Ultracryotomy of biological tissues to preserve membrane structure.

A vacuum transfer stage is described which permits visualization of ultracryotome sections without the considerable distortions found in sections which have either thawed or rehydrated after freeze drying. Membrane structure-nucleus, nucleolus, mitochondria with their cristae and plasma membrane-was observed only in cells at the surface of the tissue which had undergone the fastest freezing rates. Inspection of knife damage in sections through these superficial cells showed that glass or diamond knives which are perfectly adequate for sectioning resin-embedded tissues are less suited to sectioning frozen biological tissue. Deeper in the tissue, where the freezing rates were slower, ice crystal cavities destroyed all membranous structures.