Freezing sperm makes it possible to preserve genetically modified animal lines. Cryopreservation thus reduces the need to keep animals alive in animal houses. at the University of Lausanne’s Faculty of Biology and Medicine, Michelle Blom looks after these precious genetic resources.
Sperm cryopreservation, in vitro fertilization or "IVF", embryo transfer - these are familiar terms when talking about assisted reproduction techniques in humans. What is less well known is their application in the field of animal experimentation. Today, these procedures have become essential levers for applying the 3R principles: replacing, reducing and refining the use of animals for research.
Since 2018, the Faculty of Biology and Medicine (FBM) at the University of Lausanne (UNIL) has had a unit dedicated to cryopreservation. Its main objective: to store, in liquid nitrogen tanks, the genetic heritage of elaborate lines. at the head of this platform, Michelle Blom , animal facility manager, ensures the proper conservation of the material. She is also committed to convincing skeptical scientists of the usefulness of this approach. Interview.
Why is cryopreservation essential to limit the number of animals bred for research?
This method makes it possible to preserve, for decades to come, the genetic material of animals bred for a specific research purpose - without having to maintain living colonies. Creating a genetically modified line can take years: it involves numerous crosses over several generations, until we obtain an animal model, such as mice, adapted to a very specific biological question. This process is long and costly, and the animals obtained are therefore precious.
When a project comes to an end and a line is less used, the question arises: should we continue to breed these mice, "just in case"? But this means housing individuals that may never be used, with all the costs, workload and ethical implications that this entails. In addition, there are risks. For example, an infectious disease could decimate a colony, and over time, the line may undergo unwanted genetic changes. Cryopreservation offers a safe alternative: by freezing the sperm, we can preserve the essential genetic baggage and reactivate it if necessary. This is why we advise scientists to freeze their lines as soon as possible.
How does cryopreservation work in practice?
at the BMF, we mainly freeze sperm, as this is the most robust, efficient and space-saving method. A tiny volume of biological material contains a gigantic quantity of genetic material. this means that with a single tube containing ten microliters of sperm, i.e. one hundredth of a milliliter, we can revive a large number of animals. Indeed, in such a tube, called a "straw", there can be up to a million spermatozoa, each of which can potentially fertilize an oocyte and give birth to an embryo.
Of course, not all survive freezing and thawing, but thanks to techniques developed by Japanese experts - the world leaders in this field - we limit losses considerably. For each line, we prepare around twenty straws. That’s enough to revive a colony. And logistically, it takes up very little space: storing straws in the freezer doesn’t take up much room.
What happens when you want to "reactivate" a mouse line?
The method we use is in vitro fertilization (IVF), followed by embryo transfer - the principle is similar to that used in humans. First, we induce what we call "superovulation" in a female, by injecting her with a cocktail of hormones that stimulate oocyte production. The resulting oocytes, together with thawed spermatozoa, are then placed in a culture dish. The next day, we observe the number of two-cell embryos, a sign that fertilization has worked. Several embryos are then transferred to the uterus of a female carrier. Around a third implant and develop. Three weeks later, the offspring are born.
Are these offspring identical to the animals whose sperm was frozen?
Not immediately. In general, we use wild females, i.e. not genetically modified. The offspring of this first generation therefore have 50% of the father’s genetic material. The other half of the genes come from the mother. Individuals from this first generation must then be crossed to find, according to the laws of genetics, individuals with the desired genetic profile. This may seem tedious, but in mice, the reproduction cycle is rapid: in less than three months, a complete line can be reconstituted.
Does this process easily convince scientists?
In the past, there was some reluctance, often linked to the fear of wasting time or the idea that revival was risky. But our experience shows that it’s a fast, reliable technology. Above all, it makes a real difference to animal welfare. By reducing the number of animals bred that may not be used, we are taking concrete steps towards more ethical research. Since the creation of our unit at the BMF, demand has continued to grow. This is why we are planning to expand the platform to meet growing needs.
