Laboratory mouse
The laboratory mouse or lab mouse is a small mammal of the order Rodentia which is bred and used for scientific research. Laboratory mice are usually of the species Mus musculus. They are the most commonly used mammalian research model and are used for research in genetics, psychology, medicine and other scientific disciplines. Mice belong to the Euarchontoglires clade, which includes humans. This close relationship, the associated high homology with humans, their ease of maintenance and handling, and their high reproduction rate, make mice particularly suitable models for human-oriented research. The laboratory mouse genome has been sequenced and many mouse genes have human homologues.
Other mouse species sometimes used in laboratory research include the American white-footed mouse and the deer mouse.
History as a biological model
Mice have been used in biomedical research since the 17th 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, Harvard undergraduate Clarence Cook Little 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 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 at birth, are hairless, and have closed eyelids and ears. Pups are weaned at 3 weeks of age when they weigh about. 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 perinea.
Genetics and strains
Mice are mammals of the clade Euarchontoglires, which means they are amongst the closest non-primate relatives of humans along with lagomorphs, treeshrews, and flying lemurs.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 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, 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 -- Mice resulting from ordinary breeding and inbreeding:
- * Non-obese diabetic mice, which develop diabetes mellitus type 1.
- * Murphy Roths large mice, with unusual regenerative capacities
- * Waltzing mice, which walk in a circular pattern due to a mutation adversely affecting their inner ears
- * Immunodeficient nude mice, lacking hair and a thymus: these mice do not produce T lymphocytes; therefore, they do not mount cellular immune responses. They are used for research in immunology and transplantation.
- * Severe combined immunodeficiency mice, with an almost completely defective immune system
- * FVB mice, whose large litter sizes and large oocyte pronuclei expedite use in genetic research
- Transgenic mice, with foreign genes inserted into their genome:
- * Abnormally large mice, with an inserted rat growth hormone gene
- * Oncomice, with an activated oncogene, so as to significantly increase the incidence of cancer
- * Doogie mice, with enhanced NMDA receptor function, resulting in improved memory and learning
- Knockout mice, where a specific gene was made inoperable by a technique known as gene knockout: the purpose is to study the function of the gene's product or to simulate a human disease
- * Fat mice, prone to obesity due to a carboxypeptidase E deficiency
- * Strong muscular mice, with a disabled myostatin gene, nicknamed "mighty mice".
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 of them; 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 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.
Husbandry
Handling
Traditionally, laboratory mice have been picked up by the base of the tail. However, recent research has shown that this type of handling increases anxiety and aversive behaviour. Instead, handling mice using a tunnel or cupped hands is advocated. In behavioural tests, tail-handled mice show less willingness to explore and to investigate test stimuli, as opposed to tunnel-handled mice which readily explore and show robust responses to test stimuli.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 per of body weight per day; water intake is approximately per 100 g of body weight per day.Injection procedures
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:| Route | Recommended site | Needle gauge | Maximal volume |
| subcutaneous | dorsum, between scapula | 25-26 ga | 2-3 ml |
| intraperitoneal | left lower quadrant | 25-27 ga | 2-3 ml |
| intravenous | lateral tail vein | 27-28 ga | 0.2 ml |
| intramuscular | hindlimb, caudal thigh | 26-27 ga | 0.05 ml |
| intracerebral | cranium | 27 ga | - |
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 plus xylazine, 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 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 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.Legislation in research
United Kingdom
In the U.K., 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 Act 1986. U.K. regulations are considered amongst the most comprehensive and rigorous in the world. Detailed data on the use of laboratory mice in research in the U.K. are published each year. In the U.K. 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 U.S., laboratory mice are not regulated under the Animal Welfare Act administered by the USDA APHIS. However, the Public Health Service Act 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, however, not a prerequisite for federal funding, unlike the actual compliance.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 disease, 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.Mice differ from humans in several immune properties: mice are more resistant to some toxins than humans; have a lower total neutrophil fraction in the blood, a lower neutrophil enzymatic capacity, lower activity of the complement system, and a different set of pentraxins involved in the inflammatory process; and lack genes for important components of the immune system, such as IL-8, IL-37, TLR10, ICAM-3, etc. Laboratory mice reared in specific-pathogen-free conditions usually have a rather immature immune system with a deficit of memory T cells. These mice may have limited diversity of the microbiota, which directly affects the immune system and the development of pathological conditions. Moreover, persistent virus infections are activated in humans, but not in SPF mice with septic complications and may change the resistance to bacterial coinfections. “Dirty” mice are possibly better suitable for mimicking human pathologies. In addition, inbred mouse strains are used in the overwhelming majority of studies, while the human population is heterogeneous, pointing to the importance of studies in interstrain hybrid, outbred, and nonlinear mice.
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 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.