At each stage in its development, the cell is presented with a limited set of options, so that its developmental pathway branches repeatedly, reflecting a large set of sequential choices. Like the decisions we make in our own lives, the choices made by the cell are based on its internal state—which largely reflects its history—and on current influences from other cells, especially its close neighbors. To understand development, we need to know how each choice is controlled and how it depends on previous choices. Beyond that, we need to understand how the choices, once made, influence the cell’s chemistry and behavior, and how cell behaviors act synergistically to determine the structure and function of the body.
Figure 21–1 The four essential cell processes that allow a multicellular organism to be made.
As cells become specialized they change not only their chemistry but also their shape and their attachments to other cells and to the extracellular matrix. They move and rearrange themselves to create the complex architecture of the body, with all its tissues and organs, each structured precisely and defined in size. To understand this process of form generation, or morphogenesis, we will need to take account of the mechanical, as well as the biochemical, interactions between the cells.
At first glance, one would no more expect the worm, the flea, the eagle, and the giant squid all to be generated by the same developmental mechanisms than one would suppose that the same methods were used to make a shoe and an airplane. Remarkably, however, research in the past 30 years has revealed that much of the basic machinery of development is essentially the same in all animals—not just in all vertebrates, but in all the major phyla of invertebrates too. Recognizably similar, evolutionarily related molecules define the specialized animal cell types, mark the differences between body regions, and help create the animal body pattern. Homologous proteins are often functionally interchangeable between very different species. Thus, a human protein produced artificially in a fly, for example, can perform the same function as the fly’s own version of that protein (Figure 21–2). Thanks to an underlying unity of mechanisms, developmental biologists have been making great strides toward a coherent understanding of animal development.
We begin this chapter with an overview of some of the basic mechanisms that operate in animal development. We then discuss, in sequence, how cells in the embryo diversify to form patterns in space, how the timing of developmental events is controlled, how cell movements contribute to morphogenesis, and how the size of an animal is regulated. We end by considering the most challenging aspect of development—the mechanisms that enable a highly complex nervous system to form.
misexpression of Eyeless/Pax6 in misexpression of wild type wing precursors Eyeless/Pax6 in leg precursors
Figure 21–2 Homologous proteins can function interchangeably. (a–c) the eyeless protein (also called pax6) controls eye development in Drosophila and, when misexpressed during development, can cause an eye to form in an abnormal site, such as a wing (B) or a leg (c). the scanning electron micrographs show a patch of eye tissue on the leg of a fly resulting from misexpression of Drosophila Eyeless (e) and of squid Pax6 (F). the homologous protein from a human or practically any animal possessing eyes, when similarly misexpressed in a transgenic fly, has the same effect. the entire eye of a normal Drosophila is shown for comparison in (a) and (D). (B–c, courtesy of Georg halder; D–F, from S. I. tomarev, et al. Proc. Natl Acad. Sci. USA 94:2421–2426, 1997. With permission from National academy of Sciences.)