Cell Growth and Differentiation

Model Organisms

Model organisms, such as the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the zebrafish Danio rerio, have given scientists many insights into the mechanisms that underlie embryological development.

A model organism is a species that is studied because of characteristics that make it easy to learn from and easy to apply that knowledge to other species. Model organisms are easy to maintain and breed in a laboratory setting and have a comparatively simple system of interest, such as a smaller genome or a simple nervous system. Several model organisms have been studied to gain a better understanding of genetics, animal development, neurobiology, and evolution. There are now hundreds of model organisms, ranging from bacteria to mammals, being used. By observing these species, scientists have gained knowledge that can be applied to other organisms, including humans.

Caenorhabditis elegans, or C. elegans, is a free-living soil nematode. It offers many advantages for many areas of study, such as genetics and development. It demonstrates a relatively short life cycle, a small size, a small genome, and the ability to self-fertilize. Furthermore, its body is transparent, and once scientists began investigating development in the worm, they learned that all individuals have the same number of cells and that all of those cells align in the same positions. These traits enabled scientists to map the paths of development of each cell in the animal's body, from conception to death.

Another common model organism is the fruit fly species in the genus Drosophila. The most common species studied is Drosophila melanogaster. Drosophila offers many of the advantages of C. elegans—short life cycle, small size, and small genome. In addition, Drosophila species lay eggs outside the body, which can then be genetically manipulated. Furthermore, many phenotypes in the fly are easily observed, including both physical and behavioral phenotypes. Much of modern understanding of the influence of genes on development is the result of studies of Drosophila.

The zebrafish, a type of small fish in the genus Danio, most commonly Danio rerio, offers similar advantages to those of C. elegans and Drosophila. The zebrafish has the additional advantage of being a vertebrate organism, offering insights into vertebrate development not possible with invertebrate model organisms. The embryo of the zebrafish is transparent and relatively large and can develop outside its mother. Additionally, many behaviors in zebrafish are associated with particular genes. Zebrafish are able to regenerate their heart and lateral line hair cells during embryonic development and also are able to regrow amputated limbs, such as fins. Further inquiry into these phenomena has given scientists a deeper understanding of stem cells and cell development.

Organism Genes Discovered Gene Function
C. elegans xbx Forms cilia involved in sensory reception
clk Controls timing of organism development
D. melanogaster hox Controls body plan along the head-tail axis
hedgehog Controls neuronal development
Danio sp. slc24a5 Controls skin pigmentation
ntl Controls limb development

Model organisms, such a C. elegans, Drosophila, and Danio (zebrafish), have been used to study development. Specific genes have been identified and matched with their roles in development. These studies have informed genetics research involving other organisms.