High subzero cryofixation: A technique for observing ice within tissues.

While various fixation techniques for observing ice within tissues stored at high sub-zero temperatures currently exist, these techniques require either different fixative solution compositions when assessing different storage temperatures or alteration of the sample temperature to enable alcohol-water substitution. Therefore, high-subzero cryofixation (HSC), was developed to facilitate fixation at any temperature above -80 °C without sample temperature alteration. Rat liver sections (1 cm2) were frozen at a rate of -1 °C/min to -20 °C, stored for 1 h at -20 °C, and processed using classical freeze-substitution (FS) or HSC. FS samples were plunged in liquid nitrogen and held for 1 h before transfer to -80 °C methanol. After 1, 3, or 5 days of -80 °C storage, samples were placed in 3% glutaraldehyde on dry ice and allowed to sublimate. HSC samples were stored in HSC fixative at -20 °C for 1, 3, or 5 days prior to transfer to 4 °C. Tissue sections were paraffin embedded, sliced, and stained prior to quantification of ice size. HSC fixative permeation was linear with time and could be mathematically modelled to determine duration of fixation required for a given tissue depth. Ice grain size within the inner regions of 5 d samples was consistent between HSC and FS processing (p = 0.76); however, FS processing resulted in greater ice grains in the outer region of tissue. This differed significantly from HSC outer regions (p = 0.016) and FS inner regions (p = 0.038). No difference in ice size was observed between HSC inner and outer regions (p = 0.42). This work demonstrates that HSC can be utilized to observe ice formed within liver tissue stored at -20 °C. Unlike isothermal freeze fixation and freeze substitution alternatives, the low melting point of the HSC fixative enables its use at a variety of temperatures without alteration of sample temperature or fixative composition.