Cryopreservation is crucial to long-term maintenance of cells. Not even the most diligent and dedicated scientist is capable of maintaining a cell line day in and day out for years. Additionally, cells can change properties, senesce, or, ooops (!) become contaminated as the length of time in culture increases. Freezing and resuscitating your cells can be done easily with just a handful of reagents. Final storage of cells in liquid N2 will ensure a reliable, career-long source of cells.
Special materials needed: cultured cells, cell growth media, solutions for detaching cells from the plate if using adherent cells (e.g. balanced salt solution, trypsin/EDTA), tissue culture grade dimethylsulfoxide (DMSO), fetal bovine serum (FBS), cryovials, cell freezing chamber, -80°C freezer, and liquid N2 tank for long term storage.
1) Passage cells. Healthy, actively growing cells should be used for cryopreservation. I always try to freeze large numbers of cells that have not been passaged in culture too many times. I passage the cells 1-2 days prior to freezing to ensure that they are in a log rhythmic phase of growth at the time of freezing. Some labs recommend changing the growth media 24 hours prior to freezing if the cells have not been passaged at that time.
3) Resuspend cells in freezing medium and aliquot to cryovials. Freezing cells can be lethal; therefore a cryoprotectant is used to lower their freezing point. DMSO is the most common cryoprotectant used and is used as a 10% stock solution.
Caution: Wear gloves when working with DMSO as it easily penetrates the skin.
The most common freezing medium is 90% FBS/10% DMSO. For less finicky cells and for tissue culture on a budget, 10% DMSO in cell growth medium can also be used. After centrifugation, resuspend the cell pellet in 1mL of freezing medium/cryovial. Make sure you have cryovials designed for liquid N2 storage. I typically plan on 1 100mmdish of cells/cryovial. Vials should be labeled using a lab marker that will withstand alcohol and liquid N2. I am careful to include passage/lot numbers when labeling cryovials.
4) Freeze cells. To allow water to move out of the cells before freezing, freeze cells slowly. This is accomplished using some type of cell freezing chamber. Pricey freezing chambers pulse in liquid N2 periodically to control the freezing rate. Less expensive options include chambers that use room temperature isopropanol. Vials are placed in the chamber, isopropanol is added and the chambers are placed at -80°C for at least 4 hours. Those of us on a tight budget (like me!) use homemade chambers. I keep the Styrofoam™ racks that come with 15 mL conicals, place a cryovial in each hole, cover with a second rack, tape the racks together and place at -80°C.
5) Remember to move your cells to the liquid N2 tank. Probably one of the hardest things in this protocol is remembering to move your frozen cryovials to the liquid N2 tank! I usually allow my cells to freeze overnight and then quickly place the cryovials into the liquid N2 tank.
Special materials needed: frozen cells, cell growth media and a 37°C waterbath.
1) Remove cells from the tank and thaw. For the greatest cell viability, it is important to freeze the cells slowly. The opposite is true for thawing – thaw quickly! Remove cryovials from the liquid N2 tank and immediately place them in a 37°C waterbath. Keep the cryovials in the waterbath until just the tiniest ice crystal is left in the cryovial.
2) Transfer, spin (?) and plate. Immediately transfer the cells to a large volume of pre-warmed cell growth media (~10mL/1mL aliquot cells). For the next step, you have a decision to make. There are two schools of thought on whether the cryoprotectant should be immediately removed from the cells. In some labs, the cells are centrifuged and the cell pellet is resuspended in fresh cell growth media prior to placing cells in a culture dish. This is the way I first learned to thaw cells. However some data suggest that cells are extremely fragile after thawing and centrifugation may increase cell death. Therefore it is recommended to plate the cells and change the media at a later time. For adherent cells you can change the media as soon as the cells have attached to the dish. I switched to this method and I generally take my cells out just before leaving the lab for the night and change the media first thing the next morning.
3) Check your cells. About 24 hours after thawing cells I look them over carefully under the microscope to make sure they are healthy and behaving normally.
Knowing how to correctly freeze and thaw your cells will save you the embarrassment of asking a colleague for yet another aliquot of their cells or the expense of purchasing new cells.
If all else fails, remember to freeze slowly and thaw quickly!