Cryofixation

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Cryofixation is a technique for fixation or stabilisation of biological materials as the first step in specimen preparation for the electron microscopy and cryo-electron microscopy. [1] Typical specimens for cryofixation include small samples of plant or animal tissue, cell suspensions of microorganisms or cultured cells, suspensions of viruses or virus capsids and samples of purified macromolecules, especially proteins. [2] [3]

Contents

Types of cryo-fixation are freezing-drying, freezing-substitution and freezing-etching.[ citation needed ][ clarification needed ]

Plunge freezing

The method involves ultra-rapid cooling of small tissue or cell samples to the temperature of liquid nitrogen (−196 °C) or below, stopping all motion and metabolic activity and preserving the internal structure by freezing all fluid phases solid. Typically, a sample is plunged into liquid nitrogen or into liquid ethane or liquid propane in a container cooled by liquid nitrogen. The ultimate objective is to freeze the specimen so rapidly (at 104 to 106 K per second) that ice crystals are unable to form, or are prevented from growing big enough to cause damage to the specimen's ultrastructure. The formation of samples containing specimens in amorphous ice is the "holy grail" of biological cryomicroscopy.[ citation needed ]

In practice, it is very difficult to achieve high enough cooling rates to produce amorphous ice in specimens more than a few micrometres in thickness. For this purpose, plunging a specimen into liquid nitrogen at its boiling point (−196 °C) [4] does not always freeze the specimen fast enough, for several reasons. First, the liquid nitrogen boils rapidly around the specimen forming a film of insulating N
2 gas that slows heat transfer to the cryogenic liquid, known as the Leidenfrost effect. Cooling rates can be improved by pumping the liquid nitrogen with a rotary vane vacuum pump for a few tens of seconds before plunging the specimen into it. This lowers the temperature of the liquid nitrogen below its boiling point, so that when the specimen is plunged into it, it envelops the specimen closely for a brief period of time and extracts heat from it more efficiently. Even faster cooling can be obtained by plunging specimens into liquid propane or ethane (ethane has been found to be more efficient) [5] cooled very close to their melting points using liquid nitrogen [6] or by slamming the specimen against highly polished liquid nitrogen-cooled metal surfaces made of copper or silver. [7] Secondly, two properties of water itself prevent rapid cryofixation in large specimens. [8] The thermal conductivity of ice is very low compared with that of metals, and water releases of latent heat of fusion as it freezes, defeating rapid cooldown in specimens more than a few micrometres thick.

High-pressure freezing

High pressure helps prevent the formation of large ice crystals. Self pressurized rapid freezing (SPRF) can utilize many different cryogens has recently been touted as an attractive and low cost alternative to high pressure freezing (HPF). [9] Cold pressurized nitrogen substitutes ethanol at temperatures roughly 123K. The warm ethanol is then expelled by the freezing LN2 and most likely produces an ethanol-nitrogen mixture that gradually becomes colder and colder. [10]

Freeze-drying

Drying times are reduced by up to 30% with proper freeze drying. [11]

See also

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References

  1. ↑ Pilhofer, Martin; Ladinsky, Mark S.; McDowall, Alasdair W.; Jensen, Grant J. (2010). Bacterial TEM. Methods in Cell Biology. Vol. 96. pp. 21–45. doi:10.1016/S0091-679X(10)96002-0. ISBN   9780123810076. ISSN   0091-679X. PMID   20869517.
  2. ↑ Echlin P (1992). Low Temperature Microscopy and Analysis. New York: Plenum Publishing Corporation.
  3. ↑ Dubochet J, Adrian M, Chang JJ, Homo JC, Lepault J, McDowall AW, Schulz P (1988). "Cryo-electron microscopy of vitrified specimens" (PDF). Quarterly Reviews of Biophysics. 21 (2): 129–228. doi:10.1017/s0033583500004297. PMID   3043536. S2CID   2741633.
  4. ↑ Battersby BJ, Sharp JC, Webb RI, Barnes GT (1994). "Vitrification of aqueous suspensions from a controlled environment for electron microsocopy: an improved plunge-cooling device". Journal of Microscopy. 176 (2): 110–120. doi:10.1111/j.1365-2818.1994.tb03505.x. S2CID   95926972.
  5. ↑ Ryan, Keith P. (1992). "Cryofixation of tissues for electron microscopy: a review of plunge cooling methods" (PDF). Scan. Microsc. 6 (3): 715–743.
  6. ↑ Bald WB (1984). "The relative efficiency of cryogenic fluids used in the rapid quench-cooling of cryogenic samples". Journal of Microscopy. 134 (3): 261–270. doi:10.1111/j.1365-2818.1984.tb02519.x. S2CID   97233738.
  7. ↑ Allison DP, Daw CS, Rorvik MC (1987). "The construction and operation of a simple and inexpensive slam freezing device for electron microscopy". Journal of Microscopy. 147 (Pt 1): 103–108. doi:10.1111/j.1365-2818.1987.tb02822.x. PMID   3305955. S2CID   112876.
  8. ↑ Bald WB (1987). Quantitative Cryofixation. Bristol and Philadelphia: Adam Hilger.
  9. ↑ Leunissen Jan L.M. and Yi H. (2009). "Self-pressurized rapid freezing (SPRF): a novel cryofixation method for specimen preparation in electron microscopy". J. Microsc. 235 (1): 25–35. doi:10.1111/j.1365-2818.2009.03178.x. PMID   19566624. S2CID   205342519. Archived from the original on 2013-01-05.
  10. ↑ Studer, D (September 1995). "Vitrification of articular cartilage by high-pressure freezing". Journal of Microscopy. 179 (3): 321–322. doi:10.1111/j.1365-2818.1995.tb03648.x. PMID   7473694. S2CID   32347571.
  11. ↑ Silva, A. C. C.; Schmidt, F. C. (2019-10-01). "Vacuum Freezing of Coffee Extract Under Different Process Conditions". Food and Bioprocess Technology. 12 (10): 1683–1695. doi:10.1007/s11947-019-02314-x. ISSN   1935-5149. S2CID   201644501.
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