Cryofixation methods for ultrastructural studies of Dictyostelium discoideum

Cryopreservation methods, including rapid freezing, freeze-substitution, and low-temperature embedment, lead to superior ultrastructural preservation compared with traditional fixation procedures. This is particularly true for the multicellular stages of Dictyostelium discoideum, in which the hydrophobic sheath that surrounds the structures causes delayed penetration by the already slow-acting aqueous chemical fixatives, resulting in cell shape changes, loss of cell-cell contacts, and changes in cell-matrix interactions. The surface tension effects of traditional fixation methods can also result in disruption of the delicate structures. Depth of freezing is often greater than expected because of the relatively dehydrated state of the multicellular structures. Variations in freeze-substitution solvents and embedment media can be employed to allow for antigenic preservation. Commercial instruments exist for most of the procedures, but excellent results can be obtained using inexpensive hand-crafted apparatus.