served as models for intensively investigating the molecular mechanisms of these and other functions unique to eukaryotic cells, such as the cdk-cyclin-based cell cycle, mitosis, meiosis, ribonucleic acid (RNA) splicing, regulation of chromatin structure, secretion, dynamics of the cytoskeleton, stress pathways, checkpoint pathways, and, to some degree, intercellular signaling and differentiation, the last two associated with yeast mating. Most of these cellular functions have been highly conserved during eukaryotic evolution, so that knowledge gained from yeast research is directly applicable to understanding human cell processes. However, understanding the interactions of cells and tissues in development and physiology of higher eukaryotes requires study of metazoans (i.e., multicellular animals). It should be appreciated, though, that as the processes are understood in metazoa, the components of each process can be introduced into yeast and the individual processes reconstituted there for further detailed study. For example, it has been found that a number of human cell-cycle proteins function well in the yeast cell cycle, when replacing the yeast cell’s components.

Utility of Model Animals

Much has been learned about human development and physiology through the study of model animals, a small set of diverse metazoans that have particular advantages for laboratory research. There are several reasons for their utility. Research on humans and other primates is expensive and limited by ethical considerations. The most commonly studied model animals are relatively inexpensive to maintain and are well suited for experimental manipulation. Most important, as outlined in Chapter 6, recent research has shown that there is a remarkable degree of similarity in the developmental mechanisms of all animals. Not only individual genes and proteins but also entire pathways of signaling and response and their functions in developing embryos appear highly conserved throughout evolution. This means that, although the embryology of simpler animals might appear superficially very different from that of humans, knowledge gained from those models can often be applied directly to understanding human developmental mechanisms.

On the other hand, there are important developmental and physiological attributes that can be investigated only in vertebrates, such as the adaptive immune system, or in mammals, such as placentation and lactation. Therefore, it is useful to study a representative range of model animals—from invertebrates that are only distantly related to humans but have particular experimental advantages, to rodents and other mammals that are less convenient but more closely related to humans.

Model Animals for Study of Development

For study of development, the currently most intensively investigated model animals, in order of increasing complexity, are the free-living soil roundworm

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