Core breeding at the Wallenberg laboratory

The core breeding unit provides a unique possibility to reduce breeding of animals in the research units. The core breeding unit is intended for the maintenance of transgenic mouse colonies only.

Here, a small number of cages (4-7 cages) can be kept for long periods of time. When needed, the colonies can be expanded in the research unit and thereafter used in research at the facility or further transported to other facilities. 

In the Djurhälsoprojekt (Dnr 1-349/2013) report from Comparative Medicine (KM), different barriers are described based on several parameters. Read more about Barriers

The main idea of this classification is that the implementation of different barrier levels at KI could create better conditions for animal research. That includes the possibility to improve migration of animals between different units without violating the health status of the different areas. One of the defined barriers is the core breeding barrier (Barrier B2) at the KM Wallenberg Facility (KM-W). 

Purpose of core breeding 

Maintaining live mice free from infections and genetically unchanged is the major purpose for keeping these animals in a protected and enclosed environment. This can be achieved by keeping a small set-up of breeding pairs for a specific strain in an area that is separate from all other barriers and to which only a limited amount of persons have access to. In this way the core breeding unit serves as a live back-up repository for the most valuable and regularly used mice.

The core breeding is used as the basis (foundation stock colony) for each strain and is therefore a basic principle for producing animals for the expansion and production (research) colonies. It is not allowed to place mice inside the barrier without prior re-derivation. Even access of researchers is prohibited regarding the B-barrier.

Both the core breeding and the expansion breeding are based on a strict inbreeding schedule (brother sister mating). The size of a core breeding nucleus should be around 10 to 50 breeding pairs, whereas 10 to 15 breeding pairs are normally sufficient (depending on the breeding performance of the strain). The core breeding pairs should be refreshed every 6 month, to avoid a decrease of the breeding performance. The thorough genetic control and the refreshing of the gene pool of the strains are major prerequisites to maintain the strains in the core breeding unchanged. A genetic drift can result in the loss of the genetic modification, a changed phenotype due to mutations or loss of the initial phenotype. 

Fig. 1: The pyramid is showing the standard steps in a breeding scheme. The top of the pyramid shows the core breeding standard with refreshing of the gene pool.

a. Core breeding standard

Core breeding standard normally starts with the cryopreservation (embryo- or sperm-freezing) and re-derivation of the strain to be placed into the core breeding barrier (see figure 1). This standard way of core breeding should be done:

  • For valuable animals 
  • For animals that are frequently used in research  
  • For all newly produced or backcrossed animal  

b. Expansion breeding 

To expand the stock, breeding pairs from either the core breeding or directly after re-derivation are transferred into an expansion colony. 
The characterization of this colony is: 

  • Strictly inbreeding 
  • Physical separation from the core breeding area Housing of the mice in IVC 
  • No or only reduced researcher access (depending where the bredding is situated) 
  • A larger amount of cages 

​c. Production/Research breeding

 The next step after the expansion breeding is the production/research breeding. This colony is characterized by: 

  • Random breeding of the animals (no inbreeding necessary) 
  • Physical separation from the core and expansion breeding 
  • Housing of the mice in IVC 
  • Full researcher access 
Animal experiment