MUS MUSCULUS
MORPHOLOGY
Musmusculus, commonly known as house mice, originated in Asia, probably in India, and then spread throughout Europe and Africa and to the rest of the world. It belongs to family Muridae.
Most laboratory mice are hybrids of different sub-species, which include, Musmusculusdomesticus and Musmusculusmusculus. The long history of breeding, together with their short life-spans and breeding cycles has made mice particularly useful for studying mammalian genetics.
House mice are 6-9 cm long, with 6-10 cm long tail. Colour varies from light brown to black with lighter shades on the ventral side. Weight ranges from 12 to 30 grams. Many lab forms of mice have been developed that vary in color from white to black and with spots. Lab mice and pet mice are often white, with ears and tail having little hair, weighing 12-40 gms and length 15-19 cms long.
Young males and females are not easily distinguished; Males have testicles which are usually hairless and can be retracted into the body. Females have 5 pairs of mammary glands with nipples.
BIOLOGY
Mus musculus breeds throughout the year, although wild mice may have a reproductive season extending from April to September. Males court females by emitting characteristic ultrasonic calls in the 30kHz-110 kHz range. Following copulation, female mice develop a vaginal plug that prevents further copulation. The estrous cycle is 4-6 days long. Females generally have 5-10 litters per year if conditions are suitable. Gestation period is 19-21 days. Litters consist of 3-12 offspring, which are born naked and blind. They are fully furred after 10 days, open their eyes at 14 days, are weaned at 3 weeks, and reach sexual maturity at 5-7 weeks. Average life span is about 2 years in captivity, but individuals have lived for as long as 6 years. In the wild, most mice do not live beyond 12-18 months.
SPECIAL FEATURES
Mice are convenient in research because their physiology is similar to that of humans and their short life cycle makes breeding easy. They are mainly used to model human diseases, to develop new drugs and to test the safety of proposed drugs.
Mice are the most commonly used animal research models with hundreds of established inbred, outbred, and transgenic strains. They are common experimental animals in the studies in biology, behaviour and psychology because they are mammals, and thus share a high degree of homology with humans.
It consists of a complex of several separate lineages and nearly 3000 different strains of natural mutants, and transgenic and targeted mutant mice.
The mouse genome has been sequenced, and virtually all mouse genes have human homologs. The mouse genome is about the same size as the human genome, and the organization of genes is strikingly similar. Mouse genome sequencing indicates that mice and humans share about 95 percent DNA sequencing similarity. This means that any gene in humans is likely to have an identical or very similar counterpart in the mouse genome.
Mice can also be experimented with in ways that would be considered unethical for humans.
Mice are small, inexpensive, easy to maintain and have high reproductive rate. Several generations of mice can be completed in a relatively short time. They become docile if raised from birth in captivity and given sufficient human contact.
Mice possess different coat colours that includes agouti, black and albino, which helps in making genetic studies. Many laboratory strains are inbred to make them genetically almost identical. The different strains are identified by a letter-digit combination, for example, C57BL/6 and BALB/c. The first such inbred strains were produced by Clarence Cook Little in 1909, who promoted mouse as a laboratory organism.
The behavioural patterns of laboratory mice are significantly different from those of most common house mice and wild races due to years of lab breeding. They are docile and can be handled easily.
Besides primates, such as monkeys and chimpanzees, which are costly as well as protected animals, the most closely related animal to humans is the mouse, Musmusculus.
GENOME
Sequencing of the mouse genome was completed in late 2002. The haploid genome is about 3 billion bases long (3000 Mb distributed over 20 chromosomes) and therefore equal to the size of the human genome. The current estimated gene count of mice is 23,786 while humans are estimated to have 23,686 genes, which is strikingly similar.
CONTRIBUTION OF SCIENCE
Mice were first used for genetics research by the French biologist Lucien Cuénot in 1902.
Research on the mammalian genome is now being conducted by using mice. Much of this research is aimed at genes and DNA sequences and documenting the phenotypic expression of genes.
A technology has been developed to enable engineering of knockoutstrains in mice, in which a single known gene has been selectively deleted from the genome of every cell. For human disease genes, knocking out the homologous gene in mice can provide an excellent model system for studying the disease. The knockout mouse may show disease conditions similar to that of human disease. Learning how the elimination of the gene in the mouse contributes to the growth of mouse disease may give important clues about the involvement of homologous genes in humans. The disease model mouse strains are now available for cancer, Alzheimer’sdisease, arthritis, diabetes, heartdisease, cystic fibrosis, and obesity.
The autosomal variation and the genetic control of GPI have been determined by a comparison of electrophoretic patterns of F1 and backcross progeny of three inbred strains of mice.
Construction of a small library of recombinant plasmids containing Musmusculus repetitive DNA inserts has been completed.
A precision stereotaxic procedure for mouse brain research has been developed. A new design in mouse stereotaxic head holder and a new device used to guarantee accurate alignment of the skull in the stereotaxic device has been designed.