Laboratory mouse

History as a biological model

Mice have been used in biomedical research since the 16th Century when William Harvey used them for his studies on reproduction and blood circulation and Robert Hooke used them to investigate the biological consequences of an increase in air pressure. During the 18th Century Joseph Priestley and Antoine Lavoisier both used mice to study respiration. In the 19th Century Gregor Mendel carried out his early investigations of inheritance on mouse coat color but was asked by his superior to stop breeding in his cell "smelly creatures that, in addition, copulated and had sex". He then switched his investigations to peas but, as his observations were published in a somewhat obscure botanical journal, they were virtually ignored for over 35 years until they were rediscovered in the early 20th Century. In 1902 Lucien Cuénot published the results of his experiments using mice which showed that Mendel's laws of inheritance were also valid for animals — results that were soon confirmed and extended to other species.

In the early part of the 20th century Clarence Cook Little, a Harvard undergraduate was conducting studies on mouse genetics in the laboratory of William Ernest Castle. Little and Castle collaborated closely with Abbie Lathrop who was a breeder of fancy mice and rats which she marketed to rodent hobbyists and keepers of exotic pets, and later began selling in large numbers to scientific researchers. Together they generated the DBA (Dilute, Brown and non-Agouti) inbred mouse strain and initiated the systematic generation of inbred strains. The mouse has since been used extensively as a model organism and is associated with many important biological discoveries of the 20th and 21st Centuries.

The Jackson Laboratory in Bar Harbor, Maine is currently one of the world's largest suppliers of laboratory mice, at around 3 million mice a year. The laboratory is also the world's source for more than 8,000 strains of genetically defined mice and is home of the Mouse Genome Informatics database.

Reproduction

Breeding onset occurs at about 50 days of age in both females and males, although females may have their first estrus at 25–40 days. Mice are polyestrous and breed year round; ovulation is spontaneous. The duration of the estrous cycle is 4–5 days and lasts about 12 hours, occurring in the evening. Vaginal smears are useful in timed matings to determine the stage of the estrous cycle. Mating can be confirmed by the presence of a copulatory plug in the vagina up to 24 hours post-copulation. The presence of sperm on a vaginal smear is also a reliable indicator of mating.

The average gestation period is 20 days. A fertile postpartum estrus occurs 14–24 hours following parturition, and simultaneous lactation and gestation prolongs gestation by 3–10 days owing to delayed implantation. The average litter size is 10–12 during optimum production, but is highly strain-dependent. As a general rule, inbred mice tend to have longer gestation periods and smaller litters than outbred and hybrid mice. The young are called pups and weigh 0.5–1.5 g (0.018–0.053 oz) at birth, are hairless, and have closed eyelids and ears. Pups are weaned at 3 weeks of age when they weigh about 10–12 g (0.35–0.42 oz). If the female does not mate during the postpartum estrus, she resumes cycling 2–5 days post-weaning.

Newborn males are distinguished from newborn females by noting the greater anogenital distance and larger genital papilla in the male. This is best accomplished by lifting the tails of littermates and comparing perineums.

Genetics and strains

Mice are mammals of the Glires clade ( a group consisting of an ancestor and all its descendants), which means they are amongst the closest relatives of humans along with lagomorphs, treeshrews, flying lemurs and other Primates.

Laboratory mice are the same species as the house mouse, however, they are often very different in behaviour and physiology. There are hundreds of established inbred, outbred, and transgenic strains. A strain, in reference to rodents, is a group in which all members are as nearly as possible genetically identical. In laboratory mice, this is accomplished through inbreeding. By having this type of population, it is possible to conduct experiments on the roles of genes, or conduct experiments that exclude genetic variation as a factor. In contrast, outbred populations are used when identical genotypes are unnecessary or a population with genetic variation is required, and are usually referred to as stocks rather than strains. Over 400 standardized, inbred strains have been developed.

Most laboratory mice are hybrids of different subspecies, most commonly of Mus musculus domesticus and Mus musculus musculus. Laboratory mice can have a variety of coat colours, including agouti, black and albino. Many (but not all) laboratory strains are inbred. The different strains are identified with specific letter-digit combinations; for example C57BL/6 and BALB/c. The first such inbred strains were produced in 1909 by Clarence Cook Little, who was influential in promoting the mouse as a laboratory organism. In 2011, an estimated 83% of laboratory rodents supplied in the U.S. were C57BL/6 laboratory mice.

Genome

Sequencing of the laboratory mouse genome was completed in late 2002 using the C57BL/6 strain. This was only the second mammalian genome to be sequenced after humans. The haploid genome is about three billion base pairs long (3,000 Mb distributed over 20 chromosomes), therefore equal to the size of the human genome. Estimating the number of genes contained in the mouse genome is difficult, in part because the definition of a gene is still being debated and extended. The current count of primary coding genes in the laboratory mouse is 23,139. compared to an estimated 20,774 in humans.

Mutant and transgenic strains

Various mutant strains of mice have been created by a number of methods. A small selection from the many available strains includes -

Since 1998, it has been possible to clone mice from cells derived from adult animals.

Appearance and behaviour

Laboratory mice have retained many of the physical and behavioural characteristics of house mice, however, due to many generations of artificial selection some of these characteristics now vary markedly. Due to the large number of strains of laboratory mice, it is impractical to comprehensively describe the appearance and behaviour of all these, however, they are described below for two of the most commonly used strains.

C57BL/6

C57BL/6 mice have a dark brown, nearly black coat. They are more sensitive to noise and odours and are more likely to bite than the more docile laboratory strains such as BALB/c.

Group-housed C57BL/6 mice (and other strains) display barbering behaviour, in which the dominant mouse in a cage selectively removes hair from its subordinate cage mates. Mice that have been barbered extensively can have large bald patches on their bodies, commonly around the head, snout, and shoulders, although barbering may appear anywhere on the body. Both hair and vibrissae may be removed. Barbering is more frequently seen in female mice; male mice are more likely to display dominance through fighting.

C57BL/6 has several unusual characteristics which make it useful for some research studies but inappropriate for others: It is unusually sensitive to pain and to cold, and analgesic medications are less effective in this strain. Unlike most laboratory mouse strains, the C57BL/6 drinks alcoholic beverages voluntarily. It is more susceptible than average to morphine addiction, atherosclerosis, and age-related hearing loss.

BALB/c

BALB/c is an albino, laboratory-bred strain from which a number of common substrains are derived. With over 200 generations bred since 1920, BALB/c mice are distributed globally and are among the most widely used inbred strains used in animal experimentation.

BALB/c are noted for displaying high levels of anxiety and for being relatively resistant to diet-induced atherosclerosis, making them a useful model for cardiovascular research.

Male BALB/c mice are aggressive and will fight other males if housed together. However, the BALB/Lac substrain is much more docile. Most BALB/c mice substrains have a long reproductive life-span.

There are noted differences between different BALB/c substrains, though these are thought to be due to mutation rather than genetic contamination. The BALB/cWt is unusual in that 3% of progeny display true hermaphroditism.

Nutrition

In nature, mice are usually herbivores, consuming a wide range of fruit or grain. However, in laboratory studies it is usually necessary to avoid biological variation and to achieve this, laboratory mice are almost always fed only commercial pelleted mouse feed. Food intake is approximately 15 g (0.53 oz) per 100 g (3.5 oz) of body weight per day; water intake is approximately 15 ml (0.53 imp fl oz; 0.51 US fl oz) per 100 g of body weight per day.

Injection procedures

Routes of administration of injections in laboratory mice are mainly subcutaneous, intraperitoneal and intravenous. Intramuscular administration is not recommended due to small muscle mass. Intracerebral administration is also possible. Each route has a recommended injection site, approximate needle gauge and recommended maximum injected volume at a single time at one site, as given in the table below:

To facilitate intravenous injection into the tail, laboratory mice can be carefully warmed under heat lamps to vasodilate the vessels.

Anaesthesia

A common regimen for general anesthesia for the house mouse is ketamine (in the dose of 100 mg per kg body weight) plus xylazine (in the dose of 5–10 mg per kg), injected by the intraperitoneal route. It has a duration of effect of about 30 minutes.

Euthanasia

Approved procedures for euthanasia of laboratory mice include compressed CO₂ gas, injectable barbiturate anesthetics, inhalable anesthetics, such as Halothane, and physical methods, such as cervical dislocation and decapitation. In 2013, the American Veterinary Medical Association issued new guidelines for CO₂ induction, stating that a flow rate of 10% to 30% volume/min is optimal for euthanasing laboratory mice.

Pathogen susceptibility

A recent study detected a murine astrovirus in laboratory mice held at more than half of the US and Japanese institutes investigated. Murine astrovirus was found in nine mice strains, including NSG, NOD-SCID, NSG-3GS, C57BL6-Timp-3^−/−, uPA-NOG, B6J, ICR, Bash2, and BALB/C, with various degrees of prevalence. The pathogenicity of the murine astrovirus was not known.

United Kingdom

In the UK, as with all other vertebrates and some invertebrates, any scientific procedure which is likely to cause "pain, suffering, distress or lasting harm" is regulated by the Home Office under the Animals (Scientific Procedures) Act 1986. UK regulations are considered amongst the most comprehensive and rigorous in the world. Detailed data on the use of laboratory mice (and other species) in research in the UK are published each year. In the UK in 2013, there were a total of 3,077,115 regulated procedures on mice in scientific procedure establishments, licensed under the Act.

United States

In the US, laboratory mice are not regulated under the Animal Welfare Act administered by the USDA APHIS. However, the Public Health Service Act (PHS) as administered by the National Institutes of Health does offer a standard for their care and use. Compliance with the PHS is required for a research project to receive federal funding. PHS policy is administered by the Office of Laboratory Animal Welfare. Many academic research institutes seek accreditation voluntarily, often through the Association for Assessment and Accreditation of Laboratory Animal Care, which maintains the standards of care found within The Guide for the Care and Use of Laboratory Animals and the PHS policy. This accreditation is voluntary, not a prerequisite, for federal funding.

Limitations

While mice are by far the most widely used animals in biomedical research, recent studies have highlighted their limitations. For example, the utility of rodents in testing for sepsis, burns, inflammation, stroke, ALS, Alzheimer’s, diabetes, cancer, multiple sclerosis, Parkinson’s disease and other illnesses has been called into question by a number of researchers. Regarding experiments on mice, some researchers have complained that “years and billions of dollars have been wasted following false leads” as a result of a preoccupation with the use of these animals in studies.

An article in The Scientist notes, “The difficulties associated with using animal models for human disease result from the metabolic, anatomic, and cellular differences between humans and other creatures, but the problems go even deeper than that” including issues with the design and execution of the tests themselves.

For example, researchers have found that many mice in laboratories are obese from excess food and minimal exercise which alters their physiology and drug metabolism. Many laboratory animals, including mice, are chronically stressed which can also negatively affect research outcomes and the ability to accurately extrapolate findings to humans. Researchers have also noted that many studies involving mice are poorly designed, leading to questionable findings.

Some studies suggests that inadequate published data in Animal testing may result in irreproducible research, with missing details about how experiments are done are omitted from published papers or differences in testing that may introduce bias. Examples of hidden bias include a 2014 study from McGill University in Montreal, Canada which suggests that mice handled by men rather than women showed higher stress levels. Another study in 2016 suggested that gut microbiomes in mice may have an impact upon scientific research.