Cryo-planing of frozen-hydrated samples using cryo triple ion gun milling (CryoTIGM™).

Cryo-SEM is a high throughput technique for imaging biological ultrastructure in its most pristine state, i.e. without chemical fixation, embedding, or drying. Freeze fracture is routinely used to prepare internal surfaces for cryo-SEM imaging. However, the propagation of the fracture plane is highly dependent on sample properties, and the resulting surface frequently shows substantial topography, which can complicate image analysis and interpretation. We have developed a broad ion beam milling technique, called cryogenic triple ion gun milling (CryoTIGM™ ['kri-ə-,tim]), for cryo-planing frozen-hydrated biological specimens. Comparing sample preparation by CryoTIGM™ and freeze fracture in three model systems, Baker's yeast, mouse liver tissue, and whole sea urchin embryos, we find that CryoTIGM™ yields very large (∼700,000 µm(2)) and smooth sections that present ultrastructural details at similar or better quality than freeze-fractured samples. A particular strength of CryoTIGM™ is the ability to section samples with hard-soft contrast such as brittle calcite (CaCO3) spicules in the sea urchin embryo.

Large Area Cryo-Planing of Vitrified Samples Using Broad-Beam Ion Milling.

On account of its excellent resolution and high throughput, cryoSEM imaging has recently seen resurgence. In this work, we report on the development of cryogenic triple ion gun milling (CryoTIGM™), a broad ion beam milling technique for cryo-planing of vitrified, "frozen-hydrated" specimens. We find that sections prepared with CryoTIGM™ are smooth over exceptionally large areas (~700,000 µm2), and reveal ultrastructural details in similar or better quality than freeze-fractured samples.