Step-by-Step Cryopreservation: Pooling, Freezing, Thawing, and IVF Protocols


1. Collect sperm and pool in extender E400G
2. Measure cell density
3. Dilute with E400G. Target: 4×108 - 2×109cells/ml
4. Add RMMB Cryoprotectant solution (1V sperm + 3V RMMB)
5. Aliquot into cryovials (20 µl) and cap
6. Freeze
7. Store in LN2
8. Squeeze eggs, pool or split clutches as needed; store max. 10 minutes in humid chamber
9. Retrieve & thaw sample
10. Add SS300 with or without milk
11. Activate Sperm
12. Fertilize
13. Do NOT swirl or mix fertilized eggs/dish. Wait 2 minutes
14. Add embryo medium; rear embryos

A. Collection and pooling of sperm

1. Prepare a 0.5 mL microcentrifuge tube with E400G for each fish line (see Note1) to estimate E400G starting volume). Use a separate microcentrifuge tube for each fish line.
To make 1 L of E400G, combine the following in:
- 800 mL dH2O:
- 9.70 g KCl,
- 2.92 g NaCl,
- 2.0 mL of 1.0 M CaCl2 (or 0.29 g CaCl2 * 2H2O),
- 1.0 mL of 1.0 M MgSO4 (or 0.25 g MgSO4 * 7H2O),
- 1.8 g D-(+)-Glucose,
- 7.15 g HEPES, and
- 1 g gelatin from cold water fish skin (Sigma #G7041).
- Add dry components first, stir to dissolve, add liquid components, and stir.
- Adjust the pH to 7.9 with 5 M KOH and bring the final volume to 1000 mL with dH2O.
It is advisable to check the osmolality of E400, which should be close to 400 mmol/kg. The solution should be filter sterilized and stored at 4 ºC. The resulting E400G contains 130 mM KCl, 50 mM NaCl, 2 mM CaCl2, 1 mM MgSO4, 10 mM D-(+)-Glucose, 30 mM HEPES-KOH (pH 7.9), and 0.1 % w/v gelatin.

2. Keep E400G and the collected sperm at room temperature throughout the collection and prefreezing procedures.

3. Sedate or preanesthetize males as needed for at least 10 min before the procedure (see Note2)). Prepare and store a tricaine (MS-222) stock solution as follows:
4 g/L in dH2O, pH adjusted to 7.0 with Tris-HCl (pH 9.0). To prepare the MS-222 preanesthesia solution of 48 mg/L, dilute 12 mL MS-222 stock solution in 1000 mL fish water.

4. For anesthesia, dilute 4.2 mL tricaine stock solution in 100 mL fish water (168 mg/L). Anesthetize two to three males in 168 mg/L MS-222.

5. Briefly rinse a male in phosphate buffered saline (PBS) isotonic fish rinse, dry it by rolling gently on a soft and absorbent paper towel (see Note3)), and place it belly up in a dampened sponge/foam holder (in a 35 * 10 mm Petri dish). The isotonic PBS fish rinse is prepared from PBS (pH 7.4) powder packets (Sigma #P3813) that are dissolved in 870 mL dH2O. The final osmolality should be approximately 315 - 325 mmol/kg. Place the sponge holder under a dissecting microscope with incident lighting.

6. Place the end of a calibrated 10 mL borosilicate glass microcapillary (Drummond #2-000-010) on the urogenital opening.

7. Use rubber-tipped Millipore forceps (Millipore #XX6200006P; rubber tips made from heat-shrink tubing) to apply gentle abdominal pressure to the sides of the male. Move the forceps gently from anterior to posterior toward the urogenital opening. Collect sperm into the microcapillary.

8. Expel sperm immediately into the E400G solution in the microcentrifuge collection tube.

9. Transfer fish into fresh system water for recovery from anesthesia.

10. Continue collecting sperm from all males from the same family/stock and pool into the same E400G microcentrifuge tube. Repeat steps 4 - 8 for each male.

B. Estimation of sperm cell densities and sample dilution

At ZIRC, we use a NanoDrop 2000 spectrophotometer to determine the cell density by light absorption at 400 nm (see Note4)). The worksheet we use to determine NanoDrop cell counts is available in the Cryopreservation and IVF section on the ZIRC site: (see Note5)).

1. Estimate the volume of pooled sperm in E400G in the microcentrifuge tube for each line using a Pipetman. Draw the sperm into the tip and adjust the pipette volume until all of the solution fills the tip. Record the estimated volume for later use. While measuring, gently pipette the sperm to mix completely.

2. Prepare a 1:10 dilution of the sperm solution in E400G for a density measurement. If the collected sample appears less opaque (i.e., less concentrated), a 1:5 dilution can be used. Pipette 9 mL (or 4 mL for a 1:5 dilution) E400G into a 0.6 mL microcentrifuge tube and add 1 mL of the sperm suspension.

3. Mix the diluted sample by flicking the tube and keep the tube at room temperature. Optional: Use separate (colored) microcentrifuge tubes for each line (see Note6)).

4. Calibrate A400 absorption of a blank E400G sample using the NanoDrop’s Cell Cultures menu. Set the Absorbance Cursor to 400 nm and add 1.5 mL E400G to the spectrophotometer. Read the E400G blank sample to verify that it has set the instrument’s readings close to 0. If a blank sample reads higher than 0.005, recalibrate with a fresh 1.5 mL aliquot of E400G. When prompted, save the NanoDrop data with the date of the freeze event and line name(s).

5. To measure the absorption of a sample, enter the sample ID on the Measure Cell Cultures page.

6. Mix the sample well using a vortex mixer set at intermediate speed (~1300 rpm).

7. Immediately load 1.5 mL of the diluted sperm and read the cursor absorbance (AOD400; select the measure button in the left-hand corner). Readings above 0.3 are acceptable, and you can proceed with the dilution. Any reading at or below 0.2 should be diluted 1:5 and repeated.

8. Measure each sample three to five times and calculate the average A400 for all samples. A minimum of three successful readings is optimal.

9. Clean the NanoDrop using a clean Kimwipe and deionized water. Wipe the top arm (mirror) and the bottom lens with a moistened Kimwipe and then dry completely with a Kimwipe before closing the arm.

10. Calculate the cell density for each sample with the averaged A400 using the Excel cell density calculator.

11. Dilute the sperm with E400G according to the desired number of samples or sperm concentration. Optimal sperm cell densities should be between 4.0*108 and 1.6*109 cells/mL. This density range will result in samples with 2.0*106 - 8.0*106 cells/sample (see Note7)).

C. Sperm freezing

1. Prepare RMMB cryoprotective medium (100 mL) in the following order:
- Combine 20.0 g D-(+)-raffinose pentahydrate (Sigma R7630 or 83400) and 70 mL dH2O in a 250 mL beaker.
- Then, place the beaker in an evaporating dish (Pyrex 3140) or large beaker containing hot water (~70 ºC) on a stir plate.
- Stir the mixture until the raffinose is completely dissolved.
- Add 2.5 g skim milk (Difco #232100) and stir until it is completely dissolved.
- Let the solution cool to room temperature and add 3 mL of 1 M Bicine-NaOH (8.0).
- Add 6.67 mL absolute methanol (acetone-free, absolute, Certified ACS Reagent Grade), transfer the mixture to a 100 mL volumetric flask, adjust the final volume to 100 mL with dH2O, and mix by inversion three to four times.
- Transfer the solution to two 50 mL conical tubes and centrifuge at 15,000g for 20 min at 25 ºC.
- Transfer the clear supernatant into a clean beaker and aliquot into 1.5 mL microfuge tubes, 1 mL each, or a different convenient volume for daily use.
- Store the RMMB solution frozen at -20 or -80 ºC until use.
The resulting RMMB cryoprotective solution will contain 20 % (w/v) D-(+)-Raffinose pentahydrate, 2.5 % (w/v), Difco skim Milk, 6.67 % (v/v), Methanol, and 30 mM Bicine-NaOH.

2. Fill a Styrofoam container or cooler (z30 cm   30 cm) with powdered dry ice made from liquid CO2 (Carmichael et al., 2009) (see Note8)).

3. Prepare 15 mL Falcon tubes (Falcon 352,096) with an empty Matrix cryovial tube (0.5 mL Matrix Screw Top Storage Tubes, Thermo Scientific, Item #3745-BR or 2 mL Corning vials, Item #430488) functioning as a spacer in each along with the Falcon tube caps so that the tubes are ready to hold samples immediately after the cryovials have been capped.

4. Prepare labeled sample cryovials as needed prior to freezing (all samples should be labeled with the freeze date and allele number and/or line identification number). Additional vial color coders (colored caps or cap inserts specific for vial type) are helpful to distinguish freeze events or different stocks.

5. If you are using a multicapper, prepare it with the appropriate caps already attached before adding cryoprotectant (RMMB) to sperm.

6. For each sample, determine the volume of RMMB (RMMB volume = 3 x the sperm volume, see Note9)).

7. Add RMMB to sperm and mix by pipetting.

8. Immediately aliquot 20 mL into the prelabeled 0.5 mL cryovials.

9. Without delay, cap the cryovials (use an automated capper for rows of eight tubes, if available) and place the cryovials into the 15 mL conical tubes (containing a Matrix cryovial spacer).

10. Cap the conical tubes and drive the tubes into the dry ice until the caps are flush with the surface (see Note10)).

11. Freeze the samples in dry ice for 20 - 40 min and then quickly transfer them to a cryogenic freezer box submerged in LN2.

D. Egg collection

1. Place females in a tank with preanesthesia solution at least 10 min before full anesthesia. However, fish can be held in a preanesthesia solution for several hours (see Note11)).

2. Anesthetize females in tricaine/MS-222 solution.

3. Rinse fish in isotonic PBS and blot dry by gently rolling on a paper towel.

4. Place fish on its side in a small Petri dish (35 or 60 mm).

5. Dampen fingers with PBS fish rinse.

6. Expel eggs by applying gentle (light) finger pressure on the ventral abdomen and move your finger from anterior to posterior. Eggs will be expelled readily if the female is ready (see Note12)).

7. Transfer the female to a recovery tank.

8. Combine several clutches of eggs if needed by gently moving eggs to another dish with a fine-tipped paint brush dampened with isotonic PBS (see Note13)).

9. Maintain pooled eggs in a closed dish in a moisture chamber at room temperature no longer than 5e10 min before IVF.

E. Sperm sample thawing

1. To prepare 1L Sperm Solution SS300,
- combine 8.2 g NaCl,
- 5 mL of 1 M KCl (or 0.37 g KCL),
- 1 mL of 1 M CaCl2 (or 0.15 g CaCl2*2H2O),
- 1 mL of 1 M MgSO4 (or 0.25 g MgSO4*7H2O),
- 1.8 g D-(+)-Glucose,
- 20 mL of 1 M Tris-Cl (pH 8.0)
- in 800 mL dH2O.
- Add all dry ingredients first and stir until they are dissolved. Then add liquid ingredients and mix.
- Bring the final volume to 1000 mL with dH2O and check the osmolality (should be approximately 300 mmol/kg).
- Filter sterilize the solution and store it at 4 ºC.
The resulting SS300 solution will contain 140 mM NaCl, 5 mM KCl, 1 mM CaCl22, 1 mM MgSO4, 10 mM D-(þ)-Glucose, and 20 mM Tris-Cl (8.0). It is used when the cryopreotective medium already contained skim milk. The milk helps to prevent sperm tails from sticking and tangling and is thought to contain antioxidants that protect against oxidative damage during cryopreservation and thawing.

2. To prepare Sperm Solution SS300 with 2 mg/mL Difco Skim Milk (SS300 + milk),
- Add 100 mg Difco Skim Milk to 50 mL SS300 and stir or vortex to dissolve.
- Aliquot the solution into microcentrifuge tubes and store frozen at -20 ºC.
- Thaw and use at room temperature.

3. Remove the cryovial from the LN2 and quickly open the cap to vent any LN2 in the vial (see Note14)).

4. Thaw the cryovial in a 38 ºC water bath until the frozen pellet is less than 3 mm in diameter (takes approximately 10 - 15 s.

5. Immediately add 150 mL room-temperature SS300 solution to the cryovial. If you are thawing sperm that was frozen without milk (see Note15)), add 2 mg/mL Difco Skim Milk (Difco #232100) to the SS300 solution. See Note16) for an optional postthaw motility assessment.

F. In vitro fertilization

1. Add 200 mL dH2O to the cryovial to activate the sperm.
- Gently mix the sperm 1 - 2 times with a micropipette and transfer the sample to the eggs:
- slide the pipette tip sideways along the bottom of the Petri dish from the edge of the pile of eggs into the center.
- Expel the activated sperm into the mass of eggs (not on top of the eggs).

2. Start a 2-minute countdown timer.

3. Do not move, mix, or swirl the dish; let it sit completely undisturbed for 2 minutes, then flood the dish with embryo medium.

4. Determine the fertilization rate 2 - 4 h postfertilization or as soon as cell divisions are recognizable.
- Count embryos and remove unfertilized eggs (see Note17)).

G. Optional sperm motility assessment

A good way to assess the quality of sperm is to observe its motility. Computer-assisted sperm analysis software systems provide an objective and comprehensive quantification of the density and motility parameters, but a manual, subjective assessment is feasible in most labs and is sufficient for most sperm-freezing applications. A compound microscope with a 10x or 20x objective and DIC or dark field illumination is ideal. Osmolality affects both the speed and duration of sperm motility. Fresh (prefreeze) sperm will be faster and have a higher percentage of motile cells than postthaw samples.

To estimate the prefreeze motility,
1. Place 6 mL dH2O on a microscope slide.

2. Add 0.5 - 1 µL of the final sperm dilution (in E400G) to the drop and then mix the drop and spread it quickly with the pipette tip.

3. Observe immediately (see Note18)).

To estimate the postthaw sperm motility,
1. Thaw a sperm sample in a water bath as described below,

2. add 150 µL of SS300 solution to the thawed sperm, and mix gently.

3. Remove 10 - 20 µL of the sample for motility assessment (see Note19)).

4. Place 5 mL dH2O on a slide and add 4.25 µL of thawed sperm/SS300 solution (see Note20)).

5. Mix briefly with the pipette tip on the slide and observe immediately.

Protocol Notes

A few general guidelines for selecting the starting volume of E400G into which sperm is collected are provided. The use of an appropriate starting volume will ensure that the sperm concentration is as optimal as possible in the resulting samples without activating the cells. The estimated starting volume of E400G depends on:
(a) the number of available males (short-fin fish typically give more sperm than long-fin fish);
(b) the number of desired samples. To calculate a conservative starting volume of E400G, use the following equations.
- Short-fin fish: (# males-2) x 5 µl E400G
- Long-fin fish: (# males-4) x 5 µl E400G

Preanesthesia/sedation of males is recommended for lines sensitive to bleeding from the gills during anesthesia. Males should be placed in preanesthesia tricaine solution (48 mg/L MS-222 in fish water) for 10-30 min prior to anesthesia. They can remain in preanesthetic for several hours if needed.

Eggs and sperm are activated by water, and it is virtually impossible to dry fish completely. Therefore, it is important to rinse anesthetized males and females with isotonic PBS solution before gamete collection to avoid premature activation of sperm or unfertilized eggs.

The density of sperm cells in the extender and cryoprotective medium plays a crucial role in optimal cryopreservation, thawing, and reactivation. While satisfying results can be achieved without cell density estimation and adjustment, we strongly recommend measuring concentrations prior to freezing to obtain the best possible recovery results. Too few cells (or too many) in relation to the available solution ingredients will not freeze as well as a well-adjusted cell density. There are two key benefits to adjusting cell densities:
(1) further dilution of sperm (if possible) generates more samples and a longer lasting resource, and
(2) an optimal ratio of cells to available salt and buffer molecules ensures that the solution toxicity and osmotic shock are reduced, and as a result, cells suffer less damage during freezing and thawing.

This calculator helps to estimate and adjust cell densities in most cases. However, because several variables can influence the readout, it is recommended to develop a calibration curve and formula like this with your own equipment. Download the complete ZIRC cryopreservation and IVF protocol (PDF) and Excel sperm density calculator files from the ZIRC website: This worksheet/calculator has been generated with ZIRC equipment, and it should generate results within the correct order of magnitude. It may or may not be necessary to calibrate a new A400 absorption curve for your own spectrophotometer for a more accurate conversion. To generate a curve specific to your equipment, use a hemocytometer or Makler chamber to determine cell densities microscopically. Determine the A400 of a corresponding dilution series with your spectrophotometer and generate a curve from the dilution series. Calculate a best-fit curve (or line) from your absorption data and use the formula in a worksheet to determine the cell density that corresponds to the A400 of unknown cell densities.

Using a designated colored microcentrifuge tube for the NanoDrop dilution makes it easily distinguishable from samples of other stocks.

If your priority is to achieve a particular number of samples, you can dilute the sample as far as 4*108 cells/mL (2*106 cells/sample). If your priority is to ensure that a line is reactivated with only a single thaw, you may want to leave the concentration higher and add less E400G for a lower total volume. Cell densities should not be lower than 4*108 cells/mL for best results. If more than 12 samples are going to be frozen, divide the final sperm volume into two (or more) aliquots. In addition, 0.5-1 µL of the final diluted sperm suspension can be removed for an optional prefreeze motility assay. Computer-assisted sperm analysis or microscopic visualization can be used before and/or after freezing samples to assess sperm cell motility. The complete absence of motile sperm cells indicates that cells may have been irreversibly damaged. In this case, IVF can be used to determine whether a pool of samples fertilizes at all. Low fertilization rates can sometimes be obtained from low-motility/nonmotile samples.

A cooling rate between -10 and -15 ºC per minute was determined to be the optimal range for the materials and solutions used in this protocol. Using powdered dry ice with stacked cryovials in a 15 mL Falcon tube will achieve a cooling rate of 10-15 ºC/min. If possible, avoid changing the materials specified in this protocol. If the specified materials are not available (for example, the 15 mL vials or cryovials with the same wall material and thickness as specified here), you need to determine which freezing rate provides the optimal postthaw fertilization rates with the replacements.You may also have to adjust the thawing protocol to match the new freezing rate. The description for making powdered dry ice is included in the online protocol:

Before use, thaw RMMB aliquots in a water bath or heating block at 45-50 ºC. Raffinose precipitates if the RMMB solution is kept on ice. If precipitation occurs, heat the solution slightly to get it back into solution prior to use. Cool the solution to room temperature before mixing with sperm. Keep diluted sperm and RMMB at room temperature.

Because the RMMB cryoprotective medium is toxic to sperm cells, protocol steps 6-9 should be carried out swiftly with as little delay as possible once the RMMB is mixed with the sperm. However, our research suggests that cell viability is not affected significantly before freezing, when sperm is exposed to cryoprotective medium for up to 5 min. To minimize exposure time you can divide the final sperm volume into smaller aliquots (e.g., If more than 12 samples are to be frozen) and freeze them in two (or more) rounds (steps 6-9).

Egg collection and IVF of eggs should be performed early in the morning. Egg quality is optimal the first few hours after facility lights have been turned on. The resorption of eggs begins 2-3 h after daybreak.

Good eggs will be golden in color and have little fluid and no opaque or white eggs intermixed. Eggs can be gently moved away from the fish or split into different dishes using a fine water-color brush.

If females do not provide large enough clutches, or to maximize the number of embryos produced by IVF, it is helpful to combine several clutches of eggs. A soft water-color brush is an effective and gentle tool for moving and mixing eggs. Dampen the fingers and paint brush with the isotonic PBS solution before squeezing females or manipulating eggs.

Before squeezing females, sperm samples can be retrieved from the LN2 freezer and maintained in a tray of LN2 within a small Styrofoam cooler until eggs are available.

SS300 + milk is used for thawing sperm samples that were frozen with a cryopreservation medium not containing skim milk powder (other protocols). The milk helps to prevent the sperm tails from sticking and tangling and is thought to contain antioxidants that mitigate oxidative damage.

For the postthaw motility assessment, a small portion (10=20 µL) of the sperm/SS300 mix can be removed and held in a microcentrifuge tube at room temperature if assessed immediately or on ice if stored for later (see the “Motility assessment” section).

A fertilized egg will be visible by the swelling of the first embryonic cell (zygote) and the chorion or the first cell divisions. Because fertilization and the emergence of the first cell and its division are usually not perfectly synchronized, the number of fertilized eggs can be more conveniently determined during the blastula stages after several rounds of cell divisions. The fertilization rate can be obtained by dividing the number of embryos with proliferating cells (fertilized x 100) by the number of total viable eggs.

Examining the remainder of the NanoDrop dilution, if performed, is a good use of the sperm. Checking the prefreeze motility confirms the viability and concentration of the sperm being frozen.

At this point, a small portion (10-20 µL) of the sample can be removed and held on ice for motility assessment. The remainder of the sample can be used for IVF as described earlier. It is best to view the motility as soon as possible after thawing, but samples are typically stable on ice for 10-20 min.

For postthaw motility, activating the sperm on a slide in the same relative proportions as in the IVF procedure gives a consistent method and provides a good sense of sperm concentration and motility as it is applied to the eggs.

protocols/step-by-step_cryopreservation_protocol.txt · Last modified: 2021/11/01 11:51 by zoltan