Ultrastructural preservation of plasma membranes by non-lethal slow freezing to liquid nitrogen temperature.

Secondary hyphae of Lyophyllum ulmarium were shown to tolerate slow freezing, which allowed extracellular freezing, to -196 degrees C. A freeze-fracture study showed that under this non-lethal freezing condition, the plasma membrane of the secondary hyphae did not show any ultrastructural changes as compared with the control, except gross cellular shrinkage. Tertiary hyphae of Lyophyllum ulmarium, on the other hand, were completely injured by slow freezing to -196 degrees C, and the plasma membrane showed distinct intramembrane particle aggregation as a result of direct membrane contact caused by severe cellular deformation. It is suggested that the absence of freezing injury in the secondary hyphae was due to ultrastructural preservation of the plasma membrane, which resulted from avoidance of severe cellular deformation, while occurrence of freezing injury in the tertiary hyphae is considered to be due to ultrastructural changes in the plasma membrane caused by severe cellular deformation.