Model organism

A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms.^[1][2] Model organisms are widely used to research human disease when human experimentation would be unfeasible or unethical.^[3] This strategy is made possible by the common descent of all living organisms, and the conservation of metabolic and developmental pathways and genetic material over the course of evolution.^[4]

Research using animal models has been central to most of the achievements of modern medicine.^[5][6][7] It has contributed most of the basic knowledge in fields such as human physiology and biochemistry, and has played significant roles in fields such as neuroscience and infectious disease.^[8][9] The results have included the near-eradication of polio and the development of organ transplantation, and have benefited both humans and animals.^[5][10] From 1910 to 1927, Thomas Hunt Morgan's work with the fruit fly Drosophila melanogaster identified chromosomes as the vector of inheritance for genes,^[11][12] and Eric Kandel wrote that Morgan's discoveries "helped transform biology into an experimental science".^[13] Research in model organisms led to further medical advances, such as the production of the diphtheria antitoxin^[14][15] and the 1922 discovery of insulin^[16] and its use in treating diabetes, which had previously meant death.^[17] Modern general anaesthetics such as halothane were also developed through studies on model organisms, and are necessary for modern, complex surgical operations.^[18] Other 20th-century medical advances and treatments that relied on research performed in animals include organ transplant techniques,^{[19][20][21][22]} the heart-lung machine,^[23] antibiotics,^[24][25][26] and the whooping cough vaccine.^[27]

In researching human disease, model organisms allow for better understanding the disease process without the added risk of harming an actual human. The species of the model organism is usually chosen so that it reacts to disease or its treatment in a way that resembles human physiology, even though care must be taken when generalizing from one organism to another.^[28] However, many drugs, treatments and cures for human diseases are developed in part with the guidance of animal models.^[29][30] Treatments for animal diseases have also been developed, including for rabies,^[31] anthrax,^[31] glanders,^[31] feline immunodeficiency virus (FIV),^[32] tuberculosis,^[31] Texas cattle fever,^[31] classical swine fever (hog cholera),^[31] heartworm, and other parasitic infections.^[33] Animal experimentation continues to be required for biomedical research,^[34] and is used with the aim of solving medical problems such as Alzheimer's disease,^[35] AIDS,^[36] multiple sclerosis,^[37] spinal cord injury, many headaches,^[38] and other conditions in which there is no useful in vitro model system available.

Model organisms are drawn from all three domains of life, as well as viruses. One of the first model systems for molecular biology was the bacterium Escherichia coli (E. coli), a common constituent of the human digestive system. The mouse (Mus musculus) has been used extensively as a model organism and is associated with many important biological discoveries of the 20th and 21st centuries.^[39] Other examples include baker's yeast (Saccharomyces cerevisiae), the T4 phage virus, the fruit fly Drosophila melanogaster, the flowering plant Arabidopsis thaliana, and guinea pigs (Cavia porcellus). Several of the bacterial viruses (bacteriophage) that infect E. coli also have been very useful for the study of gene structure and gene regulation (e.g. phages Lambda and T4).^[40] Disease models are divided into three categories: homologous animals have the same causes, symptoms and treatment options as would humans who have the same disease, isomorphic animals share the same symptoms and treatments, and predictive models are similar to a particular human disease in only a couple of aspects, but are useful in isolating and making predictions about mechanisms of a set of disease features.^[41]