Chapter 6: Model Organisms in Development

Rather than studying these animals solely for their own sake, scientists utilize the "big six"—the mouse, chick, African clawed frog (Xenopus), zebrafish, fruit fly (Drosophila), and nematode (C. elegans)—as representative systems to uncover universal biological principles, often with the ultimate goal of understanding human development. The selection of these specific models is based on several practical and objective criteria, including year-round availability, the feasibility of laboratory domestication, and the ease of producing large quantities of embryos. While invertebrate models like C. elegans and Drosophila offer low costs and highly sophisticated genetic tools due to their rapid life cycles, vertebrate models provide varying levels of experimental access. For instance, the external fertilization of zebrafish and Xenopus allows for direct observation and microsurgical manipulation, whereas the internal development of the mouse makes early-stage access and continuous culture more challenging. Despite the high maintenance costs and slower developmental rates of mammals and birds, they remain vital due to their status as amniotes, offering closer parallels to human physiology, gene expression domains, and disease. The text also highlights the significance of modern genomic sequencing, which has simplified gene cloning across all six species by providing complete inventories of gene families. Furthermore, the chapter touches upon "ancillary" organisms such as sea urchins, which are classic models for fertilization and regulation, and planarians or urodeles that provide unique insights into regeneration and evolutionary history. Ultimately, these models are chosen because any developmental features shared among them are likely shared by the entire animal kingdom.