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Mice as Experimental Organisms

  1. Lee M Silver

Published Online: 19 APR 2001

DOI: 10.1038/npg.els.0002029

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Silver, L. M. 2001. Mice as Experimental Organisms. eLS. .

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  1. Princeton University, Princeton, New Jersey, USA

Publication History

  1. Published Online: 19 APR 2001

This is not the most recent version of the article. View current version (15 MAY 2012)

Research Utility of the Mouse

  1. Top of page
  2. Research Utility of the Mouse
  3. Life Cycle: Sexual Reproduction, Embryology and Development
  4. The Mouse Genome
  5. Manipulating the Mouse Genome
  6. Mouse Populations and Behaviour
  7. Further Reading

The common house mouse Mus musculus has played a prominent role in the study of genetics since the rebirth of the field at the beginning of the twentieth century. This birth occurred with the rediscovery of Mendel's laws in 1900. But this rediscovery, as well as Mendel's own research, was performed entirely on plants. As a consequence, there was initial scepticism in the scientific community as to whether Mendel's laws could explain the basis for inheritance in animals, and especially in human beings. The reason for this scepticism is easy to see. People, in particular, differ in the expression of many commonly inherited traits – such as skin colour, eye colour, curliness of hair, and height – that show no evidence of transmission according to Mendel's laws. We now understand that all of these traits are controlled by multiple genes that each individually segregate according to Mendel's first law, even though the ultimate trait that they control does not. But, at the beginning of the twentieth century, a demonstration of the applicability of Mendel's laws required the analysis of simple traits controlled by single genes. See also History of Classical Genetics, Mendel, Gregor Johann, and Quantitative Genetics

The house mouse has a long history of domestication as a pet, and over the centuries, mice with numerous coat colour and other easily visible mutations were selected and bred by dealers in the ‘fancy mouse’ trade, first in China and Japan, and later in Europe. In contrast to the variation that occurs naturally in wild populations, new traits that appear suddenly in captive-bred animals are always the result of single gene mutations. Early animal geneticists appreciated the importance of the genetic resource available within the ‘fancy mice’ and these animals were quickly put to use to demonstrate the applicability of Mendel's laws to mammals, and by extrapolation, to humans as well. See also Mutations and New Variation: Overview, and Evolution during Domestication

Beyond the readily available fancy mouse mutations, there are a number of other compelling reasons why the house mouse has continued to represent the mammal of choice for genetic analysis. Mice have very short generation times of just 8 to 9 weeks, they are small enough so that thousands can be housed in relatively small rooms, they have large litters of eight or more pups, they breed readily in captivity, fathers do not harm their young, and after centuries of artifical selection, they are docile and easily handled.

But why study a mammal at all when animals like the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans are even smaller and much more amenable to genetic analysis? The answer is that a significant portion of biological research is aimed at understanding ourselves as human beings. And although many features of human biology, especially at the cell and molecular level, are shared across a broad spectrum of life, our most advanced organismal-level characteristics are shared in a much more limited fashion with other animals. In particular, many aspects of human development and disease are common only to placenta-bearing mammals such as the mouse. See also Drosophila as an Experimental Organism for Functional Genomics, Caenorhabditis elegans as an Experimental Organism, and Experimental Organisms Used in Genetics

Life Cycle: Sexual Reproduction, Embryology and Development

  1. Top of page
  2. Research Utility of the Mouse
  3. Life Cycle: Sexual Reproduction, Embryology and Development
  4. The Mouse Genome
  5. Manipulating the Mouse Genome
  6. Mouse Populations and Behaviour
  7. Further Reading

Life cycle

The life cycle of the mouse is equivalent to the life cycle of human beings and all other placental mammals. It is only the timing that differs in each species. The average lifespan for humans is ∼78 years, while for mice, it is only 2 years. The period from fertilization to birth is 8.9 months in duration for humans, and only 21 days long for mice. After birth, humans must mature for another 12 years or so before they reach puberty, when they are able to conceive children of their own (although in our society, they usually wait for double that time). In comparison, mice reach puberty in just 5 to 6 weeks – 100 times sooner than human beings. Thus, it is possible to go from the birth of one mouse to the birth of its offspring in just 8 weeks. But, even with these very large differences in timing, the details of each stage of the developmental programme, before and after birth, are remarkably similar, and the overview presented below applies equally to each species. See also Reproduction in Mammals: General Overview

Male germ cell development

Once a male has reached puberty, haploid germ cells are produced continuously in large numbers for the rest of his life. The mature haploid cell is called a sperm cell or spermatozoan, and the process by which it is produced is called spermatogenesis. Spermatogenesis takes place in the seminiferous tubules, which are jumbled up like cooked spaghetti inside each testis. Packed along the outer circumference of each seminiferous tubule are large numbers of diploid spermatogonia that act as a self-renewing source of stem cells. With each spermatogonial division, one daughter cell is retained as a stem cell, while the other is sent along the pathway of spermatogenesis. This pathway progresses through four expanding rounds of mitosis, followed by meiosis, which leads to the production of immature haploid spermatids. Each spermatid matures into a single spermatozoan that is released into the lumen at the centre of the seminiferous tubule to join millions of other spermatozoa. These pass through countless passageways to reach the epididymis, where further maturation steps occur, and finally to the vas deferens, where they wait until they are needed during copulation. See also Mammalian Sex Determination

Female germ cell development

The production of haploid germ cells in the female follows a very different course. Unlike the male, a female is born with all of the haploid cells that she will ever have (∼50 000 in the mouse and one million in women). The mature haploid cell is called an egg or oocyte, and the process by which it is produced is called oogenesis. Oogenesis begins inside the newly formed ovaries of the developing fetus. Long before birth, primordial germ cells differentiate into oogonia and enter meiosis, but stop at the diplotene stage of the first meiotic prophase. These primary oocytes remain arrested in suspended animation – for weeks in mice and many years in human females – until after the time of puberty.

From this time on, the female will progress through an oestrous cycle with a ∼4 day period in mice and a ∼28 day period in women. During each cycle, primary oocytes (one in women and 8–10 in mice) are stimulated to continue the process of differentiation. Differentiation leads to the completion of the first meiotic division and the extrusion of the first polar body. The second meiotic division is begun, but stops at metaphase. The mature secondary oocyte is now released from the ovary, in a process called ovulation, and passes into an oviduct (known as a Fallopian tube in human females). For a brief period of time, known as oestrus, each mature oocyte, or egg, remains alive and receptive to fertilization. Most wild mammals die while they still have the ability to reproduce. Human females, however, usually live long enough to pass through a stage called menopause when they stop cycling through oestrus and are no longer able to reproduce. See also Reproduction in Eutherian Mammals

Fertilization

Just before and during the oestrus phase of the oestrous cycle in wild animals, females release species-specific chemical signals – called pheromones – that stimulate the interest of males in the sexual process. In response to these pheromones, a male will copulate with a female and ejaculate semen containing millions and millions of sperm into her reproductive tract. Sperm will swim from the vagina into the uterus and up the oviducts. Only one hundred or fewer sperm will actually survive this journey to the waiting eggs. See also Mammalian Pheromones

Fertilization is a multistep process. First, sperm must bind to the zona pellucida, which is a thick solid shell, made of glycoproteins, that surrounds the egg proper. In the process of binding to the zona pellucida, the sperm is induced to release special proteases that provide it with the ability to ‘burn’ its way through the zona pellucida into the space that surrounds the egg membrane. Although multiple sperm can make it into this space, only one can fuse with the egg. Fusion between egg and sperm causes rapid electrochemical changes in the egg membrane that prevent the entry of additional sperm. The fusion event also ‘activates’ the newly fertilized egg to move down the pathway of animal development. See also Sperm–Egg Interactions: Sperm–Egg Binding in Mammals

Early embryonic development is highly plastic

For the purposes of scientific analysis, mouse development is divided into two distinct stages of unequal length that are separated by the moment of implantation into the uterus. During the preimplantation phase, which lasts 4.5 days, the embryo is a free-floating object within the mother's body. Because it is naturally free-floating, the preimplantation embryo can be removed easily from its mother's body and cultured in a Petri dish. Here it can undergo genetic manipulation before it is placed back into a female where it can continue along the developmental path to a newborn animal. Once the embryo has undergone implantation, it can no longer be removed from its mother's body and remain viable. The accessibility of the preimplantation embryo provides the basis for a number of specialized genetic tools that are used to study mammalian development, as described in the next sections. See also Mouse Early Development: Molecular Basis

The preimplantation phase starts with the zygote (the special name for the one-cell fertilized egg or embryo) at the time of conception. Development begins slowly with the first 22 hours devoted to the expansion of the highly compacted sperm head into a paternal pronucleus that matches the size of the original egg (maternal) pronucleus. Once this process is completed, the embryo undergoes the first of four equal divisions, or cleavages, that increase the number of cells, over a period of 60 hours, from one to sixteen. See also Cleavage and Gastrulation in Mouse Embryos

Throughout this period, known as the cleavage stage, all of the cells in the developing embryo are equivalent and totipotent. The word totipotent is used to describe a cell that has not yet undergone differentiation, and still retains the ability, or potency, to produce every cell type present in the developing embryo and adult animal. As a consequence of totipotency, cleavage stage embryos can be broken into smaller groups of cells that each have the potential to develop into individual animals. The outcome of this process can be observed in humans with the birth of identical twins or, much more rarely, identical triplets. In the laboratory, scientists have obtained completely normal mice from individual cells that were dissected out of the four-cell-stage mouse embryo and placed back into the female reproductive tract. This experimental feat demonstrates the theoretical possibility of obtaining four identical clones from a single embryo of any mammalian species. See also Whole Animal Cloning

It is important to contrast the early developmental programme of all placental mammals with that of other animals including the two model organisms Caenorhabditis elegans and Drosophila melanogaster. Identical twins can never be obtained from a single nematode or fly embryo. During nematode development, individual embryonic cells from the two-cell stage onward are highly restricted in their developmental potential or ‘fate’. The fly egg is polarized even before it is fertilized and different cytoplasmic regions are devoted to supporting different developmental programmes within the nuclei that end up in these locations. Thus, half a nematode embryo or a half a fly embryo could never give rise to a whole animal. See also Caenorhabditis elegans Embryo: Determination of Somatic Cell Fate, and Twinning

Embryonic differentiation and postimplantation development

During the 16-cell stage of mouse embryogenesis, the first differentiating event occurs, and the developmental potency of individual cells finally becomes restricted. The cells on the outside of the embryo turn into a trophectoderm layer that will eventually take part in the formation of the placenta. Meanwhile, the cells on the inside compact into a small clump that remains attached to one spot along the inside of the trophectoderm sphere. This clump of cells is called, appropriately enough, ‘the inner cell mass’ or ICM. The fetus will develop entirely from the ICM. At this stage of development, the embryo is called a blastocyst. Two more rounds of cell division occur during the blastocyst stage before the embryo implants. See also Mouse Early Development: Molecular Basis

Throughout the process of normal preimplantation development, the embryo remains protected within the inert zona pellucida. Thus, there is no difference in size between the one-cell zygote and the 64-cell embryo. To accomplish implantation, the embryo must first ‘hatch’ from the zona pellucida, so that it can make direct membrane-to-membrane contact with the cells in the uterine wall. Implantation initiates the development of the placenta, which is a mixture of embryonic and maternal tissue that mediates the flow of nutrients, in one direction, and waste products, in the other direction, between the mother and embryo. The placenta maintains this intimate connection between mother and fetus until the time of birth. The process of internal uterine development is a unique characteristic of all mammals other than the primitive egg-laying platypus. See also Reproduction in Monotremes and Marsupials

With the development of the placenta, a period of rapid embryonic growth begins. Cells from the ICM differentiate into all three germ layers (endoderm, ectoderm and mesoderm) during a stage known as gastrulation. The foundation of the spinal cord is put into place, and the development of the various tissues and organs of the adult animal is initiated. With the appearance of organs, the embryo is now called a fetus. The fetus continues to grow rapidly in size and birth occurs at ∼21 days after conception. Newborn animals remain dependent upon their mothers during a suckling period which can last another 18 to 25 days. By 5 to 6 weeks after birth, mice have reached adulthood and are ready to begin the reproductive cycle all over again. See also Human Parturition and Birth: Regulation

The Mouse Genome

  1. Top of page
  2. Research Utility of the Mouse
  3. Life Cycle: Sexual Reproduction, Embryology and Development
  4. The Mouse Genome
  5. Manipulating the Mouse Genome
  6. Mouse Populations and Behaviour
  7. Further Reading

To modern geneticists, the most important feature of the mouse genome is its close resemblance to that of our own. The haploid genomes of humans and mice (as well as all other placental mammals) contain approximately 3 billion base pairs of DNA and nearly every human gene has a corresponding homologue in the mouse genome. This is not to say that the two genomes are equivalent in content. But most differences appear to result from species-specific additions to gene families that already existed in the common ancestor to mice and humans. A further caveat to bear in mind is that humans and mice have been evolving apart for 75 million years, and during this time, some gene homologues may have become functionally distinct. In general, however, the mouse genome provides a powerful model system for investigating the genetic basis of both simple and complex human traits, especially those related to development and disease. See also Human Disease: Mouse Models, Genome Organization: Human, and Genome Evolution: Overview

The human genome is distributed among 22 autosomes and two sex chromosomes, while the mouse genome is contained within 19 autosomes and two sex chromosomes. When one examines and compares human and mouse karyotypes under the microscope, there is no evidence of any chromosome banding homologies between the two. But with the mapping of thousands of the homologous genes in both species, a remarkable pattern has emerged. Genes that are closely linked in one species are usually found to be closely linked in the other. When any two or more loci are found to be linked in one species, they are said to be syntenic. When the same set of loci are also found to be linked to each other in a second species, they are said to exist in a state of conserved synteny. When genetic maps that comprise the whole mouse genome are compared to genetic maps that comprise the whole human genome, it becomes apparent that regions of conserved synteny extend across nearly the complete lengths of both. The average size of each individually conserved syntenic region is approximately 15 megabases (Mb). The implication of this finding is that during the 75 million years that mice and humans have evolved apart from a common ancestor, their genomes have broken apart and become rearranged some 200 times (15 Mb × 200 = 3000 Mb = the size of the mammalian genome). Conversely, if the proper genome scale scissors and glue were available, one could break apart the mouse genome into ∼200 pieces and glue it back together again – like a puzzle – in the form of the human genome. See also Gene Mapping: Comparative, Genome Mapping, and Human Chromosomes

Conserved synteny has powerful evolutionary implications, but just as importantly, it serves as a critical tool for practising geneticists. Once a locus has been mapped in one species, a scientist can look at a homology map and immediately identify its likely map position in the other species. This trick is not very important for genes that have already been cloned, since one can always use DNA–DNA hybridization to pick out gene homologues directly from the other species. But, for loci that are characterized only by their phenotypic expression, conserved synteny provides geneticists with the ability to move back and forth between the analysis of a particular trait in humans and the analysis of models for the trait in mice. See also Plant Synteny, Colinearity and Genome Evolution, and Genetic and Physical Map Correlation

Manipulating the Mouse Genome

  1. Top of page
  2. Research Utility of the Mouse
  3. Life Cycle: Sexual Reproduction, Embryology and Development
  4. The Mouse Genome
  5. Manipulating the Mouse Genome
  6. Mouse Populations and Behaviour
  7. Further Reading

Nuclear injection to add genes to the embryonic genome

The 1981 development of a protocol for inserting foreign DNA into the germline of mice changed forever the complexion of mammalian genetics. A strictly observational science was suddenly thrust into the realm of genetic engineering with all of its vast implications. Yet, the incredibly powerful transgenic technology is based on a very simple process. To create a transgenic mouse – with a novel DNA sequence integrated into one of its chromosomes – a scientist simply injects the foreign DNA into a pronucleus of a one-cell embryo, and then places that embryo back into a female to allow development to proceed. Up to 50% of the mice born from injected embryos will have the foreign DNA stably integrated into their genomes and will transmit this DNA on to their children. See also Transgenic Animals, Nuclear Transfer from Cell Lines, and Microinjection into Xenopus Oocytes

There are no limits to the type of DNA that can be injected. It can come from any natural source – animal, plant or microbial – or directly from a DNA synthesizer. It is very common for investigators to ‘construct’ DNA molecules composed of parts taken from different sources. For example, a DNA construct could have a coding region that is a composite of human and Escherichia coli sequences flanked by an upstream regulatory region that is a composite of mouse and synthetic sequences. The transgenic technology can be used to explore many different aspects of mouse biology and gene regulation. The goals of most transgenic experiments can be classified in one of three categories: (1) determining the function of gene products, (2) characterizing genetic regulatory regions, or (3) establishing links between mutant phenotypes and particular transcription units. See also Transgenic Mice Production

Targeted mutagenesis: taking genes away

The embryonic nuclear injection technology just described represents just one-half of the repertoire of genetic engineering tools used by mouse geneticists for the analysis of gene function and regulation in the developing animal. While this form of transgenic technology is very powerful, it has two serious limitations. First, it can only be used to add – not subtract – genetic material. Second, the insertion of genetic material cannot be targeted to particular genomic locations. In genetic terms, this means that transgenic mice produced by embryonic nuclear injection are only useful for the analysis of dominant phenotypes.

By 1989, a second independent transgenic technology – known as gene targeting or targeted mutagenesis – was developed to circumvent these limitations. Targeted mutagenesis provides researchers with the ability to eliminate, or knockout, any cloned gene. The same technology can even be used to replace single amino acids or larger regions of a gene to obtain an allele with an altered function. This ultimate tool of genetic engineering can be used in experiments with two different kinds of objectives: first, as a means for determining gene function by examining the phenotypic consequences incurred in a developing embryo or animal that does not express a particular gene, or expresses an alternative form of that gene; and second, as a means for creating mouse models for human diseases, such as cystic fibrosis, that are caused by the loss of gene function. See also Mutagenesis: Site-Specific, Knockout and Knock-in Animals, and Mouse Knockouts: Modifying the Mouse Genome by using Embryonic Stem Cells

Mouse Populations and Behaviour

  1. Top of page
  2. Research Utility of the Mouse
  3. Life Cycle: Sexual Reproduction, Embryology and Development
  4. The Mouse Genome
  5. Manipulating the Mouse Genome
  6. Mouse Populations and Behaviour
  7. Further Reading

The house mouse, Mus musculus, can live in an extraordinary variety of different habitats. Commensal animals can live in all types of human-made structures including houses, buildings, barns, haystacks, ruins, and in coal mines, 1800 feet below the ground. But the possibilities are virtually unlimited – animals have been found in climates as different as frozen-food lockers and central heating ducts. Feral animals can live in agricultural fields, meadows and scrublands.

Mice can eat almost anything – cereals, grass, seeds, roots and stems of various plants, adult insects, and even larvae. Animals can also subsist with very little water, especially if their food is high in moisture content. In many locations, the morning dew can probably provide much of the daily water requirement. These traits provide the house mouse with great adaptability and have played an important role in their dispersion among many different habitats, both commensal and feral.

The paradigm population structure for animals living under commensal conditions is that of independent, relatively stable demes, or families. The classic deme will have a single dominant male who patrols a well-defined home range and sires most of the young; up to ten breeding female members of the deme will confine their own ranges to that of the single dominant male. Different dominant males will have mutually exclusive territories. Males will tolerate their own offspring, but will kill offspring born to females that belong to other demes. See also Population Structure

Under optimal environmental conditions with plenty of food and nesting material, commensal mice living inside temperature-controlled buildings can breed throughout the year. In strictly feral populations in temperate climates, breeding activity tends to be seasonal, from spring to early autumn. The average litter size has been found to vary from as few as three pups to as many as nine. Although mice from some laboratory lines can survive as long as 3 years, free-living wild animals are likely to die much earlier from disease, competition or predators.

The usual structure of feral populations may be very different from that of commensal populations in that animals living outdoors appear to move over much larger distances, and deme structures appear to be much less stable. However, the ability and desire of mice to migrate over long distances is complex and highly variable. Many animals appear to live their entire lives in very small and well-defined home ranges (defined as the area in which an animal spends the vast majority of its time) of less than 10 metres across. Others will move constantly over much larger distances, travelling kilometres daily, and some will migrate long distances between home ranges that are very small. All permutations are possible, and the distribution of animals in each class varies greatly among different populations. The lifestyle of the house mouse has been aptly described by Berry and Bronson (see Further Reading): ‘The house mouse is a weed: quick to exploit opportunity, and able to withstand local adversity…A consequence of the repeated formation of new populations by small numbers of founders is that every population is likely to be unique.’

The incredible adaptability of M. musculus to new environments can be accounted for almost entirely by the enormous plasticity that exists in its behavioural traits. In the case of nearly all other species, specific behaviours are highly defined by genomes, and adaptability to new environments can occur only slowly with changes in behaviour as well as physiology and/or morphology driven by natural selection. In contrast, the house mouse can disembark from ships in subantarctic islands or in equatorial Africa, and adapt immediately to survive and prosper. ‘To be introduced into a radically new environment is one thing; to be able to reproduce there and so to establish a new population is quite another. The planet-wide spread of the house mouse in both manmade and natural habitats suggests an extreme reproductive adaptability, probably the most extreme among the mammals.’ Only humans are as adaptable (some would say less so). See also Adaptation: Study, and Adaptation and Constraint: Overview

Thus, the defining characteristic of the species M. musculus is the decoupling of genetics and behaviour. At some point during evolution, the ancestral house mouse population broke away from its previous behavioural constraints and once this occurred, the success of the species was assured. With men and women as chauffeurs and guides, the global conquest of the house mouse began.

Further Reading

  1. Top of page
  2. Research Utility of the Mouse
  3. Life Cycle: Sexual Reproduction, Embryology and Development
  4. The Mouse Genome
  5. Manipulating the Mouse Genome
  6. Mouse Populations and Behaviour
  7. Further Reading
  • Berry RJ and Bronson FH (1992) Life history and bioeconomy of the house mouse. Biological Reviews of the Cambridge Philosophical Society 67: 519550.
  • Hogan B, Beddington R, Costantini F and Lacy E (1994) Manipulating the Mouse Embryo: A Laboratory Manual, 2nd edn. Cold Spring Harbor: Cold Spring Harbor Laboratory Press.
  • Lyon MF and Searle AG (1989) Genetic Variants and Strains of the Laboratory Mouse. Oxford: Oxford University Press.
  • Silver LM (1995) Mouse Genetics: Concepts and Application. New York: Oxford University Press.