For the Love of Mouse! How to Start Working with Mice

Researchers have always been in search of model organisms that can be used to study and explore biological phenomena to make discoveries that can be extrapolated to more complex higher organisms like humans. Of the various model organisms developed and used, starting from the ubiquitous E. coli and S. cerevisiae to the humble D. melanogaster and C. elegans, we have come a far way to using the mouse as a common laboratory model organism.

Here are some helpful bits of information about this good old labmate:

Some Basic Information and Demographics

• Scientific name: Mus musculus
• Number of chromosomes: 20+20
• Adult mouse weight: 20 to 40 grams (depending on age and sex)
• Life span: 1 to 3 years
• Reproductive life: about 8 months (longer in males than females)
• Sexual maturity: 4 to 7 weeks of age (depending on strain)
• Female estrous cycle: 4 to 6 days
• Gestation period: 18 to 20 days
• Weaning age: 21 to 28 days after birth
• Typical litter size: 4 to 12 pups (depending on strain)

Why the Mouse?

Mice are placed further than humans in the phylogenetic tree and no evident similarities exist in regards to size and shape between mice and humans, raising the question of why mice are the most commonly used laboratory animal.

This is why:

• Though it may not look like it, mice are biologically similar to humans, sharing 95% of the genome, with many homologous genes
• They get the same diseases as humans and have a compatible immune system
• They are mammals, having reproductive and breeding physiology similar to humans
• Mice are inexpensive to maintain and small and easy to handle
• Mice are fast and prolific breeders. Thus, a large number of inbred animals are easily available for reproducible and defined research experiments
• Mouse genetics, physiology, anatomy, and behavior are well studied and documented with a large number of supporting reagents and resources
• The mouse genome has been completely sequenced and is easy to manipulate, thus, providing opportunities for a wide variety of genetic modifications

Getting Started

If you are interested in taking your in vitro experiments further to explore in vivo methodologies, here are some basic steps to find your way to begin animal work:

1.  Mouse Training and Certification

Federal authorities regulate animal research in most countries. You must obtain permission, training, and certification before beginning animal work. Follow institutional guidelines and policies to get the right certification and training in handling mice and understanding the ethics related to animal research.

2.  Get Your Mice

You can obtain mice either from a commercial animal supplier or from a collaborating laboratory. The advantage of obtaining mice from commercial suppliers is that the background, genetics, and phenotype of the mice are clearly stated, and the mice come with a clean bill of health. This makes it easy to import them into your animal facility. However, commercial mice can be expensive and options may be limited. If you use a commercial supplier to obtain mice, it is best to stick to that supplier for all your future requirements. Even though all mice of a particular inbred strain can be traced back to a common ancestor, small genetic drifts occur during breeding in different facilities that could lead to variability in results.

For those special mice with multiple genetic modifications or already crossed into a unique genetic background, contact collaborators to obtain a breeding pair or two. Beware: this could be disadvantageous since the mice may not be completely healthy and source and information may be incomplete or untrustworthy.

3.  Importing Mice

If you are importing mice, contact your facilities director or animal technicians. Every institute sets strict rules about importing animals from the outside. Commercial suppliers provide detailed health reports that test for various common infectious agents, and the mice are generally of an acceptable health quality to be imported immediately for use. Mice from non-commercial facilities also need health status reports for the past few months and may need to be placed in quarantine at the recipient facility depending on presence, type, and severity of any infection. Give your mice an acclimatization period to adjust to their new surroundings before you start breeding or using them for experiments.

4.  Food and Water

Mouse food is of two types – chow or purified diets. Chow diets include rough pellets that are cereal based with added vitamins, minerals, and fat depending on nutritional requirement and manufacturer formula. They are inexpensive and easy to produce, but the exact composition is based on the cereals used in making them. This leads to variations in levels of nutritive components as well as non-nutritive components, like phytohormones.

On the other hand, purified ingredient diets are chemically defined diets in which each component is a purified single ingredient, thus, making them reliable and consistent. Purified diets are easy to modify as per research purposes, too. Diets rich in fat, cholesterol, certain nutrients or components are used in specific metabolism and nutrition research areas.

Provide mice with tap, acidified, or chlorinated water, which can be purified by reverse osmosis, distillation, or autoclaving. Water is provided through individual bottles or through a centralized supply system with water tubes and valves.

5.  Cage and  Bedding

Cage size standards are set by authorities and the number of mice per cage is fixed; anything above constitutes overcrowding. You can house female mice from different litters together to conserve space. In contrast, male mice only from the same litter will get along with each other. Give your mice easy access to food and water and soft bedding made of dust free wood chips, corncob or paper shavings. Additionally, provide mice with nesting material like soft tissue, paper boxes, cardboard rolls or egg cartons for comfort.

6.  Rooms and Housing

Room temperature, humidity, ventilation, and illumination need to meet regulatory standards. Small vibrations, noise, disturbances, or harsh light and temperature conditions affect mouse health, breeding performance, and behavior.  They can also affect experimental results. Commonly, a 12-hour day/night cycle is maintained. Cages are placed on racks or shelved in individual ventilation systems with a filtered air supply.

Keep cages, wire racks, lids, water bottles, valves, bedding and food pellets clean and autoclaved at all times. Change them at regular intervals to keep your mice healthy and comfortable

Most important of all – maintain detailed and meticulous records of your mice breedings, genotypes and mouse IDs. It is easy to mix them up, and let’s face it – they all look the same.