Organisms with large-celled embryos that develop outside the mother’s body (e.g., frogs, sea urchins, fish, and chickens) are extremely useful for tracing the fates of cells as they form different tissues, as well as for making extracts for biochemical studies. For instance, a key protein in regulating cell division in all eukaryotes, including humans, was first identified in studies with sea stars and sea urchin embryos and subsequently purified from extracts prepared from these embryos (see Chapter 19).

Studies of cells in specialized tissues make use of animal and plant model organisms. Neurons and muscle cells, for instance, were traditionally studied in mammals or in creatures with especially large or accessible cells, such as the giant neural cells of the squid and sea hare or cells in the flight muscles of birds. More recently, muscle and nerve development have been extensively studied in fruit flies (Drosophila melanogaster), roundworms (Caenorhabditis elegans), and zebrafish (Danio rerio), in which mutations in genes required for muscle and nerve formation or function can be readily isolated (see Figure 1-22).

Mice have one enormous advantage over other experimental organisms: they are the most closely related to humans of any animal for which powerful genetic approaches have been available for many years. Mice and humans have shared living structures for millennia, have similar nervous systems, have similar immune systems, and are subject to infection by many of the same pathogens. As noted, both organisms contain about the same number of genes, and about 99 percent of mouse protein-coding genes have homologs in the human genome, and vice versa.

Using recombinant DNA techniques developed in the past few years, researchers can inactivate any desired gene, and thus abolish production of its encoded protein. Such specific mutations can be introduced into the genomes of worms, flies, frogs, sea urchins, chickens, mice, a variety of plants, and other organisms, permitting the effects of these mutations to be assessed. Using the Cas9 experimental system described in Chapter 6, this approach is being used extensively to produce animal versions of human genetic diseases, in mice as well as in other animals. As an example, people with autism spectrum disorder often have mutations in specific protein-coding genes. To understand the role of these mutations, these genes have been inactivated in mice; in many cases, the mice exhibit symptoms of the human disease, including repetitive actions such as excessive grooming, strongly suggesting that the human mutation indeed has a role in triggering the disorder. Within the past year, similar techniques have been used to produce monkeys in which the targeted gene has been inactivated. Such approaches can be useful in uncovering the role of specific genes in higher-order brain tasks such as learning and memory, or in studies of viruses that infect only humans and nonhuman primates. Once animal models of a human disease are available, further studies on the molecular defects causing the disease can be done and new treatments can be tested, thereby minimizing the testing of new drugs on humans.