Concerning the nature of the cryosectioning process

Intentional melting of frozen-thin-sections induces ultrastructural alterations. Cross sections through frozen dried and embedded cryosections revealed that even at the surface of such a section no melting artefacts are visible. One surface of the cryosection (obtained at 168 K) has a saw-teeth appearance whereas the other surface is rather smooth. The rate of freeze-drying in the cryochamber of the microtome was estimated to be in order 3.3 nm/min at 183 K and 22.5 nm/min at 193 K for sections, about 120 nm thick, of rat kidney frozen in freon 22 without any chemical fixation of cryoprotection. Although freeze-drying will normally not interfere with the cryo-sectioning process itself, it is a process which cannot be neglected during handling of sections. The available evidence indicated that frozen-thin-sections represent the frozen tissue with respect to the ultrastructure an probably also with respect to the chemical composition.