Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1146 Chapter 21: Development of Multicellular Organisms
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.
wild type
misexpression of
Eyeless/Pax6 in
wing precursors
misexpression of
Eyeless/Pax6 in leg precursors
(A) (B) (C)
wild type
misexpression of Drosophila
Eyeless/Pax6 in leg precursors
(D) (E) (F)
100 µm
50 µm
misexpression of
squid 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.)