The kindness and generosity of Dr. Kent McDonald of UC Berkeley are greatly appreciated. We thank an anonymous reviewer for very helpful suggestions. The Burch-Smith lab is supported by start-up funds from the University of Tennessee.

NOTE: The QFS procedure requires extreme care and caution by the user and we highlight these safety precautions here as Cautions and Notes where applicable.
1.Preparation for HPF Run
<ol>
	<li>Before beginning sample preparation, turn on the high-pressure freezer following manufacturer&#8217;s instructions. 
	NOTE: The HPF unit used in this protocol is a Wohlwend Compact 02 unit (Figure 1A), and it takes approximately one to one-and-a-half hours of start-up procedure be...

<ol>
	<li>Studer, D., Humbel, B. M., Chiquet, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electron+microscopy+of+high+pressure+frozen+samples+bridging+the+gap+between+cellular+ultrastructure+and+atomic+resolution.">Electron microscopy of high pressure frozen samples bridging the gap between cellular ultrastructure and atomic resolution.</a> Histochemistry and cell biology. 130, 877-889 (2008).</li><li>Studer, D., Hennecke, H., Muller, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-pressure+freezing+of+soybean+nodules+leads+to+an+improved+preservation+of+ultrastructure.">High-pressure freezing of soybean nodules leads to an improved preservation of ultrastructure.</a> Planta. 188, 155-163 (1992).</li><li>Bowers, B. a. M. M., Crang, R. F. E., Klomparens, K. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ch+2.">Ch 2.</a> Artifacts in Biological Electron Microscopy. 2, 13-42 (1988).</li><li>Mollenhauer, H. H., Crang, R. F. E., Klomparens, K. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ch+3.">Ch 3.</a> Artifacts in Biological Electron Microscopy. , 43-64 (1988).</li><li>Kiss, J. Z., Giddings, T. H., Staehelin, L. A., Sack, F. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparison+of+the+ultrastructure+of+conventionally+fixed+and+high+pressure+frozen/freeze+substituted+root+tips+of+Nicotiana+and+Arabidopsis.">Comparison of the ultrastructure of conventionally fixed and high pressure frozen/freeze substituted root tips of Nicotiana and Arabidopsis.</a> Protoplasma. 157, 64-74 (1990).</li><li>Moor, H., Steinbrecht, R. A., Zierold, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Cryotechniques in Biological Electron Microscopy. , 175-191 (1987).</li><li>Vanhecke, D., Graber, W., Studer, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Close-to-native+ultrastructural+preservation+by+high+pressure+freezing.">Close-to-native ultrastructural preservation by high pressure freezing.</a> Methods in cell biology. 88, 151-164 (2008).</li><li>Dahl, R., Staehelin, L. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-pressure+freezing+for+the+preservation+of+biological+structure+theory+and+practice.">High-pressure freezing for the preservation of biological structure theory and practice.</a> Journal of electron microscopy. 13, 165-174 (1989).</li><li>McDonald, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-pressure+freezing+for+preservation+of+high+resolution+fine+structure+and+antigenicity+for+immunolabeling.">High-pressure freezing for preservation of high resolution fine structure and antigenicity for immunolabeling.</a> Methods in molecular biology. 117, 77-97 (1999).</li><li>Hippe-Sanwald, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+of+freeze+substitution+on+biological+electron+microscopy.">Impact of freeze substitution on biological electron microscopy.</a> Microscopy research and technique. 24, 400-422 (1993).</li><li>Kiss, J. Z., McDonald, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electron+microscopy+immunocytochemistry+following+cryofixation+and+freeze+substitution.">Electron microscopy immunocytochemistry following cryofixation and freeze substitution.</a> Methods in cell biology. 37, 311-341 (1993).</li><li>Segui-Simarro, J. M., Austin 2nd, J. R., White, E. A., Staehelin, L. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electron+tomographic+analysis+of+somatic+cell+plate+formation+in+meristematic+cells+of+Arabidopsis+preserved+by+high-pressure+freezing.">Electron tomographic analysis of somatic cell plate formation in meristematic cells of Arabidopsis preserved by high-pressure freezing.</a> The Plant cell. 16, 836-856 (2004).</li><li>Kobayashi, K., Otegui, M. S., Krishnakumar, S., Mindrinos, M., Zambryski, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=INCREASED+SIZE+EXCLUSION+LIMIT+encodes+a+putative+DEVH+box+RNA+helicase+involved+in+plasmodesmata+function+during+Arabidopsis+embryogenesis.">INCREASED SIZE EXCLUSION LIMIT encodes a putative DEVH box RNA helicase involved in plasmodesmata function during Arabidopsis embryogenesis.</a> The Plant cell. 19, 1885-1897 (2007).</li><li>Austin, J. R., 2nd, L. A., Staehelin, <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-dimensional+architecture+of+grana+and+stroma+thylakoids+of+higher+plants+as+determined+by+electron+tomography.">Three-dimensional architecture of grana and stroma thylakoids of higher plants as determined by electron tomography.</a> Plant physiology. 155, 1601-1611 (2011).</li><li>McDonald, K. L., Webb, R. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Freeze+substitution+in+3+hours+or+less.">Freeze substitution in 3 hours or less.</a> Journal of microscopy. 243, 227-233 (2011).</li><li>Taiz, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Plant+Vacuole.">The Plant Vacuole.</a> The Journal of experimental biology. , 172-1113 (1992).</li><li>Wilson, S. M., Bacic, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+of+plant+cells+for+transmission+electron+microscopy+to+optimize+immunogold+labeling+of+carbohydrate+and+protein+epitopes.">Preparation of plant cells for transmission electron microscopy to optimize immunogold labeling of carbohydrate and protein epitopes.</a> Nature protocols. 7, 1716-1727 (2012).</li><li>Ding, B., Turgeon, R., Parthasarathy, M. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Substructure+of+freeze-substituted+plasmodesmata.">Substructure of freeze-substituted plasmodesmata.</a> Protoplasma. 169, 28-41 (1992).</li><li>McDonald, K. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+Embedding+Methods+into+Epoxy+and+LR+White+Resins+for+Morphological+and+Immunological+Analysis+of+Cryofixed+Biological+Specimens.">Rapid Embedding Methods into Epoxy and LR White Resins for Morphological and Immunological Analysis of Cryofixed Biological Specimens.</a> Microscopy and microanalysis. 20, 152-163 (2014).</li><li>McDonald, K. L., Auer, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-pressure+freezing,+cellular+tomography,+and+structural+cell+biology.">High-pressure freezing, cellular tomography, and structural cell biology.</a> BioTechniques. 41, 137, 139-141 (2006).</li><li>Spiegelhalter, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=From+dynamic+live+cell+imaging+to+3D+ultrastructure+novel+integrated+methods+for+high+pressure+freezing+and+correlative+light-electron+microscopy.">From dynamic live cell imaging to 3D ultrastructure novel integrated methods for high pressure freezing and correlative light-electron microscopy.</a> PloS one. 5, e9014 (2010).</li><li>McDonald, K. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+review+of+high-pressure+freezing+preparation+techniques+for+correlative+light+and+electron+microscopy+of+the+same+cells+and+tissues.">A review of high-pressure freezing preparation techniques for correlative light and electron microscopy of the same cells and tissues.</a> Journal of microscopy. 235, 273-281 (2009).</li></ol>

The authors have nothing to disclose.

The success of the protocol presented here depends heavily on the user. First, advanced preparation is required to ensure that all necessary materials are readily available and in sufficient quantity to complete an entire HPF-QFS run. Second, the user must work quickly, moving from step-to-step in an efficient manner that minimizes sample handling, thus minimizing changes to the native state of the tissue. Once samples are frozen and before they are dehydrated it is imperative that they be kept cold, so care must be take...

Our knowledge of cell ultrastructure comes mainly from electron microscopy, which can resolve details in the range of a few nanometers 1. Despite being so powerful in resolution TEM is not considered user-friendly, as sample preparation requires time-consuming and laborious protocols, and demands some expertise from the practitioner. Traditional fixation of samples has combined the use of aldehydes and osmium tetroxide before further processing that includes dehydration, embedding in resin and then sectioning to produce ultra-thin sections that are then stained with heavy metals. However, it is known that chemical fixation can produce artifacts including protein aggregation and loss of lipids 1, and changes to membranes that ultimately affect several cellular compartments 2. These artifacts are largely attributed to the slow rate of fixation and dehydration at room temperature 3,4,5.
Cryofixation by high pressure freezing (HPF) avoids most of the artifacts caused by chemical fixation. The principle of cryofixation is that it lowers the freezing point of water by 20 degrees, slows down the nucleation and growth of ice crystals and increases the viscosity of water in a biological sample so that cellular constituents are essentially immobilized 6, 7. HPF decreases a sample&#8217;s temperature to that of liquid nitrogen, under very high pressure (210 MPa or 2,100 bar) in milliseconds. When done properly HPF prevents formation of large ice crystals that can cause major damage to cell ultrastructure. HPF can be used to fix samples of 100-200 &#956;m thickness at typical concentrations of biological solutes 7. There are numerous reviews on the physics and principles underlying HPF, e.g.1,7,8.
After HPF, samples are incubated at low temperature (-78.5 &#176;C to -90 &#176;C) in the presence of liquid organic solvent containing chemical fixatives like osmium tetroxide, generally for a few days. At this low temperature, the water in the sample is replaced by the organic solvent, typically acetone or methanol 1,9. Thus, this process is called freeze substitution (FS). The sample is then gradually warmed and during this time is fixed, usually with osmium tetroxide and uranyl acetate 9. Crosslinking at low temperatures has the advantage of fixing molecules that are immobilized 1. FS therefore produces samples of superior quality compared to those fixed by conventional chemical fixation at room temperature, in particular it results in improved ultrastructural preservation, better preservation of antigenicity and reduced loss of unbound cellular components 10,11.
Most FS is carried out over long time periods, typically up to several days. This is particularly true for plants samples 12,13,14. A recent protocol developed by McDonald and Webb greatly reduces the time for FS from several days to a few hours 15. In their quick freeze substitution (QFS) procedure, FS is carried out over 3 hours, while in the super quick FS (SQFS) samples are processed in 90 minutes. The quality of samples produced by these methods is comparable to those yielded by traditional FS protocols. We have adopted the QFS protocol for downstream processing of plant samples after HPF. This has proven to save not only time but also money, as QFS and SQFS use common lab equipment instead of the costly commercially available FS machines.
Plant tissues are often very challenging to prepare for TEM. On average, plant cells are bigger than either bacterial or animal cells. The presence of hydrophobic waxy cuticle, thick cell walls, large water-filled vacuoles containing organic acids, hydrolases and phenolic compounds that may occupy up to 90% of the total cell volume 16, and the presence of aerated spaces severely decreases heat conductivity of the system 17. Further, in the case of plants, the sample thickness almost always exceeds 20 &#956;m, the limit for use of chemical fixation. At these thicknesses, the low heat conductivity of water prevents a freezing rate more than &#8211;10,000 &#176;C/sec in the center of the sample. That rate is required to avoid damaging hexagonal ice formation (ice crystals with a lower density and bigger than 10 to 15 nm) 8. Together, these present challenges to both proper freezing of the sample and subsequent FS. Nonetheless, cryofixation is the best method for fixing plant samples. Here a protocol for HPF-QFS of plant tissue samples is presented. It focuses on the model species Arabidopsis thaliana, but has also been used with Nicotiana benthamiana. The typical results demonstrate that HPF-QFS produces samples of comparable quality to traditional HPF-FS in a fraction of the time. With proper adjustments, this protocol may also be used for other relatively thick biological samples.

<table border="0" cellpadding="0" cellspacing="0" width="1070">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Name of Material/ Equipment</td>
			<td style="width:245px;">Company</td>
			<td style="width:119px;">Catalog Number</td>
			<td style="width:167px;">Comments/Description</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Wohlwend HPF Compact 02 High Pressure Freezing Machine</td>
			<td style="width:245px;">Technotrade International, Inc</td>
			<td style="width:119px;">HPF02</td>
			<td>With integrated oscilloscope to display freezing and pressure curves; PC (not included) is required for display&#160; of freezing parameters</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Holder for DN 3 x 0.5 mm aluminum apecimen carriers</td>
			<td style="width:245px;">Technotrade International, Inc</td>
			<td style="width:119px;">290</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Specimen carriers, P=1000, DN 3 x 0.5 aluminum, type A</td>
			<td style="width:245px;">Technotrade International, Inc</td>
			<td style="width:119px;">241-200</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Specimen carriers, P=1000, DN 3 x 0.5 aluminum, type B</td>
			<td style="width:245px;">Technotrade International, Inc</td>
			<td style="width:119px;">242-200</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Storage Dewar 20.5 L, MVE Millennium 2000 XC20</td>
			<td style="width:245px;">Chart</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Baker&#39;s yeast</td>
			<td style="width:245px;"></td>
			<td style="width:119px;"></td>
			<td>The older the better, to avoid excessive gas (CO2) production</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Tooth picks</td>
			<td style="width:245px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Thermocouple data logger EL-USB-TC</td>
			<td style="width:245px;">OMEGA Engineering Inc.</td>
			<td style="width:119px;">OM-EL-USB-TC&#160;</td>
			<td>Replacement battery purchased separately</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Temperature probe</td>
			<td style="width:245px;">Electron Microscopy Sciences</td>
			<td style="width:119px;">34505</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Heater block 12/13 mm</td>
			<td style="width:245px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Rotary shaker</td>
			<td style="width:245px;">Fisher Scientific</td>
			<td style="width:119px;">11-402-10</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Leaf punch - Harris Uni-core 2.00</td>
			<td style="width:245px;">Ted-Pella Inc.</td>
			<td style="width:119px;">15076</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Pink dental wax</td>
			<td style="width:245px;">Electron Microscopy Sciences</td>
			<td style="width:119px;">72660</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Cryogenic vials 2 mL</td>
			<td style="width:245px;">Electron Microscopy Sciences</td>
			<td style="width:119px;">61802-02</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Methanol</td>
			<td style="width:245px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Blow dryer</td>
			<td style="width:245px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Dry ice</td>
			<td style="width:245px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Liquid nitrogen</td>
			<td style="width:245px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Acetone</td>
			<td style="width:245px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:539px;">Forceps</td>
			<td style="width:245px;"></td>
			<td style="width:119px;"></td>
			<td>Several pairs</td>
		</tr>
	</tbody>
</table>

tandem high-pressure freezing and quick freeze substitution of plant tissues for transmission electron microscopy

Since the 1940s transmission electron microscopy (TEM) has been providing biologists with ultra-high resolution images of biological materials. Yet, because of laborious and time-consuming protocols that also demand experience in preparation of artifact-free samples, TEM is not considered a user-friendly technique. Traditional sample preparation for TEM used chemical fixatives to preserve cellular structures. High-pressure freezing is the cryofixation of biological samples under high pressures to produce very fast cooling rates, thereby restricting ice formation, which is detrimental to the integrity of cellular ultrastructure. High-pressure freezing and freeze substitution are currently the methods of choice for producing the highest quality morphology in resin sections for TEM. These methods minimize the artifacts normally associated with conventional processing for TEM of thin sections. After cryofixation the frozen water in the sample is replaced with liquid organic solvent at low temperatures, a process called freeze substitution. Freeze substitution is typically carried out over several days in dedicated, costly equipment. A recent innovation allows the process to be completed in three hours, instead of the usual two days. This is typically followed by several more days of sample preparation that includes infiltration and embedding in epoxy resins before sectioning. Here we present a protocol combining high-pressure freezing and quick freeze substitution that enables plant sample fixation to be accomplished within hours. The protocol can readily be adapted for working with other tissues or organisms. Plant tissues are of special concern because of the presence of aerated spaces and water-filled vacuoles that impede ice-free freezing of water. In addition, the process of chemical fixation is especially long in plants due to cell walls impeding the penetration of the chemicals to deep within the tissues. Plant tissues are therefore particularly challenging, but this protocol is reliable and produces samples of the highest quality.

Results presented below have been obtained using a Wohlwend Compact 02 for HPF (Figure 1A). One major advantage of this instrument is the ease of use of the specimen carriers and its holders. When using other instruments, McDonald recommends that two users should carry out the sample preparation and HPF, one preparing the samples while the other does the freezing and transfer to the FS cryovials 9. However, the Wohlwend specimen carriers and holder are easy enough for a single user to manipula...

Watch this Scientific Journal Video about Tandem High-pressure Freezing and Quick Freeze Substitution of Plant Tissues for Transmission Electron Microscopy at JoVE.com

Tandem High-pressure Freezing and Quick Freeze Substitution of Plant Tissues for Transmission Electron Microscopy

Obtaining high-quality transmission electron microscopy images is challenging, especially in the case of plant cells, which have abundant large water-filled vacuoles and aerated spaces. Tandem high-pressure freezing and quick freeze substitution greatly reduce preparation time of plant samples for TEM while producing samples with excellent ultrastructural preservation.

Since the 1940s transmission electron microscopy (TEM) has been providing biologists with ultra-high resolution images of biological materials. Yet, ...

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16.3K vues.  University of Tennessee, Knoxville. Obtaining high-quality transmission electron microscopy images is challenging, especially in the case of plant cells, which have abundant large water-filled vacuoles and aerated spaces. Tandem high-pressure freezing and quick freeze substitution greatly reduce preparation time of plant samples for TEM while producing samples with excellent ultrastructural preservation.