ice crystals were also observed within a salivary gland cell which deformed and finally invaded the nucleus. In cell strands it was observed that intracellular freezing is indeed a two-step event in which ice developed in cytoplasm several seconds before invading the nucleus.