Every research animal facility eventually reaches the same point. The corridor is full of cages holding genetically altered mouse lines that nobody is actively using, each one costing staff time, cage space and animals every week, simply so the line is not lost. Freezing those lines as sperm or embryos in liquid nitrogen solves the problem: the line stops being a breeding colony and becomes a set of straws that can be recovered years later. The science of mouse sperm and embryo freezing is well established. What decides whether an archive actually works, years later when a group needs a line back, is far more ordinary: the straws, the way they are sealed and labelled, how they are arranged in the tank, and how they travel.

This guide is written for facility managers, transgenic core staff and technicians who are building or tidying up a cryopreserved line archive. It focuses on the consumables, because that is where most recoverable problems start.

Why facilities archive lines rather than keep breeding them

Keeping a line alive only to preserve it uses animals that are never used in an experiment, which runs against the reduction principle of the 3Rs. A frozen archive removes that breeding pressure, protects a line against disease outbreaks, breeding failure and genetic drift, and allows lines to be shared between institutions as frozen material rather than live animals, which avoids the welfare and biosecurity issues of transporting mice. Once a facility decides to archive, the volume builds quickly. A single line becomes dozens of straws, and a busy core can be holding thousands of straws across hundreds of lines within a few years.

The straw is the sample

Mouse sperm is most commonly frozen in 0.25 ml French type straws, with each straw holding a small aliquot, typically around 10 µl of sperm suspension, separated by air and medium. Embryos are also commonly frozen in straws. That makes the straw itself the most important consumable in the archive: it has to seal reliably, survive plunging into liquid nitrogen, survive years of storage and survive thawing without cracking.

For general work, CBS sterile clear PETG straws are available in 0.25 ml and 0.5 ml. Where a facility wants a closed, leak proof format, often because lines are shared with external groups or stored alongside material from other sources, CBS High Security straws come in 0.3 ml and 0.5 ml with a sliding cotton safety plug and are sealed with the CBS SYMS sealer. Whichever straw is chosen, the most useful decision is to standardise on one format across the whole archive. Mixed straw types mean mixed visotube diameters, mixed sealing methods and mixed thawing procedures, and that is where mistakes creep in.

How many straws, and how to split them

An archive should be planned around how many recoveries each line might need, not around a single attempt. One large US repository asks investigators for at least 25 straws per line when it freezes sperm, and many facilities freeze a similar number so that a line can be test thawed for quality control, recovered more than once and shared without running the stock down.

Those straws should never all sit in one place. The safest practice is to divide each line between at least two storage vessels, ideally in separate rooms or buildings, so that a single vessel failure or a missed top up cannot remove a line entirely. Our breakdown of the timeline from vacuum failure to sample loss shows how much time a team really has when a vessel starts to fail, and why a second copy elsewhere matters more than any alarm.

Visotubes, goblets and canes: giving every line an address

Loose straws in a tank are lost straws. Each line should be held in its own container so that it can be found, counted and removed without disturbing anything else. CBS Round Visotubes are made from PETG, come in diameters from 7.1 mm to 17.5 mm and accept 0.25 ml, 0.5 ml and CBS High Security straws, which makes one visotube per line, or per split of a line, an easy rule to follow. Visotubes sit in CBS Classic Goblets on aluminium cryocanes, protected by cryosleeves, or inside canisters in a canister vessel. Facilities that prefer flat storage often use storage cassettes, which come in a range of colours and lengths.

The internal layout of the tank should follow from that choice, not the other way round. Our guide to how the consumable chain decides your tank furniture explains the logic. For a straw based mouse archive, a canister vessel such as the CryoCan 47-6 lets each canister be given to a research group, a project licence or a range of line numbers.

Identification that survives liquid nitrogen and staff turnover

The person who thaws a line is often not the person who froze it. Every straw needs to carry its line identifier, the freezing date and a straw number, and every visotube and goblet should be readable at a glance while the operator is working quickly over an open tank. Colour coding helps: coloured ID rods for visotubes and identification tabs for canes and visotubes can be assigned to research groups or to the primary and backup copies of each line.

Handwritten marking needs ink made for the job. The fine tip Cryomarker uses alcohol resistant ink that stays legible down to around minus 200°C and comes in four colours. For printed labels, the Brady BMP21 label printer works with Brady B-595 low shrink vinyl labels, which are designed to stay attached through temperature changes, moisture and handling. Physical labels should always mirror a digital inventory that records every straw’s vessel, canister, goblet and position, and that inventory should be backed up away from the facility.

Moving lines between institutions

Sharing frozen lines is one of the main reasons to archive, and it is also the point where straws are most at risk. Frozen straws should travel in a charged dry shipper, which holds the samples in vapour phase with no free liquid and is accepted by couriers for this work. The compact CryoStork V2 suits shipments of a few lines, and a cryogenic data logger with a PT1000 probe gives the receiving facility a temperature record of the whole journey. Pack each line in its own visotube inside the shipper so the receiving team can identify and transfer it quickly. Our guides to shipping frozen material internationally and what goes wrong in cryogenic transport cover the paperwork and handling side, and dry shipper hire or purchase helps decide whether to own a shipper or hire one as needed.

PPE and the room itself

Freezing and archiving involves frequent work over open liquid nitrogen: plunging straws, sorting visotubes and transferring material between vessels and shippers. Staff should have cryogenic gloves sized correctly, a full face shield and suitable aprons for decanting and transfers. Archive rooms in animal facilities are often small internal rooms, so ventilation and oxygen monitoring need checking against the volume of nitrogen held there. Our oxygen depletion calculator and guidance on liquid nitrogen hazards are a sensible starting point, and running costs are set out in our guide to the annual cost of liquid nitrogen.

Keeping the archive honest

An archive is only as good as its last check. Spot checking a sample of lines against the inventory, confirming both copies of a line still exist, reviewing nitrogen levels and replacing cracked visotubes or worn labels should be part of a routine, not a reaction to a lost line. Our annual cryo storage self audit gives a structure that works well for line archives as well as clinical stores.

Setting up or tidying a mouse line archive

If you are planning a new archive or bringing order to an existing one, send us the number of lines you hold, how many straws you freeze per line, the straw type you use, whether you keep a backup copy in a second vessel or building, and how often you send lines to other institutions. We will come back with a matched set of straws, visotubes, goblets or cassettes, identification, PPE and storage, drawn from our storage accessories range, with next day delivery on standard UK stock and shipping to research facilities worldwide.